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Testing a LatheBy J.

THERE is an old saying that a badworkman blames his tools ; the

converse of this is that a good workmanis one who knows how to put his toolsright. I don’t propose to explain how tocorrect an inaccurate lathe, but only toshow how it should be tested for faults,which, after all, is the essential prelimin-ary to making up your mind to dosomething about it.

Few amateurs are in the happyposition of being able to pay the priceof a first-class machine, the accuracyof which the maker will guarantee toSchlesinger limits; nevertheless, quitecheap lathes are often surprisinglyaccurate as regards the straightness ofthe bed and slides, and the true runningof the mandrel, but fall short becausethe alignment of the various parts oneto another has not been made withsufficient care. Careless handling intransit, or faulty setting up, can alsocause trouble. If any errors in theserespects can be put right, the time spentwill be well worth while.

Now it might be thought that thequickest way to test a lathe for truthwould be to check the work produced,but anyone who has tried this way willagree that it is slow and unreliable. Itis seldom realised how much springthere is in the work itself, even with thelightest of cuts. For example, if theheadstock is adjusted so as to turnparallel a piece of bar held in the chuck,

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a cylinder bored immediately after-wards will be found to be most un-accountably large at the back end. Theerror may not be great if the mandrelis a stiff one, and the work rigid, but itwill be there all the same, and repeatedattempts to get things just right by thismethod generally end in the unfor-tunate owner wishing he had left wellalone.

Setting a headstock in perfect align-

ment is a slow job at the best of times,and the advice sometimes given toraise the headstock on packing so as tobe able to swing an outsize job is mostunwise. It is a long and tedious processgetting it lined up again, and oncecorrectly set, it should not be disturbedlightheartedly.

To check up a lathe reliably andspeedily, a test indicator of some sortis almost essential, and if this mostuseful instrument is not in the workshopkit, perhaps one can be borrowed to dothe job.

I need hardly say that before anytesting is begun, the lathe should becarefully cleaned, all slides adjustedand oiled to work smoothly but withoutany slackness, as it is easy to get falsereadings due to swarf and grit.

The main requirements are not many,

Fig. 2. Testing the cross-slide for accuracyof facing

Left : Fig. 1. Testing the alignment of theheadstock

and can be summarised as follows:(1) The headstock spindle should be

parallel with the ways of the bed.(2) The cross-slide should move at

right-angles to the headstock spindle.(3) The axis of the tailstock spindle

and centre should be in line with theaxis of the headstock when set in anyposition on the bed.

(4) If a boring table is fitted, itssurface must be parallel to the bed andto the cross-slide:

(5) The taper holes for the centresshould be concentric with their re-spective spindles.

The first item is much the most im-

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portant, as on this depends the parallel-ism of all work done in the chuck,which probably covers 95 per cent. ofall the model engineering done in a smalllathe. Years ago, this was not the case,and between centre work was almostthe rule, but nowadays a good chuckseems to cover nearly everything.

Assuming then that you have the useof an indicator, a test bar is needed tocheck the headstock. For a lathe of3-1/2 in. centres, a convenient length isabout 6 or 7 in.. and about 2 in. dia.It should have a’flange on one end sothat it can be bolted to the faceplate;a piece of steam pipe with a flangescrewed or welded to it makes an excel-lent bar, or it may be possible to findsomething suitable from the junk box.The bar shown in Fig. 1 is a piece ofcast iron scrap of unknown origin,which shows the sort of thing wanted.

The advantage of a flange to bolt tothe faceplate is that the bar can havethe maximum of length for the minimumof overhang from the front headstockbearing. But if nothing with a flangecan be obtained, a piece of stout pipeheld in the chuck can be used instead.

Having got your bar, and set it upto run reasonably true, turn two bandson it about 3/8 in. wide, smooth and

parallel, so that they can be sized witha micrometer. The bands should beas far apart as possible, consistent withbeing able to measure the inner oneproperly.

In the bar shown in the photograph,the two bands or collars stand proudfrom the surface, and this is an advant-age, but is not essential. What is im-portant is that the diameter of the twobands should be as nearly the same asyour skill in turning and measuring will

allow. The exact size does not matter ahoot, but they must be the samediameter.

If you have no micrometer, you mustuse calipers, but the stiff frame of amicrometer makes it much easier todetect a small difference in size. Owingto the overhang, only very light cutscan be taken when turning the outerband, and, if thesurface is at all roughto start with,, it may be advisable tosupport it with the tailstock until asmooth surface is obtained. However,the final cuts must be made without anysupport from the back centre.

