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NOVICES WORKSHOP

I a n B r a d l ey t a l k s a b ou t

L a t h e Tool s

THE NUMBER OF tools necessary to perform worksatisfactorily in the lathe is really very few. At onetime very comprehensive sets of tools were offeredfor sale, many if not most of which the eventualpurchaser never used. These sets also had the dis-advantage that the shanks of the tools themselveswere left in the rough forged condition, and notground flat on their underside which is generallyaccepted as being best for ensuring a firm mountingin the toolpost.

Today, however, one can buy tool bits that areground all over and, as they are bodily shaped, maybe used as they are, or clamped in a holder if need

be, the firmness of their mounting is assured, pro-vided of course that the machine tool itself has noshortcomings in this respect.

Varying types of steel are used in the makingof lathe and shaping machine tools. Originallycarbon steel was the main source of material butwith the advent of more difficult machining prob-lems, and the need from the commercial aspect tocarry them out rapidly with the minimum of re-sharpening, high-speed steels were introduced.Tools made from this material are obtainable in theforged condition if needed, in addition the toolbits already referred to are made from high-speed

steel and are well worth the inevitable extra costinvolved.

In view of the advantages to be obtained fromthe use of high-speed steel, it is perhaps not sur-prising that tools made from carbon steel are losingtheir favour even in the amateur workshop. Oneform of carbon steel still fmds a use, however, thisis silver steel, available in round bright bar of vary-ing diameters each 13 in. long. The material findsan application in the making of cutters for variouspurposes, since in the annealed state it is readilymachined and subsequently hardened and tempered

by quite primitive means. On the other hand, thehardening and tempering of high-speed steel callsfor the employment of equipment most unlikelyto be found in the average small workshop, so toolforming there is by a grinding process only.

AS stainless steel is now often found in the smallworkshop, the employment of tools made fromhigh-speed steel is virtually essential. It is possibleto machine stainless material with carbon steeltools, but their durability is then much reducedand they rapidly lose their shape.

Carbide-tipped toolsThe amateur workshop for the most part cannot

make the best use of carbide tools with oneexception, that is the tool used for machiningcast-iron. The reason for their failure, when usedon the light type of lathe possessed by mostamateurs, is that the carbide tip itself has littlemechanical strength, so, when it is not possible totake a heavy cut, the chip produced by the turningoperation impinges directly on the cutting edge,causing it to crumble.

When turning cast-iron, these conditions do notapply. The chip produced breaks up immediatelyon impact with the cutting edge and has then noill effect upon it. Carbide tools suitable for use inthe small workshop will be discussed later.

F ig . 1:Nomenclatureof lathecutt ing tools.

Before dealing with specific tool shapes the termscommonly applied to their angles must be con-sidered. These are shown in Fig. 1. From thisillustration, where the tool is depicted in sections,it will be seen that, in order to allow the tool tocut in the direction shown in the diagram A, sideclearancemust be given or the tool will rub on thework. Additionally to provide the correct cuttingangle for the material being machined, side rakemust be given to the tool point. Where viewedfrom above the point of the tool will be seen to have

an angle ofr e l i e f ;

this is needed to preventchatter or vibration of the tool, the frontal areaof the tool in contact with the work being reducedto the minimum consistent with obtaining a goodfinish to the work surface. Front clearance mustalso be given to the tool as shown at B, in order toprovide a satisfactory cutting angle.

Before leaving the subject of tool angle termino-logy, one further condition needs to be considered.This is the provision of negative rake to the toolpoint as illustrated in Fig. 1 at C.

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From this diagram it will be seen that the topsurface of the tool is inclined at an angle above thecentre-line of the work. This practice is oftenadopted industrially, but in the small workshop canbe employed with success when machining theharder bronze alloys.

