0G

0H

3 JANUARY 1957

TAPERS GEOMETER gives a few hintson machining and checking

THE DESIRABLE true running ofa pulley or flywheel taper-fitted on a shaft is generally

best ensured by finishing faces andoutside diameter with the c o m -ponent mounted on a mandrelr u n n i n g between centres in thelathe. All important surfaces are _thus finished at one setting andthe wheel is both parallel andconcentric-a condition difficult toachieve by chucking and re-chuck-ing no matter how carefully thismay be done.

The preliminary roughing out isadvisedly done in a chuck (whichcari be a four-jaw independent typewith the jaws reversed if necessary,since heavier cuts can be taken in a

chuck than on a mandrel). leavingabout 1/64in. surplus for finishing.In this way, scale or any hard spotsin the casting can be successfullydealt with in the rough machining,using a slow rotational speed andtaking cuts deep enough to be every-where well below the surface.

Diagrams A and B illustrate typicalchuck set-ups for rough machining asmall flywheel, the material removedat each being shown by the shadeda r e a s At each set-up the wheel ispushed back to the jaws for facialalignment and the jaws are regulatedfLthpenpheral or general spinning

Although it is not vital to do soit is generally best to machine the taperbore on the first set-up, and also toturn along the outside diameter. as faras possible. On the second set-upit is. then practicable to fit a tapermandrel in the bore and employ itsend for checking and truing-if itshould happen that it is difficult toapply the pointer. of a surface gaugeto a portion of the outside diameter.

Moreover, should a small errorresult from the-setting, cleaning cutscan easily be taken on the mandrelset-up C since the particular faceswill be towards the tailstock.

TAPER UNIFORMITYWhen the shaft is available on

which the wheel is to fit, it,can be

11

tried in the taper as this is machined(or reamed) in order to locate thewheel endwise correctly-in whichrespect, should the taper bore be madeslightly too large a reducing cut canalways be taken over the face.

Alternatively, the bore can be sizedfrom a reamer or mandrel, as at D,which may be necessary if the com-ponent is a replacement, or one isrequiring to standardise tapers forwheels to be fitted on different shafts.In the case of a mandrel, a shouldercan be left in machining or a sleevecan be fitted for a distance X to obtainwhen the taper is at correct size;in the case of a reamer, a sleeve isessential when the distance can bemeasured with a rule, or a smallgauge made just to push into thespace.

A common type of gauge for thismethod of sizing tapers is as E, wherethe taper portion ends in a step on oneside Xl. On the tool being pushedtightly into the bore to be tested, thestep should-go just below the surfacewhile the full diameter just standsproud-showing the bore to be withinits particular tolerance.

Should the gauge enter too fara light correcting cut can always betaken across the face-assuming thereremains sufficient material on otherfaces to machine them into relation-ship-which is as good a reason asany for finishing the taper early in .the proceedings.

The principle also applies to ashaft F where a ring gauge (corres:ponding to the component) is used.This may have a step X2 to locatethe position where the taper finishesat the full diameter, or at the oppositeend on the small diameter, though abetter way i s to take the distance Yfrom the face to a shoulder or the endof the shaft.

If a keyway is required in a wheelits cutting should be the fmal opera-tion. From square silver steel a toolis made as G, turning the shank, filingthe surplus to tool shape, then harden-ing and tempering. Planing cuts aretaken from the saddle with the chucksecured against rotation, as at H,by a holding strip from backplate toheadstock.

MODEL ENGINEER

A SOUND lathe-work principleis to machine as much aspossible of a component at

a single chucking in order topreserve alignment between facesand concentricity on diameters andto save time in resetting.

Many chucks do not hold reallytruly, particularly after a period ofuse, and there are components whoseslenderness or fragility renders theirrechucking a problem. Often, ofcourse, a second set-up is inevitableand consideration must then be givento ensuring truth, avoiding distortion,and performing the work with theminimum of trouble.

A diameter, bore, or face on mater-ial gripped and machined in thechuck is obviously true until thematerial is moved, so that any suchfeature employed for alignment willresult in a component being true on asecond set-up.

Stub mandrels, as at A, machinedfrom material held in the chuck,provide for the true setting up ofbushes, sleeves, small wheels andpulleys., previously finished in thebore either by accurate boring in thelathe or drilling and reaming. Frictiondrive is sufficient for light cuts, anda mandrel can be turned with a slighttaper, or a taper produced by a Swissfile and/or abrasive cloth. Pushing acomponent on by hand is sufficientand in some cases, a smear of oil isadvisable to prevent seizing. If the

DRIVING/BAR

SLEEVE/

10 JANUARY 1957

Second operationset-ups

component is tight a clamp or pro-tected-jaw pliers may be needed for

the case of wheels or pulleys withspokes or holes, a solution is toemploy a driving bar bolted to themandrel and shaped and “ set ” asrequired to pass between spokes orthrough a hole. The mandrel shouldbe partly finished, the bar fitted, thenremoved while the locating diameterand face are machined. Again supportfrom the tailstock may be necessaryin use, as at B.

removal.Using a nut and washer, a com-

ponent can be gripped on a mandrel;and when overhang is considerablesupport can be given from the tail-stock. Mandrels may also be runbetween centres-but, naturally, moretime is needed for making them.For ordinary stub mandrels scrapsand off-cuts of most common mater-ials can be used-mild steel, brass,aluminium alloy: and they can begiven a reference dot mark to No 1jaw for rechucking when necessary-or set up in an independent chuck.

A problem may arise when a largediameter must be turned with thecomponent on a mandrel, since afriction grip, or even a mountingwith a nut on the end of the mandral,will not ensure a slip-free drive. In

0E.

On more elaborate mandrels torun between centres an expandingsleeve or opposed cones may be usedin the bores of components, as atC-the cone type somewhat wantingin accuracy. For the expanding sleevethe mandrel is tapered and the sleeveslit lengthwise to be displaced by anut. A second nut at the large end isadvisable when the mandrel taper isslight-to free the sleeve by screwingback against it.

Provided with threads externally orinternally stub mandrels in the chuckafford means of setting up screwedcomponents, as at D, for operationson the outer ends-turning, facing,threading, etc. To ensure truth,threads on the mandrels should bescrewcut or cut with dies or tapsfrom the tailstock.

Spindle-type components or thosewith shanks for location can be setup in various sorts of clamping orcontracting mandrels or collets. Asimple and effective type can be madefrom flat stock,. as at E, by bolting ona cap and setting in an independentchuck for drilling and boring--thecap then being eased on the jointface for a grip to be obtained on theshank of the component.

Collets to contract and hold com-ponents can be as at F. For theclamping type the stub mandrel isbored, turned, and a small undercutmade at the shoulder. Without remov-ing from the chuck the end is slitlengthwise with a fine saw. Forclosing, a clamp as shown is betterthan a collar with a screw. Thetype with a nut can have a taperthread by not running the die onfully, while the nut can be undersizefrom not passmg the tap right through

MODEL ENGINEER

By GEOMETER

I

NTRIC

MODEL ENGINEER 94

E C C E N T R I C M O T I O N SAND SET-UPS

MOTIONS employing eccentricsare common in appliedmechanics and can be

arranged in several ways.First, in conjunction with a con-

necting rod, an eccentric gives straight-line motion to a spindle or plunger.Second, without any intermediary,an eccentric gives straight-line motionto a spring-loaded tappet in a guide.Third, using a pivoted lever, aneccentric gives circular motion to itsfree end through a small arc-andsuch a lever may be spring-loaded likethe tappet for return, or this may bearranged by employmg a connectingrod between the eccentric and thefree end of the lever.

The first application is normal foroperation of the slide valve of a steamengine, or the plunger of a feed pump;while the other two applications arenormal for operating the petrolpumps of car engines.

Basically, eccentric motion is thesame as that of a crank-connecting-rod system, with the difference,however, that for most practicalpurposes the mechanism is non-reversible.

This is to say that, while a crank-shaft can be turned to operate a piston,or a piston pushed to turn a crankshaft,only an eccentric can be turned tooperate a spindle or plunger. Neithercan be pushed to turn the eccentric,owing to the friction involved in thedisproportion of the considerablediameter to the small throw.

Setting up barSince an eccentric is circular, a

loose one to fix on a shaft witha grubscrew can be machined in astraightforward manner in a lathe.A four-jaw independent chuck isemployed to set up the round steelbar for the outside to be turned tosize and the bearing surface machined.Then the whole bar is displacedeccentrically in the chuck for drillingand reaming the bore-or drillingand finishing with a boring tool.

The off-set of the bore (or throw)in relation to the outside diameter isnormally obtained in one of two ways.With the bar held in the chuck, as at

A, the off-set required is X, and isobtained by regulating the chuckjaws. Rotating chuck and bar, andbringing a flat-ended tool or pointerjust to touch the high spot, thenrotating a half-turn to find the lowspot, a gap, Y, is left, and this is twiceX.

Thus, Y can be measured with asmall rule-or a piece of material ofsuitable thickness or diameter can beused as a gauge (a drill shank, forexample).

This is true whatever the size oroff-set, as at B. If the shaded bar isdisplaced so its side is on the chuckaxis, the displacement is equal to theradius X, and when the chuck isrotated the gap is equal to the dia-meter Y. At a smaller off-set of thebar, Xl is half Yl.

The second way of obtaining theoff-set or throw is to mark the barwhile laid in V-blocks, centre-punchthe position, then bring this spinningtrue in the chuck.

If the off-set is kept in line with apair of chuck jaws, the position ofthese on the chuck face serves as aguide in preliminary setting-the facemay be ringed for reference, or a smallrule may be used to check movement of the jaws.

For a considerable off-set, packingis advisable between two jaws andthe bar, as at C, to avoid side thruston the jaws and admit of properadjustment. Strips of paper betweenthe packing and surfaces, in conjunc-tion with firm tightening, will avoidslip. Packing may also be used whenfouling of jaws would otherwise occur.

A bar may be set up in a V-angleplate, D, for machining or boringeccentrically, the angle plate beingmounted on the lathe faceplate andadjusted accordingly. Again, a singleeccentric can be bored clamped to aflat plate, E, this held in a four-jawchuck.

.

An eccentric mandrel, F (top), canbe used for machining outsides afterboring when several- eccentrics arebeing made; and for facing ends,eccentrics can be pressed or held bya grubscrew on a simple mandrel(bottom). q

17 JANUARY 1957

b.

USINGHE NORMAL faceplate suppliedT with a lathe provides means

of setting up componentswith flat bases or ends, the com-ponents being held by bolts throughthe plate and clamps on the frontor by studs or setscrews passingthrough the plate and screwinginto the components. For specialjobs additional holes may be drilledin the faceplate and spigot anddowel holes can be used forlocating purposes.

