United StatesDepartment ofAgriculture

Forest ServiceFinger-Jointed

ForestProductsLaboratory Wood ProductsResearchPaperFPL 382

April 1981

A B S T R A C T

An overview of the literature onfinger jointing of wood products in-dicates the basic information requiredto produce strong, durable fingerjoints is available. With existinglimitations, the finger joints describedare the best that can be produced.Between-joint variability Is seen as amajor production problem. Develop-ment of a nondestructive test methodwill make possible evaluation of alljoints; proof loading is the onlyevaluation method now available.Development of new adhesivesystems and bonding techniques aswell as developments in machiningcould affect manufacturing methodsand improve joint performance.

United StatesDepartment ofAgriculture

Forest Service

ForestProductsLaboratory1

ResearchPaperFPL 382

Finger-JointedWood Products

BYRONALD W. JOKERST

INTRODUCTION

Splicing two pieces of woodtogether endwise has always beenchallenging and at times difficult.Wood exhibits its greatest strengthparallel to the grain; development ofend joints that can transmit a signifi-cant proportion of this strength hasbeen the goal of many research pro-grams. The problem is that wood can-not be bonded sufficiently well, endgrain to end grain with existingadhesives and techniques to be ofany practical importance. However,wood can be bonded quite effectivelywith most adhesives side grain toside grain and generally quite easily.Thus, the approach historically hasbeen to modify ends of pieces to bejoined in a manner so that adhesivejoints are primarily side grain; at thesame time the bond area is sufficient-ly increased so that the total load ajoint withstands in shear approachesthe load it can withstand in tension.

Many types of end joints have beendesigned, tried, and discarded (62).2

Some joint forms were too difficult tomake, too difficult to bond, or mostoften did not prove effective. Foryears the standard of comparisonwas, and to some extent still is, theplain scarf joint. This joint is formed

1 Maintained at Madison, Wis., in cooperationwith the University of Wisconsin.

2 Italicized numbers in parentheses refer toLiterature Cited at the end of this report.

by cutting a slope, or incline, usuallythrough wood thickness-thus expos-ing wood that approaches side gram.Considerable work has been done onthis type of end joint. Joints withslopes of 1 in 10 or 1 in 12 werefound to attain tensile strengthsequal to 85 to 90 percent of thestrength of clear wood. At a slope of1 in 20, the average was approximate-ly 95 percent of the strength of clearwood in tension (50, 92).

Scarf joints, however, also haveproblems. First, they are wasteful ofwood; in joining two pieces of1-1/2-inch-thick wood with a joint hav-ing a slope of 1 in 10, about 15 inchesof length are lost. Secondly, the ac-curacy at which the scarf is machinedand the alinement and bonding of thetwo surfaces are also critical in deter-mining how well joints will perform.Third, under production-line condi-tions maintaining necessary accuracyto form consistently good scarf jointshas proved difficult; thus, perfor-mance can be quite variable (27).These factors have resulted in adecline in use of plain scarf jointsand they are being replaced by thefinger joint.

The finger joint is not a new type ofend joint; it has been used for manyyears. In the literature finger jointsare mentioned as being used in theautomotive industry in wood steeringwheels and spokes of wood wheels.Figure 1, an automobile steeringwheel of the midtwenties, contains

four finger joints. In 1955 Norman (76)discussed finger jointing of small cut-tings, or scrap, for use in cabinetsand door and window units; he statedin 1947 his company went tomelamine adhesive and high-frequency curing to get away fromcold clamping and the dark color ofresorcinol. This indicates the com-pany had been making finger jointsprior to 1947.

The first reference found in whichfinger joints were used in a structuralapplication was by Egner and Jagfeldof the Otto Graf Institute in Stuttgart,Germany (30). They discussed resultsof tests on finger-jointed bridgemembers after 10 years of use in abridge constructed in the early for-ties. Since that time the use of fingerjoints has steadily increased in bothstructural and nonstructural uses.

Although finger joints have beendescribed in a variety of ways,basically they are a modification ofthe plain scarf joint. They are madeup of a series of short scarfs,sometimes separated by a bluntfingertip. In some finger joints a fold-ed scarf joint could more exactlydescribe their configuration (19). Afinger joint is diagramed and labeledin figure 2.

Classifying finger joints as struc-tural and nonstructural is based onintended use and to the extentgeometry affects ability of a joint totransmit stress, on its shape or ap-pearance. Nonstructural finger joints

generally are short with blunt tips;structural joints generally are longerwith relatively sharp tips.

Nonstructural finger joints are usedif strength is not a primary concern.They are used to join pieces ofvarious lengths end grain to end grainfrom which natural, but unwanteddefects have been removed and tojoin short lengths of material intolengths long enough to be useful (14,36, 38, 59, 124). Nonstructural fingerjoints are primarily found in moldingstock, trim, siding, fascia boards,door stiles and rails, window frames,and similar millwork material (23, 74).

If strength is the primary objectivestructural finger joints are used.These joints may be used in struc-tural dimension lumber, and for end-jointing laminae for large, laminatedbeams in which the length of thebeam may exceed the length ofavailable lumber by several times.Finger joints may also be used toupgrade lumber by removing defectsthat limit the grade of the lumber;then the defect-free pieces are finger-jointed back together (67).

End-jointed structural lumber 2 in-ches or less in nominal thickness andup to 12 inches in width is acceptedfor use interchangeably with lumbernot end jointed by the InternationalConference of Building Officialsunder Research RecommendationsReport 1837 (47); by the Building Of-ficials Conference of America underResearch and Approvals CommitteeReport 339 (13); by the Southern Stan-dards Building Code (106); and by theFederal Housing Administration underUse of Materials Bulletin UM-51a (34).This acceptance is subject to thematerial having been manufacturedunder a program of structural lumberend-joint certification and quality con-trol and shown to be in complianceby a grade stamp containing the markof a recognized grading associationor inspection bureau (35). The cer-tification and quality-control pro-grams of these organizations areclosely alined with that outlined inthe appropriate sections of U.S.Department of Commerce ProductStandard PS 56-73 for StructuralGlued-Laminated Timber (121).

Figure 1.�Wood Automobile steering wheel from mid-20�s contains four fingerjoints.

(M 147 757)

DESIGN OF

FINGER JOINTS

The geometry of a finger joint large-ly dictates potential strength of ajoint (fig. 2). The elements that

describe the geometry are so relatedthat changing any one elementautomatically changes another. Thisinterrelationship of the elements of ajoint complicates investigating the ef-fect on strength of any one element.

The effects of joint geometry onstrength have been investigated anddiscussed by several authors;generally their findings agree (6, 77,79, 88, 103). All authors indicate the

importance of keeping finger tips asthin as practical to obtain maximumstrength. Two primary reasons forthis are indicated: (1) blunt tips arebutt joints incapable of transmittingstress, and (2) finger tips introduceabrupt changes in section that causestress concentrations that, in turn,result in lower than expected loads atfailure.

In a comprehensive study of the ef-

2

Figure 2.�Finger joint and identifying nomenclature.

(M 123 727)

feet of joint geometry on tensilestrength of finger joints, Selbo (103)concluded that:

1. Finger joints in general gave in-creased tensile strength withdecreasing slopes, but rate ofincrease decreased as slopedecreased. Gain in strength wasvery small as slope was de-creased from 1 in 12 to 1 in 16.

2. With slope and tip thicknessheld constant, joint strength in-creased with increase in pitch,but at a decreasing rate.

3. Correlation was good betweenjoint strength and effective (orsloping) glue-joint area. This in-dicated that, to obtain high

strength, fingers must be suffi-ciently long and slope sufficient-ly low to provide an effectiveglue-joint area large enough towithstand a shear load ap-proaching tensile strength of anuncut or net effective section.

4. If the first three conditions weremet, tip thickness became thedeciding factor for jointstrength. The thinner the tip, thehigher the strength.

5. Stress developed in the net sec-tion of a finger joint (total sec-tion minus area of fingertips)did not greatly depend on slopeof fingers in a range of 1 in 10to 1 in 16 but did depend onsloping joint area or ratio of

finger length to pitch (L/P), reaching a maximum for L/P greater than about 4. This max-imum stress in Sitka spruce wasapproximately 17 percent lessthan the strength of the material(probably caused by stress con-centrations at tips). Thus, thestrength of a finger jointdepends on the area of the netsection and the strength of thescarf joints in the net section.

6. The data Indicated conclusivelythin joint tips (thinner than inthis study) will developsignificantly higher strength,and if maximum strength isneeded, such as in tension or inbending, as thin a tip as prac-tical should be used.

In related work in Australia, Page(79), stated bending and tension testson joints of constant pitch and tipthickness indicated that, at slopessteeper than 1 in 8, small reductionsin slope produced marked increasesin strength. Beyond 1 in 8 or 1 in 9further reduction of slope resulted inonly slightly stronger joints. Reducingthe slope from 1 in 4 to 1 in 6 in-creased strength 50 percent and fur-ther reduction to 1 in 8 added 20 per-cent, whereas slopes of from 1 in 10to 1 in 16 produced the same strengthvalues-all about 75 percent strongerthan 1 in 4. Page was increasingfinger length and increasing the UPas discussed by Selbo (103).

