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The following guidelines and technicalinformation outline what an investmentcasting is capable of offering. It will coverdimensional and structural topics. Theintegrity of a part, both dimensional andstructural is proportional to the cost of thefinished part. This is true of investmentcastings, as well as fabricated andmachined parts. Investment castings aredesigned to minimize the cost of producingclose tolerance parts.

Maintaining close dimensionaltolerance in an investment casting isaffected by many factors. Most of thesefactors can be controlled by the foundry,although minor “lot to lot” variations willoccur. The tolerance bands provided forinvestment castings are determined bythese variations in the process.

Machining can achieve closertolerances than available in an investmentcasting. A review of the design will oftenpermit expansion of some tolerances,undercuts, blind holes, etc. to allow thehigher production yields and lower piececosts possible with investment castings. Ifcloser than cast tolerances are necessary,the machining required on an investmentcasting will still be substantially less thanon conventional cast or fabricated pieces.An added benefit of an investment castingis the ability to add radii and other detailthat would be more expensive withtraditional manufacturing processes.

VARIABLES

Casting size and shape determines the tolerancerequired to allow for process variables. Manyprocess factors affect investment casting tolerances,such as:

1. wax temperature2. wax die temperature3. the amount of pressure applied to inject wax

into a die4. firing temperature of the ceramic shell5. ceramic mold refractory composition6. cooling rate of the metal7. lot to lot variation of raw materials

STANDARD LINEAR TOLERANCES

As a general rule normal linear tolerance on aninvestment casting can be +/- .005 of an inch per inchor +/- .125 of a mm per 25mm. The following table gives a partial listing ofdimensions and tolerances.

LINEAR TOLERANCE

Dimension Inches Dimension MM(English) (Metric) (Metric)

0" up to 1" +/- .005 0 up to 25.02 +/- .13

1" + up to 2" +/- .010 25.02 + up to 50 +/- .25

2" + up to 3" +/- .015 50.02 + up to 75 +/- .38

3" + up to 4" +/- .020 75.02 + up to 100 +/- .50

4"+ up to 5" +/- .025 100.02 + up to 125 +/- .63

5" + up to 6" +/- .030 125.02 + up to 150 +/- .75

6" + up to 7" +/- .035 150.02 + up to 175 +/- .88

7" + up to 8" +/- .040 175.02 + up to 200 +/- 1.0

8" + up to 9" +/- .045 200.02 + up to 225 +/- 1.13

each inch +/- .005 each 25 MM +/- .125after after

Investment Casting Design Parameters

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FLATNESS

Flatness and straightness are so closely related thatconfusion may arise unless the foundry andpurchaser clarify the definitions before production.Flatness tolerance is a tolerance zone defined bytwo parallel planes within which the surface mustlie. In measuring (see Fig. 1), the parallel planesmust be the minimum distance apart.

Figure 1

Degree of flatness in an investment casting isalmost always determined by the volumetricshrinkage of the wax pattern and then by the metalduring cooling. Shrinkage usually occurs in thecenter of a mass, this shrinkage is called “sink”.Sink can be controlled by specialized techniques,but it will always occur to some extent. Generalflatness tolerances cannot be quoted because theyvary with the configuration of the part and alloyused. The following is a rough guide derived fromdata collected from standard test procedures.

Section Volume of PossibleThickness Section Sink per

Face of Casting

Inches In3

.25 .5 not significant

.5 1 .0051 2 .0122 8 .014

Millimeters mm3

6 13 not significant

12 25 .1325 50 .350 200 .36

Allowable sink is in addition to the basic tolerance.For example (see figure 2), on a block 1" +/- .005thick (2.5mm +/- .13mm) the allowable sink mayaffect the tolerance of the part an additional .008":

Figure 2The method of measuring flatness should bespecified by the purchaser. It may vary from asimple surface plate and feeler gage to full layoutwith equalization and dial indicators.

STRAIGHTNESS

Straightness tolerance is a tolerance zone withinwhich an axis or the considered element must lie.Thus, to correctly measure axial straightness of ashaft, bar, or plate, (see Fig.3 & 4) the tolerancezone within which the axis or axial plane lies mustalso be measured.

