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IN THE

WORKSHOPby “ Duplex ”

46-The Dial Thread Indicator

THOSE whose experience goes back forsome years will remember that the recog-

nised method of ensuring correct engagement ofthe clasp-nut, when screwcutting, was to use thelathe tailstock as a back-stop for the saddle, andto mark the mandrel carrier plate and the lead-screw with chalk lines to indicate the relativelycorrect positions of the work and the leadscrew.

In this way, if care was taken, the clasp-nutcould be closed at the right moment, when takinga succession of cuts over the work, withoutdanger of cross-threading’; that is to say,cutting a second thread on top of the first whenthe thread pitch is not a multiple of that of theleadscrew.

Nowadays, the fitting of a dial thread indicatorto even the less expensive lathes has renderedthese elaborate precautions unnecessary, andchalk marks and a saddle stop are apparently. Fig. I. Modified Myford thread indicator

Fig. 2. The component parts of the thread indicator

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now seldom used by newcomers to lathe work.Before dealing with the construction of this

device it may be as well to describe its mannerof working and its method of use.

Clearly, if the screw thread to be cut is amultiple of the 8 t.p.i. leadscrew, that is to say16, 24, 32 and so on, then the clasp-nut may beengaged with any thread of the leadscrew, withthe knowledge that the tool will register with thework thread on all occasions when a series of cutsis taken.

Should, however, a thread of, say, 12 t.p.i.have to be cut, then the work will make 1-1/2 turnsfor each revolution of the leadscrew, or three

0A

turns of the work for every two turns of theleadscrew ; therefore, were the clasp-nut en-gaged successively with each leadscrew thread,the resulting thread on the work would bealternately correctly cut and cross-threaded.

division line.In the same way, any

even-numbered thread iswholly contained in a half-inch, and so any divisionon the circular scale willshow when the clasp-nutcan be closed. As hasalready been stated, theclasp-nut can be engagedat 1/4 in. intervels on theleadscrew for threads, suchas 12 t.p.i., which are divi-sible by 4 , but to avoidconfusion it is customaryto divide the dial into fourmain divisions only, to andto make use of these for alleven-numbered threads.Nevertheless, in the caseof a square-thread leadscrewwhere engagement of theclasp-nut is rather moredifficult than when a V-shaped or Acme thread isused, it may be found anadvantage to subdivide thedial into sixteen divisions,denoted by short lines, toenable the clasp-nut to bemore readily engaged duringordinary turning operations.

To recapitulate, whenscrew cutting, take the

least linear distance that will contain a wholenumber of work threads and use the appropriatedial divisions corresponding to 2 in., I in., or1/2 in. as the case may be.

Although it is customary to number thedivisions on the dial, this is hardly necessary andmay even be confusing ; for the great majorityof threads cut are even-numbered and any ofthe longer lines will then serve ; in the case ofan odd-numbered thread, two opposite linesshould be marked with the grease pencil, andfor cutting a half-thread a single division issimilarly marked.

Fig. 3 . Working drawings ofindicator parts

From what has been said, it will be evidentthat in the above instance some means is requiredfor determining with certainty the exact point atwhich the clasp-nut can be engaged with thealternate threads of the leadscrew. This, andmore, is exactly what the thread indicator isdesigned to do.

Where the leadscrew has 8 threads to the inch,the pinion engaging with it and driving theindicator dial usually has 16 teeth ; this meansthat one complete turn of the dial correspondsto a distance of 2 in. on the leadscrew or to2 in. of saddle traverse along the work. More-over, as the dial has four main divisions, thesegraduations will indicate a distance of 1/2 in. onthe work or leadscrew.

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SEPTEMBER 22, 1949

The easiest way, perhaps, to understand themethod of using the indicator is to find theshortest linear distance which will containexactly a whole number of threads cut on thework.

Thus, for an odd-numbered thread such asII t.p.i., which has 5-1/2 threads in each half inchof its length, a full inch is required, correspondingwith a half turn of the indicator dial and denotedby either pair of diametrically opposite gradua-tions . Similarly, a half-thread, such as 11-1/2 t.p.i.,will require two whole turns of the leadscrew toensure correct re-engagement of the clasp-nut ;this, of course, corresponds with two turns of the

dial as indicated by any one

The Myford Thread IndicatorAs a result of having fitted new indexes to the

feedscrews of a friend’s lathe, we were askedto alter the thread indicator ; the graduationlines were to be more finely cut to be in keepingwith the new indexes, and their number increasedto sixteen.

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THE MODEL ENGINEER

-REMOVE. FIBRE WASHERS TO SET

/INDEX LINE

LATHE SADDLE VIEWED FROMTAILSTOCK

Fig. 4. The Myford-Drummond thread indicator

When the indicator had been dismantled it wasdecided to rebuild it, at the same time making thezero setting readily adjustable without the useof fibre spacing washers.