Tool shapes for external workThe basic tool shapes required are not many but

the few there are can be applied to a variety of toolsfor both internal and external work. The first ofthese is the roughing tool which probably needsless power (despite its ability to take heavy cuts)than any other. As an example of its effectivenessFig. 3 shows a roughing tool removing 7/16 in.from a 3 in. billet of stainless steel. This machin-ing operation was carried out at a mandrel speedof 60 r.p.m. taking a cut of 0.005 in. per revolutionusing a copious flow of neat cutting oil (GulfMetsil E) as a coolant. The lathe used was a 4 in.

Myford, now some 40 years old, and the drivingmotor fitted was a 1/3 h.p. machine of a standardcommercial type.

- - - - D I R E C T I O N O F C U T -

-FRONT

Fig. 2: Roughing and kni fe tools.

The knife toolThe most important weapon in the turners

armoury is the knife tool. It is found in both right-handed and left-handed forms. The right-handed

knife tool cuts from right to left, that is to say it isfed towards the headstock of the lathe, whilst itscounterpart the left-handed tool cuts in the opposite

direction. Fig. 2 depicts the salient characteristicsof a right-handed knife tool.

It is usual to grind a small flat land at the tipof the tool. This area is at right-angles to the cuttingedge, helping to promote a good finish to the work.As it is sometimes necessary to form a roundedcomer on shoulders machined by the knife tool,the flat land can be replaced by a radial point, Forthe amateur worker the radius of this point can

probably be standardised at some convenient figureand it will therefore be convenient to have tworight-hncded knife tools to avoid the wastage ofmaterial and the time involved in altering the pointof a single tool. An enlarged view of both forms ofpoint is seen in Fig. 4.

Fig. 3.

The parting toolOnce the worker has completed the turning of

a component he will need to sever it from the stockmaterial. This he does with the parting tool, shownin Fig. 4. This is the form usually supplied in setsof turning tools, but experienced workers modifythe point to ensure that the work is removed cleanlyand this alteration is shown at A. This is the toolpoint commonly used in automatic lathes where,of course, it is absolutely essential that the partsproduced are parted off cleanly.

Where much bar material turning is undertakenthe parting tool is best mounted in a toolpost seton the cross-slide behind the work, thus leaving thetop-slide itself for holding outer turning tools eithersingly or in a capstan head that may be indexedaccurately to bring a succession of tools to bear onthe work as required. It is perhaps worth em-phasising that the back toolpost, as it is called,is the ideal holder for parting tools in a light lathebecause the forces then acting tend to force the

faces of the cross-slide into closer contact. In thisway vibration sometimes encountered in the front-of-work placing of the tool can usually be elimin-

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FLATPOINT

ROUNDED

POINT

O

FOR STEEL 3 O RAKE

L 8

Fig. 4.

ated. Long ago Duplex advocated this positionand designed a toolpost that itself possessed acapstan-head. carrying a pair of tools, one forchamfering and one for parting-off, two operationsregularly needed. A typical set-up embracing front

and rear tool placings in capstan-heads is illustratedin Fig. 5.

The screwcutting toolThe last of the basic tools needed for external

work is the screwcutting tool whose salient featuresare seen in Fig. 4 above.

The point angle depends upon the thread formto be cut. In this context the handbook ScrewThreads and Twist Drills published by Model &Allied Publications may be consulted. It shouldbe noted however that as supplied, commercial

single-point threading tools are not necessarilyground to any particular angle. One must be care-ful, therefore, to check this angle using the appro-priate setting gauge for the purpose. This gauge,illustrated in Fig. 6, is provided with male andfemale cones of the correct angle so that the accur-

WORK

GAUGE

Fig. 7

ROUGHING FINISHING

acy of the tool point can be checked. In additionthe sides of the device have angular notches, againof the correct angle, machined in them to enablethe turner to set the threading tool squarely withthe work. This he does by placing the gauge againstthe side of the part and engaging the tool point

with one of the notches in the manner shown bythe illustration Fig. 7.

A piece of white paper or card placed below thetool point will enable the correct engagement ofthe tool point with the notch to be observed moreeasily. To be conti nued.

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