Nevertheless, to work convenientlyon a faceplate, components need tobe above a certain minimum size. Ifthey are small-but still of a type bestset up on a faceplate-they cannot bebolted direct owing to the size of theboss and spindle, so clamps must beused and these are necessarily largeand often out of proportion, and may

SUB-FACEPLATESeven cause serious obstruction to theuse of tools.

Again, there are many old latheswith solid spindles on, which it isnot possible to employ the drawboltprinciple--using a long bolt rightthrough the spindle to hold a com-ponent on to a faceplate or jig at thefront. Consequently, here too thereis a handicap on some classes of work.

In both cases, however, the solutionis to use a sub-faceplate, which inprinciple is merely a flat plate onwhich components can be attached inwhatever is the most convenient way-the plate itself either being held inthe chuck, or mounted on the normalfaceplate. And with the sub-faceplateprojecting from the normal faceplatethere is a flat face and access to therear for fitting and tightening boltsand nuts used for holding com-ponents.

Most normal materials can be used

SUB-FACEPLATE

COMPONENT

-24 JANUARY 1957 131

for sub-faceplates in the form ofparallel discs for holding in the four-jaw independent chuck, or flat rec-tangular bars-for similarly mounting-or attaching to the normal faceplate.Aluminium alloy, brass. and mildsteel can be used; and the mostconvenient thicknesses are 1/4in. to3/8 in. If desired, a simple casting couldbe obtained from a wood pattern andmachined.

At A is shown a parallel discmounted in the independent chuck-jaws reversed-for a small componentto be clamped by its flange to thefront. To ensure true running thedisc should be faced after mountingfor which reason soft material likealuminium alloy is best.

Countersunk screws for the clampsminimise obstructions at the front;and instead of the screws screwinginto the plate they can be spacedbetween the chuck jaws, pass throughclearance holes in the plate and beprovided with nuts on the back. Insetting up the clamps are only partiallytightened until the component isrunning truly.

Small component set-upAt B appears a plate mounted on

the normal faceplate. Two flat barsabout 3/4 in. square are attached to thefaceplate with countersunk screws, X.Then the sub-faceplate is attached tothe front with screws, Y, which passthrough the bars. and the normalfaceplate (holes being drilled) andare provided with nuts at the back.Again, the sub-faceplate can be facedtrue for mounting components.

At C, D and E, where the sub-faceplate is marked Z, are typicalset-ups of small components. For anypedestal-type part, the plate can becentred and drilled from the tailstock,the hole chamfered at the front, burrsfiled off the back; then the componentcan be mounted with a nut. For acomponent with a tapped base, thepreparation can be the same-then asetscrew used for holding, or a shortpiece of studding and a nut.

For a small engine cylinder theplate can be bored through, thenholes drilled for mounting the cylinderby its flange-when the bore is to bemachined or ground. For a smallpiston, the drawbolt principle can beadopted. A locating spigot should befitted to’ the plate, then a stud screwedinto a block which is drilled crosswiseto take the gudgeon pin.

MODEL ENGINEER

. I

Accurate length machining

A S IN THE CASE Of diameters,there is not always the needfor precision on width and

length measurements-but when itis demanded accuracy is no lessimportant on the one than theother. Given a micrometer, dia-meters can easily be checked aswork proceeds; but if width andlength measurements have habit-ually been made with a rule-withall the variations that that implies-the need for greater precisionmay find one unprepared. Yetaccuracy when machining widthsand lengths is relatively easy toachieve.

On a lathe having a topslide feedwith graduated collar readings canbe taken from this for positioningtools for facing cuts, and in someinstances longer lengths can beobtained using the leading screw.

HOLDER - G A U G E

31 JANUARY 1957

BORING’ TOOL

B y GEOMETER

Again, very accurate work can bedone using simple gauges for settingtools, and this is often practised asoccasion demands even on lathesfitted with feed collars.

For old-type lathes without feedcollars, or on which screws are worn,the use of gauges is virtually imperativeto ensure precision and speed upproduction.

The principle is as illustrated at A,where a piece of material is turnedwith two shoulders. The lengths couldbe measured with a rule,; but a muchmore precise and speedier way is tomachine the lengths slightly oversizethen employ two gauges Xl, X2, toset the tool for finishing cuts. Withthe lathe stopped, a gauge is held toits respective shoulder and the toolset just to touch; then the gauge isremoved and the cut taken at theprecise position.

In most workshops there arenumerous objects of reasonable pre-

0E

155

cision, such as drill shanks, silver-steel rod, pieces of ground tool steel,etc., in standard sizes which can beused as gauges. If a micrometer isavailable material may also be turnedor filed to size, and then it is possibleto add to or delete from nominaldimensions for particular fits.

For example, if a flange is nominally1/8in. wide, but for a clearance fit0.002 in. endplay is desirable, thenthe gauge could be made 1/8 in. minus0.002 in. Again, if necessary, widthsand lengths can be obtained fallingbetween inch fractions, which woulddemand estimation on a rule.

The chuck face or a jaw can be usedas a datum for gauges, but one extrais required since the two gauges, Yl,Y2, only locate the end face and oneshoulder; another would be neededfor the second shoulder. Besides this,the gauges are in general longer andwould require to be made specially.

On second operations, however,measurements can be made from thechuck face or jaws, though when aholder is used it is best to work fromits face which, as at B, should be flatand large enough to take the gaugessquarely.

As at C, a recess can be machinedto depth by finishing the interiorover-length, then using a gauge toset the tool for the outside facing cut.Where a groove must be accuratelylocated from the end face, either oftwo methods can be employed. Thetool edge may be set flush with theend face, then taken in by feed collarreading, or a step gauge can be used, C.

The principle is applicable to back-facing flanges, as at D, where thegauge is a simple adjustable type setby using a block of suitable thicknessin conjunction with a straight-edge.

A variation of such a step gauge-snipped or sawn, then filed fromsheet metal-is as at E for obtainingthe over-all dimension of a pair offlanges on a shaft machined betweencentres. Beyond the flanges, theshorter lengths can be obtained withother gauges.

Locating from the saddle, as at F,an adjustable stop can be fitted to theheadstock to take gauges in thespace, Z.

MODEL ENGINEER

Various types of gauge that canbe readily made in the workshop

7 FEBRUARY 1957

Simple gaugeconstruction

By GEOMETER

IN THE LAST article was describedthe principle of obtaining ac-curate lengths in lathe work.

For the most part, the gaugessuggested were standard-size ob-jects such as drills, silver-steel rod,ground tool shanks which areconvenient in the sizes they cover.Larger sizes, however, requirespecially-made gauges, which forsimple length dimensions can bein round or flat material.

A micrometer or vernier caliper isdesirable for measuring gauges, thoughthe fact that the instrument may havea fixed limit of 1 in. does not debar itsuse in making longer gauges. Forexample, to obtain a 2 in. dimension,two 1 in. gauges can be made andplaced end to end for a careful checkby ordinary calipers to make a single2 in. gauge. The same applies tofractions? such as 1 3/4 in., when one ofthe possible combinations would bea 1 in. gauge and a 3/4 in. one placedend to end.

In fact, with no other measuringdevices than a steel rule and ordinarycalipers quite accurate gauges can bemade on the above principle. Severalgauges are made which all measureas near as possible alike on thecalipers,. and which together total agiven dimension on the rule-whenthe total error will be divided betweenthe gauges.

Silver steel is bestFor end gauges, material 1/4in. to

3/8 in. dia. is convenient. For occasionaluse, it can be left soft, though harden-ing and tempering are desirable ifpermanent or durable equipment isthe aim. Silver steel is ideal in thisrespect. Pieces to make gauges shouldbe faced over-length in the lathe, thenif they are to be hardened and temperedthese processes follow.

Finishing accurately to length is bylapping, as at A. The process requiresa guide bush or block for the gaugeguide block -having been drilled(preferably also reamed) and facedat a single setting in the lathe,

195

for the end to be square with thebore.

The lap is a piece of flat material,such as cast aluminium, brass or castiron, and may have some file groovesacross its face. For convenience, it isheld in the vice, its face charged withfine grinding paste, then the blockand gauge rubbed on it. Very smallcontrolled amounts can be removedfrom gauge faces by this means, andthe faces maintained or broughtsquare. Cleaning can be in parahin.

For a plate gauge, as at B, where afew thou, X have to be removed fromone edge, the principle is similar. Awide block is clamped each side withthe assembly stood on a flat surface,the wide blocks being packed up bystrips of paper to leave the edge of theplate gauge slightly protruding.

Clamp-type holderIn making a long end gauge from

several in line, they can be placed ina piece of tubing, or joined in pairsby sleeves with grubscrews. Forindividual handling, either of themeans at C can be employed. Usinga screw-m handle for a set of gauges,each must be drilled and tapped inmaking before hardening and temper-ing. With a clamp-type. holder,however, as can be made from flatstock, the gauges are simply pushedin and gripped. As shown, the jawsshould allow the side of the gauge tolie on the work, to ensure accuracyin setting tools.

Numerous gauges can be avoidedby employing adjustment, as at D .The screw portion is made from roundstock, its thread kept square by tail-stock die-holder, and flats are filedto hold in the vice. The tapped sleevepiece is drilled cross-wise for a tommy-bar and set by locknut. Alternativelythe end of the sleeve can be turneddown, slit lengthwise, and a clampfitted. Accurate adjustments are thenmore easily made.

Diagrams E and F s h o w h o wadjustable depth gauges can befurnished with plain clamping or across-drilled screw and nut. Depth Yis set from rule or gauge block. q

MODEL ENGINEER

Geometer discusses the toolsand procedure necessary for

Measuring internal diameters

THERE ARE numerous ways ofmeasuring inside diameters,depending on the accuracy

necessary and whether the checkis to verify an identifiable size,test s ize exactly, or comparediameters for dimensions or generaltruth.

For simple checks of identifiablesizes a rule can be used, or a ruleand callipers, or for more precisework callipers and a slide gauge oroutside micrometer. Thus,. to checkthe bore of a piece of tubmg a rulecan be used direct, or the size can betaken on friction grip callipers andthese checked on a rule.

Where a rule cannot be used, asfor the bore of a ball race which hasradiused ends? callipers are essential,with verification made on a rule,since the size will be standard and notsubject to variation of a few thou.

On the same principle, callipers andoutside micrometers can be used tohigher accuracy. For example, a

. check on the diameter of a cylinderwhich could be standard, or plus0.010 in., would need to be madewith some care, and with a micrometerto verify the calliper setting. Thesame is true also if there is a possibilityof confusion between millimetre andinch dimensions, when checking on arule could leave the issue in doubt.

Friction grip callipers are subjectto limitations in setting, but the screwtype, as at A, will check a diameterto within about 0.002 in. when usedcorrectly with a light touch. Toobtain the size of a bore, one leg ispositioned at X, and the callipers arerocked to carry the other leg to andfro across the shortest distance andon a diameter, Y-Y 1.