Page (79) also said �...as the widthof the fingertip is increased, both ten-sile and bending strength are re-duced. This effect becomes lesssevere as the slope is reduced.� In-creasing the tip width, he notedcauses a greater reduction instrength at a slope of 1 in 8 than at aslope of 1 in 16. This is to be ex-pected because increasing the tipthickness on a joint with a 1 in 8slope will reduce finger length further;thus the effective glue-joint area isdeceased more than in a joint with aslope of 1 in 16.

In much of the early work on endjoints, and in some countries even yet(118), the strength of joints is com-pared to the strength of similar clearmaterial stressed in the same mannerto evaluate the joint potential; it iscalled joint efficiency. This approachhas been used with all types of endjoints. Plain scarf joints have beenevaluated extensively on this basis.With a slope of about 1 to 20, a flatenough scarf can attain tensile

3

strengths of about 95 percent thoseof clear, unjointed material. Rajcanand Kozelouh (88) developed a systemusing the joint efficiency datadeveloped for scarf joints as a basisfor predicting strength of fingerjoints. With their method, joint effi-ciency of a scarf joint of the sameslope was reduced by a factor ob-tained by dividing the area of thecross section in fingertips by the areaof the cross section of the piece tobe joined. The authors state �...if theefficiency of a simple scarf joint isknown, the geometric slope of a jointof required efficiency can be deter-mined.� Conversely, from thegeometry of a joint it is possible tocalculate the efficiency or permissiblestress for the joint in relation to clear,unjointed wood.

Pavlov (81) was among the first toinvestigate the effect of jointgeometry on strength. He found that,with a tip of about 0.012 inch and apitch of 0.315 inch, slopes of 1 in 8through 1 in 16 were about equal instrength. If he increased tip thicknessto about 0.078 inch, slopes had to be1 in 14 and 1 in 16 to maintainstrength at the same level.

At a slope of 1 in 8 increasing tipthickness from 0.012 inch to 0.078inch reduced finger length, and the ef-fective bond area by about 50 per-cent. To increase bond area to obtainan equivalent stress level, the slopemust be reduced to 1 in 14 or 1 in 16.

Richards (94) also investigated ef-fect of tip thickness on tensilestrength of finger joints. He found bycutting the joints so that the male tiphad a feathered edge and by cold-forming the female tip with a wedge,tensile strength was increased by anaverage of 47 percent. This increasewas greater than would have beenpredicted based on the change ingeometry; a large portion of the in-crease could be attributed to a reduc-tion in stress concentration at thetips.

Richard�s research (94) indicatessharp-tipped fingers have a signifi-cant positive effect on strength.However, with conventional finger-joint cutters there are practical limita-tions on how thin tips can be. If tipsof knives in cutting heads are toothin, they rapidly overheat. Thisresults in either permanent damageto the knives or dulling so that theymust be resharpened. Thus, most pro-files for conventional finger joints arethe thinnest tips commensurate witha practical volume of production.

Several systems have been devel-oped specifically for producing afinger joint with a sharp tip. Examplesof these are Strickler�s impressionjoint (108), a system marketed byWadkins in England (122), andMarian�s Mini-Joint (64, 65).

The impression joint is formed witha heated die or a combination ofpreliminary forming with a sawfollowed by a final forming with aheated die. Although the jointsformed are strong and economicalbased on wood loss, the system isconsidered too slow for many produc-tion applications (60, 110).

The Wadkins system (122) is also adie-forming process, but the dies arenot heated. This joint is not con-sidered suitable for structural use;because of the force required to im-press the cold die into the wood, it islimited to low-density softwoods (15,122).

The Mini-Joint system (64, 65),unlike the other two, cuts the fingersusing a series of special saw blades,mounted one for each finger, on anarbor. The arbor is offset to the longaxis of the pieces being jointed byone-half the pitch of the fingers. Thisarrangement allows both halves ofthe joint to be formed in a singlepass. The adhesive is then applied,and the two halves forced togetherwith high pressure and held about 2seconds. The frictional forcesdeveloped when the joints are forcedtogether are reported sufficient tohold the joints so that they can behandled carefully; the adhesive com-pletes its cure at room temperature.

In addition to the effects of jointgeometry on strength, other factorsmight also be classified as jointdesign. Conventionally made struc-tural joints must be cut to fit togetherproperly. The fit is proper if the tipsof the fingers do not quite �bottomout� when pressure is applied. Thisensures sufficient pressure for goodbonding is being applied to theglueline.

If appearance is of primary concernfor bonding conventionally formednonstructural joints, a joint should fittightly together with no gaps at thetips or elsewhere.

A factor that reportedly can affectperformance is orientation of a fingerjoint relative to width and thicknessof a piece (26, 74, 83, 111, 118) (fig. 3). Vertical finger joints to be stressed inbending or tension are said to per-form better than horizontal fingerjoints. The reasoning is that, with the

profile of the joint on the edge, thetwo outer fingers carry most of theload and their integrity is very criticalto the performance of the joint. Withthe profile on the wide face thestresses are more evenly distributedacross all of the fingers of the joint. Itis not uncommon in finger jointing toapply only end pressure. If this isdone the outer fingers tend to spreadout; the result is thick gluelines andlow-strength joints at face or edge. Asthese weak joints appear on the outersurfaces of the edges or faces theyresult in areas of high stress concen-trations, and the reduction in strengthis greater than would be expectedbased on the reduction in bond areaalone. This problem can be reducedeither by applying lateral pressure atthe time of bonding or by machiningoff the poorly bonded outer fingers.Pellerin (82) found the tensilestrengths for horizontal and verticaljoints were almost equal. It is possi-ble that, in carefully made fingerjoints, differences in strength due toorientation are not significant (3).

A finger joint has been developedin Finland with the primary purposeof reducing the problems discussedabove. This joint is formed by cuttingfingers at an angle of 45° to theplane of the board (fig. 3). Thus no.thin, flexible fingers are at the sur-faces. All fingers are sufficiently rigidto resist spreading, and strong bondsare obtained throughout the jointwithout applying lateral pressure.Roth (95) claims these joints performsignificantly better in bending andsimilarly in tension to vertical orhorizontal joints.

MANUFACTURE OF

FINGER JOINTS

Five steps are basic in manufactur-ing finger-jointed wood products: (1)Selection and preparation of material,(2) formation of joint profile (3) ap-plication of adhesive, (4) assembly ofjoint, and (5) curing of adhesive. Thereare variations within these steps,depending on the particular systemused, but all are necessary and impor-tant in producing good joints.

Even under the most favorable con-ditions, strength of a finger joint willbe lower than strength of clear wood.Therefore, manufacturers should payparticular attention to controllablefactors to prevent additional un-necessary strength losses, or equallyimportant, higher than expectedvariability in strength between joints.

4

Figure 3.�Orientations of finger joints.

(M 149 050)

Selection and Preparationof Material

Developing quality finger jointsnecessarily begins with selection andpreparation of material to be end-jointed. No adhesive joint can bestronger than the wood being bonded.Therefore, if strength is of primaryconcern, only material with potentialto develop needed strength should beselected. Instead of listing all thedefects and abnormalities to avoid, itis probably best to say that the jointprofile must be in normal, clear,straight-grained wood of average orhigh density. In most grading rulesand specifications, definite limita-tions are placed on how close a knotor other strength-reducing defects

can be to a finger joint. These limita-tions reduce the interactions ofdefects with the joints, and keepknots far enough removed from ajoint so that the associated graindeviations will not cause problems.

Wood to be used for finger jointsnormally should be dried to amoisture content (MC) suitable forgluing. The usual recommendation isto dry the wood to about the averageMC it will attain in service. Satisfac-tory adhesion is obtained with mostadhesives if wood is at an MC be-tween 6 and 17 percent. With someadhesives it is possible to go beyondthis range (49), with the precise upperand lower limits varying with theadhesive type and formulation (45).Satisfactory joints have been made

experimentally with resorcinol-typeadhesives with woods at or abovefiber saturation point, and with caseinglues at very low MC. The usual prob-lem encountered with gluing wood athigh MC is that the adhesive is ab-sorbed by the wood, and results in astarved joint. Wood at a high MC orthat contains wet pockets can alsocause problems if radio-frequency(RF) energy is used to cure theadhesive. If MC is too high, energy isexpended to heat the water instead ofthe glue, and the result might beundercured gluelines.

Several attempts have been madeto develop systems for finger-jointinggreen or high MC wood. Then shortscould be salvaged and some lumberproduced could be upgraded (78). Theshort lengths could be upgraded bycutting out grade-limiting defects andend-jointing the pieces back together.if this could be done before drying, itwould be much more attractiveeconomically than if done after drying(57, 58).

The systems reported in theliterature for end-jointing high MCwood (above 20 pct) have one featurein common-they all employ heat insome manner to accelerate adhesivecure. Both Raknes (90) and Currier(20) used heated presses, Strickler(109) used heat retained in the jointfrom the heated forming die, andChow (16) heated joints in an oven for10 minutes at 150° C. All four authorsreported excellent joint strength anddurability. Heat apparently ac-celerates adhesive cure, thus reducesexcessive penetration of the resin in-to the wet wood. In both Strickler�sand Chow�s systems it is quite prob-able the MC at the immediate sur-faces being bonded is actually quitelow in relation to the remainder of thepiece being joined. This, plus the ac-celerated cure, would increase pro-bability of success.