This adjustment was provided by fitting thedriving pinion on a plain shaft where it wasfrictionally held by means of a clamp-nut. Inpassing, it may be noted that, as the spindlecarrying the dial at its upper end is quite freeto turn in its bearings and is subjected to verylittle wear, all that is required by way of lubri-cation is to apply some thin grease at the time ofassembly ; in fact, oil is best avoided in thissituation, as it tends to seep on to the surface ofthe dial and obscure the graduations.

The reconstructed indicator is seen in positionin Fig. I, and its component parts are illustratedin Fig. 2.

The body is first machined by mounting it ona stub mandrel and boring out the upper end toa diameter of 3/8 in. and to a depth of 17/32 in. Atthe same time, the upper end is faced and theexternal surface of the head machined, as willbe seen in Fig. I. The work is then reversed onthe mandrel and the lower end is also bored3/8 in., but to a depth of 7/16 in.

To complete the work on the body, the lugbearing the index mark should be carefullyfiled flush with the surface of the boss providedfor the attachment bolt, and the filed surface isafterwards painted to match the rest of the body.

The cap-piece, Fig. 3 (A), is machined from alength of I in. diameter round mild-steel to thedimensions given.

The material is gripped in the chuck and thespigot is turned to a firm press fit in the upper endof the body.

The bore is formed centrally by first drilling

SEPTEMBER 22, 1949

with a centre drill having a body 1/4 in. in diameter;and the body portion is entered in the work forat least 1/8 in. This is to give a true bearing forthe 1/4in. D-bit which is used to finish the boreafter an A-size drill has been fed in for a depthof 7/8 in.

After the capping has been parted-off slightlyin excess of the finished length, it is reversed inthe four-jaw chuck and set with the bore runningtruly with the aid of the test indicator.

The cavity to receive the head of the dial shaft(B) is then turned with a small, pointed boringtool, and by reference to the leadscrew indexthe work is finally faced to bring the recess tothe exact depth required. As shown in thephotograph, Fig. 2, the bore is countersunk orcounterbored for a short distance to provideclearance for the neck of the dial spindle.

An alternative method of boring and recessingthe capping, and one which will more readilyensure concentricity, is to defer the boringoperation until after the part has been turned onits external surface and parted-off. The work isnow gripped by its spigot in the four-jaw chuckand set to run truly with reference to its flange ;the rest of the machining is then carried out asbefore.

The index line, which is also apparent in Fig. 2,is cut on the diameter by mounting a 45 deg.V-tool on its side at centre height in the lathetoolpost, and then feeding it outwards almost asfar as the edge of the capping.

Fig. 5. Thread indicator fitted to Drummond lathe

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THE MODEL ENGINEER SEPTEMBER 22, 1949

It will be found that a line cut with this tool ball, less the difference between the two micro-to a depth of 4 thousandths of an inch will have meter measurements.a good appearance.

The bushine fitted to the lower end of the dialThe next step is to cut the division lines, and

it will be seen in the photograph that in thisspindle (B) carries the pinion and is also arunning fit in the bcshown in Fig. 3(C), and in Fig. 2

with the pinionmounted in place.By this construc-tion, the pinionis secured to thedial snindle whenthe clamp-nut istightened, and atthe same time abearing is pro-vided in the bodyfor the lower endof the shaft.

To make thebushing, a lengthof 1/2 in. diameterround mild-steelis gripped in thechuck, and afterit has been facedand centre-drilled,a No. 13 drill isentered to a depth

instance the dial has been indexed into sixteenre in the body ; this part is divisions ; this is to enable

Fig. 6. Thread indicator. made for Drummondlatheof I in. prepara-

tory to reamingthe bore to 3/16 in. Both the pinion seating andthe bearing portion are turned, but are leftoversize for finishing at a later stage whenmounted on the dial spindle. The work is thenparted off a few thousandths longer than thedimension given so that it can be rechuckedand the upper surface faced.

The dial spindle (B) is made from 3/4 in. dia-meter mild-steel gripped in the chuck andsupported at its outer end by the tailstock centre.

The machining is quite straightforward, but the1/4 in. diameter portion should be made a closerunning fit in the part (A) ; in addition, the headshould be carefully turned to fit the recess in thecapping, so that. although it is free to revolve.the finished joint line should be hardly visible:At this stage, the bushing (C) is secured to thespindle by-means of its clamp-nut, but a shortspacing collar must be fitted between the nutand the bush to enable both the pinion seatingand the bearing portion to be turned to their

finished diameters. By adopting this method ofmachining, the outside diameter of the part ismade truly concentric with the bore.