Adjustments are made until thereis just-detectable friction across theshortest distance, then the callipersare verified in a micrometer, adjustingthis until similar friction obtains onthe callipers, when the size can beread in the normal way on the micro-meter.

The principle is applicable when aninternal diameter is being bored ina lathe; but the micrometer should beinitially set several thou. undersize,and care exercised in trial cuts so asnot to overrun the dimension. As

14 FEBRUARY 1957

the bore nears size, all cuts should berun right through, since this avoidsvariations in depth of cut and springof the tool, which could result in abore being bell-mouthed.

If a very large diameter is to betested, the tailstock centre can berhn up, a rule laid on the point, anda diameter marked on the face of thework with pencil or chalk so thatthe callipers can be kept across thediameter, as at Y-Yl.

For checking machined bores, alter-natives to callipers are telescopicgauges and adjustable ball gauges, Band c, the latter for very smalldimensions. A telescopic gauge isentered in the bore, expanded, thenits sliding plunger locked from thehandle; a ball gauge is expanded inthe bore from the handle. Both arechecked for size (or set) with an out-side micrometer.

233

A gauge with a slight taper, as atD. affords means of checking a borebeing finished in a lathe. Mild steelcan be used for the gauge, the topdiameter (0.750 in.) turned to finishedsize and the taper carried downseveral thou. less (0.740in.) in anyconvenient length, Z. A flat is filedand divided according to the differ-ence in thou. between the ends, sothat for each mark that enters, thebore increases by 0.001 in.

End gauges employed for boresmust be provided with radii, as at E(right), not with flat ends (left), thecomers of which would prevent anaccurate check. This applies to theoutside ends of gauges with slidingplungers, as at F, the inner ends beingflat for feeler gauges to be placedbetween them, checking over a range.Bodies of such gauges can be mildsteel; plungers hardened silver steel. q

H E R E

MODEL ENGINEER

THE LATHE

I T IS A great convenience, evenon a simple lathe, to be ableto mark round components

with the commonly-required num-bers of divisions, such as four orsix, as are necessary when makingsquares and hexagons; and to makeother numbers of equal spacingsin a neat and regular manner, suchas to provide serrations on theedges of small bosses and knobsby which a finger grip can beobtained on fittings.

It is true, square and hexagonmateterial can be obtained in rangesof standard sizes; but these do notcover all requirements-such as anexceptionally large size, or if a fittingis needed with a circular flange largerthan the hexagon, or if a square orhexagon is required on the end ofa shaft for key or spanner manipu-lation.

To make squares or hexagons to ahigh degree of precision, milling is,of course, necessary.; but for generalpurposes careful fihng-checking bymicrometer if desired is quite satis-factory. When the lathe has meansof dividing, the material is machined

. slightly larger than the size over thecorners, a pointed tool being mountedsideways in the toolholder and settouching the work.

Using four-jaw chuckAt each located position the tool

is traversed by saddle or topslide,leaving a scribed line. To producethe flats, the material between thelines is filed away, the job beingremoved from the chuck and held inthe vice, still on the bar or in softjaws to avoid damage.

When equipment includes a four-jaw chuck and the jaws overlap theflat surface of a bed, squares can bemarked by holding each jaw to asupport bar, as at A. Such a bar canbe from round mild steel, say about5/8 in. dia., its length having beenobtained by checking with insidecalipers from the bed to the under-

21 FEBRUARY 1957

sides of a pair of iaws when these arehorizontal: The bar should be reason-ably to length, and it can be usedeither side of the bed, but on one sideonly for one job.

Another method, applicable in theabsence of a four-jaw chuck and alsoto work between centres is to clampa straight bar to the work, as at B .Distances X-Xl can be equalised witha surface gauge or scribing block anda mark made on the work with thetoo!; then the bar can be set horizontalagain, after rotating half a turn, formaking the second mark.

Quarter markings are made withthe bar set vertically with a squarefrom the bed. Pressure can be kepton the work from the tailstock toprevent movement.

Quicker methodMuch more speedy and wider in

scope, however, are drilled backplates,as at C, in conjunction with a simpleplunger device for holding. Indexingfor drilling can be done from achange gear which can be mountedon a mandrel in the chuck, key-pinnedand held by a setscrew. A locatingbar in the toolholder fits between thegear teeth, as at D.

When there are two chucks, 12spacings on one backplate and 40on the other will give the followingdivisions most commonly needed: onthe first, 2, 3, 4, 6, 12; on the second,2, 4, 5, 8, 10, 20, 40. The first can beobtained from a 48 or 60 tooth gear;the second from a 40 or 80 tooth gear.

The guide for a drill about 1/8 in. dia.should be from silver steel, andhardened. A countersunk screw fixesit to a mild-steel bar which is mountedfrom the back on a block or wall, orfrom the bench or stand at the frontof the bed, as at E.

For a bench lathe, indexing can beas at F, a cross-bracket fixed behindthe lathe carrying a pivoted bar witha silver steel pin. Normally, the barholds by its own weight, or a wing-nut and screw can be fitted at Y fortightening.

269

AMPED

ON

LOCATING

M O D E L E N G I N E E R

IN LATHE WORK, an angle platemounted on the faceplate isthe means of setting up types

of components which have flatbases or sides but which cannotbe conveniently held in a chuckor attached direct to the faceplate,In effect, the angle plate provides afairly extensive platform on whichcomponents can be s tood andclamped, at the same time withalignment assured in one plane-often an important factor.

Frequently, some initial work isrequired on components to prepareflat surfaces to stand on the angleplate. In production engineering,components are often first planed orground on bottom or sides. But inmodel work, careful filing to removemajor roughnesses is satisfactory,followed if necessary by packing upon the angle plate to avoid distortion.

Alternatively, the initial operationmay be performed with componentsheld in the chuck, or even clampeddirect to the faceplate. And of course,once a surface has been machinedfrom an angle plate a component canalways be turned on to it for machin-ing another at right angles.

A typical mounting for a smallangle plate is as at A, left, wherealignment is obtained from the twooutside faces of the plate, one abutting

COMPONENT

28 PEBRUARY 1957

l-lMOU

I AN T I N GNGLI

to the faceplate, the other forming theplatform for components-the ortho-dox way, which is quite satisfactorywhere off-sets from the centre arerelatively small. Space to swing suchan arrangement is available on manylathes by provision of a gap in the bed.

The orthodox way of mounting anangle plate has, however, a number ofserious drawbacks and only onefeature slightly to recommend it,which is that the whole width of theplatform is available to mount com-ponents. From every other aspectthe mounting at A, right, is superiorand is generally adopted on toolroomlathes with straight beds.

With the orthodox mounting, the

_l

0C

0D

E PLATESBY GEOMETER

angle plate must necessarily be smallin relation to the diameter of the face-plate, which in itself can preventcomponents being set up; and as theangle plate is moved away from thelathe centre there is a reduced holdon the faceplate and the cornersseriouslyMoreover,

overhang,. as at B, Y-Yl.there IS an increasing

problem of out-of-balance, whichalways demands a balance weight.

With the reversed angle platemounting, however, a much largerplate can be used without the cornersoverhanging; the plate may often betwice as long as a normal type andis normally about three-quarters thediameter of the faceplate.

A very firm mounting can beobtained as at B, and the back of the plate over the centre line helps to actas a counterbalance. Generally, witha reversed mounting, a larger objectcan be swung on a straight bed lathethan with an orthodox mounting ona gap bed type.

A reversed angle plate only re-quires a simple wood pattern toobtain a cast-iron casting, which thenhas to be planed on the back andthe mounting face-not expensivework to have done out. Alternatively,for small lathes, two pieces of flatsteel plate could be electrically weldedat right angles.

Setting of any angle plate fromcentre is checked with a componentstraight-line marked the distance frombase, as at B, X-Xl. Clamps holdthe component, though these arenot shown. A piece of flat plateacross the bed supports a scribingblock for its pointer to be adjusted tothe line, when this is horizontal. Then,rotating the faceplate half-a-turn, theline should recheck the same;. other-wise, the angle plate needs adjusting.

Components may be clamped invarious ways, using a straight-acrossbar, as at C, or separate clamps, asat D; and packing or reaction blocks,instead of being loose, can be fixedto clamps by a length of studding ora rivet, as at E. Also, for convenience,clamps can be slotted. E l

MODEL ENGINEER

“, “- - 1 _. __

BY GEOMETER,j-j

Testing and truing angle plates

A FEATURE of la the work-ingeneral irrefutable-is thatcomponents tend to reveal

the accuracy or otherwise of themachine and its equipment. Thus,the platform for components whichan angle-plate provides whenmounted on the faceplate must, ofnecessity, be in correct alignmentwith the lathe axis.

If the platform tilts so as to makean angle with the axis inaccuracy isclearly present, and a componentmachined on the plate will carry asimilar error.

Given, however, that the angle-plateis true and mounted the correct dis-tance from the axis, two importantconditions of setting up are fulfilled.In confidence of the result, a com-ponent can be stood on the plate,positioned sideways, squared-per-haps from the front edge of the plate-and clamped. Any face machining

will then be square on the component,while the axis of a bore will run parallelto its base. The latter condition isshown at A, where the distance fromthe centre of the bore to the base isthe same each end, X being equalto Xl.

This type of accuracy depending onalignment chiefly affects overall dimen-sions-and through them assemblyof a component with others. Forexample, the bore and piston-rod-cover face of a small steam cylindermay be true; but if they are out ofalignment with the base mounting-face, trouble will be experienced withthe cross-head binding when thecylinder is attached to the bedplate.Again, flat-base bearings bored out ofalignment will result in tilt and tight-ness of the crankshaft.

Faults mainly occur when there isan error in alignment of the angle-plate on the faceplate, as at B, sincefor practical purposes most faceplates

MODEL ENGINEER 340

are sufficiently flat and true-running.In any case, they can be easily checkedand the methods to be suggestedcounteract even a wobble error.

Theoretically, of course, the func-tional faces of an angle-plate shouldbe at right-angles, but it should notbe taken for granted they are unlessthe plate has been checked and provedto be accurate. Usually there is someerror, though it may be small; andeven should the error be considerableit can be eliminated when mountingthe plate-which, incidentally, is oftennecessary in production shops.

The common method of testing isas C. The angle-plate is attached tothe faceplate in approximately theposition it will occupy, then an in-dicator is mounted on the topslide,for its plunger or lever attachment tobear on the plate. Saddle movementto and fro should then produce nodifference in indicator reading.

Making correctionsTests should be made with the plate

horizontal and in two vertical attitudes,near to and away from the operator,to search any error in spindle align-ment, for which allowance could thenbe made. An error being present onthe plate, as at B., packing is intro-duced, as at C, until a uniform readingobtains on the indicator.

On occasion a small cheap or home-made indicator will serve almost aswell as a dial type, and with carefulobservation a bar or pointer willoften suffice. Packing may be shim-stock or tinplate, or strips of paper.

