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BULLETIN NO. 36A

How |o Cut Screw Threads®

ln the Lathe

Cutti^g a Screw Thr.ad.

Price lO CentsPostpaid toAnyAddressCoin or StlmPS Of Any Country Accept.d

SOUTL! BEND LATHEWORKS456 NILESAVE. SOUTH BEND, INDIANA, u.S.A.

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Screw Threads

The history ()i the origin ancl (le\.elopment of the

screw thread is lost in antiquity. IIo\\-e\.er)it iskno\\-I

that scre\\. threads \|.ere used more than 2000 \,earsago alrd that screws and bolts \\.ere used for i:lsten-

ing together \.arious parts of the armor \\-om by

ancient Roman \\-arriors. Thl-ended cle\.ices \`,ere

also used to open and close heat.), doors and

dra\\.bridges of ancient castles.

The de\.elol)ment of modern mechanical (Ie\,ices

has been made possible by the im.ention and (le\el-

opment of the scre\\, thread. The most importantuse of threads is for fastening parts of machineryand equipment together. \1'ith screws and bolts

parts can easily be assembled and taken apart\\,ithout damage) but \`.hen fastened by othermethods, such as welding. ri\.ets. nails. etc.. this

is not possible.

screws are also used for transmitting motion)such as the lead screw, cross feed screw) etc. of a

lathe, also for let,erase as in jack scre\`.a. \,ises and

clamps. The screw is also used in I)recision measur-

ing instruments. such as micrometer caliI,erS. Cross

feed of lathe) etc.

South Bend Lathe Works

col)yrighI, |938. by South Bond Lalho Works. All rights reBOrVOd.


Fig. 1' Cutting a Screw Thread in the Lathe.

How to Cut Screw Threadsln the LatLle

Cutting screw threads in the lathe is accomplished by connecting the headstock

spindle of the lathe with the lead screw by a series of gears so that a positive carriage

feed is obtained and the lead scre\y is dri\.en at the required speed with relation to

the headstock spindle.

The Searing between the headstock spindle and lead screw may be arranged so that

any desired pitch of the thread may be cut. For example) if the lead screw has eight

threads per inch and the gears are arranged so that the headstock spindle revolves

four times \\.hile the lead screw revolves once) the thread cut will be four times as fine

as the thread on the lead screw or 32 threads per inch.

The cutting tool is ground to the shape required for the form of the thread to be

cut, that is "y," Acme, square, etc. The de])th of the thread is determined by ad-

justing the cross slide.

Either right hand or left hand threads may be cut by reversing the direction Of

rotation of the lead screw. This may be accomplished either byadding or removing one

idler gear in the change gear train, or by shifting the tumbler gears on the headstock.

i


FOR}I|'I.AI

\-u Th'd>, Per ln.

I)-I)epth-I x-61952PF=FhI--8

Fig. 2. American National Screw Thread Fol.m (Formerly U. S. Standard Screw Thread).

American National Screw ThreadsThe National Screw Thread Commission in l928 \\.as authorized by Congress to

establish a Standard system Of SCre\1. threads for use in the I_nited States. As a resultthis commission estal)lished the.American i_ational Screw Thread System \\.hich hasbeen approved by the secretar_\. of \\-ar) secretary of the r\-avy) and-congress) and isnow generally used by all shops in the United States.

The form of the thread adopted is shorn abc)\.e and tables for both the FineThread Series and Coarse Thread Series are given on the opposite page. The tabu-lation at the bottom of page 3 lists a number of special pitches of scre\\- threadsthat are commonly used. However, the standard screw threads should all+.ays beused) if possible.

A report of the National Screw Thread Commission applying to screw threadslbolts, machine screws, etc. defines the follorving terms.

Terms Relating to Screw Threads

screwThread. A ridge of uniform section in the form of a helix on the surface ofa cylinder or cone.

ExternalandlnternalThreads. An external thread is a thread on the outside ofa member. Example: A threaded plug. An internal thread is a thread on the insideof a member. Example: A threaded hole.

MajorDiameter (formerly known as "outside diameter')). The largest diameterof the thread of the screw or nut. The term "major diameter" replaces the term<(outside diameter" as applied to the thread of a screw and also the term "full

diameter" as applied to the thread of a nut.

Minor Diameter (formerlv known as "core diameter''). The smallest diameter ofthe thread of the screw or nut. The term "minor diameter" replaces the term "corediameter" as applied to the thread of a screw and also the term "inside diameter"as applied to the thread of a nut.

Pitch Diameter. On a strait`oht screw thread. the diameter of an imaginary cylinder)tlle surface of \l.hich \+-ould pass through the threads at such points as to make equalthe width of the threadsandthe\1-idth Of the SPaCeS Cut bV the Surface Of the CVlinder.

Pitch. The distance from a point on a screw thread t() a corresponding point onthe next thread measured parallel to the axis.