Forming Finger-Joint Profile

Before actually forming a joint, onelast preparatory step remains�squaring the ends. If lumber is roughsawed this last step must be pre-ceded by machining at least one edgeto serve as a reference plane. Squar-ing of ends is usually done by a trimsaw just before a piece reaches ajoint-forming head. The piece isalready clamped into position for cut-ting the joint, and the location of thetrim saw is important because itdetermines the length of the fingers.

5

If a trim saw is not properly located,fingers will be too short or too long(77). If fingers are too long, they willbottom out when pressure is applied,and good contact between adjacentsurfaces will not be obtained. Theresult could then be a thick, weakglueline. If fingers are too short, therewill be gaps at tips of the fingers,possible splitting at roots of thefingers, or maybe excessivesqueezeout of the adhesive. In jointsin which appearance is a primaryneed the gaps could cause rejection.

Essentially three methods are usedto form a profile of a finger joint. Themost common method employs cut-ting tools, the second uses dies, andthe third combines both cutting toolsand dies.

The cutting tool is normally arevolving head made up of a series ofstacked knives, shaped saw blades,or a head that holds replaceable bits.The knives, blades, or bits are groundto the desired shape of the joint. Thesize and the number of cutters in ahead depend on the geometry of ajoint and the thickness or width ofmaterial being joined.

If stacked knives are used to formstructural end joints, the tolerancesin the knives and their assembly inthe head is extremely critical. A fewthousandths of an inch variation canresult in poor fitting, low-strengthjoints.

Die-formed joints are produced byforcing squared ends of pieces ofwood against a metal die the shapeof a desired joint. The die may or maynot be heated. In die forming, thespecific gravity (SG) of a wood is alimiting factor in sizes and shapespossible (108). The cell wallsubstance of woody material has anSG of about 1.5; this represents theupper limit of compressibility. If theSG of a wood is 0.5, ordinary pressuremethods can reduce the volume toabout one-third of normal. The higherthe SG, the less a wood can be com-pressed without seriously weakeningfibers. This then limits size andgeometry of joints that can be formedwith only dies. Because of size limita-tions, joints made in this manner areconsidered suitable for only nonstruc-tural uses.

To make a joint suitable for struc-tural uses and still retain some of theadvantages claimed for die-formedjoints, joint must be preshaped by us-ing a cutting tool. This preformingoperation removes a portion of thematerial so that the remaining

material can then be shaped with thedie to the desired geometry. This two-step method effectively removes themajor limitations on sizes and shapespossible with die forming.

Application of Adhesives

Several approaches are available toapply adhesive to a surface of a joint.The simplest and least precisemethod is dipping a joint into a con-tainer of adhesives. However, mostfinger-jointing operations are moresophisticated than this and make useof some type of mechanical ap-plicator. A common type is a revolv-ing metal drum with a surface havingthe same profile as the finger joint.The surface of the drum is con-tinuously coated with fresh adhesive.Then as the joint passes the revolvingdrum, the adhesive is wiped onto thejoint surface. A variation of this is touse a shaped brush that revolves andwipes the adhesive onto joint surface.

A second type of applicator is astationary extruder nozzle, alsoshaped with a matching profile of ajoint. As the joint passes the nozzle, aquantity of adhesive is extruded ontothe joint surfaces.

None of the systems is completelysatisfactory. The control of theamount of adhesive applied is notalways precise, and the nozzles andspreaders may become clogged withdebris, requiring frequent checkingand maintenance.

A method used by Strickler (108) inhis impression-joint system wassuspending a piece of tissue paperthat had been dipped in adhesive be-tween the two joint halves just beforethey were pressed together. No infor-mation was found in the literature toindicate whether this method wasever tried on joints other than the im-pression joints.

Another method described in theliterature is the separate applicationof hardener and resin by spraying.This system allows highly reactiveadhesive systems to be used thatmight otherwise have a too short aworking life to be useful (33).

Assembly of Joints

After an adhesive has been applied,the next step is alining the joint andapplying bonding or mating pressure.Again, several systems are availableby which pressure can be applied. Inone system, the required pressure is

applied by having the infeedmechanism in the bonding or curingarea move at a faster rate than theoutfeed end of the line. This is com-monly called a crowder. The crowdersystem is often used if material beingjoined continues to move as it passesthrough an RF curing tunnel.

In another system a stationaryclamp and a movable clamp are used.The stationary clamp grabs onto apiece on one side of the joint andholds it in place while the movableclamp grips the piece on the oppositeside of the joint and moves forward,forcing the two halves together. Thissystem, sometimes called a stop- andgo-system, also is used with RF cur-ing, but here the joint is stopped andheld in position between the elec-trodes of the RF generator.

in the Mini-Joint system, thepressure applied is high and, after be-ing held for 2 or 3 seconds, it isreleased. The frictional forcesdeveloped between the two halvessuffices to hold them together andallow normal handling while theadhesive cures at ambient conditions(64).

Regardless of how pressure is ap-plied it is important it be of sufficientmagnitude to force the two halvestightly together and the pieces be pro-perly alined while the adhesive cures(100).

Based on the author�s findings inthe literature, apparently there is adivergence of opinion on the amount,and to some degree the necessity, ofpressure in gluing finger joints.

Pavlov (81) noted tests have shownthat, because of friction developed incompression on lateral surfaces, afinger is restrained from longitudinalmovement. Therefore, when gluing itsuffices to apply an initial endpressure, but further curing of a piececan be done out of the press. The re-quired end pressure on a finger jointdepends on viscosity of glue and thequality of fitting of the fingers. Well-fitted fingers have shown a high jointstrength at insignificant endpressures. For close contact offingers, end pressure must be main-tained between 3 to 6 kilograms persquare centimeter (kg/cm2) (43 to 85lb/in.2). At smaller taper angles theend pressure can be lower; at largerangles, higher. Pavlov was workingwith joints that had a 0.012-inch tip, apitch of 0.315 inch, and slopes of 1 in8 through 1 in 16.

Madsen and Littleford (63), workingwith a joint with a finger length of

6

2-5/8 inches, a 1/2-inch pitch, and a3/64-inch tip thickness investigatedthe effect of load pressures from 0 to600 Ib/in.2 They found an end pressureof 400 lb/in.2 adequate to facilitatecuring and to develop optimum ten-sile strength. They used both caseinand phenol-resorcinol adhesives; thejoints tended toward production-lineaccuracy. They stated above 600lb/in.2 splitting was likely at the rootsof the fingers.

Cook (19) has noted the endpressure necessary to producereliable joints is very important. Hedid not offer any supporting data, butindicated the minimum pressure forsoftwoods is 300 Ib/in.2 Cook told ofthe impossibility of being veryspecific because of various factorsthat will affect the pressure that canbe applied, such as splitting at roots.He noted splitting results from move-ment of finger ends while machiningthe longer joints. A tip is wider than aroot and splitting occurs when a jointis forced together. With the smallerminiprofiles, a much higher endpressure is required, in the range of1,000 to 2,000 Ib/in.2 High endpressures do not cause splitting of aroot in minijoint.

The German specifications DIN68-140 (24) (October 1971) include thefollowing admissible minimums forpressures: 120 kg/cm2 (1,705 lb/in.2) forfinger lengths of 10 mm (0.394 in.),and 20 kg/cm2 (285 Ib/in.2) for fingerlengths of 60 mm (2.36 in.). in no caseshould pressure be less than 10kg/cm2 (142 Ib/in.2).

In a related but somewhat differentapproach to the effect of gluingpressure on finger-joint strength, Mur-phey and Rishel (73) investigated theeffect of glueline thickness onstrength of finger joints of yellow-poplar. They used a polyvinyl resin(PVA) adhesive, and the joint dimen-sions were: finger length, 0.375 inch;pitch, 0.131 inch; and tip thickness,0.005 inch. They controlled gluelinethickness by adjusting the cutters todevelop a shoulder onto which a shimof known thickness was placed. Threeshims, 0.02, 0.04, and 0.08 inch, wereused. A no-shim series was used toserve as controls. The joints weretested in bending with the jointsoriented both vertically and horizon-tally. The results indicate that con-trolling glueline thickness producedstronger joints in this test. With thejoints oriented vertically, the jointsmade with the 0.08-inch shim weresignificantly stronger than were the

others. if the joints were orientedhorizontally, all three controlledglueline thicknesses were significant-ly stronger than those of the controlgroup.

The information in the literature onamount and duration of pressure re-quired to form strong, well-bondedjoints is confusing and at times con-tradictory. The author concludes areason for this is the limited scope ofmuch of the work reported. Thesereports fail to give a complete pictureof the relationships of the manyvariables that can affect the requiredpressure.

Curing the Adhesive

The final step in manufacturingfinger-jointed wood products is curingthe adhesive. Most of the adhesivescommonly used in finger jointing willcure at room temperature (70° andabove). However, the time required tocure these adhesives can be greatlyreduced by adding heat. Thus, tospeed up production, most systemsfor finger jointing employ somemethod of supplying heat to theadhesive.