Once trued in this, manner-whichalso allows for faceplate wobble-the angle-plate can be finally adjustedfor position; and if the job is to be arecurrent one, dowels can be fitted asat D, removing the faceplate from thespindle for drilling and reaming.

Tests of angle-plates alone, thoughless important than on the lathe, canbe made on a surface plate, as at E.A parallel cylinder with a square endis clamped to the angleplate, andchecks at Y-Y1 with dial indicator orsurface gauge should reveal the cylin-der to be horizontal.

Instead of a cylinder, a steel mandrelcan be turned, as at F, pressed into abase, then this is faced and the testdiameters turned with the mandrelrunning between centres.

7 MARCH 1957

.1

HEN an angle-plate for mounting components hasbeen correctly set on the

lathe faceplate, a considerable stephas been taken in accurate setting-up. But there still remain theproblems of sideways location andsquare alignment since in theabsence of positive locating means,the position of a component canbe varied sideways. It can also beslewed at an angle to the plate,and inaccuracy of this latter type,while quite small, may be un-suspected if markings on the frontface of the component are beingrelied upon.

Locating onangle-plates

By GEOMETER

LOCATING

An ordinary sideways setting isobtainedby marking a vertical lineup the component face from the base.Then with the component clamped,the angle-plate is turned into a verticalattitude, and the scriber point of asurface gauge is set to the line. Tum-ing the faceplate through 180 deg.and testing again, the line shouldcoincide with the scriber point whenthe setting is accurate.

If the front edge of the angle-plateis true it can be employed to checksquareness of the component setting,testing with a straight edge along thefront of the plate or presenting asquare to it for the blade to runalong the side of the component.14 MARCH 1957

Again, a semi-rough check can bemade with a flat-pad centre in thelathe tailstock, or a pointer or toolcan be set near the component faceand the faceplate revolved.

When a component has been markedwith vertical lines on both front andback faces, another method of settingcan be employed, as at A, where aline has been scribed centrally on theplate and down the front edge.

.Setting the front and back lines onthe component to this one--perhapsusing a small mirror. for the back-squares the component and at thesame time locates it sideways.

If a component is to be followed onthe angle-plate by another exactly

385

like it, a line scribed on the plate atthe edge of the first component willserve to set the second in the sameposition. Should there be severalcomponents the principle can becarried a stage further by fixing alocating strip as at A-which i ssometimes done in production work.Dowels may also be employed insuch a manner, as at B, pushing thecomponent sideways against them tolocate it; if the component has holesin its face these may be used toengage on dowels.

A component with a circular borein its base may be set up in either oftwo ways, depending on its type.Should it be an i.c. engine pistonwhich is to be mounted for boringgudgeon pin bosses scribed lines canbe provided each side and the pistonstood on the plate for the lines tolocate to the centre line-A. Alter-natively, a spigot can be fixed in theplate over which the piston can beplaced to locate from its bored-outskirt-a method useful for locating acomponent like an elbow, as at C,for the second face to be machined inalignment to the first.

Accuracy of setting of a spigot canbe verified as at D, a test bar havingbeen machined between centres fortruth, and its stem made a good fitin the plate. A check is made bysurface gauge with the angle-platevertical, then again at 180 deg.

The preliminary setting and boltingof an angle-plate is simplified withthe faceplate laid flat. Then, if thereare scribed lines, as at E-X-XI-sideways location is assured. Andallowing for the thickness of theangle-plate, Y, a depth gauge can beused from the edge of the faceplatefor Z.

For guided boring bars generallyused from the tailstock, an angle-plate. at its setting may be providedwith a bush, as at B and F, though thiswould normally be for a special com-ponent or in production work. q

MODEL ENGINEER

G E O M E T E R d i s c u s s e smethods employed in usingthe lathe for milling work

SETTING VERTICAL SLIDES

AN IMPORTANT ASPECT of thelathe, from the amateur’spoint of view? is that with

the minimum of equipment it canbe employed for light millingoperations.

Taper-shank end-mills can bemounted in the spindle, while parallel-shank mills and facing cutters can beheld in a chuck. Using a stub mandrelin a chuck, or a long mandrel withone end in the chuck and the otherby the tailstock, various types ofcircular cutters can be run for sawing,slotting and similar operations.

For very simple operations, thework can often be clamped to the top-slide and traversed past the cutter bycross-slide and longitudinal feeds. Insome instances, the topslide can beremoved and an angle-plate sub-stituted, thus providing a largersurface for mounting. The mostversatile fitting, however, is a verticalslide which can be fitted in place ofthe topslide.

Given such a slide, the basic accuracy of the work produced on it

will largely depend on the setting.Usually, the bases of slides aregraduated to provide approximatesettings, but these are not nearlyaccurate enough and should not berelied upon-except, perhaps, forvery simple operations.

For more precise settings, involvingnormal parallel and right-angle faces,the face of the slide must be adjustedto correspond with the cross-slide orsaddle feed. This can be done eitherwith a round-ended pointer, orpreferably with an indicator gaugeto show errors in setting by the move-ment of its hand.

Referring to diagram A, showing aplan view of a slide set at right-anglesto the lathe axis,, a round-endedpointer could be mounted on thedriving plate, and the slide fed near toit. The gap between pointer end andthe face of the slide should be equalacross the face as the slide is fed, andmay be checked by observation orusing feeler gauges.

If an indicator is mounted as shown,any error will be revealed as a varia-tion in the reading of the indicator,and when the reading is steady theslide is correctly set. With eithermethod, should an error be shown theslide must be suitably re-adjusted.21 MARCH1957

Referring now to diagram B, wherethe slide has been turned through90 deg., a round-ended pointer orindicator can be mounted on amandrel, held one end in the chuck,the other supported by the tailstock.Similar principles of setting are thenemployed in conjunction with saddlefeed along the bed.

gap or reading should be uniform asthe work is raised and lowered. Inevery case of inaccurate setting thework must be adjusted on the slide.

These two basic settings cover themajority of set-ups; and for adjustingwork true on the slide, the sameprinciples apply. Thus, should it berequired for the edge of a componentto run parallel with the lathe bed,the round-ended pointer or indicatoris moved into a vertical attitude.

In regard to accuracy of the slideitself, any error shown by the tableagainst a fixed indicator when thevertical slide is operated would suggestan error in the table. This method,however, would not reveal an errorin the guideways of the slide. To dothis, a test would have to be made,either on the lathe or’a surface plate,against a true angle-plate, as at c, orusing a test mandrel, as at D, machinedbetween centres. An inaccurate slidecan be packed at its base.

With crossfeed for diagram A, or The setting of an angle-plate cansaddle feed for diagram B, a level be checked as at E, with saddle andsetting should then be obtained. If cross-slide feeds. Inaccuracy on thefeed is to be from the vertical slide plate can be corrected by packing,itself, the pointer or indicator should and the other way by suitably adjustingbe set to the edge of the work, and the the plate on the slide table.

,

r

.IDE

419 MODEL ENGINEER

By GEOMETER

FEEDS-in lathe milling

IN SETTING UP work for millingin the lathe, an importantinfluence is the manner in

which the feed will be applied.The lathe spindle runs in the normaldirection when driving a millingcutter-that is, the top of thecutter rotates towards the operator;or looking from the headstocktowards the tailstock, the cutterruns in a clockwise direction.

Drills and end-mills rotate in thismanner ; and although a saw orslotting cutter could be reversed itwould mean running the lathe spindlethe opposite way, with the possibilityof the chuck or driving plate un-screwing.

With the work mounted on thevertical slide, feed can be appliedeither from the cross-slide or from thevertical slide with the longitudinalfeed of the saddle on the bed em-ployed for putting on cut or locatingthe work in relation to the cutter,though this arrangement may bemodified according to circumstances.

Having regard to cutter rotation,and the way in which feed may beapplied, the next point is the mannerin which the cutter will contact thework. In ordinary milling, the feedforces the work on to the cutter, asat A, and the thrust of feed and cutteris always in opposition.

This means that if the cross-slidefeed is used the work should bemounted above the cutter and astart made near the operator for thework to be traversed away from him.To maintain the same conditions withcross-slide feed the work could bemounted to run under the cutter andbrought from the far side towardsthe operator. But in many instancesthe cross-slide feed is too limited forthis.

On occasion, from some conveni-ence there may be in setting up orbecause the hazards are overlooked,the conditions of operation may bereversed and the down-cut millingprinciple introduced, as at B, withthe possibility of breaking the cutteror spoiling the work from chatter ordigging-in.

This is because, as the cutter rotates,the tooth which is nearing the endof its cut has a tendency to carry the

28 MARCH 1957

work with it, taking up the slack awayfrom the feedscrew. When thishappens the next tooth to strike thework has a greatly augmented cut tocontend with, and in a bad case thetooth may be chipped or the cutterbroken in halves; while there is alwaysthe danger of the work being movedor the finish spoilt.

Even with the slide gibs tightenedso that the feed is stiff the risk remains.Consequently, whenever possible the down-cut principle, as at B, should beavoided in milling in the lathe.

Using vertical-slide feed in a down-wards direction, conditions are satis-factory when the work is on the farside of the lathe away from theoperator, as at C. With the workmounted near to the operator thefeed should be upwards, as at D,as when slotting a piece of materialto make a fork. A slight tendency to

dig in may be noticed on commencinga cut if this is to be very deep. Theremedy is to take a series of light cuts.

The principles apply in end millingwhen’ the cut is on the side of a bar,as at E (top), and in machining a slotor channel (bottom), the cut isbalanced. Where a slot is to bewidened the method at F should beadopted. Cutting on the top of theslot, cross-slide feed should be awayfrom the operator; cutting on thebottom, it should be towards hi.The same applies when milling adovetail, as at G, where feed conditionsshould be as F.

,

A keyway for a Woodruff or “ half-round ” key is machined as at H(top) with a direct feed on to thecutter; usually downwards from thevertical slide. A long keyway bottomcan be cut with cross-slide feed underconditions similar to A. q

FEED

0E

0F

0G

0r-1

453 ENGINEER

4 APRIL 1957

S I M P L E M l L L l N GCUTTERS

By Geometer

FOR a large number of millingoperations, as performed bythe amateur in the la the

a surface gauge or height gauge, overthe cutter tips.

when rate of production is notimportant, s imple cut ters con-trived at very small cost in theworkshop provide results equalto those obtaining from boughttools.

For milling soft materials like aluminium or brass, cutters of silversteel, hardened and tempered, aresatisfactory.; but for cast iron and ,steel, cutters can be made from shortpieces of round high-speed or alloysteel tools.

It is not, of course, essential toemploy multi-tooth or multi-bladecutters for many milling and similaroperations. With care, and by regu-lating the speed and feed, the samework can often be done with a single-point tool. In production work,multi-cutting-edges permit of a highrate of feed; and wear is distributedover all the cutting edges-whichtogether mean faster production andlonger runs on set-ups.