Lead. The distance a screw thread advances axially in one turn. On a single-thread screw, the lead and pitch are identical: on a double-thread screw the lead istwice the pitch; on a. triple-thread screw) the lead is three times the pitch, eta.

2


American National Standard Screw Threadsand Recommended Tap Drill Sizes

Coarse Thread Series (N.C.)Formerly U. S. Standard

Fine Thread Series (N.F.)Formerly S. A. E. Th'e.d

American National Special Screw Threads (N. S.)and Recommended Tap Drill Sizes


Fig' 3. Change Gear Equipment Used on Standard Change Gear Lathes.

Setting Up Standard Change Gear Lathes*

for Cutting Screw Threads

Screw threads are cut on Standard Change Gear Lathes ty engagillg theapron hllf rlutS \\.ith the lead screw. The pitch of threacl to be cut is de-terminecl lIV the number Of teeth in the Change gears uSeCI Om the re\-erse StuCland the lead screw, also the compound gears \\.hen used-

To set up the lathe for cutting a screw threa(1, first dc.tcrmine the numbl.rof threads per inch to he cut. By referring to the cha.ngre gear chart attacheclto the lathe (Figr. 4) the chant,cre goal-S required Can be determine(1. The threa(Ito I)e cut should be located in the first cc)lumn under ttle lleading "Threadsper Inch.,, Ill the SCCOlrd COlumn under the Ileadillg "Stu(I Gear" is listed thenumber of teeth in the chang-e gear \\.hich should be placed on the rc,versestud (A) of the lathe. (See Fig. 3.) In tile third COlu-1 under the headlng"Idler Gear" is listed the fig.ure number of the cliagrlm on the index chat-t

showint,a. the correct arrangement of i(ller gear an(I cl""p(""d gears. Ill tllefourth column uncler the hea(ling "Scrc\\r Gear" is listed the llumbCr Of teethill the gear tObe I)laced On the lead Sere". (C).

After sclccting. the aha"ge a,crears necessary for cuttimg the desi1-ed thread.

place them on the reverse stu(I ancl lea(I screw respccti\-cl}. and co-ect thenl\\,ith the idler gear an(1 conlPOund gears, aS She"m On the Change t.a-ear Chart

I Procedure for 9-inch I.Workshol," I-athe i9 Same aS for Standard Change Gear Lathe.

4


Fig. 4. Change Gear Chart for |0" Standard Change Gear Lathes.SiTnilar Ct`arts at.a used On Other Sizes Of Standnrd Change Gear Lathes'

The spacing colla1. On the lea(I scre\\. nlust bc place(I olltSi(1c the scre\\.

gear) as sho\yn in 1.`ig.- 5. \\.hen simple gc.arillg. iS uS(.(1. ancl inside tllc sc1.e\r

gc.ar, as sh(""I ill Fig- 6. \\.hc.n c.("lil")uml grt.:lt.ing is tlSe(1.

|CRLW

o i)g

F'ig. 5. PositioT' Of Spacing Collarfar Simple Gent.ing.

_0

S"G"COLLAISCnE*ce^t\ g I

I I

Fig. 6- Position of Spacing Collarfor Compound Gearing.


Adjusting Levers of Quick Change Gear Box for

Cutting Various Pitches of Screw Threads

The only difference bet\+.eon the Quick |'hange Gear Lathe ancl the Standardchange Ge'ar Lathe is that the Quick l,ha-nge Gear Lathe is tired \\.ith a gear boxwhich permits obtaining \,arious pitches of screw threads l\ithout the u=-e ()i loosechange gears. (See Fig. 7.)

The scre\\. thread chart is attached to the gear bo-i. aS Shout-n in 1.'ig. 8 below.This chart reads directly in threa(lS Per inch. 1l is onl_\. necessal.y to arrani!i thelevers of the gear box as in(licate(I all the ill(let pl:lie ill OrCler to obtain \-alious

_ccrew threads and fee(ls

The I)itch Of the threilCl tO IJe Cut

is determine(1 by smtini! the sli(ling

gear A. top let.er B :m(I tumblerle\.er C of the quick change i!carbox so that they confl)rm \\.ilk thethrea(I clltting chart. For examl)let

to cut 3 threa(ls per inch the sli(ling

gear A is pushe(I in. the top le\-erB on the gear box is 1)llShed tO

the extreme left p()sitioIl an(I the

tumbler lever C is I,lace(1 ju-.I be-low the column in


Lathe Tools Used for Cutting Screw Threads

The point of the cutter bit mustbe ground to an angle of 6Oo for

cutting screw threads in the lathe.as shown in Fig. 9 at right. Acenter gage or angle gage isused for grinding the tool to theexact angle required. The top ofthe tool is usually ground flat,with no side rake or back rake.However, for cutting threads insteel, side rake is sometimes used.