Some exceptions to the addition ofheat must be noted however. One isthe Mini-Joint system that does notsupply additional heat to the glueline.In the system, fingers are about 7.5 to15 mm long (0.3 to 0.6 in.) (18, 49, 65);fingers that are forced together undera high pressure, held from 2 to 3seconds, and the pressure released.The frictional forces developed in thejoint are high enough to hold the twohalves together well enough to allowlimited handling and machining whilethe adhesive continues to cure. It issaid advisable to stockpile thematerial for at least 8 hours beforeusing (116). Another method reportedin the literature is using a highly reac-tive adhesive system and applyingresin and hardener separately. Thistechnique prolongs pot life of thesystem that otherwise would be tooshort for practicality (33).

The heat necessary to accelerateadhesive cure can be supplied eitherbefore or after application of theadhesive to the joint (51). The use ofheat supplied before applying anadhesive is commonly called storedor residual heat gluing. Applying theadhesive to a preheated surfacelimits the amount of time allowable toget joints under pressure. The amountof heat energy available to cure anadhesive is also limited; this may

restrict the types of adhesives thatcan be used. With this approach caremust be taken not to expose joint sur-faces to a too-high temperature fortoo long or the wood will be per-manently damaged.

On the positive side, in certainsituations heat can be obtained atvery low cost. An example would beusing heat remaining after a joint hasbeen formed with a heated die.Another possibility would be takingadvantage of heat retained in woodafter drying.

Using residual or stored heat ap-parently can remove some of thelimitations related to wood MC at thetime of gluing. Both Strickler (109)and Chow (16) have shown thesesystems will effectively bond wood athigh MC�s.

Applying heat to a joint after apply-ing an adhesive is somewhat moreflexible than using the stored-heatmethod. The amount of heat energyavailable to cure a resin is not limitedas it is with preheated material.Allowable assembly time is alsomuch less sensitive; therefore, unex-pected minor delays will not result inreject material. However, the equip-ment required to apply heat after jointassembly is expensive to purchase,operate, and maintain.

Probably the most common methodof accelerating cure of adhesivebonds in finger joints is with RFheating. This method of rapidly curingadhesive joints has been in use formany years. Briefly, the joint to becured is placed in position betweentwo electrodes attached to an RFgenerator. The polarity of the elec-trodes is changing rapidly, commonlybetween 10 and 27 megahertz (MHz).Polar molecules in the field try toaline themselves with the rapidlychanging magnetic field, but are re-strained by internal frictional forces.The extent of these frictional forcesgoverns both rate and amount of heatgenerated. The stronger the field andthe higher the frequency, the higherthe rate of heating (29, 56, 91, 117).

The RF curing cycle depends onsuch factors as generator capacity,type of adhesive, glue-joint area, andarrangement of electrodes in relationto the glue joint. The level of MC inthe wood is also an important factor.The higher the MC, the more conduc-tive the wood; thus, more energy isdissipated throughout the wood in-stead of being concentrated at theglue joint (105).

Close control of the variables in a

gluing operation is required for suc-cessful RF curing. Accurate machin-ing, uniform MC in the wood, anduniform glue spread are highly impor-tant. Considerable work is reported inthe literature on RF curing ofadhesives (119, 120). Apparently prop-erly made RF-cured finger joints areas good as those made by any otherprocedure.

Most other systems for curing anadhesive involve some method of con-ductive heating to accelerate cure.Examples of these include heatedplatens, electrical resistance heaters,and infrared lamps. The primary prob-lem with them is that they are slow.In wood, a good insulator, conductiveheating rapidly decreases in efficien-cy as thickness of wood increases.Heating rate is approximately propor-tional to the square of the thickness;doubling thickness causes heatingtime to increase four times.

A D H E S I V E S

F O R B O N D I N G

F I N G E R J O I N T S

Any adhesive suitable for bondingwood technically could be used forbonding finger joints. However, cer-tain factors limit choices. Factorsthat may be considered are intendeduse of a product, mechanical andphysical properties of an adhesive,speed at which a bond must beformed, curing method, cost, andsometimes color of the adhesive. Inevery situation, not all of these areconsidered. Often one overriding fac-tor is limiting; then the choicebecomes automatic.

The intended end use will quicklyeliminate several adhesives. Whenkind of exposure and structural re-quirements are known, choice may belimited to perhaps three types ofadhesives. For example, if anadhesive must bond a finger joint fora structural member to be used in anexterior exposure, the choice islimited immediately to resorcinol,phenol-resorcinol, or melamine. Themechanical and physical properties ofan adhesive also have a strong bear-ing on where it can be used. Urearesins are excluded from structuralusage as are polyvinyls; urea resinsbecause they deteriorate if exposedto heat and humidity, and thepolyvinyls because their plasticbehavior permits creep under a sus-tained load. If the adhesive is to be

8

cured with RF energy, straightphenolics and some epoxies would beeliminated because they are not com-patible with this method of curing.Resorcinol and phenol-resorcinolmight not be suitable for use withfinger-jointed trim and moldingbecause their dark-red color wouldnot be desirable.

Adhesives most commonly used infinger-jointing wood products arephenol-resorcinol, resorcinol,melamine, melamine-urea, urea, andboth thermosetting and thermoplasticpolyvinyl adhesives (PVA�s). Themelamine-urea, urea, and the PVA�sare used only in nonstructural ap-plications

All of the adhesives mentioned aresynthetic resins, and can be dividedinto two categories, thermoplasticand thermosetting. The thermoplasticresins never harden permanently butsoften or melt when the temperatureis raised and harden again whencooled. This reversible hardening pro-cess involves no actual chemicalreaction. The thermosetting resins,however, undergo irreversiblechemical reactions either at room orat elevated temperatures to developtheir strength and durability. Afterthis reaction has occurred, the resincannot be dissolved or again meltedwithout degrading.

Most of the synthetic woodworkingadhesives in use set or cure bychemical reaction. Rate of curing, likethat of all chemical reactions,depends on temperature. Raisingglueline temperature speeds the rateof curing as well as the rate ofstrength development of a joint. Thisproperty is used to advantage in RFand other means of heating in high-speed production processes.

Resorcinol, phenol-resorcinol, andmelamine adhesives are not affectedby the commonly used preservativetreatments. They will also bondtreated wood, making it possible totreat the wood and then glue theassemblies. However, wood treatedwith creosote or pentachlorophenol inheavy oil are difficult to bond and fewif any end joints are made in lumberso treated.

Resorcinol Resins

Adhesives based on resorcinol-formaldehyde resins were first intro-duced in 1943; these resins bearmany resemblances to phenol resins.A principal difference is the greater

reactivity of resorcinol, which permitscuring at lower temperatures. Resor-cinols (105) are supplied in two com-ponents as a dark-reddish liquid resinwith a powdered, or at times with a li-quid, hardener. These adhesives cureat 70°F or higher but usually are notrecommended for use below 70°Fwith softwoods and generally requirehigher cure temperatures with densehardwoods. Straight resorcinol resinadhesives have storage lives of atleast a year at 70°F. Their workinglife is usually from 2 to 4 hours at7O°F.

Assembly periods are not toocritical on softwoods as long as theadhesive is still fluid when pressureis applied. On dense hardwoods theassembly period must be adjusted(usually extended) to give a ratherviscous glue at the time pressure isapplied. On dense hardwoodsassembly times that are too short willresult in starved joints.

Phenol-Resorcinol Resins

Phenol-resorcinols are modifica-tions of straight resorcinol resinadhesives produced by polymerizingthe two resins (phenol-formaldehydeand resorcinol-formaldehyde). Theprincipal advantage of the copolymerresins over straight resorcinol resinsis their significantly lower cost; theprice of phenol is much lower thanthat of resorcinol. This cost advan-tage apparently is achieved withoutany significant loss in joint perfor-mance. For wood gluing, the volumeof phenol-resorcinol used far exceedsthat of straight resorcinol. Propor-tions of the two resin components inthe copolymer generally are notrevealed by manufacturers. Like theircomponents, the copolymer resins aredark-reddish liquids, and are preparedfor use by adding powderedhardeners. The hardeners generallyconsist of paraformaldehyde andwalnut shell flour, mixed in equalparts by weight.

Phenol-resorcinol resins generallyhave shorter storage lives than dostraight resorcinol, at 70° F, usuallyless than 1 year. Many manufacturerscan adjust the reactivity of phenol-resorcinols to fit prevailingtemperature conditions; fast for coolweather, slower for warm, and stillslower for hot. Phenol-resorcinols canbe specifically formulated for use inRF curing.

Melamine Resins

Straight melamine, like straightresorcinol, is not used extensively infinger jointing, principally because ofhigh cost. In its uncatalyzed formmelamine requires relatively hightemperatures to cure, 240° to 260° F.The melamines that use hardeners orcatalysts will set much more rapidlyat lower temperatures. Indicationshave been, however, the catalyzedmelamines do not have the sameresistance to weathering as do theuncatalyzed.

Pure melamine resins are almostwhite, but adding fillers usuallymakes them a light-tan. Mostmelamine adhesives are marketed aspowders to be used by mixing withwater and preservative with aseparate hardener. As a group,melamine resin adhesives generallyhave a pot life of at least 8 hours at70° F, and tolerate rather long openand closed assembly periods.Melamine resins have been usedsucessfully with RF curing if adurable, colorless glueline is required.