For milling hollow surfaces orradii, a single-point tool set in amandrel can be used,. as at B. Themandrel should be driven by holdingone end in the chuck and supporting the other at the tailstock, since a set-up between centres driving the mandrelby a carrier is too choppy and chatter-inducing. A double-edged cutter canbe used on this set-up, checked forlength over its tips and set centrallyin the mandrel.

For facing operations in productionwork, a large end mill or slab facecutter would be used; but in lathemilling, a single-point cutter or toolas used for turning can be employed.If the surface is small, a tool off-setin the independent chuck is all thatis required; and such a tool will alsocut a slot.

If the surface is large, however, aholder is necessary for the tool, thisbeing mounted in the independentchuck. In such an event, an improveddouble-blade cutter can be made, asat A, where a piece of rectangular mildsteel has been drilled (and, if possible,reamed) to take two round tools heldby grubscrews.

Small end cutters for narrow slotsin work can be made from silver steel,as at C. The piece of rod should beturned to the diameter, X, of thecutter. Then the diameter is filed torectangular section backed off behindthe cutting sides; and the end face isbacked off oppositely to form a

pair of cutting edges. The tool ishardened and tempered in the normalway and should be run at fairly highspeed with light cuts.

A bought slotting cutter or saw’needs to be mounted on a mandrel,as at D, where there is a driving pinfitting in the keyway, and the sleeveholding the cutter up to the shoulderis slotted to fit over the driving pin.

True setting in the chuck with thecutter tips rotating in the same planecan be easily checked by allowing thetips to scrape past a f i x e d bar onthe slide or other mounting, Should thesetting be incorrect, the holder canthen be tapped or packed as required;and the cutters can be adjusted bythe chuck jaws to spin on the samecircle. A preliminary check forprojection of the cutters from theholder can be made by laying this onits back-on a surface plate, and using

As distinct from saws, slottingcutters may be built up m a mild-steel holder, as at E and F. Four ormore cutters or tools may be used,clamped by grubscrews or setscrews,the tools being flattened on the sides if required. Grooved cotters, how-ever, admit of a narrower holder andavoid the need for tapped holes. Theholes for the cutters having beendrilled, they should be temporarilyplugged (pieces left projecting forremoval), then the cotter holes can beeasily drilled through the holder. I

‘491 MODEL ENGINEER

N A C C E S S O R Y c o n s i d e r a b l yA extending the range of workon a centre lathe is the

fixed steady which is mounted onthe bed to provide intermediate support for lona shafts and similarslender components in conjunctionwith the tailstock, or to supportthe outer ends of componentshaving a lengthy projection from

FIXED LATHE STEADIESthe chuck.

Without such support, a shaft ofany length even runnmg betweencentres, is hkely to wobble and would B y G E O M E T E Rcertainly be unstable and subject tochatter under cutting stress.

The normal steady supplied with alathe is provided with three equally-spaced jaws which ‘are individually-adjustable to the work, and tipped or,capped with brass to obviate scoring.Each jaw is set just to touch andsupport the work? then locked by anut or similar device.

Frequent oiling is necessary andadjustments must be made as thejaws wear and bed to the work. Inthe absence of a steady, either as anaccessory or for a particular job, atemporary one can usually be con-trived from a wood block with an

angle-iron mounting to the bed.How a steady extends the use of a

lathe to work which could not other-wise be performed is shown at A.

An axle too large to pass throughthe lathe spindle-even if this ishollow-and too long to run betweencentres is required to be machinedwith parallel concentric ends. Withthe tailstock removed from the bed,a piece of over-length shafting is heldin the chuck, while the free end issupported. in the steady and theturning-and any screwcutting-isdone close to the chuck.

Afterwards the surplus pieces arecut off-as far as possible by partingtool, then finishing by hacksaw forsafety. A set-up for facing the ends ofa long tube can be made in the sameway if a mandrel is mounted in thechuck for the tube to be pushed on.

a CENTRE, DRILL

11 APRIL 1957 ‘525

On the same principle, shafts canbe faced and centred, as at B. Thisis necessary when large billets are tobe run between centres;. or when it isdesired to centre material accurately,there being a minimum to machinefrom the outside afterwards.

,

When a centre in a shaft has beendamaged, such a set-up is necessary;and for truing, a fine boring orpointed tool may be required on theslide to avoid the swing and con-tinuing wobble to which the centredrill would be subject. A setting forthe steady jaws may be obtained byadjusting them with the ‘steady closeto the chuck, then bringing it backto the working position. -Lifting andriding of the centre drill-from badsetting-is to be avoided on new work.

Support for hollow work, such asfor boring a large bush, can beprovided as at C, when the adjustmentof the steady jaws has an effect onthe parallelism or otherwise of thebore-so checks and adjustments arenecessary well before finished sizeis reached.

When prolonged use is to be madeof the steady and frequent adjustmentsto jaws would be necessary-apartfrom the possibility of marking the work from restricted local contact-abush provides larger and more durable support, It can be from brass tofit the work and mount in the steady’jaws, as at D. A screw in the sidecounteracts a tendency to rotate? andadjustment can be made by slittinglengthwise.

To reduce wear, steady jaws ingeneral use may be given a radius, asat E, using a reamer or boring tool inthe chuck and adjusting the jaws toit. Using a boring tool, the steadycan bc traversed by lightly gripping itto the bed, then pushing it along bysaddle feed.

A built-up steady, as at F, can bea wood block bored on the faceplate,or as at E, and bolted to a piece ofangle iron which has a guide tonguefor the bed riveted to the underside.A slit and a screw can provideadjustment.

MODEL ENGINEER

i. .

T U R N I N G ___-_____

SPHERICAL

RADII . . . . . . . By Geometer

W HEN HIGH PRECISION is notessential there are severalways in which the spherical

radii of ball-ended components canbe machined in the-lathe with theminimum of equipment.

fitOn parts that are not required to

others-such as ball handles orknobs-neat appearance, good finishand reasonable adherence to size arethe most important features andshould occasion occur, a fair degreeof precision can be obtained withcare even by simple methods, usinggauges for checking in the course ofworking. Form tools can be used ofcourse in the smaller sixes and whenthe numbers of components justifytheir production.

Although it is in the nature of an, impossibility to machine a good ball-

end solely by manipulation of the topand cross-slides of a lathe, familiaritywith its working can greatly improveon the result that might at one timehave been considered possible. Fora feature like that at A, for example,the ball would originally be left in a“ square ” by turning down in theneck with a parting tool then chamfer-ing the left side out to the full diameter.

Cheek with plate gaugeWith a round-nosed turning tool,

a start is then made on the comers ofthe square, the top and cross slidehandles bemg manipulated to swingthe tool in an arc. Progress can bechecked by plate gauge, made bydrilliig or boring a hole of required

, size in’ flat material then cutting thesurplus away. When the ball has beenroughed-out and shaped-up reason-ably by the tool, the surface is finishedby files and emery cloth.

For an internal hemisphere, as atB, similar principles apply-exceptthat it is not possible to file the sur-face for finishing. In place of this, around-nosed hand scraper can be usedon a support bar close to the work, ora form tool can be employed for thefinal scraping cuts, followed by polish-

18 APRIL 1957

ing with emery cloth on the end of afinger or a rounded piece of wood.

Owing to the propensity for chatterto develop on broad cuts with formtools, the depth of cut should be keptto a mere scrape, with the work revolv-ing slowly-the chuck pulled roundby hand if necessary, succeeded byhigh-speed polishing. Again, a plategauge can be used for checking, theend being filed to a scribed radius, orby clamping a suitable disc to theplate if the disc is not used itself. Inmost instances it is unnecessary toemploy a gauge of the full half-circle type as at B; either half to thetop or bottom of X-Xl suffices.

A form tool for a ball-end is madein the manner of a plate gauge bydrilling or boring a hole throughmaterial such as silver steel or caststeel, then sawing or filing the surplusaway, as at C. Below the cuttingedge the tool should be given clear-ance, either by careful hling or byfinishing the full hole with a taperwhen it is bored.

Harden and temperHardening and tempering are essen-

tial before use; and finish on the ballis much enhanced when the tool isgiven top rake by carefully grindinga groove behind the cutting edge.Preparation of the work as for A isadvisable, keeping the form tool forfinishing-at a slow speed and with aflow of suds or oil. Tools of this typeshould be set with the cutting edgeflat and at centre height.

Simple round tubular tools withtrue ends chamfered like those ofwasher punches can be used forfinishing ball-ends, and will eachcover a range of sixes. Round silver-steel rod faced off square, drilled up,chamfered and heat treated can beused for small tools with a supportbar on the slide, as at E. Suitablehandles should be provided, of course.

A support bar again may be usedfor a pivoted tool for finishing ball-ends with curved necks, as at F.

561 MODEL ENGINEER

By Geometer

SIMPLE FORM TURNING

FEEDS of cross-slide, topslideand saddle provide for ordin-ary facing and turning opera-

tions, while small irregular featurescan be machined with form tools,and radius tools or radius-turningattachments enable regular radiito be produced.

There remain, however, larger formswhich none of these processes properlycover, since there is a limit to thesize a form tool can be made-apartfrom the chatter which occurs witha broad cut. For machining to besmooth and clean, the single-pointtool principle must be adopted, andthis means the tool must be guided insome manner, ‘whether the cut is tobe over the end of a piece of work oralong the side.

At A is shown how a conical endcan be provided on work by settingthe topslide at an angle, for theline, X-X to be produced with theslide travelling along Y-Y. If, however,it is necessary to finish the work witha curved end, the method shown atB is about the simplest that can becontrived.

It might in some circumstances, ofcourse, be possible to machine thework free-hand to a plate gauge; andagain, on occasion, a radius turningattachment might be used-if thecurve is a proper radius and not toolarge.

The method shown employs asliding tool in a holder mounted onthe topslide in the normal manner.A collar is attached to the rear endof the tool against which bears aspring to keep the tool in contact witha form plate. This plate is mountedon a pad centre in the tailstock, anda cut is applied to the tool by tailstockfeed. Ordinary cross-slide feed thencarries the tool over the work.

WORK,PLPTE

In any type of form turning, theform plate should provide a solidbacking for the tool and be capable6f feed so the tool can be advancedto the work for succeeding cuts.Here, this is conveniently done fromthe tailstock. The plate- can be ofangle iron held by screws or boltsto the pad centre-or mounted in anyother convenient manner, such as bybrazing it to a taper shank.

25 APRIL 1957 595 MODEL ENGINEER

The required profile can be marked,then with the surplus metal sawnaway, it can be filed and scraped toa finish for the rear end of the toolto slide smoothly over it-with a dropof oil. Too great a movement shouldnot be attempted for the tool.

The above provides for machining acurved crown on a piston, or forfinishiig the outside of a shallowbowl-the inside being turned with aform plate of opposite shape. Fortool application to the side of work,such as for crowning a pulley orflywheel rim, or machiiing handles,candlesticks or shaped table legs, theprinciple is the same and a tool set-up can be made as at C.