Figure 3

Figure 4

Straightness may be a real problem with certaintypes of castings. A relatively thin, short part maybend while a long, heavy part may not. Experiencetells us that a given design may bend, but it cannotindicate to what extent. Ribs and gussets willinhibit warpage, but will also hinder mechanicalstraightening of whatever warpage did occur.

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PARALLELISM

Parallelism is the condition of a surface equidistantat all points from a datum plane or an axisequidistant along its length to a datum axis.

Parallelism is difficult to control in the as castcondition and may require a straighteningoperation. Parallelism is a feature that is a functionof the complexity of the part. As the part is beingdesigned, consult your foundry for tolerances.

Figure 5

For example, casting parts with parallel prongssupported at only one end (see Fig. 5) and yokecastings (see Fig. 6a) are very specializedproblems which should be discussed fully with thefoundry before production.

Figure 6a

In Figure 6a, point X is the thickest section, it isthe ideal gate point. It is also where the greatestvolumetric shrinkage will occur. Dimension Y,however, will be restrained by the rigid mass ofrefractory used to produce the mold. Parallelism istherefore difficult to maintain. It can be improvedby control techniques and sizing.

This condition will also affect any through holesusually found in yokes (see Fig.6b). Whenspecified, such holes should carry considerablefinish stock for machining if they are to befinished truly concentric.

Figure 6b

ROUNDNESS

Roundness tolerance specifies a tolerance zonebounded by two concentric circles within whicheach circular element of the surface must lie. It isthe total indicator reading (TIR) when the part isrotated 360º, or calculated by taking half thedifference between the maximum and minimumcondition. The latter is usually preferred becauseit is quicker to determine. The actual inspectionmethod, however, should be specified by thepurchaser.

CONCENTRICITY

Concentricity is a condition in which two or morefeatures ( cylinders, cones, spheres, hexagon, etc.)in any combination have a common axis. Forexample, any two cylindrical surfaces sharing acommon point or axis as their center areconcentric. Any dimensional difference in thelocation of one center with another is the extent ofeccentricity.

Figure 7Figure 7 shows that out of roundness in eitherdiameter does not affect concentricity becauseconcentricity is determined by the centers or axis.

(Continued)

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CONCENTRICITY (Continued)Out of roundness is variance from a true circle.Straightness does influence concentricity if thecasting has a shaft or tube feature.

Figure 8

In Figure 8, diameters A and B may be true circles,but the out of straightness affects concentricity.

When the length of a bar, a casting, or tube is notmore than 2 times its component diameters, thediameters will be concentric within .005" per 1" ofseparation (see Fig. 9 below).

Figure 9

5"OD x 2"ID x 2" longThe 5"OD x 2"ID will be concentric within .015" TIR5"OD - 2"ID = 3" separation.

When the length is more than two times thediameters, out of straightness should be added toeccentricity (see Fig. 10 below).

Figure 10

3"OD x 2"ID x 4" long3"OD x 2"ID will be concentric within .005" TIR 3"OD-2"ID = 1" separation.

ANGULARITY

Angularity is the condition of a surface, axis, orcenter plane which is at a specified angle (otherthan 90 degrees) from a datum plane or axis (seeFig. 11 below). As a designer you should be awarethe angularity of a component part to bemaintained may require the part to be mechanicallystraightened or the die reworked. In heavy sections,the die may need to be reworked, and in thinsections, the casting may be straightened to achieveangularity tolerances. Angular tolerance dependson the configuration forming the angle. Sketch“A” cannot be sized, but in certain cases aftersufficient data review, the die can be reworked tobring the part closer to optimum. Sketches “B” and“C” represent castings that can be reworked to +/- 1degree, depending on the alloy and its condition.

Figure 11POSITIONING

Position tolerance is dependent on the parentcasting configuration. A casting that is lesssymmetrical in confirguration yields greaterdeviation in position tolerancing. A part that issymmetrical, with consistent cross section, willyield the best positioning tolerances. Features suchas holes, pockets or slots that can restrict thecasting’s ability to shrink properly will effect theposition features around the restricting feature. Alinear dimension may be affected approximately5% because of a restricting feature. Features thathave heavy sections near, can be effected by thevolumetric shrinkage of the heavy section. Thiswill pull or distort the feature in the direction of theheavy feature. It is difficult to predict the exactamount of the pull or distortion, and the foundrymay wish to rework tooling to minimize its effectand to improve the position tolerance.