After the spindle has been parted-off, it iscentred. in the four-jaw chuck and the head isfaced to a thickness one thousandth of an inch inexcess of its given length to allow for finishinglater. To ensure that the finished length of thehead is exactly equal to the depth of the recessin the cap, the latter dimension can be checkedin the following manner ; put a 3/16 in. diametercycle ball in the recess and measure with themicrometer the length of the flange of the cap,first directly and then over the ball. The depthof the recess is then equal to the diameter of the

engagement of theclasp-nut to bereadily made at all

it will be seen times, but, if pre-ferred, four divi-sions only can becut.

To index thesixteen divisions,a 40-T. wheel issecured to thetail of the man-drel, and mountedon the stud, a25-T. wheel iskeyed to a 50-T.wheel so that theformer meshes

with the mandrelwheel. and thelatter is engagedby the detent fixedto the quadrant.

Sixteen divi-sions are then ob-tained by index-ing: from evervfifth tooth of the50-T. wheel. The

backlash in the gears must here be taken up bymeans of a suspended weight attached by a cordto the lathe chuck, as previously described.

Each line is formed to a depth of 5 thousandthsof an inch by taking three successive cuts, and thefour long lines should be made of sufficient lengthto afford easy reading. The lines are, of course,cut towards the centre so that they end sharply.

As the head has been turned one thousandthin excess of its reauired thickness. it is nowreduced by this amount to impart a’good finishto the division lines and, at the same time, tomake it lie exactly flush with the cap.

After the cap-piece has been pressed into thebodv with its index line correctly positioned.and the pinion has been pressed on to its bushing;the thread indicator should be assembled with thepinion clamp-nut turned finger tight. When oneof the long division lines has been registered withthe zero line on the capping, a toolmakers clampis applied over pieces of thin card to retain theparts in this position. A hole to receive a smalltommy bar, made of 12 gauge wire, is then drilledwith a No. 37 drill 5/64in. from the uppersurface of the capping and to a depth of 5/16 in.With the tommy bar in place, the pinion clamp-nut is fully tightened and the end-float of thespindle in the body is then tested. Shouldthe end-clearance be found to be excessive, thepinion bushing is mounted on a stub mandreland the requisite amount is turned off its upperface.

If, on the other hand, there is insufficient end-clearance to allow the shaft to revolve freely, thena facing cut is taken over the upper surface of thepinion itself when mounted in a similar manner.

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THE MODEL ENGINEER SEPTEMBER 22, 1949

Finally, with the tommy bar in place and the co the rear face of the saddle apron by meanspinion clamp-nut slackened, the pinion is meshedwith the leadscrew to afford a small amount of

of the arm (A) which carries the bracket (B) forholding the indicator body (C). The body in

working clearance between the teeth ; the bracket turn serves as a bearing for the rotating spindleclamp-nut is then tightened to secure the indicator (D), graduated on its upper surface and carryingfirmly in place. the pinion (E) at its lower end.

1II3/4’DlA I-- 1

DRlLL 1/8"ON ASSEMBLYOF PARTS C AND D

Fig. 7

The leadscrew is then turned in the forward The arm (A) is formed from a length of 1/2 in.direction to allow the clasp-nut to be closed, and,with the parts in this position, the pinion clamp-nut is tightened to complete the setting.

Fitting a Thread Indicator to the Myford-Drummond Lathe

The thread indicator made for the Myford ML7lathe can also be fitted to the Myford-Drummondlathe by means of a bracket of the form shown inFig. 4 designed by the makers.

On taking delivery of a 3-1/2 in. Drummond lathesome years ago, one of the first additions madewas to design and fit a thread indicator of thetype illustrated in Fig. 5. In the photographthe tubular leadscrew guard fitted to the right ofthe saddle has been removed to give a clearerview.

Reference to Fig. 6 and to the working drawingsin Fig. 7 will show that the indicator is attached

square mild-steel and is secured to the apronby two 2-B.A. screws.

The projecting cylindrical portion on whichthe bracket fits is drilled to a depth of 3/4 in. andtapped a-B.A. To allow this part to be ex-panded by the coned screw (G), thus securingthe bracket in place, the nose is countersunk witha centre drill, and four longitudinal cuts are madewith a slitting saw.

The bracket (B) is drilled and reamed to renderit a firm sliding fit on the arm, and, in addition,a 1/2 in. diameter hole is formed to accommodatethe indicator body (C), which is secured in placeby the clamp-screw (F) closing the saw-cutshown in the drawing.