Melamine-Urea Resins

Melamine-urea resins are a specialgroup of heat-curing adhesives pro-duced either by dry blending urea andmelamine resins or by blending thetwo separate resins in liquid solu-tions and then spray drying the mix-ture. Manufacturers supply the resinsas powders, prepared by adding waterand a catalyst. Reportedly, theadhesive produced by spray drying amixture of the two resins producessomewhat more durable bonds thandoes the adhesive produced by blend-ing the two powdered resins. Themost common combinations are saidto contain 40 to 50 percent by weightof melamine resin and 50 to 60 per-cent of urea resin on a solids basis.

In finger-jointing lumber for struc-tural laminated timbers, a 60:40melamine to urea ratio is used. Inthese operations they are generallycured with RF heating. These joints, ifproperly produced, are consideredadequate for normal dry interior ser-vice but are not recommended if long-term exterior or high-humidity ex-posures are involved.

Urea Resins

Urea formaldehyde resin adhesivescame on the market in the mid to late

1930�s. By using different types andamounts of catalyst, they can beformulated either for elevated-temperature or room-temperature cur-ing. They are compatible with variouslow-cost extenders or fillers, thusthey permit variations in both qualityand cost. Being light-colored or slight-ly tan, urea resins form a rather in-conspicuous glueline. But exposureto warm, humid surroundings leads todeterioration and eventual failure ofurea resin adhesive bonds.

Urea resins are generally marketedin liquid form (as water suspensions)where large-scale use is involved andshipping distances are not excessive.They are also marketed as powders,with or without the catalyst incor-porated. The powdered ureas areprepared for use by mixing with wateror with water and catalyst if thecatalyst is supplied separately. Ingeneral, powdered urea resins withseparate catalyst have longer storagelives than do the liquid urea resins orthe powdered types with catalyst in-corporated. Special formulations havebeen developed for use in RF curing.

Polyvinyl Resin Emulsions

Polyvinyl resin emulsions are ther-moplastic, softening if temperature israised to a particular level andhardening again when cooled. Theyare prepared by emulsion polymeriza-tion of vinyl acetate and othermonomers in water under controlledconditions. Because individual typesof monomers are not identified bymanufacturers, this adhesive group issimply referred to as PVA�s. Inemulsified form, the PVA�s aredispersed in water and have a con-sistency and nonvolatile contentgenerally comparable to thermoset-ting resin adhesives. PVA�s aremarketed as milky-white fluids for useat room temperature in the form sup-plied by manufacturers, normallywithout additives or separatehardeners.

The adhesive sets when the waterof the emulsion partially diffuses intothe wood and the emulsified resincoagulates. There is no apparentchemical curing reaction as occurs inthermosetting resins.

Setting is comparatively rapid atroom temperature, and for some con-struction it may be possible toremove the gluing pressure in 1/2hour or less.

Thermosettlng PolyvinylEmulsions

Thermosetting polyvinyl emulsions,also identified as catalyzed PVAemulsions and cross-linked PVA�s, arealso available. They are modified PVAemulsions and are more resistant toheat and moisture than are ordinaryPVA�s, particularly when cured atelevated temperatures.

Room-temperature cure of theseadhesives does not always developtheir full potential for resistance tocreep, heat and moisture. Even if theyare heat cured, their properties arenever quite equal those of resorcinolor phenol-resorcinol. These ther-mosetting emulsions are, however,markedly superior to ordinary PVA�sin resisting moisture, and they shouldperform well in most nonstructural in-terior and protected exterior uses.These resins can also be cured withRF energy.

E F F E C T O FF I N G E R J O I N T SI N S T R U C T U R A L

M E M B E R S

In much of the literature on fingerjoints, strength is expressed as apercentage of the strength of a pieceof clear, unjointed wood of the samespecies, and is referred to as joint ef-ficiency. The efficiency of a joint canvary, depending on species (11, 26, 42,89, 91), the quality of the wood,whether the stress applied is in ten-sion, bending, or compression, and insome cases, the orientation of thejoint.

Finger Joints inStructural Lumber

In 1941 Erickson investigated thestrength of finger-jointed 2 by 4�s tobe used as studs in light-frame con-struction (32). He did not include adescription of the joint used;however, the studs were tested instatic bending both edgewise andflatwise and in compression parallelto the grain. Before testing, thefinger-jointed 2 by 4�s were subjectedto several high and low humiditycycles; some were exposed to out-door weathering for 14 weeks.

Analyzing the data on these finger-jointed studs from a strength stand-point resulted in the following values:

9

Average efficiency in edgewise bend-ing was 59 percent (range from 49 to79 pct), in flatwise bending 33 percent(range 23 to 42 pct), and in compres-sion parallel to the grain 89 percent(range 80 to 100 pct).

Erickson (32) pointed out that, onthe basis of the largest knot admittedin a 2 by 4, the efficiency of theStandard Grade of coast-typeDouglas-fir in edgewise bending wasabout 20 percent. A 2 by 4 in the Con-struction Grade, which admits a1-1/2-inch knot, was 34 percent. Bycomparison, the lowest efficiency inedgewise bending found in this seriesof spliced studs after exposure toweather and large moisture changeswas 49 percent. Erickson concludedthat, except for flatwise bending, theeffect of the joint on the strength ofstuds as manufactured was less thanwas the effect of knots ordinarily per-mitted in material for this use. SinceErickson�s findings, the same conclu-sions have been reached by severalother authors.

Stanger (107) reported on sometests on finger-jointed 2 by 4�s ofradiata pine. Each piece containedone centrally located finger joint with1-1/16-inch-long fingers. They weretested in both flatwise and edgewisebending and in compression parallelto the grain. The average modulus ofrupture (MOR) of the edgewise testswas 58 percent and that in flatwisebending was about 50 percent that ofclear material. The joints tested incompression gave values about 90percent that for clear wood. Themodulus of elasticity (MOE) did notdiffer greatly from clear material.

Brynildsen (12) reported on bendingstrength of finger-jointed 2- by 4-inchand 3- by 8-inch Norway spruce.These were tested in edgewise ben-ding with fingers vertical. The MOEwas not affected by the finger joints.The 2 by 4�s could be produced in allstructural classes up to and includingClass T-390. The 3 by 8�s were notsufficiently strong to make classT-390. Brynildsen also reporteddefects such as knots in or nearjoints could not be allowed.

Finger Joints inLaminated Timbers

In a comprehensive investigationSaunders (96, 97) examined the effectof finger joints in single laminationsand in small laminated beams. Heconcluded the following:

(a) Behavior of finger-jointedsamples in tension and com-pression was similar to that ofdefect-free wood in tension andcompression, except that in thedefect-free wood stress at pro-portional limit and failure in ten-sion were considerably higherthan in compression, whereas inthe finger-jointed wood this dif-ference was not as great.

(b) MOE of finger joints in tensionand compression was verysimilar.

(c) Proportional limit strain in ten-sion in finger joints was approx-imately 45 percent greater thanit was in compression.

(d) Photoelastic observations on afull-sized model of the fingerjoint in tension showed stressconcentrations were confined toabout 1/2 inch from the finger-tips.

(e) Within the true proportionallimit in bending, strain distribu-tion across the depth of thedefect-free beams (those with ajoint in bottom laminae andthose with a joint in the top andbottom laminae) was verysimilar although strain in thejoints in the bottom tended tobe slightly higher than that forjoint-free wood in this position.Using the E-modulus in bending,within this limit the correspond-ing stress distributions weredetermined to within 7.0 percentfor all beams.

(f) Tensile and compressive strainin finger joints of laminatedbeams could be predictedreasonably accurately (within 7.0pct) in the elastic range by us-ing the stress-strain relationshipin pure tension or compressionof a matched portion of a joint.

(g) Tensile strain measured acrossfinger joints in beams beforefailure always exceeded thatmeasured at failure on cor-responding tension samples.Finger joints were thus able todeform more in the beam beforefailure in direct tension probablybecause of the support given byadjacent lamina.

(h) Occurrence of well-made fingerjoints in the compression zoneof a glue-laminated beam willprobably have little or no effecton strength of a beam becausestrength and elastic properties

of a joint in compression arevery similar to those of defect-free wood.

Noren (75) reported on bendingtests on 45 laminated beams contain-ing finger joints at midspan in outerlaminations. Each beam was made of8 laminations and was 17.6 cen-timeters (cm) deep, 6.6 cm wide, and420 cm long. Figure 4 shows the loca-tion of finger joints in each seriesand the bending strength and stiff-ness as a percentage of unjointedcontrol beams.

The work of Noren (75) showeddeformation of beams was not in-fluenced by the finger joints untilloaded above two-thirds of theultimate load. A significant decreasewas obtained only in series 1, 3, 4, 6,and 9, in which the quality of thelaminae was as high as T-130, andwhen end joints were exposed to ten-sile forces. The reduction of bendingstrength (MOR) was about 20 percent,i.e., although strength grade wasreduced from T-130 to T-100 by thejoints. Consequently, no reduction ofstrength resulting from joints of thesame type was to be expected ifT-100 was used for the beams, as inseries 7 and 8. This was confirmed bythe test results. However, the failurein all beams but two in series 7 and 8apparently was related to the joints,indicating joints have some influenceon strength.