Having completed all this to the bestof your ability, fix the indicator in thetoolpost as shown in Fig. 1, so that byracking the saddle along the bed, itcan be brought into contact with eachband in turn. The advantage of havingthe bands raised above the surface of thebar now becomes obvious, as the saddlecan be moved to and fro to try each endof the bar, without having to wind backthe cross-slide each time. If the cross-slide has to be moved, it must be broughtback to the same position each time bymeans of the index on the screw.

There are some precautions that mustbe taken to ensure correct readings. Itwill probably be found that when theindicator is in contact with the bar, andthe saddle is moved slightly, a smallvariation in the reading occurs, this maybe due to a small fault in the gauge, butis more likely to be caused by the saddlerocking slightly on the bed, magnifiedperhaps by the fact that the gauge mayhave to stick out quite a bit in order toreach the band near the headstock.This can be avoided by always movingthe saddle in the same direction alongthe bed when bringing the indicator

Fig. 3. Checking the surface of the boring table

into position. If these precautions areobserved, and the readings on bothbands exactly agree, then the headstockis perfectly lined up and you can restcontent.

However, it is more than likely thatthere will be some discrepancy, and thequestion arises as to what is a reasonableresult.

To begin with, it will be realised thatany error found will be doubled inactual work, in the same way thatfeeding in a turning tool 0.001 in. takes0.002 in. off the diameter of the job.At the same time, if the bands on thebar are 6 in. apart, any errors found willbe proportionally less over a shorterdistance, and most jobs turned or boredin the chuck are likely to be very muchshorter than 6 in. on a 3-1/2in. lathe, so,if you are careful, the test is a revealingone. However, something must beallowed for errors in testing, and it isprobable that few will be able to getthe diameters of the bands on the baridentical to better than a tenth; somewho have to work with calipers onlywill have a hard job to get within half athou. Then there may be further smallerrors in the test indicator, or in settingthe cross-slide screw if the slide has hadto be moved back. All these smallerrors may tend to cancel each other out,or they may be cumulative.

Personally, I would say that if theindicated difference over a length of6 in. was less than a thousandth of aninch, I would be very satisfied, but if itwas a good deal more than this, I wouldthink that it would be worth while totry and correct it by setting over theheadstock with the adjusting screwsusually provided.

Before dismantling the test bar, afurther check should be made to find outwhether the spindle is parallel with thebed in a vertical direction. This iscarried out in the same way, but withthe indicator contacting the top side ofthe bar.

Any error here is not nearly so im-portant as in the first test, as it has onlya small effect on the narallelism of thework, but, if it is much out, it may causetrouble when drilling deep holes fromthe tailstock.

Our next test is to find out whetherthe cross-slide is at right angles to theheadstock. and this is easily carried outwithout making up anything special.First find out the full travel of the cross-slide; it will be at least half the diameterthat can be swung in the gap, and isgenerally much more.

Put on the big faceplate, set the cross-slide to mid travel, and mount the testindicator on the toolpost of the top-slide so that the contact point is oppositethe centre of the mandrel. Now windout the cross-slide to its full travel, orto the extreme edge of the faceplate ifthe slide is a long one and will reach thatdistance. Then bring the indicatorforward with the top-slide until it makescontact with the faceplate, and notethe reading; also, note the reading of the

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index of the top-slide screw. Make asmall circle in pencil or chalk aroundthe point where the indicator madecontact with the plate. Now windback the top-slide a trifle so thatthe indicator is clear of the plate, andthen move the cross-slide right over sothat the gauge can make contact withinthe pencil circle when the faceplate isgiven half a turn.

If the reading here is the same as thefirst one, the cross-slide is at right-anglesto the mandrel. This is a very accuratetest, and does not depend on the truerunning of the faceplate, provided thesame marked spot on the plate is used totake both the readings. Fig. 2 shows thistest being carried out, and it will benoted that in this case the top-slide isnot used, the saddle being brought upby the leadscrew instead.should be repeated with the saddlelocked, to make sure that the lockingarrangement provided does not twist thesaddle slightly; this may happen if thepinch bolt is not quite in the centre ofthe gib. Check also the effect of lockingand unlocking the saddle when theindicator is touching the faceplate.There should be no appreciable move-ment ; if there is, then the saddle is slackon the bed, or the locking screw isbadly positioned, and pulls the saddleout of line when it IS tightened.

It is usual to have a lathe adjustedso that it faces work slightly hollowrather than convex, but it should onlybe very slight, and not more than athousandth of an inch over the fulltravel of the slide.

The next on the list for checking is thetailstock. Usually the surfaces on thebed which position this are separatefrom those which guide the saddle, butin some cases they are the same, e.g.the 3-1/2in. Drummond. Where the guidesfor the tailstock are not the same asfor the saddle, wear may put them outof line with the rest of the bed, so thatthere is trouble when turning parallelbetween centres with the tailstock set indifferent positions.