A similar type of spring-loadedsliding tool is used with the rearend bearing on a form bar carrying theprofile. The form bar is backed bya guide in which it is free to slide,

and the whole is mounted on a pieceof flat steel plate which has a clampinghole to fit over the stud of the top-slide for holding down with a distancepiece and nut.

The form bar has a hole at the endfor attachment to a link, which isitself fixed to a bracket on the end ofthe lathe, or a wall at the end-theprinciple being to hold the form barfrom movement endwise but permitthe cross-slide to be fed in to applythe tool to the work.

By altering the form bar, as at D,long curves can be machined, as arenecessary on work of the type men-tioned. For end work of the type atB, the tool mounting must be at right-angles, and the form bar anchoredat the back of the lathe.

The form plate mounting on thetailstock pad centre for B is shownin the bottom diagram.

WORh

CLAMPING/ H O L E

-- -- I

Constructing

sliding

tools*

By GEOMETER*

A S DESCRIBED in the previousarticle, certain kinds ofsimple form-turning are best

accomplished by means of single-point sliding tools passing overthe contours of form plates orform bars which control the move-ment of the tools for the cuttingtips to machine the work toclosely-resembling or exactly-simi-lar profiles.

Providing the changes of contourare not too abrupt, and high precisionis not essential, this method ensuresa clean normally-machined surface,free from chatter, and requiring theminimum of smoothing and polishingwith glass paper if the material iswood, and with emerycloth if it ismetal.

A major condition, of course, witha sliding tool is that, while it movesfreely endwise, it must not tip over ordeflect unduly; and if the tool is ofround material, as it can well be foramateur use, when of silver steel, itnaturally follows some means ofradial location is essential. This, asshown at A, where is depicted atypical small sliding tool, can be aflat extending far enough to permitthe tool the necessary movement inits holder.

Denending on the type of holder,the flat o n the tool c a n engage aflat-ended screw, or a flat plate whenthe holder is in two parts; or again,a special .washer With a D-hole forthe tool to slide through can beattached to the front of the holder.The tendency for the tool to overturnwhen cutting can be kept to a mini-mum by maintaining the cutting tipcentral, similar to an ordinary turningtool with a small radius-and itshould also have top rake and sideclearance. At the opposite end, thetool should be squared, then cham-fered equally from each side, and the

SURPLUSMATERIAL

CUTTING

T A P E R

0D

ADJUSTING$@- S C R E W

0E

corners removed to leave a small radius.

The flat on the tool is produced byfiling. and a check kept on its parallel-ism-by calipers, or a micrometer forpreference. Alternatively, a small jigof the type shown at B can be used,which is advisable when several toolsare required, or they have to bereplaced quickly.

Such a jig is made by dowellingtwo pieces of flat material, drillingand reaming on the centre line, thenmachining away the surplus materialso that a piece of round stock grippedin the jig can be filed doivn to thesurface and provided with a uniformflat. The jig material can be hardenedand tempered if it is cast steel, orcasehardened if it is mild steel.

The final feature essential for asliding tool is means for attaching acollar to take the thrust of the springmaintaining the rear end in contactwith the form plate or bar. Choicein this rests between a collar whichis threaded, one drilled for a taperpm, or one provided with a grubscrew, as at C-the tool in each casecorresponding, then being hardenedand tempered.

When a jig, as at B, is used in filingflats on tools, its accuracy mustessentially be beyond question, andcan only be ensured by a machiningprocess-of which D shows theprinciple of the set-up in a four-jawindependent chuck.

The two blocks, finished exceptfor facing off the surplus material, areheld in the chuck with a bar throughthe bore, this bar locating back totwo of the jaws, while the blocks aresecured, then being pushed out.Assummg the jaws to be true, theblocks will be faced off squarely;but if the jaws cannot be relied upon,the bar should stand slightly awayfrom them? and be tested to runtruly at pomts, X-Xl, as the blocksare tightened.

At E is shown how a simple toolholder can be furnished with a flat-ended adjusting screw to locate thetool and allow it to slide, while at Fappears the body and top plate of atwo-part holder for bolting together-with shims interposed if necessary@

Practical know-how in simple terms andwith good illustrations-Workshop Hints andTips. by Geameter. Percival Marshall and Co.Ltd. price 3s. 6d. (postage 3d.) U.S. and CanadaSl.00.

MODEL ENGINEER2 MAY 1957 633

THESIMPLEST TYPE of crank-shaft for a single-cylinderengine consists of a main-

shaft, a web, and a crankpin, andmay be known variously as asingle-web crankshaft, a cantilevercrankshaft, or an overhung crank-shaft. Single-web by

For the model engineer and theconstructor of small commercial en-gines-Moped two-strokes in partic-ular-such a crankshaft has several

CRANKSHAFTS GEOMETERadvantages’: it can be built up withmuch greater facility than the doubleweb type and the parts can be hardenedto resist wear, with the minimum riskof warping in the process.

Even as a one-piece-from-the-solidconstruction it is stilleasier to machinethan the double-web type. The singlemainshaft involves no major problemsof bearing alignment; and one longmain bearing may be used insteadof two separate ones in engines notrequiring auxiliary drives.

In two-stroke engines, a single-web crankshaft actually improvesefficiency by avoiding waste volume inthe crankcase which would occurin the region of the second web.This is because a two-stroke engineinduces gas via the crankcase, andsuction or reduction of pressure asthe piston ascends and compressionor rise of pressure as it descends aredependent on the volume of the crank-case. The smaller the volume, the

greater the difference between thesepressures.

In its most elementary form asingle-web crankshaft may consist ofa long and a short length of steelrod, soldered into holes drilled in adisc or strip of steel. Where equip-ment is available for silver solderingor brazing, a stronger job can bemade. In a lathe, mainshaft andcrankpin may be shouldered downthen brazed into the web from theends, or simply riveted in, as at A .If the web is fairly wide and the holeshave been reamed, mainshaft andcrankpin may be slightly ‘oversizeand pressed in, after hardening, ifrequired.

A satisfactory crankshaft can bemade by screwing the ends of main-shaft and crankpin and tapping theweb to accept them. For this, threadpitches between 26 t.p.i. (cycle rate)and 30 t.p.i. are the most suitable.

0A

0B

LOCK-NUT

CLAMPING

COTTERS

0F

The web should be fairly wide andthe parts secured by locking screwsor locknuts, as at B and C. Preferably,the direction of turning should be toscrew the web on to the mainshaft.In assembly, mainshaft and crankpinare screwed as tightly as possible intothe web; then, to take locking screws,holes are drilled and tapped on the -joint line of each. After drilling andtapping the parts can be dismantledand hardened if required.

A clamped construction can bearranged, as at D.. For this, themainshaft and crankpin may be srmplyshouldered down parallel and grippedin a split web by clamping bolts.Both may be hardened. The web ismade by drilling two pieces of steelfor bolts then clamping together fordrilling, boring, or reaming the holes.On occasion the clamping bolts mayintercept grooves on mainshaft andcrankpin. The two web portions canbe filed on abutting faces for clearanceand to grip firmly.

Cotter security for screwed-in main-shaft and crankpin is possible as at E,where the view is of a section lookingfrom the crankpin end, the action oftightening the cotters tending to pullthe parts together. After drilling andtapping the web, the holes are pluggedwith threaded rod for drilling thecotter holes. Then the plugs areremoved and filed-out cotters fittedand the tap rotated in the holes asthe cotters are tightened to formthreads.

A very strong petrol engine crank-shaft can be made as F. with thecrankpin and web machined from thesolid and taper-fitted to the main-shaft, the step on the web accom-modating a ball-race and the shaftbeing drilled for oil feed.

Connecting rod location on single-web crankshafts can be as G: (1)by a shoulder machined on the crank-pin; (2) by a loose collar fixed by ataper pin; (3) by a loose washer heldby a stud and nut; (4) by a steppedwasher and countersunk screw.

Containing hundreds of practical hints formodel engineers is Workshop Hints and Tips, byGeomerer. oublished by Percival Marshall andCo. Ltd. b&e 3s. 6d.‘(postage 3d.). U.S.A.and Canada: $1.00.

9 MAY 1957 667 MODEL ENGINEER

DOUBLE-WEB

C R A N K S H A F T SBy GEOMETER

HILE the double-web orfull crankshaft is a morecomplicated component

than the single-web type, it admitsof greater choice in eng ine layoutboth for essential components andauxiliary drives.

Thus, for a steam-engine, eccentricsfor valve-gear and water-pump opera-tion can be on the side of the crank-shaft, away from the flywheel; andsimilarly for a four-stroke internalcombustion engine there is freedomto set out the valve gear in severaldifferent ways, according to what maybe considered desirable or expedient.

For these advantages, the penaltiesare more work in producing the crank-shaft; material retained in the softcond itron-since it is impossible toharden the crankshaft, and splitbig-ends to the connect ing rods-un-less a complicated built-up crankshaftis used. In most instances the choicefor a simple single-cylinder or multi-cylinder engine in model sizes is amachined-from-the solid o r perman-ently built-up crankshaft of steel, andconnecting rods with split big-endsof brass, bronze, or white metal.

Types of crankshaftDiagrams A, B and C show types

of crankshaft employed in single andtwin-cylinder engines. That at A maybe used in a single-cylinder double-acting steam-engine, or in a single-cylinder internal combustion engine-when it usually has balance weights.That at B is employed in flat twin orhorizontally opposed internal com-bustion engines, where the cylindersfire alternately. The crankshaft at Cis the type required for twin-cylinderdouble-acting steam-engines where thecrankpins are at 90 deg.

At D appear end views of the crank-shafts, and it can be seen that typesA and B can if desired be made fromsolid rectangular material, but thattype C would require a square section.Hence, it might be decided that whiletypes A and B could be made from thesolid without too much labour, typeC would be better built up.

A crankshaft for a vertical twin-cylinder single-acting steam-enginecan be made from solid rectangularmaterial as at E. This crankshaft is

similar to that at B for the flat twin

16 MAY 1951

internal combustion engine, but thecrankpins are further apart and thecentre web is at an angle instead ofbeing straight.

The method of preparation foreither of these crankshafts (and forsingle-cylinder types) is as shown.The steel bar is cut and faced tooverall length-plus. On a surfaceplate, the ends are marked with throwcentres for these to be centre-drilled.Surplus material is cut away bydrilling and ha c ksawing, then thecrankshaft is rough-machined-ad-visedly gripping one end in the in-dependent chuck for a firmer driveand more rigid support than can beobtained between centres. Finally, ofcourse, the crankshaft is finishedbetween centres and the throw “plates”sawn off for the ends to be turned.