A formed threading tool is usu-ally used if considerable threadingis to be done. Figs. ll and l2 il-

lustrate two good types of formedthreading tools. The formed

Fig.9. Center gnge used for checl{ing angle ofcutter bit ground lot. cutting screw threads.

Fig. 1O. Side View of lathetool cutter bit ground for cut-

tir'g screw threads.

threading tools require grinding ontop edge only to sharpen and therefore always remaintrue to form and correct angle.

For cutting American \_ational Standard ScrewThreads finer than 10 per inch) the point of the toolis usually left sharp or u.ith a very small flat to pro-\'ide clearance at the bottom of the thread. Hc)wever'for cutting coarser I)itches of threads and when maxi-mum strength is desired) the point of the tool ma),be ground flat) as shown in Fig. 13. The flat on the

point of the tool should be one-eighth of the pitch.(See Fig. 2, Page 2.)

The illustration at the bottom of the page, Fig. l4)

shows a gage \+-hich

mav be used for grind-

ing threading tools tothe exact angle re-

quired fc)I standardscrew threads. r\-otethat the gage sho\`.sthe flat for the pointof the tool as well as

the exact an.gle anddepth of thread.

FIB. |4. Standard screw threadtool gage for grindlng

threading tools.

Fig. |0-A. End Viewof lathe tool cutterI.it ground for cut.ting screw threads.

F'ig. ll. Formed threading tool,solid type.

Fig. l2 Fol.mad threading tool,spring type.

T`.-.._'-`

EEEEEFig. l3- Forged steel threading tool.

T|'].en cllt[illU.SCrelU ll".C(I(lS in SIC(>l llSC I)lenlU 0/ ()i[ an(i a

snlO(Jl1\ lhrc(Ill ll)ill I.e.Sl|ll.


Correct Position of Lathe Tool for

Cutting Screw Threads

For External Threads

The top of the threading tool

should be placed exactly on center.as sho\`.n im Fig. 15 at right) for cut-

ting extemal screw threads. Note

that the top of the tool is ground

flat and is in exact alignment with

the lathe center. This is necessary |o

obtain the correct angleofthethread.

The threading tool must be set

square with the work) as shown in

Fig. l6. The center gage is used tO

adjust the point of the threading

tool and if the tool is carefully set a

perfect thread will result. Of course)if the threading tool is not set per-

fectly square with the work, theangle of the thread will be incorrect.

For lntemal Threads

The point of the threading tool is

also placed exactly on center as

shown in Fig. l7 at right for cuttinginternal screw threads. The point of

the tool must be set perfectly square

with the work. This may be accom-

plished by fitting the point of thetool into the center gage) as shown

in Fig. l8 at right.

When adjusting the threading toolfor cutting intemal threads. allow

sufficient clearance bet\+-een the tot)I

and the inside diameter of the hole

to I)ermit backing out the tool when

the end of the cut has been reached.

However the boring bar should beas large in diameter and as short

as possible to prevent springing.

Fig. l5. Top of cutter bit should be set oI` Center forcutting screw threads.

Fig. 16. Cutter blt should be set square with worl{, asshown above, for cutting exterllal screw threads.

Fig. l7-Forcutting in-ternal screw threads|top oF cutter bit shouldbe setexactly on center.

Fig, 18- Cutter bit should be set square with work. asshown above for cutting into-Elf Screw threads.

8


Fig. l9. Compound rest of lathe set at 29O anglc` for a"tting screw thl.Cads.

PositI.On Of Compound Rest for

Cutting Screw ThreadsIn manufacturing plzmts \\hcre maximum pro(luclioll

is desired it is customary to place the c"mpound rest of

the lathe at an angle of 29o for cutting screw threads.

The compound rest is s\1.uns around lO the right) aS

shown in Figs. l9 and 20. \\_hen the c()mpound rest isset in this I)osition and the compound rest screw is used

for adjusting the depth of cut. most of the metal is re-

moved by the left side of the threading tool. (See Fig.2l.) This permits the chip to curl out of the \\.ay better

than if tool is fed straight in) and preyents tearing the

thread. Since the angle on the side of the threading toolis Boo, the right side of the tool \+.ill sha\.e the thro:Ill

smooth and produce a nice finish) although it does notremove enough metal to interfere \\iththe main chip which is taken by theleft side of the tool.

Fig. 2l. Action of thread cutting tool when com.pound rest is sot at 29O angle.

Fig. 20. Correct angle of com-pound rest for thread cutting.


I_

II

i]llrlF-I- I \Fig. 23. Latt'e dog should be tight when work is mounted between the lathe

centel.s for cutting screw threads.

Mounting the Work\|'hen mounting `\.orb between the lathe cen-

ters for cutting screw threads.makesurethelathe

dog is securely attached before starlin.g to cutthe thread. If the do(,cr should slip the thread \`,ill

be ruined. \_ever remo\.e the lathe dog from thework until the thread has been completed, andif it is necessary to remo\.e the `\.ork from tile

lathe before the thread is fmished makesurethatthe lathe dog is replaced in the same slot in

the face plate.