In general, a consistent cross section with gradualchanges from heavy to light sections that leaves theheavy section open for gating purposes will yieldthe best over all casting and position tolerance.

0.005" TIR

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HOLE TOLERANCE

Standard linear tolerances will apply(see page 1).But a cast hole’s roundness is affected by thesurrounding metal. If an uneven mass is adjacent,the hole will be pulled out of round. If thesurrounding metal is symmetrical, holes up to 0.5"(1.3cm) diameter can be held to +/-0.005"(0.013cm).

Figure 12

The longer the hole or the more mass in the sectionaround it, the more pronounced the pull effect.“A” (see Fig. 12) shows the effect of holeshrinkage concavity which will be present to someextent in all castings. The top and bottomopenings will be approximate dimensions, whilethe center will be a larger diameter. Thru holesthat require clearance can be held to fairly closetolerances if the large diameter (bow) in the centeris ignored. If the sidewalls of the hole are used asbearing surfaces, simple reaming should besufficient. “B” shows how a heavier section closeto the hole distorts the shrinkage pattern. The holediameter is distorted by the additional shrinkage ofthe heavier section. The figure graphically showsthe pattern of distortion present to a greater orlesser degree in every casting with heavier massaffecting shrinkage.

CURVED HOLESSince curved holes are formed by either solublewax or preformed ceramic cores, normal tolerancerequirements tend to double (see Fig.13).Multiply the tolerance on all dimensionscontrolling such a feature by two. Since such holescannot be sized, a diameter tolerance of anadditional +/- 0.005" per inch may be required.

INTERNAL RADII, FILLETSInternal radii and fillets improve the strength andintegrity of the casting. Shrinkage and crackingare reduced. Internal radii and fillets require thewidest tolerance possible. They are difficult to

control and can receive approximate checks byradius gages.

Figure 13

CONTOURS, RADII AND CAMS

When dimensioning a casting radii, it must beremembered that volumetric shrinkage occursduring cooling which affects external radii andcontours. Most of the volumetric shrinkage willoccur in the center of the radius.

In regards to a flat casting, the designconsiderations for concavity are discussed in theFlatness section, page 2. The same concavity alsoaffects castings that have contour, but with moredramatic results. In this case, it is advised thatwhen dimensioning your drawing that a basicdimension along with a profile tolerance be used .

Figure 14

In a concave radius, the greatest shrinkage occursin the center and outer extremities. This tends todecrease the radius. When dimensioning adrawing of a concave radius (see Fig.14), a basic

(Continued)

“A” “B”

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CONTOURS, RADII, AND CAMS (Continued)

dimension for the radius should be used with aprofile tolerance. The fit to a matingconfiguration is then controlled by using atolerance band on the radius.

Figure 15

In a convex radius, the greatest shrinkage alsooccurs in the center. This tends to increase the castradius (see Fig.15). The drawing should bedimensioned accordingly. When dimensioning adrawing of a concave or convex radius, a basicdimension for the radius should be used with aprofile tolerance.

SURFACE TEXTURE

“Surface texture includes roughness, waviness, lay,and flaws” as defined in ASNI-ASME B 46.1-1985. Values stated are in Ra (Roughnessaverage), as measured by a profilometer. Typical cast finishes of ferrous alloys range from125 to 150 Ra. Aluminum castings will range from180 to 200 Ra.

All lay is multi-directional except in ground areas.Secondary operations will improve surface texture,giving results comparable to the equivalentwrought alloy.

PERPENDICULARITY

Perpendicularity is the condition of a surface, axis,or line which is 90 degrees from a datum plane ordatum axis. When specifying perpendicularity, usethe longest plane for reference, establishing thedatum plane with three tooling points. The shortestsurface will be perpendicular to the longest within.005" per inch of length of the shortest surface.

In Figure 16 (see below), for example, surface Bwill be perpendicular to surface A within 0.005"per inch (0.13cm per cm) of length of surface B.

Figure 16EXAMPLE:Length of B = 3"0.005" x 3" = 0.015"

THEREFORE:Surface B should be perpendicular to surface Awithin 0.015 TIR.

Some improvement on a tolerance can be made bymechanical straightening.

CASTING INTEGRITY

The integrity of a casting may vary depending onthe customer requirements of the part. In theinterest of producing a casting with the greatestdetail possible, thus reducing overall cost, castingintegrity can be sacrificed. These issues shouldalways be thoroughly discussed with the foundry.The following guidelines will help to insure thesoundest casting possible.

