UNIQUE METHODS USED IN THE

MANUFACTURE OF TWIST DRILLS, TAPS AND DIES

by H. H. TAYLOR,

Works Manager, Patience and Nicholson, Ltd., Melbourne.

Presented to the Melbourne Section of the Institution, Yith April, 1955.

HP HIS Paper describes some of the manufacturingA methods and processes used in the manufacture of

twist drills, centre drills, screw extractors, taps, andfinally, dies.

TWIST DRILLSOur first item is the manufacture of twist drills

which are defined as end cutting tools having one ormore cutting edges and having helical or straightflutes or grooves adjacent thereto for the passage ofcuttings or chips, and used for originating orenlarging holes,

A drill of some sort has been used throughout theages and it is said that samples are in existence fromthe Stone Age. The advance from stone to metalmust have been to early peoples as great an advance-ment as the change from carbon to high speed steelsand then to carbides in our own industries. The paceof modern life, with its jets and rockets, is forcing thedrill makers to keep abreast of developments.

The twist drill was developed during the 19thcentury after the flat or spade drill reached the limitof usefulness and as late as 1919, in the fifth editionof " Machinery" Handbook, it is recorded that" straight shank drills f" in diameter and less areusually not ground after heat treatment".

Materials and DesignWith the exception of specials and drills of large

diameters, all taper shank drills are made from twoseparate steels ; the body or pod portion from highspeed or a cutting steel to do the work, while theshank, used only for driving3 is invariably made froma medium carbon steel (0.4-0.5% carbon) to allowthe tang to be toughened to prevent burring overwhen being removed from socket or spindle by means

of a drift. The shanks on smaller drills up to 21/64"are knurled after fluting ; heat treatment follows, andthey are then pressed into a prepared shank, thecentreless grinding operations taking place afterwardsto ensure concentricity.

The larger taper shank drills are made from shankand body material flash butt welded together. Onlywhen it suits us do we weld size to size metal, asmachining costs are reduced by welding a shank witha -fa" allowance for machining to the high speed endstepped down to shank metal diameter. A tapershank is then generally reduced to within 0.012" forgrinding allowance by one cut for a No. 1 or 2 morsetaper shank, one or two cuts for a No. 3 morse taperand two cuts for a No. 4 morse taper shank. Thelargest we regularly weld is for 2£" diameter drills.

Easily the most popular drill is the two-fluted type,and the lead of helix for these flutes is generallyconstant for the whole length, although some aremade with a gradually increased lead from the pointto the shank. However we do not know of anyvirtue claimed because of this design. The standarddrill has a lead of helix approximately 6£ to 7times the drill diameter and can be measured on theoutside diameter from a point on the land to asimilar position when the flute completes one turn(Fig. 1). The helix angle is taken from along the lineof spiral flute and the drill axis. Except for specials,the web or core thickness between the flutes increasesfrom the point of the drill to the shank and varieswith different manufacturers from 0.010" to 0.020"per inch. This taper core is necessary even fornormal work and essential for high speeds and feeds.The web thickness is approximately one-sixth of thediameter for very small drills and one-eighth for thelarger sizes, this being a rough guide only.

583

Fig. 1.

SteelsThere are two types of high speed steel in general

use for drill manufacture ; the 1 8 : 4 : 1 (0.75%carbon, 18% tungsten, 4% chromium, 1% vanadium)and a tungsten-molybdenum type also known as a5 : 6 : 2 (0.85% carbon, 5% molybdenum, 6%tungsten, 4% chromium, 2% vanadium). Since agood tool must be made from a good steel, no steelis issued until tested and approved by the laboratoryand heat treatment departments.. Specifications aresupplied to the steel maker and every heat in ashipment is tested to ensure that the steel is withinspecification by taking samples from two bars atrandom and testing for composition, decarburisationand condition.

For composition, the elements checked in highspeed steel are carbon, tungsten, molybdenum,chromium and vanadium. The carbon figure is veryimportant when it is realised that many thousands oftools are being heat treated each day and the treat-ment must be of a standard nature, although there aresome variations in hardening temperatures speciallyapplicable to differences in composition.