\\'hen threading \`,orb in the lathe chuckmakesure the chuck ja\\.s are tight and the work is\+.ell supported. The chuck must be tight enough

on the spindle to pre\.cnt unscre\\.ing when thelathe is reyersed. \-e\.er remo\.e the \\-()rk from

the chuck until the thread i< fmished.

\1'hen threading long slender shafts. use a fol-lo\+-er rest, as show,n in I..ig. 26. The center rest

may be used for su]lporting hng \1.Ork Such aS

pipe \`-hich is to be tllreade(I nn the inside.

(See Fi..c,. 27.)

Fig. 26. Cutting a long slender screw thread' with theaid of a follower rest.

1O

in the Lathe

Fig. z4. Chuck jaws should be tight whel`cutllng screw threads.

Fig. 25. \Vork should be pushed bacI{ inchuck and well supported.


Fig. 28. Thread cuttil`g stop attached to dovetail of saddle.

Adjusting the Thread Cutting Stop

On account of the lost motion caused by the play necessary for smooth operationof the change gears) leadscre\+,) half nuts, etc., the thread cutting tool must be With-

drawn quickly at the end of each cut) before the lathe spindle is reversed to returnthe tool to the starting point. If this is not done, the point of the tool will dig

into the thread and may be broken off. The thread cutting stop may be used forregulating the depth of each successi\.e chip.

The point of the tool should first be set so that it just touches the w.Ork) then lock

the thread cutting stop and turn the thread cutting stop screw until the shoulder istight against the stop. when ready lo take the first chip run the tool rest back byturning the cross feed screw to the left se\.eral times and move the tool to the pointwhere the thread is to start. Then turn the cross feed screw to the right until thethread cutting stop screw strikes the thread cutting stol).

The tool rest is now in the original position) and b),

turnint.c, the compound rest feed screw in.002" or.003,,

the tool will be in a position to take the first cut.

Micrometer Collar May Be Usedin Place of Stop

The micrometer collar on the cross feed screw of thelathe may be used in place of the thread cutting stop)

if desired. To do this, first bring the point of threadingtool up so that it just touches the \+,orb, then adjust themicrometer collar on the cross feed screw- to zero.

Fig. 29. Micrometer collar oncross feed screw of lathe.

All adjusting for obtaining the desirecl cleplh of cut should be done \\.ith the com-

I)Ound rest Screw. |\-ithdra\Y the tool at the end Of each Cut by turning the Cross feedscrew to the left one complete turn. return the tool to the starting point andtum the cross feed screw to the right one turn. stop])int,c' at zero. The compound restfeed screw may then be adjusted for any desired depth of chip.

ll


Taking the First Cut on a Screw Thread

Take Ligllt Trial Cut to Check

Lathe Set-up

After settillc30.uP the lathe. aS eXPlaimed onthe preceding pat,rres. arrange the lathe for aslow, back-Seal-i.d SPin(lie SPeed an(1 take avcr)I light trial cut jtlSt CICCI) enOugll toscribe a_ line on the s|ll.fz|ce of the \yo1.k-. aSshowll in Fig. 30. The purl)ose of this trialcut is to make.sllre that tlle lathe is art-zlllg-ed for cutting the desired pitcll Of thread.

To check the number of threads per inch)

place a scale against the \t-orb) as sho\\.n in Fig.31 so that the end of the scale rests on the pointof the thread or on one of the scribed lines.Count the spaces between the end of the scaleand the first inch mark, and this \\-ill giye you

the number of threads I)er inch.

It is quite difficult to accuratelv count fine

pitches of screw threads. as described above. Ascrew thread gage as illustrated in Fig. 32 isveryconvenient for checking the finer pitches of screwthreads. This gage consists of a number of sheetmetal plates in which are cut the exact form ofthe various pitches of threads and each plate is

stamped with a number indicating the numberof threads per inch for which it is to be used.

The flnal check for both the diameter and

pitch of the thread may be made with the nutthat is to be used or with a ring thread gage, ifone is ayailable. Fig. 33sho\\.s how the nut maybe used for checking the thread. The nut shouldfit snugly \\-ithout I)lay or shake but should notbind on the thread at any point.

If the an.gle of the thread is c()rrect and the

thread is cut to the correct depth it \`.ill flt the

nut perfectly. Hot+,e\.er, if the angle of thethread is incorrect or the lead is incorrect,

the thread may appear to fit the nut but willonly be touching at a fe\\. points. For thisreason the thread should be checked by othermethods in addition to the nut or ring gage.

12

Fig. 30. Trial cut to check set.up forthread cutting.

Fig. 32. Screw thread pitcl` gage.