For built-up crankshafts, over-widthflat or rectangular-section materialis used for webs, and over-size roundmaterial for crankpins and mainshafts.For machining the crankpin of a single-cylinder crankshaft, as is usuallynecessary, the crankpin rod can be aslong as the mainshaft and centred asat F. After the brazing or welding,the mainshaft is sawn away betweenthe webs and the crankpin machined.Then the surplus material is cut offoutside the webs and the mainshaftturned.

When there is more than onecrankpin, plates must be fixed to themainshaft, either brazed, welded orclamped, on the principle at E. O noccasion however, with careful braz-ing crankpins need only be cleaned upwith emerycloth.

For brazed crankshafts, crankpinsand mainshaft can be parallel andthe webs slightly countersunk; butfor welding,. a stepped crankpin isadvisable with the ends chamferedand the webs deeply countersunk.Penetration as shown should be aimedat in welding.

Webs may be extended to formbalance weights and should be a goodfit for crankpin and mainshaft mater-ial. Boring should be with themclamped on a faceplate, or sub-face-plate, with a plug locating the firsthole, as at G.

operations in an M.E. handbook Workshop Hintsand Tips, by Geometer, Percival Marshall andCo. Ltd. price 3s. 6d. (postage 3d.). U.S. andCanada $1.

703 M O D E L ENGINEER

Machining

s imple .

crankshafts

By GEOMETER

DEPENDING on its constructionand the type of engine inwhich it will be fitted, the

single-web crankshaft can be madein various ways. In a first-attemptsmall steam-engine everything maybe arranged in the easiest manner,taking what might be regarded asa mechanical liberty, but excusablein the circumstances.

At the other extreme, in the case ofa fast high-compression petrol engine,“ correct ” engineering principles inregard to the type of component mustbe followed-for to do otherwisewould be courting disaster.

Making the crankshaft as simply

l

rCRANKSHAFT,

as possible, flat plate material can beemployed for the web, holes can bedrilled and, if necessary, tapped forthe mainshaft and crankpin. Thenthese two parts, to fit in the web,can be shouldered by turning in alathe-from which it follows theessential aliment of the finishedcrankshaft will depend partly on thetruth of the holes? but mostly onthe shoulders abuttmg firmly to theparallel web material.

Using thick plate material for theweb, basic alignment of mainshaftand crankpin holes is vital; so theholes should be drilled and bored withthe material clamped to the lathefaceplate, or sub-faceplate--and thisprinciple is extended to the morecomplicated constructions with separ-ate webs.

But when for any reason, whetherto further mechanical design or as anessay in turning skill, it is desired tomachine a crankshaft from solidmaterial, or true a brazed-up orwelded-up type by light machiningcuts, then the problem of mountingon the lathe finally reduces to achiev-ing a suitable set-up for machiningthe crankpin in alignment with the

FACEPLATE

.mainshaft-since the mainshaft canbe dealt with by centring the materialeach end and turning between centres.Thus,. using round material for amachined-from-the-solid construction,there is after the first operation thefinished mainshaft with a thick “ web ”at the end in which the crankpin iscontained.

”

For the second operation, tomachine the crankpin, either themainshaft or the flat back of the webcan be employed for setting up, theone requiring an angle-plate on thefaceplate, as at A and B, the other asub-faceplate, as at C and D.

For the angle-plate set-up, twopieces of plate material can be used,comprising the base and the cap.The base should be bolted separatelyto the angle-plate and the cap attachedby two or four studs to the base. Toaccept the mainshaft, the cap shouldbe fitted with paper to the base (thepaper finally. being left out for thecap to grip) and the two drilled andbored on the joint line, as at A.

.

Where the crankpin will come, theangle-plate or base can be drilled fora small driving pin fitting in a holein the back of the web. Then, withthe crankpin centre marked on thefront of the web, the shaft can befitted and the angle-plate adjusted onthe faceplate, as at B.

For the sub-faceplate set-up, anysquare or disc of plate material canbe used-not less than about 1/4in.thick. Projection from the mainfaceplate can be achieved by equal-height metal blocks, or pieces of stifftubing faced off to the same lengthfor bolts; but for preference, blocksor parallel packing should be used.The sub-faceplate is bored to acceptthe mainshaft-and may also be facedfor the web. As before, a driving pin.can be fitted-where the crankpin willcome, the crankpin centre marked,and the shaft fitted. Then the sub-faceplate can be appropriately ad-justed.

The shaft mounting can be in eitherof two ways. Parts of the web to beremoved can be drilled for counter-sunk screws, as at C, X, Y, and nutsused to the sub-faceplate. When theshaft has a thread, however, this canbe used, as at D. El

Invaluable for the workshop-a collection ofarticles on practical know-how: Workshop Hintsand Tips, by Geometer, published by PercivalMarshall, price 3s. 6d. (postage 3d.). U.S. andCanada % I .OO.

MODEL ENGINEER23 MAY 1957

By Geometer

Machining double-web crankshafts

IN setting-up methods, the choicefor double-web crankshafts issomewhat larger than for single-

web type, and the main influencesin choice of a particular methodare the construction adopted forthe crankshaft, its size, and whetherit has one crankpin, two, or severalas in the case of a multi-cylinderengine.

If the crankshaft is being machinedfrom the solid, has only one crankpinand is modest in dimensions, a set-upcan be made on an angleplate or asub-faceplate as for a single-webcrankshaft. The same is true if thecrankshaft is a forging and not toolarge, the mainshafts being machinedoversize for setting up to machinethe crankpin, then afterwards finishedto size.

Where there are two or more crank-pins? however, the method alone isnot m general applicable. It can beused for one end of the crankshaft,but the other must have support fromthe tailstock, which means there must

PLUG ,

be a “throw plate,” either integralwith the shaft, to be cut off afterwards,or clamped on for the particularmachining operation.

With forethought, and assuming abrazed-up or welded-up constructionto be acceptable, a single-cylindercrankshaft can be machined entirelybetween centres. T h e crankpinmaterial is left as long as the mainshaftand both are centred, either beforethe building up process, or afterwards,locating the centres by marking offmethods. With the mamshaft cutaway between the webs, the crankpinis machined, as at A (upper diagram),then the mamshaft can be turned witha packing block between the webs(lower diagram).

For a double-web crankshaft notprovided’ with setting-up features,throw plates can be used at each endand a set-up more speedily effectedthan on an angle-plate or a sub-faceplate. The throw plates each havea centre and are clamped to the main-shaft-so a set-up can be madebetween centres.

-. BUSH , *_

30 MAY 1957 775

An improvement to this con-ventional arrangement, however, is toprovide one throw plate with fixingholes to bolt to the faceplate, as atB-which ensures a powerful chatter-free drive on the intermittent cutsdown the webs. Fixing holes aredrilled in the bar material, then themainshaft hole bored on centre, thematerial removed and the clampingbolthole drilled. Finally, saw cutsare made from the hole and theoutside to meet.

To set the required throw, a plugis placed in the mainshaft hole, thelathe turned for the plug to touch abar on the slide; then after rotatingthe lathe half-a-turn, a gauge oflength W, the stroke of the engine,should fit between plug and bar.Adjustments are made until thiscondition obtains.

Previously, the crankshaft shouldhave been prepared, also the throwplate for the tailstock end, except forthe centre hole, and to drill thisaccurately the throw plate can bemounted on the plug in the main one.

To “ push ” on the tailstock reason-ably for support, mainshaft ends maybe stepped for throw plates, thoughwith the present arrangement, thedriving end will abut to the faceplate.If required; deflection can be avoidedby tapping the plates and fitting studs,as at C, X Xl, with drilled holes orfiled flats as the means of turning.

General accuracy in making pairs ofthrow plates, or a second one, andalso during the actual crankshaftmachining can be enhanced by boringpairs plugged together, or the secondone on a plug on the main one, thenfitting a hardened silver-steel bush tothe second one, as at D, to run on thetailstock centre.

For a multi-cylinder crankshaft,graduated collars can be fixed eachend-points Y Yl, as at C. Aftermachining a crankpin, shaft andcollars can be turned m the throwplates to align for machining the nextone. For a twin-cylinder crankshaft,collars would have two graduations,as at D, ZZI. El

A collection of the most useful articles byGeometer has been issued in book form-Workshop Hints and Tips, published by PercivalMarshall and Co. Ltd. price 3s. 6d. (postage 3d.).U.S. and Canada: $1.00.

MODEL ENGINEER

Balancing

single-cylinder engines : by Geometer

ALLIED to crankshaft con-struction for single-cylinderengines, particularly those

set up forces along their line of travel;and if the engine is unbalanced, theforces, acting through its structureand mountings, set up vibration.

being turned at right angles there is

intended to run at high speed, isthe problem of balance, since it iswhile the crankshaft is being madethat provision can best be effectedfor subsequent balancing. That is,the crankshaft webs can be suitablyshaped or extended to avoid thenecessity for separate balanceweights-with the need for securemounting to obviate the possibilityof their working loose in service.

The need for balancing is obviouson the simple thesis that to start matterin motion requires a force, and tostop it demands another force, whilethe greater the “ weight ” of matterand the more sudden the starting andstopping, the greater the forcesinvolved.

In an engine, the major reciprocatingparts comprising piston, rings, gudgeonpin and part of the connecting rod,

Lightness in the reciprocating partsis important to keep the forces assmall as possible. Hence, in highspeed engines,. it is customary toemploy alummmm-alloy pistons, hol-low steel gudgeon pins and duraluminconnecting rods. In certain types ofengine, good balance is achieved inopposmg one set of reciprocatingparts by another. The flat-twinhorizontally-opposed petrol engineand the single-acting vertical twinsteam-engine are examples.

In a single-cylinder engine, althoughit is usual to speak of balancing,complete balance is not possible-theunbalance is shifted from one directionto another. In reference to diagramA, a balance weight could be attachedto provide a force W completely tobalance WI set up by the reciprocatingparts at top dead centreand againat bottom dead centre. But the crank

.

6 JUNE 1957

C O N N E C T I N G

no force to oppose comletely thatof the balance weight in direction X,and at a further half turn in directionXl. So the engine would be no lessunbalanced than before.

To avoid this, the unbalance isdivided, the force W opposing onlypart of that W1, and forces acting atX and Xl are thereby reduced.Usually, the balance weight corres-ponds to the whole of the revolvingmasses, which can be balanced, plushalf the reciprocating masses. Thepiston with rings and gudgeon pinare reciprocating masses, so half.these weights are taken. The smallend of the connecting rod is a recipro-cating mass, half of which is taken, too.But the big end is a revolving mass, sothe whole of this is taken.

Using a balancing bar and cotton,and steel balls or lead shot in a tissuepaper bag, the piston complete isweighed, as at B, and half taken. Theconnecting rod big end is weighed, asat C, and the whole of this taken-the small end resting on a block. Thesmall end is weighed similarly, andhalf taken.

The total number of balls or shotthen form a weight which shouldhold the crankpin horizontal whensuspended from it by cotton, as at D,with the crankshaft resting on knifeedges-which can be strips of steelabout 1/32in. thick bolted each sideof a board, and this levelled in the ,vice.