Decarburisation testing is always carried out onthe " as received " steel and the depth must be held

within certain limits, so that when the turningallowance is removed the decarburisation is alsoremoved. Our method for testing decarburisation isby giving a normal heat treatment as later describedin detail, to a section cut off a bar about £" thick.This is ground in 0.005" steps until a hardness of 63Rockwell " C " scale is reached for high speed steel,when we consider the decarburisation has been sub-stantially removed. Decarburisation is the loss ofcarbon from the surface of steel as a result of heatingin a medium (such as air) that reacts with carboncausing partial or total decarburisation. Either stateis undesirable, since if any decarburisation remains oncutting edges they will be soft after heat treatment,and will wear rapidly.

Condition, the last main item checked, coverswhether the steel is in the correctly annealedcondition with freedom from segregation and alsosurface defects.

Butt WeldingApproved steel having been issued from the store,

shank and body are parted off and stepped whererequired to suit any differences between diameters.A light sandblast before welding cleans and removes

Fig. 2. Flash butt welding with automaticupsetting, showing the * sunken bed '.

584

any burrs. The welding blocks have been bored outto If" diameter and there is a full range of suitablesleeves with an outside diameter to suit the block withbore diameters in 1/64" steps. The top half bearingis held up into position by two £" bolts ; the bottomhalf has a location pin only. Sleeves are split length-ways to give two half-bearings and it will be clearthat by changing the sleeves to the required size,the parts to be welded are automatically central.The flash butt welding machines (Fig. 2) have a" sunken bed " so that the work in the blocks is inline with the thrust, thus allowing no uplift, and aredesigned to allow manual control of preheating andflashing, whilst upsetting is carried out automaticallyby the operation of a compressed air cylinder ormanually as required. In flash butt welding the twopieces of stock to be welded form part of an electricalcircuit which carries low voltage (3-5 volts) and highcurrent characteristics.

The shank and pod are arranged adjacent to eachother with their abutting surfaces in light contact orwith a minute air gap between them ; a heavycurrent is allowed to pass through the circuit. Thiscauses a flashing action between the pieces of metal,which is allowed to continue until the metal is in asemi-fused condition ; the weld is then completed bybutting together. In making a flash weld with lf%"diameter metal we would make a \" allowance instock length for " burn off " and " push up ". The" burn off " represents the loss during flashing whilstthe " push up " represents the loss of strength duringthe upset. Welding times vary with sizes but a l^V'diameter face takes approximately 1£ minutes, floorto floor time.

The freshly welded blank, on being removed fromthe clamps, is very quickly pushed into a bin of micadust, the weld being well covered to allow the heatto be retained for a maximum time. This prevents' cold shorts', which are caused by the quenchingeffect of the cold metal and results in breakage within•fo" of the weld. The other most common weldingfault is caused by allowing the movable platen orside to ease back after the upset stroke before theclamps are released; this tends to pull apart the freshweld.

Next, the welded blanks are removed from the micaand transferred to an annealing furnace. The work isbrought to 750°C. and held for five hours ; this istermed sub-critical annealing. Rumbling in a sand-blast machine removes any scale.

MashiningThe 118° drill point is formed and the welding

fin removed ; the average speed would be376 - 400 r.p.m. for both operations, which arecompleted together in a capstan lathe. For pointingor coning, a tungsten carbide tipped tool is used, set atcentre height with a 5° negative rake, machinefeeding the tool into the end of the blank until thecorrect length is obtained, and using a roller boxto keep work central.

The shock or interrupted cutting to remove theweld is best done with a Stellite tool which is woundstraight into the work, this procedure being arrived

at after exhaustive test*. The blank after beingstraightened and centre drilled in the shank is nowready for turning.

The 118° drill point is used in a female centrewhen turning and no difficulties are experienced, theshanks being turned first using a tipped tool with a0.015" deep chip breaker, 1,440 r.p.m. and a 0.009"feed. As mentioned previously, this produces a No, 1,No. 2 and sometimes a No. 3 morse taper in one cutusing a good flow of soluble oil. A similar tool isused for turning the body running at 1,440 r.p.m.and a 0.006" feed rate, no trouble being experiencedwith the remains of the welding fin.