Fig. 33, Screw thread properly fitted\o nu|.


Fig. 34. Applying lard oil to work with small pait|t brtIShwhile cuttit'g screw threads in a tool steel thread gage.

Use Lard Oil When Cutting Screw

Threads in Steel

Lard oil should be used w.hen cutting screw threads in steel in order to producea smooth thread. If the oil is not used a very rough finish will be caused by tearingol- the steel by the cutting tool.

If lard oil is not a\.ailable any good cutting oil or machine oil may be used. If

trouble is experienced in producing a smooth thread, a little I)O\\.dered Sulphur may

be added to the oil.

The oil should be ilpplied generously preceding eacll Cut. A small paint brush isideal for applying the oil \+.hen cutting external screw threads, as illustrated abo\.e.Since lard oil is quite expensive. many mechanics place a small tray or cup just belowthe cutting tool on the cross slide of the lathe to catch the surplus oil which dripsoff the work. In manufacturing plants where the lathe is to be used exclusi\.el_v.

for cutting screw threads in steel it is customary to equip the lathe with an oil

pan and oil pump to provide a continuous flow of oil.

13


Resetting the Tool After the Thread

Has Been Started

If the thread cutting tool should need re-Sharp-ening) or if for any other reason it is necessary toremove the thread cutting tool before the threadhas been completed, the tool must be carefully re-adjusted so that it will follow the original groo\,ewhen it is replaced in the lathe. There are set.Oralmethods by which this can be accomplished.

Before adjusting the tool) Set the POint Of the

cutter bit square with the \\.ork) as described on

page 8, and take up all the lost motion in thechange gears, half nuts) etc.. by pulling the belt

_Fig.35. Adjusting I,oint of threading

tool to conform with thread.

for\`-ard by hand.

If the lathe has a compound rest) the COmPOund rest top may be Set at an angle,and by adjusting the cross feed scre\\. and compound rest feed screw simultaneouslythe point of the tool can be made to enter exactly into the original groove.

If it is not convenient to use the compound rest for readjusting the threading tool)the lathe dog may be loosened, the work tumed so that the threading tool will matchthe groove and the lathe dog tightened.

Another method that is sometimes used is to disconnect the rev.erse gears Or thechange gears. turn the headstock spindle until the point of the threading toolenters the groov.e in the work, and then reconnect the gears.

Finishing the End of a Threaded Piece

The end of the thread may be finish-ed by any one ot set.eral methods. The4So chamfer on the end of the threa(I,as shot+.n in Fig. 36) is commonly use(Ifor bolts. cap screws, etc. For machined

parts and special screws the elld iSoften finished bv roun(ling with a form-ing I"ol, as shot-1,n in Fig. 37.

It is difficult to stop the threadingtool abruptly so some p--ision is llsu-ally made for clearance at the en(I ofthe cut. In Fig. 36 a hole has beendrilled in the end of the shaft, aml inFig. 37 a neck or hrrroo`.e has been cutaround the shaft. The groo\-a is I)refer-able as the lathe must be run \-Cryslowlv in order lo obtain satisfactoryresults \\.ith the drilled hole.


Fig.38. Thread dial indicator attached to lathe carriage.

Use of Thread Dial Indicator in

CuttI,ng Screw Threads

A thread dial indicator or thro:ldin,,rr clock is usually

used for cuttinf.a lonf,or screw threads. This de\-ice permits

disengac,c,in[,cr the half-nuts at the end of the cut, returning

the carriage to the starting point bv hand, and then en-

gaginc,c, the half nuts at the correct time so that the tool willfollow the ori.qinal cut. This sat.es time on long tIlreadS :lnd

it alsc) eliminates the necessity of re\.ersinq thelathespindle.

The threading dial consists of a \+.orm \+-heel which is

attached to the lo\\er end of (i shaft and meshed with the

lead scre\+.. On the uI)Per end Of the Shaft iS the dial. --\S the

lead screw reyol\.es, the dial is tumecl ancl the numbers on the

\\.hich the hall- nuts ma}, be engaged.

iS:Fig'39 Face of tllread

dial ind|cator.

dial indicate points at

\\'hen cuttin..a, short threacls it is better lo lea\-a the half-nuts engaged with the lead

scre\\.. and \\.hen the en(I of the cut is reached. withdraw the tool and reverse the

lathe spindle to retum the tool to the starting point.

Rules for Operating Thread Dial on South Bend Lathe

For all e\.en numbered threads, close the half nuts at any line on the dial.

For all odd numbered threads. close the half nuts at any numbered line on dial.

For all threads in\-olying one-half of a thread in each inch, such as llJ/2) Close

the half nuts at any odd numbered line.

The thread dial cannot be used \yhcn cutting metric screw threads.

15


Cutting Multiple Screw Threads

Multiple screw threads are used for ad-

justing screv..s and other applications wherea coarse lead is desired. The depth of thethread is less than for a single thread ofequal lead. making a stronger screw.