Directional Solidification

After a casting is poured the molten metal willreduce in volume, changing to a solid. Thisrequires that the molten metal has a path to feedthe area of decreasing volume. DirectionalSolidification is a castings ability to successfullyfeed these solidifying areas. As the casting designmoves away from good directional solidification,

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problems will occur. A casting design with a thicksection then a thin section followed by a thicksection can pose problems (see Fig. 17a). If boththick sections can’t be properly gated thepossibility of shrink or hot tear occurs.

Figure 17a Figure 17b

Figure 17b illustrates an example of an idealdirectional solidification casting design where themetal flows from a thick section to a thin sectionproducing a sound casting. Parts without isolatedthick sections will produce sounder castings.

Wall ThicknessThe minimum wall thickness that can be achievedis dependent on material and the distance themolten metal must travel. Metal is poured at highertemperatures than the mold it is entering. Metalentering a mold is constantly losing heat. If themetal loses enough heat it will solidify before ithas a chance to fill out all the detail of the mold.

Figure 18

Since the mass of metal to mass of shell ratio isless in a thinner casting, as metal enters a thinsection it is cooled to a much greater degree thanwhen entering a thicker section (see Fig.18) andmay not fill the thin section properly. Metal

entering a thicker section of a casting has moremass, therefore a higher temperature is maintainedand parts fill better.

Wall thicknesses of 1/8" over a distance ofapproximately 1" are possible. Wall thicknesses inaluminum can be as thin as 3/32" over a distanceof 2" in length. Specific designs should always bediscussed with the foundry.

RadiiRadii improve castability and generally improvepart strength. In a casting, radii will help todissipate heat improving solidification. A radiusof a 1/64" to 1/32" or more is preferred for allinternal or external corners. If a casting requires azero radius on an internal corner (i.e. a casting that

Figure 19

must mate with another part that has a sharpoutside corner), a recessed corner as shown inFigure 19 can be used to provide relief. Therecessed corner will also provide a part withgreater strength as compared to a part with a sharp corner.

ENGRAVING

Engraving should be raised letters on a recessedpad, so the top of the engraving is below thesurface of the part. This allows for engraving thatis easier to produce without defect. Recessedengraving is less likely to interfere with thefunction of the part.

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HOLE LENGTHMaking a good hole in a casting is dependent onthe integrity of the shell. The ratio of holediameter to length is important in determiningwhat can be offered. Blind holes must have aradius at the top and the bottom of the hole toinsure that a strong core is built for castingpurposes and to insure the best possible castingresults.

Hole Type Size Range Dia.to Length Ratio

blind 3/16 and up 1 : 1-1/2

thru 1/8 to 3/16 1 : 1

3/16 to 1/4 1 : 1-1/2

1/4 to 1/2 1 : 2

1/2 to 1 1 : 3

1 and up 1 : 4

GENERAL

Large flat surfaces can be improved by addingcontour, such as holes or bosses. This can alsohelp to improve the usefulness of the casting.

For repair of minor defects, in-process welding is anormal procedure for some alloys using a parentmetal filler.

Some alloys cast better that other alloys.Sometimes changing to another alloy may makethe part more castable, assuming part design isrobust enough for such changes.

Figure 20

For example, Figure 20 was originally afabricated design in low alloy carbon steel(1020). The customer wanted to reducemanufacturing costs by redesigning it as aninvestment casting. Pouring this part out of1020 material, the part would be prone to moredefects as a casting. What was needed from acastability standpoint was an alloy with goodfluidity and the ability to resist hot tear ascompared to 1020. Upon review, a change to304 stainless steel was recommended. 304stainless steel had the cast characteristics neededwhile meeting the customers designrequirements.

This illustrates one of the main advantages ofthe investment casting process which is its broadalloy selection. No other manufacturing processoffers as broad of a group of alloys to choosefrom than the investment casting process. Thisenables the designer to choose the alloy that isbest suited for the application and will enhancethe performance of the part.

If you have any questions regarding thetechnical information that has been provided,please contact us.E-mail: wpcc@wisconsinprecision.comPhone: 866/642-7307

*Technical Information reprinted and provided with permission from The Investment Casting Institute.