Flute MillingFor milling the flutes there is a specialised milling

machine with table set at an angle to reduce oreliminate side interference when cutters are working,and suitable gears for whatever lead of helix "isrequired.

For the most common right-hand spiral flute, thetable is set from the right-hand side. Left-handfluted drills differ in the set-up by the table being setat an angle from the left-hand - side, and an inter-mediate gear added to give the change of directionwith the flute. The arrangement of the machine(Fig. 3) is such that the table carrying the workpiecedrops by a cam action as the blank is being fluted,thus giving the web thickening. For milling theflutes the cutters have two radii, this special shapebeing required so that the lips of the finished drillwill form two parallel straight lines the thickness ofthe web part.

Where possible, the machines have been convertedto down or climb milling which, of course, means thatthe cutter rotates in the same direction as the feed,and the diminishing cut per tooth obtained by thismeans gives a better flute finish than is possible fromconventional milling. Surface condition usually startsto deteriorate within the first hour on conventionalmilling, but after climb milling the first and lastblanks from an eight-hour production run had asurface finish with no detectable difference. We haveto be guided by the cutter appearance for regrinding.To show what can be obtained from the changefrom conventional to climb milling, we ran a singleflute milling machine on conventional cutting untilthe work showed the usual build up signs of end ofcutter life. The driving belt was then crossed and byclimb milling the cutter gave a first-class finish forsix hours before cutter grinding was necessary.

In the drill section, the machine developments ofinterest are semi-automatics carrying five blanks heldin suitable adaptors. On pressing a starting button,soluble "oil runs, cutters start at 210 r.p.m. and thecarriage runs forward at 70" per minute fluting at4" per minute, rapid return, auto index, advance,flute return and stop, the whole cycle being 2£minutes to complete five drills. The operator looksafter two machines thereby averaging four drills perminutes. These times apply to -JV - •&" but varyaccording to drill size.

Cutters (Fig. 4) are made from high speed steel3f" diameter, with 14 teeth. The full form is relieved

585

Fig 3. Flute milling.

Fig. 4. Cutter detail on multiple drill flute millingmachine.

on the teeth and grinding is only necessary on thecutting faces ; but care is exercised to ensure thatthe five cutters are all the same diameter, thuscontrolling the web thickness of the drills being flutedat a constant setting.

• Jobber or straight shank drills are parted off tolength, with a 118° drill point each end. Thisgreatly assists in determining the correct position forthe fluting cutter, the point shape being very similarto the finished job ; likewise, the correct shape onlyneeds relieving by grinding when finish pointing.These blanks are fluted in similar machines, theadaptors only being different from those which holdtaper shanks.

Relieving and TangingDrills ^ " and over are now machine relieved ; all

sizes under are relief ground later on, with theexception of 60 to 80 wire gauge, which are notrelieved as a standard but are done regularly down to73 wire gauge on request.

Relieving is the removal of metal leaving a narrowland or' margin up to drill size and is necessary withthe larger drills to prevent excessive friction betweendrill and walls of the drilled hole which would causedrill failure.

For taper shank drills tanging is the next operation.The drills are held two at a time in a jig and fed intotwo pairs of half round cutters correctly spaced togive the tang width, and the radius on the cuttersgives strength to the tang.

586

Heat TreatmentThe next move is to heat treatment and the

importance of this section needs no emphasis.Continual checking and cross-checking of the instru-ments for recording hardening temperatures starts theday, and the first piece through the furnace is a short| " round section which is immediately broken for agrain size inspection. A further sample is hardenedand tempered, then prepared for micro-examinationas a more complete check on the hardeningtemperature. The workpieces are on their way bythis time, but grain tests are taken every hour andfurther cross-checking of temperatures is carried outhalf-way or four hours through the day. With up to40,000 pieces from 80 wire gauge (0.0135") up to 4"diameter being heat treated in a day, it is veryimportant to make no mistakes in this department.