In series 9 (fig. 4) the beams hadsix end-jointed laminae exposed totension (75). Although failure usuallyfollowed the joints, results indicatethis was only a secondary effectwithout influence on strength afterthe joint in the bottom laminae wasbroken. Series 6, with top two andbottom two laminae jointed, showedthe greatest decrease in strength.However, the difference in com-parison with the beams in series 1, inwhich only bottom laminae werejointed, was not significant at the 95percent level of probability. Other in-vestigators have observed the effectsof finger joints in strength and stiff-ness, both in single members and inlaminated beams (46, 68, 72, 114, 115).

Effect of Wood Variables

A series of investigations wasmade at the Forest ProductsLaboratory (FPL) to evaluate the ef-fect of several lumber characteristicson strength of laminated beams. inone, Moody (71) investigated the ef-fect of coarse-grained southern pine

10

Figure 4.�Strength and stiffness (deflection) of laminated beams with finger-jointed laminae as a percentage of strength and stiffness of control beamswithout end joints. (From Noren (75).)

(M 149 223)

on strength of laminated beams. Thematerial was known to be lower instrength and stiffness than themedium grain and dense material nor-mally used. During testing of thebeams, a significant number offailures appeared to originate atfinger joints in tension laminations ofhigh-quality dense material. Quality-control tests prior to beam manufac-ture indicated that the equipment wasproducing satisfactory joints. Also,full-size tension tests of finger-jointedlumber indicated finger-joint strengthwas as good as two groups ofmaterial previously evaluated.Therefore, assuming the finger jointswere satisfactory, a possible explana-tion was the relatively low-stiffness,inner coarse-grained plies did notassume their proportionate share ofbending stress; thereby the stifferouter plies had to assume an evengreater share. Thus, beams havingmiddle laminations of low stiffnessrequire stronger outer laminae andconsequently higher quality fingerjoints for equal beam strength.

In 1970 Moody (69) reported on ten-sile strength of both jointed and un-

jointed pith-associated (PA) southernpine 2 by 6�s. This type of materialcan contain significant amounts ofPA material and still meet the denseclassification for structural lumber.From his work, Moody concluded thefollowing:

1. PA material greatly affects ten-sile strength of both finger-jointed lumber and lumberwithout joints.

2. For nonjointed lumber used ascontrols, tensile strength of PAspecimens averaged 34 percentless than those of specimenfree of PA material (NPA). Aboutone-half of this difference maybe attributed to effects of gradeand specific gravity (SG) andone-half to lower strength PAmaterial.

3. Average tensile strength offinger-jointed lumber containingPA wood was 22 percent lessthan that of finger-jointed NPAlumber.

4. Tensile strength of finger-jointedNPA lumber averaged 66 per-cent the tensile strength of

similar control lumber. For PAlumber, finger-jointed materlalaveraged 79 percent of control.

5. Finger-jointed lumber consistingof one NPA and one PA boardhad an average tensile strengthabout equal to that of finger-jointed lumber entirely from PAboards.

6. Tensile strength of nonjointedPA lumber meeting the 301 +grade was significantly greaterthan that for 301 grade PAlumber. For the finger-jointedlumber, the difference betweentensile strength of these twogrades was not significant.

The effect of two factors, fingerjoints and SG on flexural propertiesof glue-laminated southern pinebeams, have been evaluated byMoody and Bohannan (70). After con-sidering the two factors the authorsconcluded finger joints in tensionlaminations and SG in addition tovisual grading as a method of posi-tioning laminations did notsignificantly affect beam strength orstiffness compared to that of a con-trol group. The data indicate fingerjoints of acceptable strength for ten-sion laminations can be producedprovided some care is taken to limitregions of low density in PA woodand adequate quality-control pro-cedures are followed. Finger jointsare apparently as strong as tensionlaminations of near-minimum qualitygraded according to the American In-stitute of Timber Construction (AITC)301A-69 requirements because mostfailures were attributable to strength-reducing characteristics rather thanto finger joints.

Dawe (22) compared the effect offinger joints to knots on tensilestrength of 3-inch-wide pieces ofBaltic redwood. He found strengthreduction from a finger joint wasabout equal to that caused by a3/4-inch knot.

Louw and Muller (61) investigatedthe influence of knots and bolt holeson strength of finger joints. In thefirst part of their paper, they reportedon the influence of knots on bendingstrength of finger-jointed lumber in200 specimens. Knots of varioustypes and sizes occurred at odddistances from the joints. Foranalysis of the results, the knots weregrouped together according to theirdistance from the joint, irrespective oftype, size, or lateral displacement.

11

The investigators concluded thatknots within 4 to 6 inches of a fingerjoint influence bending strengthadversely, but this influence is smallenough to justify recommending theknots be allowed in the manufactureof structural finger joints inmerchantable-grade timber. Thematerial used in their study wasrather low in quality. The merchant-able grade in South Africa has abasic working stress in bending of1,070 lb/in.2

The second part of the paper byLouw and Muller (61) deals with in-vestigations designed to provide dataon permissibility of bolted joints in orclose to finger joints. Tests were con-ducted to determine the following in-fluences:

(a) A bolt hole on tensile strengthof finger joints

(b) A bolt hole on bending strengthof finger-jointed timber

(c) A finger joint on load-bearingcapacity of a bolt if loadedparallel to the grain of wood

(d) Applying load to a bolt througha finger joint on timber bendingstrength.

A statistical analysis of the resultsshowed that bolted joints located inor close to finger joints have anegligible effect on strength of thejoints. The authors recommended noprovision had to be made for ex-cluding finger joints from the zone ofa bolted joint.

Several authors have indicated thatorientation of a joint relative to thedirection of applied bending stressdoes affect the efficiency of a joint(22, 88, 113). In flatwise bending, forbest results a joint profile should ap-pear on the width of a piece. It isgenerally believed this results in allfingers in a joint being more or lessequally stressed. If a joint profile ison the edge of a piece, the outerfingers are the most highly stressed.In many joints this finger is most like-ly to be weak unless special care istaken during joint manufacture.

Fatigue Strength ofFinger Joints

In a large percentage of the in-stances where structural glued-laminated timber is used, loadingsare of a semi-dead-load classification.The load there is not cyclic nor israpidly repeated stress being imposed

12

on the member. Some applications ofglued-laminated timber members doinvolve cyclic loadings. An examplewould be laminated members for rail-road bridges; the members are sub-jected to repeated loading as trainsmove across a bridge. Information onfatigue characteristics of a joint areneeded to determine the safe designload and to predict the safe servicelife of the members.

The only published informationfound on fatigue strength wasreported by Bohannan and Kanvik (7).They investigated fatigue strength oftwo types of finger joints for end-jointing dimension lumber. One jointtype was classified as structural, hada 1.5- inch finger length, a pitch of0.313 inch, and a tip thickness of0.031 inch. The other was classifiednonstructural, had a finger length of0.875 inch, a pitch of 0.250 inch, anda tip thickness of 0.0625 inch. Allspecimens were tested in tensionparallel to the grain.

Based on results from 10specimens tested at stress levels of40, 50, 60, 70, 80, and 90 percent ofstatic strength, the authors offeredthese conclusions for clear, straight-grained Douglas-fir finger-jointedspecimens:

Effect of fatigue loading causedthe same percentage reductionin static strength in tensionparallel to the grain for bothstructural and nonstructuralfinger joints.Fatigue strength in tensionparallel to the grain of fingerjoints at 30 million cycles isabout 40 percent that of staticstrength.Fatigue strength in tensionparallel to the grain of fingerjoints at 30 million cycles isabout 30 percent that of 1 to 8scarf joints.

S T R U C T U R E F I N G E RJ O I N T I N G C R I T E R I A

A N D P E R F O R M A N C E I NT H E U N I T E D S T A T E S

In 1968 Eby (27) described the workthat preceded the first engineered useof finger joints in glued-laminatedproducts in North America anddiscussed development of the accept-

ance criteria outlined in VoluntaryProduct Standard PS 56-73 (121).

The purpose of Voluntary ProductStandard PS 56- 73 (121) is toestablish nationally recognized re-quirements for producing, inspecting,testing, certifying structural glue-laminated timber and for providingproducers, distributors, and users abasis for a common understanding ofthe characteristics of this product.Section 5.3.6�Tests of end jointsprior to use, reads as follows:

All configurations of end jointsshall be tested prior to the first pro-duction use on each species (orgroup of species which have closelysimilar strength and gluingcharacteristics)-adhesive-treatmentcombinations laminated by the plant.Straight-bevel scarf end joints with aslope of 1 in 8 or flatter shall betested in conformance with test 112.Other types of end joints and straight-bevel scarf end joints with a slopesteeper than 1 in 8 shall be tested inaccordance with test 113.The criteria for strength are asfollows:

Criterion (1) The average ultimateload value shall be atleast 3.15 times thehighest allowablebending, tension, orcompression stressvalue for normal condi-tions of loading beingused in design.

Criterion (2) Ninety-five percent ofthe test values mustexceed 2.36 times thehighest allowablebending, tension, orcompression stressvalue for normal condi-tions of loading beingused in design.

Criterion (3) All test values mustexceed 2.00 times thehighest allowablebending, tension, orcompression stressvalue for normal condi-tions of loading usedin design.