-i multiple thread hating two grooves iswho\yn as a double thread) one hat.ing three

grooves a triple thread. etc. (See Fig. 4O.)The pitch and lea(I of a multiple thread

should not be confused. The pitch is the dis-Fig. 4O. A multiple screw thread having

two grooyes (Double Thread)

lance from a point on one thread to the cor-resl)onding point on the next tbreild, `\.nile the lead i5 the distance a SC-. thread

ad\.ances ln one turn.

\\.hen cuttin|, multil)le threads in the lathe the first thread is cut to the desireddepth. The w.orb is then re\-ol\.ed part of a turn, and the Second thread Cut, etC.

In order to obtain an exact spacing it is advisable tO mill aS many equally SPaCed

slots in the face plate for the lathe dog as there are multiple threads to be cut. I.`ora double thread. t\\.o slots; a triple thread, three slots. eta. If it is not convenient

to cut slots in the face plate. equally sl)aced studs may be attached to the face plateand a straight tail lathe dog used.

Another method for indexing the work when cutting multiple threads is to dis-engage the change gears after the first thread has been completed and turn thespindle to the required position for starting the next cut. If a double thread isbeing cut and there are 48 teeth in the stud gear, the spindle Should be turneduntil 24 teeth hat,e been passed, then the gears engaged and the See()nd thread Cut.

Cutting Left Hand Screw Threads

A left hand screw is one that tums counter-clockwise when advancing (looking at head ofscrew) as shown in Fig. 42. This is just the oppo-site of a right hand screw. IJeft hand threads areused for the cross feed screws of lathes, the lefthand end of axles for a|ltomobiles and \\.ar.c'ons. one

end of a turnbuckle. some pipe threads, etc.

In cuttingleft handscrewthreads the lathe is setup exactly the same as for cutting right hand scre",threads) except that the lathe must be arranged tofeed the tool from left to right.insteadoffromrightto left) when the spindle is rev.ol\.ing for"-ard.

16

Fig. 4|. A left hand screw thread.

Fig 42. A left hand screw advar'ceSwhen turned counter_clockwise.


How to Calculate Change Gears for

Cutting Screw Threads

If it is necessary to cut a special thread that does not appear on the index chart()i a lathe or if no index chart is a\.ailable, the gears required can easily be calculate(I.All South Bend IJatheS are eV.en Seared; that iS, the Stud gear revolt.eS the Samenumber of revolutions as the headstock sl)indle, and when gears of the same size areused on both the lead screw and stud) the lead screw and spindle rev.ol\.a the samenumber of revolutions) so it is not necessary to consider the hr'earing between thl.headstock spindle and the stud gear when calculating change gears.

If simple gearing is to be used, as shot+.n in I.'ig. 43) the ratio of the number ofteeth in the chant,c'e gears used `\.ill be the same as the ratio between the thread I()be cut and the thread on the lead screw. For example. if lO threads per inch are toI)a cuton a lathe hat.ing a lead screw with 6 threads I)er inch, the ratio of the change

gears would be 6 to lO. These numbers may be multir)lied by any common multiplierto obtain the number of teeth in the change hoe:+rs that sh()uld be used.

Rule-To calculate change gears. multiply the number of threads per inch to becut alld the number of threads per inch in the lead screw. I)v the same mumber.

Example: Problem-To cut lO threads per illCh Oil lathe hat.inhc' lefld sol.eWwith 6 threads per inch.

Solution- 6 x 8 = 48 - \-o. of teeth in gear on stud.10 x 8 - 80 -\_a. of teeth in gear on lead screw.

If these gears are not to be found in the change gear set, a.ny other number maybe used as a common multiplier, such as 3) 5, 7) eta.

When compouncl Searing) as shown in Fig. 44, is used, the ratio of the com-pound i(1Ier t.a-ears must also be taken illtO COnSi(leratiOl1, 1)ut other\\.ise the ca1-culations are the same as for simple gearing. For e.T{am1)1e, if tllc coml)otmdidler gear ratio is 2 to 1. the threads cut are just twice the number per inchas when simple gearillg i-lSed.

Diagram Showing Simple and Compound Gearing

Fig. 43. Simple Searing.

l7


Using Taps and Dies to Cut Screw Threads

in the Lathe

TaI)S an(1 dies are often used for cutting sc-. threads ;n the lathe, especiallyin faclorics \\-here maximum procluction is imI-tant There are se\-Oral ".a.\.a inwhich the lathe mav be set up for this kin(I of ",ork' as illuStrateCl btlO\`,.

Fig. 45shows a common hand tap being used to thread a llut \`hiCh iS held in thelathe chuck. The handle of the tap \"ellch rests against the \- \`Zl), Of the hthe I)eel.and as the hthe spindle re\,ol\.es slo\\.ly the tail-stock hand "heel is turnecl to feedthe tap through the nut plellty of oil shoul(I be uced (m the tap if the nut is n-leof steel.