The drills are loaded into suitable jigs and con-veyed at a predetermined time cycle through a dryingchamber to remove moisture. Then through a pre-heated salt held at 850°C, followed by immersion ina high heat salt at approximately l,230°C. tol,240°C. for molybdenum steel, continuing with closeadherence to time and temperature to a quench saltat 580°C, which also acts as a solvent for the highheat salt, and finally finish cooling to roomtemperature in air. Whilst in this section, it is ofinterest to note that the lower temperature furnacesare oil-fired and temperature is controlled by indi-cating controlling instruments through angle typethermocouples. We find mild steel sheaths forthermocouple protection in low heat salt baths aremost suitable. Temperature control of the high heatbath for high speed steel hardening is controlled by aradiation type pyrometer connected to a recordingcontroller.

The high heat furnace is electrically heated, thethree electrodes being placed in the form of a triangleinside the walls of the pot. The salt acting as aconductor is heated and agitated by the flow ofcurrent from one electrode to another thus keepingthe salt mixed and at a constant temperature through-out the entire pot. Most tools from high speed steelsare given a triple tempering at 570°C. ; this consistsof placing the work in suitable containers in air-circulating tempering furnaces, heating to 570°C.held for 2£ hours, removing and allowing to cool toroom temperature, this being repeated three times.

Drills are tested for hardness upon the c lands',and a Vickers Hardness Tester must be used for thiswork because of the difficulty of supporting properlyon a Rockwell type machine. One or more drillsare checked from each batch.

The tangs of taper shank drills are toughened inmolten lead and the drills are then sandblasted toclean and give the finish that appears most attractive.

Centreless and Relief Grinding; PointingPlain and centreless grinders (Fig. 5) reduce the

drills to the required size and checkers are kept verybusy in the grinding room ; a finished cutting drillpoint is ground, and the jobber drills are thenstamped with the finished size.

Fig. 5. Loading magazine for Churchill centrelessgrinding machine to feed 30/40 drills per minute. Thecontrol wheel is cammed and grinds one drill each

revolution.

The relief grinding of smaller drills is done on aspecialised machine of very simple but effectivedesign. This machine has a light bracket that swingsfrom a location below the grinding wheel carryinga guide with a location pin in towards the centre ofthe wheel and correct position is repeated by the aidof stops. The drill having been pushed through theguide to a stop to give length of relief, the bracketswings forward to a stop and the drill is slowly with-drawn, keeping the flute, against the locating pin ;the grinding wheel removes metal to the amount themachine is set, pressure on the bracket is thenreleased, drill indexed and the cycle repeated. Therelief grinding operation is now complete. Theremust be a fine grain fairly hard wheel to give a goodfinish and not too much wheel wear, but it must alsocut freely without burning Or overheating; to achievethis, gashes are cut with a splitting wheel into thewheels cutting right through the face on the outsidediameter to about 1" deep, giving at least sevenserrations for a 6" diameter wheel and more for thelarger. This has exactly the same effect as largergrain size in causing the breaking up of the cut.

It is of interest to note that we have found amixture of common washing soda and water is thebest coolant for drill relieving. The soda waterallows a good grip to be taken for withdrawing the

587

Fig. 6. Screw extractor set.

drill and no trouble with rust is experienced if thedensity of the soda is kept constant. Twenty poundsof soda are used to 140 gallons of water, this mixturebeing constantly pumped through all machines.

For the extra small drills we heat treat blanks,grind to size, then grind the flutes from the solidtaking a number of cuts, indexing after each cut and aspark out gives us a perfectly balanced flute. Thesedrills are all machine pointed.

Whilst on this small class of work, I would referto the Paper presented by Mr. R. E. Andrews beforethis Institution last year, wherein was described amethod of using a Cincinnati tool and cutter grinderas a centreless grinding machine with the aid ofstandard equipment for extra small work. We foundthat by utilising this method we were able to usewheels 1" thick, giving us the corresponding shortwork plate that definitely overcame difficultiesexperienced with 4" wide wheels doing 80 wire gaugedrills on our smallest centreless grinding machine.The grinding wheel used was 6" X 1" X 1£" bore120 MV and for a control wheel 3" X 1" X \" borerubber-bonded ; we reduced the control wheel speedto 50 r.p.m. by using a combination of two heads ofstandard cutter grinding equipment. By this means,we have ground hardened high speed blanks to 0.004"diameter by f" long. We were starting to design asuitable small centreless grinder and Mr. Andrews'Paper came at a most opportune time.