However, it is very seldom that thereis much wrong here, and what mostlyhappens is that the tenon or other guidegets slack through wear, and no longerpositions the tailstock accurately, andin most cases there is no means ofadjustment. It can be checked byclamping the tailstock to the bed invarious positions, and bringing up theindicator held in the toolpost, to somefixed point each time, say the side of thebarrel, and noting the reading of thecross-slide index.

It is equally important to make surethat the barrel itself is in line with thebed, and this can be tried by running theindicator along the side of the fullyextended barrel, by racking the saddleto and fro. The test should be repeatedwith the barrel clamp tightened, whenit is usually discovered that this throwsit out of line slightly, even on a new lathe.Not much can be done about this beyondsorrowfully noting the error, but some-

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Fig. 4. Testing the truth of the centre hole in the mandrel

times it can be made use of whenturning between centres, to bring thecentre over a trifle, by extra tighteningof the clamp.

Alignment of the barrel with the bedin a vertical direction can be checkedin the same way, but using the indicatorover the upper side of the barrel. Need-less to say, the barrel itself must beparallel and free from bruising if thesetests are to mean anything.

On a lathe which has had a lot of use,or perhaps misuse, the base of the tail-stock, where it slides on the bed, mayhave worn sufficiently to drop the centreheight quite a bit, so that it no longermeets the headstock centre on terms ofequality,. so to speak. Unless this isvery serious, it will not affect parallelturning to any great extent, but it willaffect drilling from the tallstock, andsometimes accounts for the otherwiseinexplicable breakage of small centredrills, no matter how gingerly they maybe advanced to work held in the chuck.

The extent of the fault is not difficultto discover. Mount a centre or a Morsetaper peg having a short parallel portionin the headstock spindle; check that itruns quite true, then mount the in-dicator on a surface gauge seated on theways of the bed, and try the indicatorover the top of the peg. Remove thepeg from headstock, put it into the tail-stock, and repeat the performance; thedifference in the two readings shows theextent of any error.

I have found very few small lathesthat could be relied on to turn work deadparallel between centres every time,because there are too many small sourcesof error, and fortunately a good adjust-able tail centre will take care of the fewoccasionsis really vlt

hen accuracy in this respect1. However, to get it as near

as you can, a long bar should beobtained, carefully centred at eachend, and a spot marked on it a shortdistance from one end, so that when itis mounted between centres, theindicatormounted on the saddle can be broughtopposite the marked place when thebar is reversed end for end. The barneed not be parallel, or even run quitetrue, provided care is taken to make thereading on exactly the same spot eachtime.

Checking the Boring TableSo much for the tailstock, and you

can now proceed to check up the boringtable, if the lathe has one. This is quitea simple job, and Fig. 3 shows themethod of making this test. The pro-cedure is self-evident. The slide ismoved in all directions under the in-dicator, so as to completely explore thewhole surface. It is reasonable to expectan accuracy of within a thousandth. Ifthe error is greater than this, and youdon’t feel man enough to correct thesurface by file and scraper, at any ratemake a note of the deviations, so thatyou will know where to place a paperpacking under the corner of the next jobwhich needs to be “ spot on.”

The last item is to check the truth ofthe centre hole in the headstock spindle,and Fig. 4 shows how this is done. Asmall piece of rod is held in the drillchuck, and a short length turned truewith light cuts. The relative positions ofthe taper peg and the mandrel are thenmarked with pencil, the chuck knockedout, given half a turn, and reinserted inthe mandrel. If the test indicator isthen brought into contact with theturned part of the piece of rod, and showsno wobble when the mandrel is re-volved, the centre hole is true, and the

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July 21, 1955

tumbler reverse is fitted and hailed asa valuable asset, everyone being happyuntil people like “ Jed ” spoil the funby rocking the boat.

I prefer the steadier action of ascrewcutting train without the inter-position of a flimsy tumbler reversecluster which is, as “ Jed ” says, un-necessary.

Actually it is quite reasonably possibleto have a 5/8-in. hole through a lathespindle whilst retaining the use of20 d.p. gears without a tumbler reverse:interchangeable spigots at the left-hand end of the lathe spindle being used,one to carry a 20-tooth wheel for normaluse, and one with a 5/8-in. bore and 7/8 in.o.d. for use with a 30-tooth first driverand slightly modified screw-cuttinggear train (if required) when it is desiredto pass 5/8-in. dia. stock right through thelathe spindle.

Yours faithfully,St. Leonards on Sea. MARTIN CLEEVE.