Fig. 46 shows a self-opening die mounted in the taill=ock spin(nc ("- the lathe forcutting threads on studs. -+(ter starting the lathe spindle the tail+tuck is pusbe(I b),ham(I along the lathe be(I until thedie takes hold aml Starts tO Cut the thread, af-ter u,hich the clie "ill I)ull the tailstock along. ,i die holder can be made for thetailstock to hold comm(m button dies, but if the.y are used it is net,eSSar), tO re\er`ethe lathe spindle \\.hen tile end of the thread h:ls been reached.

Fig. 47 shows a die nlOunted On the tool rest Of the lilthe. \\_hen mOullted in thismanner the lead scre\\. and half nuts of tlle lathe may, I)e use(I to feecl the die so a

pert-eat lei"1 i> obtainecl in the thread cut.

Fig. 48sho\\-s a lathe equippecl with special fl-`lure fur hal(ling the \`-ork on the face

plate. ancl a turret for hal(ling drills. boring bar and taps for boring and threadingthehole.

Fig. 45. Using a tap for threading a nutin the lathe.

Fig- 47. Die mounted on lathe carriage forcutting accurate threads.

I;g. 46. I)ie mounted in tailstock of lathe iouthreading studs.

Fig' 48. Tap mounted in lathe turret forthreading special l`uts.

l8


Standard Screw Thread Forms

\J5/

A|lEIllC:I+ X_lTIOX.|I. SCREW TIIREAD/I.'ormerl}- U. S. Stan(llrd Scrc\\. Thr|.art)

I._OR}lt I, \

I

1) = Plllh i_ -----i-a lh'(l> I,rrI

D-lJ(.I)th-I x 6i95?I)

F==l.1'lt=-8

FREItCII _lND IXT1._,RX,1TIOX_1L SYSTEMSTJX1)ARD SCREW THEE:1D

I,'olt _\1 UI,AI-P|tchiI1 }I_\II) -=I)cpth of Ellg:`pe-

lllellt-I x.o19.5]t=-Dcptl| of 'l`hrc:|d

---=P I .(l915

I -_\ILIXllIIum lt:|dius-P x .Ol;.'33

PI' r1,\t-a.

W'IIITT+'ORTII STANDARI) SCREW TIIREAD

FOR|I|-I,AI

I-PIlch=-\-(> T1|'ds. Per ln.

D-I)t.I)th-P. x 610.?

R_-_Ra(JIu-- 1`i7 U

i SQUARE SCRI-.W THREADFOR}[|-L\

iI-Pllch=

_\\) 'Ihd> l'(.rln

D==D(.pt1\= I'.\_GOO

F==SIt.let.=-P x 50O

ACME SCRII.ll' THR1=JI)

p| ")R""\li_u lhl1\ Il(,rln

I)-I)el)t1|-,_ P + C10

F=I-lat=.;ro7 I(I--Ill,lI-=_i707 T`-rx)S_,

a SH^RPE. Zoo W'OR|I SCRE|+I TIIREAD

1.'OR}lL'L \

pJ I.-"\"` - I\_I, 1l (lh I|t.rln.

I)-IJtptI - (1866P

I.'- ll I _31 P

C=rl,ll= _135P.


Transposing Gears for Metric Screw Threads

In Fl-:"lc-c. a"(I othc.1- P"-tS ( I I.:m.o1)e thl.

lnt(.1.natI()1|al Stu|(ku-(I .\I|.tl-ic S|.1.i.\\- Threa(ls

ln(I otllt.1- +\\lC.nth OI m(.trlC tlU-i.,,I(I+ Zll.e uS|.(I.

I hcse tllH.Z-I(h art. u\(.(I i.\tt.mi\.C.l\- On mi|-ro-

ll.(J])C.i. I(.Ic`l.opt.+ "I(I oth(.I o1)tic:I-I ,"ld SCictl-

til'll. lml1-umC.nt-i :"I(1 stir.g-I(.:ll 1-t1.111mlltS.

\h.t1-iC thrC.ZL(ls al.e mC.:lt'm.Cd ill tll|. 1\Iet1.iC

S\.item imt("(I ot- "l the E"gli.h s\>lcm. a""Im ( "k.I. toc\lt thclll (m :I I:lthl. llZ"ing-"l Eng-1i`1l k.ad -.c.I"\-, m(.tliC tl.ZmÌ"lìng f,a-(,a1-I lllu-`l

I)c tls(.(I- lhl. r<ltio I".l\\-a(.n lhl. Engli,+h S\.s-

1(.m ln(I l\l(.tuc S\-_`t|.m (lf mc.asurc.me1|t- iS

+uc.ll th:lt ct-I"""I(-I trt"l.I")."g ge"I ll:l\i"g12/- tc{.t1| ,,Ill(I loo I(.(.t1| or 1_)/- teeth z|1|(1 50

tc.i.th lml}. I)i. u".(I to i.ut 1)lTh.l.t nlel1-iC ,SCre\\.

tl|r(.:I(ls.