The body (C) is first turned to an accuratesliding fit in the bore in the bracket ; it is thenreversed and set to run truly in the four-jaw

(Continued on page 376)

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leading from the boiler turret on the left side,is cut to length, and furnished with a union-nutand cone for connecting to the steam inlet ofthe valve. The right-hand pipe on the valvealso has a union-nut and cone, for connectingto the union on the brake cylinder. Theexhaust pipe is left open, the end being set sothat the exhaust steam is directed anywherebetween the rails.

The action is as follows. When the driver’shandle is moved to the left, the groove in thecock-plug moves to the position shown in the“ brake on” diagram ; steam then passes fromthe upper hole, through the groove, to the lowerright-hand bole, thence to the brake cylinder,pushing down the piston and applying brakes.Moving the handle to the right, causes the steamto be cut off, and the groove to connect the twolower holes ; the steam tben escapes via thegroove and the left-hand hole, through theexhaust pipe, to the atmosphere. The releasespring pulls the brakes off. Before operatingthe brakes for the first time on a run, open thedrain cock, put the handle to the “ brake on ”position, and blow some steam through the cylin-der to warm it. The drain valve should be leftopen when the engine is standing ; incidentallythe drain cocks on the engine cylinders should betreated likewise. I always leave them open on at all.my own engines, when same are “ dead,” justas in ful!-size practice. Instances have occurredin more than one running-shed, where an enginewith a leaky regulator valve has been lit up withthe cocks closed. The reverse gear is supposedto be always in the middle, when the engine isin the shed ; but what often happened, was, thatin the far-off days before engine-cleaners becamean extinct tribe, one of them would shift thelever, so that he would better be able to get at therods and links, especially with inside cylindersand valve-gear. Human nature being what it is,he might probably forget to put it back again.

SEPTEMBER 22, 1949

Consequently, when the engine got up steam,leakage past the defective regulator valve wouldaccumulate in the cylinders, and presently theengine would move herself; only a little, trueenough, but it doesn’t need much movement tocrush some unlucky wight between the buffers,or maim a hand or foot under a wheel or amongstthe “ works.” With the cylinder drain cocksleft open, it couldn’t happen. Some of theStroudley engines had a little snifting valveunder the steamchest, which remained openwhen the engine was standing, and automaticallydrained away any condensate water. This alsotook care of any steam leakage past the regulator,which was almost unheard of with the Stroudleytype. On the “ Gladstones,” this valve was atthe bottom of the underneath steamchest, along-side the solitary drain cock. Those engineswhich had steam sanding-gear, took steam forsame direct from the steamchest via very shortpipes, so we didn’t have to wait a month ofSundays before the sands operated. On theBillinton engines, steam had to travel all theway from the valve on the backhead, to the sandejectors below the driving wheels, via a 3/8in.copper pipe ; and by the time it got there, andthe condensate water had been blown out, wehad usually got a start, and didn’t want any sand

Speaking about sanding gear, if anybody wantsto fit a working sander to “ Doris,” it can be doneon the lines described for the “ Maid of Kent,”only making the parts smaller in proportion ;personally, I don’t think it is worth the troublewith a small six-coupled engine. I have fittedone to “ Grosvenor,” but that is only a bit ofswank, to show how much weight a single-wheeler can start, if she is put to it.

Well, all we now need to finish off little “Doris,”is the tender brake gear ; and all being well,I’ll deal with that in a final instalment of this“ serial.”

In the W o r k s h o p(Continued from page 371)

chuck for boring and reaming the bearing for thespindle (D), and at the same setting the recessfor the head of the spindle is also turned toensure concentricity. Finally, the index line iscut with a V-tool mounted on its side at centreheight in the lathe toolpost.

The machining of the spindle (D) is a straight-forward turning and threading operation, and,following this, the part is reversed in the four-jaw chuck for facing the head to the correctthickness and cutting the sixteen scale lines inthe manner previously described. Although alubricator screw is shown, this as already ex-plained is, perhaps, best omitted.

The sixteen-tooth skew pinion, which isscrewed into place and then secured by ashouldered lock-nut, was obtained as a standardfitting from the lathe manufacturers.

The spindle should now be inserted in thebody and clamped in position by placing a

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washer under the pinion and screwing it downfirmly; a 1/8 in. diameter cross-centre tommyhole is then drilled right through both the bodyand spindle, in order to facilitate locking thepinion securely on the shaft or dismantling theparts when required.

The indicator can now be assembled andattached to the lathe apron. The pinion isbrought correctly into mesh by rotating theindicator bracket on the arm, and at the same time,the body is moved backwards or forwards to setthe pinion on the centre-line of the leadscrew.With the clasp-nut closed and the leadscrewrotated in a forward direction, a division line onthe dial is set to register with the index line onthe body ; this is effected either by rotating thebody itself or by sliding the bracket along thearm, but in any case, for the sake of appearance,the index line on the body should be set to lievertically.

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