Section 5.3.6 of the Standard in-dicates any end joint that will meetthe criteria mentioned, regardless ofgeometry, method of production, ororientation is suitable for structuraluse under this product Standard. ThisStandard more closely resembles aperformance standard which specifieswhat must be accomplished but not

how it must be done. In contrast theBritish and German (10, 24) specifica-tions spell out in detail acceptablejoint designs, how much pressuremust be used in gluing, and otherdetails. It can be argued thespecification approach tends to stiflethe research and the innovation thatcould lead to significant im-provements in finger-jointing wood.

Specialized Uses ofFinger Joints

Several papers in the literaturedescribe somewhat unique or special-ized uses of finger joints. Egner andothers (31) discuss the performanceof finger-jointed window frames, Mur-phey and Rishel (73) proposed usingfinger joints in producing furniture.There are also reports in the literatureon finger-jointing plywood (37, 80);Strickler (111) has proposed a methodfor finger-jointing veneer using avariation of his impression joint.

In a paper, �High Strength CornerJoints for Wood,� Richards (93)reported on design and testing oftwo-and three-member corner joints.The joints would be suitable for avariety of end uses, including furni-ture and trusses. Pincus, Cottrell, andRichards (85) investigated stressdistribution in rigid triangular rooftrusses; the elements of the trusseswere joined with finger joints. Kolb(55) reported on the strength ofcurved laminated beams, finger-jointed across the entire cross sec-tion at midspan; the cross sectionswere 50 by 12 cm (19.7 by 4.7 in.).

Hoyle, Strickler, and Adams (43)reported on a finger- joint- connectedwood truss system; the authorsstated the system to be technicallyfeasible with performancecapabilities superior to metal-plate-connected trusses and on a par withglued plywood gusset trusses. Thedesign procedures for a simplepedestrian bridge 80 feet long with a36- inch camber and a 4-foot-widedeck are reported by Hoyle (44). A uni-que feature of the bridge is the use ofa finger-jointed structural system.

A wide variety of potential uses forfinger joints are reported in theliterature; in several they weresubstituted for existing methods ofjoining; improvements in performancewere substantial (99). Whether theseimprovements are justifiableeconomically is not known.

Quality Control InFinger-Jointed Products

The importance of quality control inany finger-jointing operation cannotbe overemphasized. Two primarygoals for quality control in anyorganization are to detect substand-ard products and prevent them frombeing put into service, and to pinpointthe cause of a problem, combat andsolve it.

The success of finger joints, eitherthe structural or the nonstructuraltype, depends on joints being of con-sistent quality. Both types will bejudged on a basis of poorest jointsproduced, not best. The quality ofjoints must never drop below aminimum satisfactory level (98).

The only way to be reasonably surethat the finger joints being producedwill meet requirements is to develop aquality-control program that monitorseach phase of the operation. Aquality-control program should in-clude visual inspection, directmeasurement of selected variables,and physical testing of selectedsamples. Visual inspections and themeasurement of selected variablesduring processing are to prevent orkeep to a minimum production ofunacceptable joints. Physical testingof joints after production is a checkon visual inspection, on measure-ments, and on procedures (4, 5, 39,54, 101).

Most of the major inspectionbureaus and associations includefinger-jointed lumber in their gradingrules. However, before a plant canmarket finger-jointed material with aninspection agency�s grademark, aplant must be certified and maintaina continuing quality-control program.The certification tests are required ofa plant to determine if productionoperations are adequate to producestructural joints. After certification,inline production tests and tests onsamples of finished production are re-quired to assure a day-today level ofquality control at or above theminimum requirements established bythe inspection agency.

The American Institute of TimberConstruction (AITC) publishes an In-spection Manual AITC 200-73 thatcontains section 201-73, �LaminatorsQuality Control System� (1). TheManual sets forth minimum quality-control requirements for all AITCmember laminators. Included in thissystem are sections pertaining to

quality control of finger joints. Thesystem used by AITC is the patternfor many of the other inspectionagency quality-control systems andfor Product Standard PS 56-73.

In the literature the physical testingaspects of quality control in fingerjointing have received the most atten-tion. Testing of finger joints can beconveniently divided into twocatagories, destructive andnondestructive.

In destructive testing, samples aretaken from production at selected in-tervals and stressed to failure inbending or tension. The load atfailure is then compared with anestablished minimum value. Aftertesting, the failed joint is visually in-spected to determine percentage ofwood failure; this also is compared toa minimum value. Of the two testmodes, bending or tension, the ten-sion is generally considered morecritical. The tension test, however,does require more effort to preparespecimens and the testing equipmentis somewhat more sophisticated.When finger joints were first con-sidered for structural use, the tensilespecimen used was the necked-downtype with the joint located at thecenter of the length (21, 102). Specialequipment and jigs were required tomanufacture this type. A testspecimen was developed by Selbo(102, 104) and evaluated by Bohannanand Selbo (8) that greatly simplifiedthe making of tension specimens. Allthat is required to produce suitabletensile specimens is a small tablesaw equipped with a well-sharpenedhollow-ground blade. Specimens arecut about 3/32 inch thick by 12 incheslong. The width is a multiple of one-half the pitch of the joint beingtested. The finger profile appears onthe wide face of the specimen, andseveral specimens must be testedfrom each joint for evaluation. Thetest machine must also be equippedwith wedge-type grips that tighten onto the specimens as load is in-creased.

The bending test commonly used toevaluate finger joints is quick andsimple. Although strength evaluationis not as critical as the tension test,much can be learned about themanufacturing process by inspectinga failure. In the bending test two-point loading is used, and the extentof preparation required is crosscut-ting a selected joint from a board to arequired length. Specimens the fullwidth and thickness of the material

13

being joined are used, and specimenlength depends on the length of thefinger joint being tested. The recom-mendation is the load points shouldbe at least 2 inches outside the tipsof the fingers (1).

Nondestructive testing of fingerjoints has always been of interest.With nondestructive testing it will bepossible to evaluate all joints pro-duced, rather than a small percent-age. Thus, the probability of a poorjoint being placed in service will begreatly reduced. It was this desire totest all, or at least a high percentageof the finger joints produced, thatresulted in development of one of theearly lumber stress-grading machines(9). Other test methods mentioned inthe literature for nondestructivetesting of finger joints are acousticemissions (86) and stress-wave at-tenuation (52).

In the work on acoustic emissionsby Porter and others (86), pieces ofDouglas-fir 2- by 6-inch materialfinger-jointed at midlength wereloaded in bending on the wide face.The predicted failure was the asymp-tote of a curve of the cumulativenumber of acoustic emissions plottedagainst the load. The accuracy of theprediction of failure increased with in-creasing load. According to theauthors, a load just beyond the pro-portional limit should provide anestimate of the failure load to an ac-curacy of ± 10 percent. In fingerjoints purposely made withoutadhesive on some of the fingers, theaccuracy of prediction decreased. Theauthors also found that the MOE ofthe finger joints was poorly correlatedwith the MOR.

Caution should be taken in stress-ing any structural member beyond itsproportional limit. When proportionallimit is exceeded there may bedamage that affects initial strength,fatigue strength, or life to failure of amember.

In work by Kaiserlik and Pellerin(52) on developing a new method ofmeasuring stress-wave attenuation, amagnetic gage was developed tomeasure particle velocity indirectly.Although no work on finger joints isindicated in the article, the authorsfeel that the qualitative results in-dicate the magnetic gage may havesome practical application forevaluating finger-joint quality.

Proof loading is another means oftesting or evaluating finger joints thathas received attention. In proofloading finger joints are subjected to

14

a stress large enough to indicate theyare capable of withstanding a designload. In a series of studies (82, 84,112) at Washington State University,it was concluded �...tensile proofloading of high-stress tension lamina-tions for glue-laminated beams offersa practical method of assuringreliability for strength in suchbeams...� Tensile proof loading af-fords reliability equally well for fingerjoints and for the wood itself.

In trying to implement a tensionproof-loading system, Eby (28)reported some difficulties. The prob-lem was in trying to develop a systemcapable of tension proof-loadingmaterial from automated continuousfinger-jointing lines operating atspeeds to 150 feet per minute. It wasdecided the only viable alternativewas to develop a system for proof-loading finger joints in bending.

Before this could be accomplishedseveral questions had to be answeredon how proof-loading affected:

1.2.3.

4.

5.

Fully cured finger joints,Partially cured finger joints,Bending strength of fully curedjoints that had been proof-loaded immediately out of theRF energy field,Tensile strength of fully curedjoints that had been proof-loaded immediately out of theRF energy field,a through d on both horizontaland vertical configurations foreach species, adhesive, and pro-cess combination.

In an investigation at WashingtonState University designed to answerthe questions 1 through 4 Pellerin (83)concluded the following:

1.

2.

3.

4.

Joint strength at 6 to 7 secondsout of the RF energy field is ap-proximately 52 percent of itsfully cured strength for bothhorizontal and vertical fingers.Bending strengths of horizontalfinger joints are about 87 per-cent that of vertical fingerjoints.A bending proof-load of 3,600lb/in.2 applied to a partiallycured joint has no apparent ef-fect on bending strength of afully cured joint for eitherhorizontal or vertical fingers.A bending proof-load of 3,600lb/in.2 applied to a partiallycured joint has no apparent ef-fect on tensile strength of a

5.