A heavy balance, weight is correctedby reducing its stze, or drilling orenlarging holes such as Y, Yl. Often,however, the difficulty is to providesufficient weight in the web. Thecrankpin can be partially drilled tohelp, and where the crankshaft sizeis determined by the crankcase, thebalance weight can be extended side-ways, as far as connecting rod clear-ance will permit, as at E.

Where the shaft runs in the open,the balance weight can be extendedradially Z beyond the circle sweptby the crankpin. For extra weight,a lead plug can be used at times, heldin by a steel screw; and where noprovision has been made, a steelbalance weight can be bolted on. q

~~No model engineer’s workshop should be

without Workshop ‘Hints and Tips, by Geometer,published by Percival Marshall and Co. Ltd,price 3s. 6d. (postage 3d.) U.S. and Canada$1.00.

MODEL ENGINEER

By GEOMETER

Machining simple connecting rods

AN ADVANTAGE of the single-web crankshaft with opencrankpin-and of cer tain

built-up types-is that the con-necting rod can have a solid big-end eye, which simplifies its con-struction, makes for strength, andreduces bulk and weight-all im-portant factors, and particularlyso for high-speed engines and fortwo-stroke engines where deadspace in the crankcase should bekept to a minimum.

In the interest of reducing theweight of the connecting rod, whileretaining a high degree of strength,duralumin is often used in modelpetrol engines, and in itself providesa good bearing on soft-steel or hard-steel gudgeon pins and crankpins.Steel may be used, but it is heavier,and the eyes require bushing withbronze, brass, or duralumin-exceptin some instances where the gudgeon--

4- _-

jw-- I- l .

.CLAMP

C O N N E C T I N G

13 JUNE 1957

-7-IREL

pin may be a fixture in the small endand oscillate in the piston bosses.

Brass or bronze may also be usedfor the connecting rods of simpleengines, of moderate speed and power,and both are heavier than steel,though neither requires bushing ex-pressly, but only to facilitate overhaulwhen wear occurs, thus avoidingrenewal of the whole connecting rod.

The simplest section for the lengthof a connecting rod is a rectangularone with rounded sides; and on aweight/strength basis it lacks littlebeside the familiar I-section, which ismilled each side to leave a narrowcentral web. The connecting rod ofrectangular section, however, can beentirely finished by turning methodsand some handwork-without theneed for milling where there are nofacilities for this.

Using a piece of suitable flat barto make the comecting rod, it isfaced flat one side by filing or machin-ing a light cut, in the four-jaw chuck.

M O U N T I N G

0C

TOOL

Then the centres of small and bigends are marked, and these boresmachined, clampmg the bar to thelathe faceplate-preferably leavingthe bores undersize for final finishinglater by machining or reaming.

For machining the web, the bar iscentralised and clamped, and facingcuts taken across. Final smoothingmay be done with a file, with the barremoved ; and for machining theopposite side of it, a piece of packingof appropriate thickness should beclamped against the faceplate, as atA, to provide support for the web.

Removing surplus materialThe small-end and big-end bosses,

which will be circular, can be pro-duced partly by sawmg and filing,partly by machining. Sawing andfiling are recommended for removingthe surplus material; and for holding.the connecting rod securely in thevice. without risk of damage, suitablehardened steel plugs should be madefor each end, as at B. Followingthis, the connecting rod is mountedon a mandrel in the chuck (onemandrel for each end), and the bossesare turned back to the web-slightlytapering them towards the outsideif required.

Surplus material along the edges ofthe web can also be removed by care-fully sawing and filing; and eventuallythere will remain only material on thebosses in line with the web. Plugscan then be used as filing guides-particularly where the outside edgesof the bosses are tapered, as withlarger plugs, the file will clear exceptat the centre.

Alternatively, material in line withthe web can be milled or “ turned ”off. For milling, as at C, the mill isrun in the chuck, and the connectingrod mounted on a bracket on thetopslide. A stop prevents the cuttergathering on to the web; with a lightfeed, the connecting rod is pulledround by hand. For “turning,” asat D, the connecting rod is on amandrel in the chuck, which is pulledround by hand for each cut.

Connecting rod centres may beaccurately obtained, a s at E, using aplug Y in one bore, for dimension Xto be measured over a hollow “button”Z, which is then set to run truly formachining the other bore.

847 MODEL ENGINEER

Split big-end connect i ng rods. . .

By GEOMETER

OR use on multi-cylinder engines and single-cylinder F types with double-web crank-

shafts which cannot be dismantled, it is essential for connecting rods t o have split and bolted big-end bearings. From the constructional point of view, this naturally adds to the work involved, but the processes can all be carried out accurately on a lathe with normal equipment.

In detail, connecting rods may vary according to the designer's ideas or in keeping with the engine being made. For open type horizontal internal combusion engines and steam engines, connecting rods may be of steel with flat ends and split brasses bolted up with steel plates at the ends, as at A left. Alternatively, steel connecting rods may be bored as at A right-in which case they are provided with brass or bronze liners-or white metal is run direct on the steel and machined to form the actual bearings. Again, brass or steel liners may be lined with white metal for fitting to the rods. Where reduction of weight is important, as in small high-speed engines, connecting rods may be of duralumin, working direct on crankpins and gudgeon pins.

Sections of connecting rods are usually round for the type at A left, and rectangular or I-section for that at A right. Bearing liners or shells may be flanged at the ends to run in the crank cheeks, or the con- necting rods themselves may thus locate and carry flush-fitting liners, as at B left and right, respectively. A simple means of locating shells is a pin screwed into the rod or cap fitting in a hole in one half.

In small model sizes, whatever the type of connecting rod being made, it is useful to employ rectangular material and bore the gudgeon-pin hole first-with the bar clamped flat 20 JUNE 1957

to the faceplate. The bar can then be held in the independent chuck and the end faced, either to take the flat brasses or to the centre line for fitting the cap.

Where flat brasses will be used. the faced end of the connecting rod. can be centred, then the bar reversed and the other end centred. Surplus material should be available for fitting a carrier or driving pin to effect a set-up as at C. The bulk of unwanted material can be sawn away, and the rod turned taper with the topslidee at an angle.

The small end may be finished after- wards, sawing off the surplus material, then rounding up the boss by filing with hardened stepped plugs in the bore-thee ends finally being tapered if desired by machining with the small end clamped on a threaded mandrel by a nut.

The steel plate which will hold the brasses should be drilled and clamped to the connecting rod for drilling the bolt holes-and may also be used as a template for drilling the brasses. On the other type of connecting rod, the cap material may be so employed;

and where the holes are long, a set-up in the independent chuck for drilling from the tailstock is advisable.

With the cap material bolted to this type of connecting rod, the big- end can be bored by clamping to the faceplate, as at D, using slotted clamps for access to the joint line, which serves as one setting-up line, while a scribed line is marked on the rod for the other direction. For machining the web of the connecting rod, the set-ups described for simple connecting rods can be employed. Then the big-end may be turned at the sides with the rod clamped on a mandrel, as at E.

For making flat brasses, the material should be machined true if necessary by careful set-ups in the independent chuck or on the angle plate; then the halves are drilledd from the steel plate and dowelled together for mounting on the angle plate, as at F, to machine the bore. Round brass liners may be machined similarly from two pieces of flat bar, centred and projecting from the independent chuck for machining the bore and outside at one setting.

CONNECTING \ . ROD

MANDREL I 1 1

BEARING I 2 PLATE

' CON N E'CT IN G I ROD

881 MODEL ENGINEER

4

Connecting rod

alignment By GEOMETER

IVENG alignment accuracy inthe major components ofan engine-crankshaft, main

bearings, cylinder and piston-anyfault must rest with the connectingrod; and this part more than theothers may be subject to defects asa result of machining, dismantlingor servicing.

In view of the comparativelyslender proportions of a connectingrod, however much care is taken toensure accuracy in machining it ispossible for slight warping to occurdue to release of stresses in the metal.In the case of a connecting rod havinga clamped gudgeon pin, inappropriatehandling when loosening or tighteningthe clamping screw can result indistortion. A badly-machined bushin a small-end will tip or twist thegudgeon bin-and the whole rod maybe put out of alignment if a re-metalled big-end is machined in-accurately.

Anart from these possible defectsit is-important on occasion, owing to

, big-end off-sets, to fit connecting rodsthe correct way round; and whennew pistons or gudgeon-pin busheshave been fitted, side clearance atthese points of the assemblies isimportant.

For checking connecting-rod align-ment, the essentials are, first, twowell-fitting parallel mandrels-one forthe small-end, the other for the big-end, each about 3 in. long for smallconnecting rods and about 6 in. longfor car sizes. The set-up can be on alathe or surface plate and, where anindicator is not available for use onthe slide rest or surface gauge, carefulobservation of a round-ended pointerwill suffice.

The two mandrels, which can bemachined in the lathe, should beparallel in both planes when fitted inthe connecting rod, as at diagrams Aand B. Vertically, the dimensions Xand Xl should be the same; whilehorizontally, as on a surface plate,Y and Yl should be equal, as shouldZ and Zl. Dimensions X and Xlcan be easily measured-particularlyon small connecting rods-but thehorizontal check requires a lathe orsurface plate, or a jig as may beused for car connecting rods.

In a lathe, with the big-end mandreheld truly in the chuck or between

centres, an indicator or pointer canbe mounted on the slide rest, then theconnecting rod turned for the small-end mandrel to be tested each end,moving the indicator or pointer bythe saddle. For the cross-check, theconnecting rod must be supported forthe indicator or pointer to pass overor under the small-end mandrel,using the cross feed. If the lathe has aflat bed, the surface gauge can be usedfor both tests. On a surface plate, thebig-end mandrel is levelled on blocksor placed in V-blocks for Y and Ylto check the same. Then Z and Zlshould be equal.

In practice, exact equality in dimen-sions is not easily attained, and areasonable degree of error must oftenbe accepted. On the short mandrels,differences in dimensions should ad-visedly not exceed 0.002 in. to 0.003 in.,and on the long mandrels 0.005 in. to0.006 in.

A bent connecting rod, with anerror in the dimensions at A., can becorrected as at C in a vice using threeblocks; where the malalignment istwist, with an error in the dimensionsat B, correction can be made as atD, holding one end of the connectingrod by the web between blocks, andusing a bending bar at the other end.Care is necessary, and “ spring-back ”must be allowed for, particularIy inthe latter operation-for which com-mercial alignment jigs often havescrew adjustment.

Loosening or tightening a gudgeon-pin clamping screw may be donesafely,. holding the gudgeon pm end-wise in the vice between suitableplugs as at E, whereas holding by theweb may cause distortion. On car ormulti-cylinder engines, observationshould be made for big-end off-setsas at F and for small-end clearancesduring assembly, using a flashlight. q

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27 JUNE 1957 9 1 9 MODEL ENGINEER