SECONDARY ELECTRIC CLOCKSDEAR SIR,-In the June 16th issue of

THE M ODEL ENGINEER , you answer aquery regarding secondary or slaveclocks. I work for a large steel companywhich uses time circuits, consisting ofmaster- clocks (four in all) controllingsome 600 clocks, time recorders andtime switches. The wall and panelclocks are manufactured by Messrs.Gents Ltd.,, Leicester. These consistof a solenoid, ratchet wheel and gearswith a half-minute impulse of approx.220 m/a current. For panel mounting,the firm purchase the movements,which consist of a plastic face platewith spindles protruding through forhands, and a plastic box-like cover.May I suggest if this is the type L.E.P.is wanting? he should contact the makerswho definitely supply movements with-out cases and faces.

Messrs. Erricson make impulse con-verters and one-minute time recorders,but I can’t say if they make one-minuteclock units.

Scunthorpe.Yours faithfully,

B. TOMBLIN.

PORTABLE ENGINE DETAILSD EAR SI R,-TO Mr. Hughes’ most

interesting notes on portable engines Ithink might be added one on exhaustpipes. Until well into the ‘60’s thesewere neatly led through the boilerbarrel: the Ransomes engine was Ithink, the only one at the 1862 exhibitionto have a external pipe along the top ofthe boiler.

This Ipswich firm issued “ Instruc-tions to Drivers,” which contained twopoints of interest. The first concernedgovernors: the balls were to be removedto avoid damage when travelling. Theother related to the Salter spring-loadedsafety-valves. Before raising steam thevalve was to be taken out and replaced,thus ensuring it was not stuck. Thespring balance was to be screwed downto about 5 lb. per sq. in.: as soon assteam began to blow off, the springcould be screwed down to the desired

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SIGNAL -FIXED CON TAC T

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pressure. The R.A.S.E. believed in nohigher pressure than 45 lb. In practice,the spring was loaded to suit the work,always the lowest pressure possiblebeing selected, though there were in-stances of the lowest pressure beingrather on the high side for the strengthof the boiler !

Mr. Hughes does not mention thevery small flywheel of the Turnerengine: it suggests, and he may be ableto confirm, that the normal speed wasmuch greater than the usual practice.This may have been adopted on accountof the crankshaft governor, though Ihave seen Turner engines with theHartnell governor which to the bestof my recollection had flywheels com-parable to other makes.

Two extra pairs of contacts are

Yours faithfully,Ruardean, Glos. R. C. STEBBING.

OUTDOOR SIGNALLINGD EAR Sir,-If anyone is thinking

of taking up “ L.B.S.C.‘s ” signallingsystem, might I suggest the followingmodification for tracks where there ispermanent electrical contact betweenrunning rails ? It obviates the continu-ous contact strip and uses instead ashort contact ramp located near eachsignal.

used, and their function is as follows:The train makes contact at A a n doperates the light sequence as in“ L.B.S.C.‘s ” circuit. As soon asthe relay closes, however, it switchesitself directly on to the supply, andthus does not fall out again whenthe train has passed over the ramp.When relay B is operated, however, thissupply to A is broken and relay Areturns to its normal position. Evenwhen this supply line is energised againafter the train passes beyond section Bthis does not have the effect of operatingrelay A until the ramp at A is passedover again by the train.

Rugby.Yours faithfully,

R.P.(Engineering Apprentice).

GALVANOMETERS

Liverpool. LEONARD M. DOUGHERTY.

DEAR SIR,-In your issue of March31st, H.L.H. of Grays, asks for adviceconcerning a sensitive galvanometer.

“ Science Masters’ Book ” (publishedby Messrs. J. Murray), Part I, Series 2,Article 176, will be of assistance to him.A desirable modification of the datagiven there is to use “ Alnico ” magnetsand high-permeability pole-pieces. Ihave done this with success.

Yours faithfully,

TESTING; A LATHE(Continued from page 82)

centre can be replaced in any positionwith the certain knowledge that it willrun truly. However, it is most unlikelythat no error will be found, thus empha-sising the oft repeated advice to alwaysreplace the headstock centre to a markedposition.

If this test reveals a gross error, it isprobable that there is some dirt m thecentre hole, and careful inspection mayshow a small chip embedded in the bore.

These tests will cover all require-ments as regards the general run of

work done on a lathe, and if carefullycarried out will give a very fair idea ofwhat faults exist. Whether they can becorrected depends on what means ofadjustment are provided, and the skill ofthe owner.

It is worth mentioning, as a final word,that many a good lathe has been pulledout of truth by bolting it down firmlyto an uneven bench. So make surethat you are blameless in this respect,before you blame the machine or themaker.

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