6.

fully cured joint for eitherhorizontal or vertical fingers.Tensile strengths for horizontaland vertical finger joints arealmost identical.Stacking of end joints in thecritical tension zone (1/8 ofdepth plus 1 lamination) has nodetrimental effect on a beam ifend joints in the two outermostlaminations are proof-loaded to3,600 lb/in.2 in bending whileonly partially cured.

Based on this information a bend-ing proof-loading system has been im-plemented in several laminatingplants. The American Institute ofTimber Construction has a qualitycontrol test specifically for bendingproof loadings (2) and tenative codeapproval has been received for theuse of this material (48). This is prob-ably one of the most important ad-vancements in the quality control offinger joints since they were first ac-cepted for structure use.

E C O N O M I C S O F

F I N G E R J O I N T I N G

A number of reports of successfulfinger-joint operations appear in theliterature (40, 53). Very few reportscan, however, be considered truly in-vestigations of the economics of theoperation. This is not surprisingbecause each operation is different,and the extent to which finger joint-ing will be profitable cannot be decid-ed in general terms. Each individualoperation must be decided on its ownmerit (17, 66).

In some operations there is noalternative to finger jointing and inthese the value of cost analysis is inpricing the product or determining theparticular operation in the processthat needs improvement. In themanufacture of large, laminatedbeams that are often many timeslonger than available lumber, endjoints are a necessity. The need is notto decide to finger-joint, but what thecost of finger-jointing will be.

In salvaging short lengths ofmaterial for use in trim and millworkcost analysis is more difficult. It isnecessary to determine the percent-age that can be salvaged, the alter-native value of the material as chipsor as fuel, and then compare its valueto the appropriate grade of lumberminus the cost of manufacturing to

determine if the operation iseconomically feasible. On theeconomics of finger jointing, Hawkins(41) states the cost of finger jointingdepends on type of machine, rate atwhich the machine is used, amount ofpreparation needed on the material,size of the material, and type andamount of auxiliary equipment usedwith the machine. He tells that fivebasic charges are to be met whenfinger jointing; they are:

1.

2.

3.4.

Equipment costs. �Capital investedplus interest, insurance, and deprecia-tion. These are fixed costs, indepen-dent of the number of joints made peryear.

Operating costs.-Labor, power,and others. Costs in equipmentoperation.

Direct costs.-Glue costs, trimmingcosts. These are variable costs direct-ly dependent on number of jointsmade.

Waste costs.-The recovery offinger-jointed products from a givenbatch of timber will depend on sizeand quality of both input material andfinished product. It is debatablewhether cost of nonrecoverablematerial should be charged to thefinger-jointing process or considereda surcharge on the cost of inputmaterial.

Profit. �If the finger-jointing pro-cess is to be economical, it isreasonable it should return about 35percent on capital before taxes, parti-cularly if consideration for generaloverhead is included.

Other authors list essentially thesame factors to be considered in aneconomic evaluation. Dobie (25)published a detailed economicanalysis of the green lumber finger-jointing process developed at theWestern Forest Products Laboratoryof Canada. In his analysis he used aninput of shorts and economy gradeDouglas-fir to produce finger-jointedlong lengths of No. 2 and BetterStructural lumber. The productionrate was conservatively set at 50lineal feet per minute, and capital in-vestments of $500,000 and $1 millionwere used in the analysis. He com-pared the published market price re-quired to obtain returns on in-vestments of 20, 30, 40, and 50 per-cent at those different rates of pro-duction. Based on these com-parisons, he concluded there weremany opportunities to recover at least50 percent return on investment even

at a capital investment of $1 mlllion.Many case-histories in the

literature are on the success variousoperations have had turning low-valuematerial suitable only for chips orfuel into usable material suitable forhigh-value products. One operation isreported capable of using material asshort as 4-1/2 inches (125). This com-pany estimates their ability to utilizematerial less than 8 inches in lengthallows them to recover 25 to 30 per-cent of clear material formerly con-sidered waste.

In another operation (87), a com-pany reported finger jointing S4S 2 by4�s 7 feet and shorter for studs andnow produce standard and betterlight framing at the rate of 48 thou-sand board feet (Mfbm) per l&hourday. The Company reported someproblems, at least initially, withcustomer acceptance, and sold at aprice less than the selling price of un-jointed material. Because the alter-native value of the shorts they usedwas so low, this was possible.

In 1970 Westlake (123) reportedthat, beginning with shop lumber inmolding and millwork, a normaloperator would have between 30 and35 percent waste. After the waste, 75percent was suitable for finger joint-ing, and 25 percent was classified assolids. The solid material sold for$150 to $200 per thousand board foot(Mfbm) more than did the finger-jointed stock. Westlake notedprefinished jointed molding helpedbring selling prices closer together;he thought most of the price dif-ference was caused by traditionalpreference.

Dean (23) reported his companybegan finger-jointing pine, white fir,and cedar in the early 60�s for panel-ing primarily to get rid of shorts. Theyhave since dropped the paneling, andare now jointing 4/4, 5/4, and 6/4lumber. The 4/4 high-grade lumber isused for fascia, cornice, siding, sof-fits, cable reel ends, and mobilehomes. The 5/4 and 6/4 is producedinto cutstock, door jambs, molding,blanks, and garage-door stock. Sizesrange from 2 to 12 inches wide andup to 24 feet long. Their production isabout 15 Mfbm per shift. The input isany grade that can be upgradedenough to make a profit. Dean reportsa 30- to 40-percent waste factor canbe expected on common grades and a20- to 30-percent, on shop grades oflumber. He listed the followingbenefits of finger jointing:

5.

6.

Greater return because ofupgrading lumber.Less lower grades for sales tocontend with.Increased sales.Increased sales offeringbecause of variety of productsproduced.Flexibility for company to followmarket.Increased utilization of rawmaterial.

In summarization, in the literaturemany advantages apparently are seento finger jointing. With the Increasingvalue and decreasing volumes ofhigh-grade raw material available,finger jointing is becoming or hasbecome an economic necessity.However, one cannot afford just toassume that finger jointing will pay. Adetailed analysis should be made tooffer reasonable assurance that thereis sufficient volume of the proper sizeand quality of material and thatmarkets exist for what can or will beproduced. It is always possible that itmay be more profitable to sell to anexisting operation than to install anew operation. Also, in these days ofincreasing fuel prices, the powerderived from the waste may be worthmore than the product produced byfinger jointing. As has been stated,each operation differs; thus, no all-inclusive statements on theeconomics of finger jointing can bemade.

C O N C L U S I O N S

The author concludes from thisreview of the literature the followingfindings on finger jointing:

No practical economic alter-native to finger joints exists forjoining wood end grain to endgrain.The effect of joint geometry onstrength has been defined.Wood quality directly affectsjoint strength. Finger joints inwood that is below averagedensity for a species will ex-hibit less strength than thesame joints in the samespecies with average or higherdensity.Adhesive bonding variableshave been found essentially thesame as in any other type ofgluing procedure.

15

5.

6.

7.

8.

9.

Several methods of formingfinger joints are available, andail have advantages and disad-vantages; however, all arecapable of producing satisfac-tory joints.Finger-jointing operations canbe mechanized or automated toabout any desired degree.In common grades of lumber awell made finger joint has lesseffect on strength thanallowable natural defects inlumber.Based on examples reported inthe literature, finger joints arenot restricted to end-grainbonding, but are possiblealternatives to other types ofjoints.An effective quality-control pro-gram that covers all phases ofa manufacturing process is thekey to consistently good fingerjoints.

10. The confidence level on fingerjoints could be improved bydeveloping nondestructive testmethods capable of rapidlyevaluating all joints soon afterthey are produced.

11. Before investing in a finger-jointing operation an economicanalysis is recommended. Eachsituation differs and values ofmaterials constantly change;thus it unwise to make generalstatements about profitabilityof finger jointing.

With perhaps a few minor excep-tions the basic knowledge required tomake strong, durable finger joints isavailable. The finger joints may notbe as strong as desired, but withexisting limitations they are the bestthat can be produced. A major prob-lem in the production of finger jointsis between-joint variability. If thisvariability can be reduced it will bepossible to increase potential designloads when finger joints are used;this could then be translated intopossible savings in wood.

At present the only procedure toreduce or control variability is carefulcontrol of the entire finger-jointingprocess, from initial selection ofmaterial and careful location of jointsin a piece relative to natural defectson through the curing of the adhesive.A major step forward will be develop-ment of a nondestructive test methodthat will make possible evaluation ofall joints produced. Proof loading isthe only method now available thatapparently has much promise.

To say there is no possibility of im-provements in finger joints or jointingwould be shortsighted. Technology isconstantly changing, and the spinofffrom seemingly unrelated fields canhave substantial effects on solvingapparently impossible problems. Infinger jointing development of newadhesive systems and bonding techni-ques and developments in machiningcould affect manufacturing methodsthat, in turn, would improve joint per-formance.

LITERATURE CITED

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17

18

19

20

21

22

23

24 U.S. GOVERNMENT PRINTING OFFICE: 1981-750-027/58 3 . 0 - 2 5 - 4 / 8 1