"\[c.tlic. tr.,m+poì"g- i:l.:"-I arc. sul)1)lie(1 for

I")tb Sl"l(kl"I |'l-lge (;l.ill- L.,LtIIC` Zlml OlliCkchz"lgc. Gc.al- I,:lthc` 1""ingr I..Jlgli.Ìl_k.[1d"`crl.".,+. ,\ melzLI ch,.".t ,ÌmiI:"- to F-ig. 51, sllO\\--

illgr tllc. '"i."li:i.-mt ot' lhc. gre<."-.s l'ol- cuttlngr\-".")u+ 1)itClll.` Of mCt1.iC .`C1.C\\. th1-ea(1s, is

:LttaChe(1 to (.ach lzlt1|e.

Thc. i-orm ol' thc. mc.tric tl|r(.a(I is sim'1Ial. to

the -\1m.riC:ln \àtiOl1<,ll Scre\\- Threzld fo1.nl,I"",ills- a r)Oo i"cltl(li"a a"grlc all(1 a fl:lt at thc.tol) ot' tlle tl"-c,I(I. I"lt :I -I-,Ill rzl(li\ls at tile

ro()t ()I- tl1(. tl"-Cad 1"-O\.i(lCS gTeatel. Clearance.

(See Figr. 50.)iI ll

FORMUI.AP=Pitch in MMD-I)cT,th Of Enpage-

m|,nt=P ;{ .¢JO.5H-Dl.!lth of Thread

-I' I .694.-)r-Maximum Radius

=P x .0633r F_Flat=£

`

Fig. 50. International Standard Metric ScrewThread Form.

Fig. 52. Metric TransposingGear Attachment for 16',

Lathe.

Fit,I 49. Metric TI-aml,OSinP GearsAttached to I_othe.

Fig. 5|. Metric Thread Cuttingcharc for I.alhe with EnglishThl.Cad Lead Screw and Metric

Transposing Gears.

International Stenderd Metric Screw Thread


Fig. 53. A Standilrd Change Gear Lathe with Metric Lead Screw.

Lathe with Metric Lead Screw

}Ietric lathes equipl)ecl \\.ith metric lead scrc`\s arc u`C.Cl \\.here -.tl-iC

scre\\, threads are cut exclusively. Tlle metr:a lathe :s i(ll."tlCal \\ith the

Ellglish lathe) excel)t that the lead scre\`',

cross feed screw and colllPOund rl.St SCre\\,

hate metric threads an(I all t,a-racl\lated

collars read in the llletriC S}.Stem instead

of the Ellglish system.

challge gear equilmlent SuPPliC.d \\,ithtlle hthe I)ermits cuttillg either 1.it,a-ht hand

or left hand metric scrc\\, thrl.acts. -,\n

index chart (Fig. 54) is zltta-ched to the

lathe to show the a-"1gCment Of thechant3O-e gea1.S for Cuttillg the Various

pitches of tllreads.

when it is necessar\, to cut both Englishand metric screw threads, an FJlgrlish leadscrew \\.ith metric trans1)OSing equiprlentis prefc1-able tO a metric leaCt screw.

21

Fig. S4. Index Chart far Standard ChangeGear Lathe with Metric Lead Screw.


At Right-No. t87-YN IO"xNe3i-Lilon:helCs1:cuttChaEiaecfay

Underneath Belt MotorDrlven. Quick Change Gear.Back-Gtared Screw CuttingPreci8|On Bench Lathe.

A pc'pular type high qual-ity precislon lathe.

At L.ft-No. 444-Y 9. _T{3' Model A South Bend"Workshop" AdjustableHorizontal Motor Driven'Back-Geared Screw CuttingPrecisiol\ Bench Lathe.

One of the finest smalllathes wc have I.vcr built.

At left--No. 8ll7-C l6rvx6' New Mnd>l South BendTool P\oom Unclerneath BeltMotor Driven. Quick ChangeGear. riack-Glared ScrewCutting Precision Lathe.

A praLCtiCOl, t;ffieient and

veloped toSquare feetBend Back-Geared Screw Cutting Precision LatheL

ular |y\otor clriven lathe.

Below---The plant of thesouth Bend hat_he Works at nc±6

3:guatnh,.::,n.dn. :na:i:.nua;a.dThiin9 'jXut:l906 and has grown and de-

adnnan:::hpli9Cg=i;:l£yair:;ga tohfC4bJ*dcirn=8d8ehvOoWigWriI;i:1hy toaVtehca nT=nru;gtcue':folf8sO;3?tho.I.f_.


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