Before leaving drills, some interesting types ofspecials are worth mentioning. A combination drilland reamer requires a narrow fluted drill body; forthe drill portion the first -J" or whatever required

length is reduced l/64th under the remainder inwhich three reamer shape flutes are milled. Thisspecial then goes through all the usual drill processes,but we grind a relief on the drill portion and grindrelief on each of the reamer lands as for a standardreamer with a 45° angle from drill to reamer forcutting purposes ; also a conventional drill point onthe end enabling the required hole to be drilledand the reamer follows through. These, whilst beingin general use for car components, etc., were alsothe answer to a maker of rotary clothes hoists, hisproblem being to get a good reamed hole afterdrilling pipe which by conventional drill thenmachine reamer, gave a very poor finish with a seriesof flats. With the combined tool, we had the drillportion long enough to finish drilling through bothwalls of the pipe before the reamer started, and thismade the drill as a steady pilot for the first reamedhole which afterwards steadied the tool for thesecond.

For a drill 4' long we weld about 6" of mild steelto the high speed and complete the drill through alloperations. We then hold the mild steel portion inone clamp and weld any length required of mildsteel to it. Since no chilling takes place, it is onlynecessary to remove the welding fin and polish, noannealing being required. Other specials includeleft-hand drill, subland, and step drills.

Centre DrillsFor centre drills, the drill and angle portion are

ground together, the machine having one cam to giveclearance to the angled portion and another to giveclearance to the drill, both operating together.

Drill TestingPerformance testing of drills is carried out by all

reputable drill makers, the first essential being that thedrill performs as well or better than required byBritish Standard 328:1950. Besides testingpenetration rates in the prescribed steel, it isnecessary to check in steel with a higher tensilestrength, this still being applied to standard drills.

Drills with special heat treatments are oftenrequired for drilling special steels, and whereproblems of this nature exist we are only too willingto assist the user by carrying out feed and speed testsand, if necessary, providing variations of flute shape,lead of helix and heat treatment. Although BS.328calls for 9.1" per minute penetration rate for a f"drill, feeds up to 24" are often obtained and 200%of the British Standard rates is our aim, regularlyachieved. Small wire gauge drills have reached 18"per minute penetration rate. The laboratory, bymicro-examination, can tell if a drill fails throughover or under heating when hardened, or if thetempering could be at fault.

SCREW EXTRACTORSOur development of manufacturing processes for

screw extractors will no doubt be of interest. Theextractor (Fig. 6) may be described as having serra-tions or shallow half-round flutes, in the form of avery coarse left-hand tapered thread which grips the

588

sides of a hole drilled into the screw to be extractedand, on being turned left, will remove the brokenscrew. The flutes were the problem for considerationand milling was ruled out because of estimated costand expected difficulty of producing clean fluteswithout a milling burr. This left the rolling method,but the machine maker at that time said that no taperrolling could be done.

Rolling was adopted, however, and our method(Fig. 7) is a displacement process. The size of theblank controls the finished workpiece, which issupported on a rest between two cylindrical rollson the same principle as a centreless grindingmachine, and while rotating is subjected to pressuresufficient to generate the correct depth of form. Tosave costs in trial-and-error making of rolls, we madetwo rolls of lead, turning, boring, etc. to our estimateof the required design. They were then mountedon the arbors of our Steinle thread generatingmachine and brought together on to an approvedmaster pattern screw extractor. After a number ofattempts, a portion of each roll had a clear impressionof the work and by using only that portion we rolleddried Plasticine which held the shape. From theresults, we were fairly sure of success and with thedata thus obtained we made our first rolls in metal,testing upon soft copper before hardening, and alter-ing until we felt the best shape was obtained. Theserolls were made from 18 :4 :1 high speed steel. Todigress a moment, the lead rolls looked like normalsteel and when they were finished with, were taken toan operator on the lead pot who duly wired andimmersed them in the lead to ' let them back a bit ' .He got the surprise of his life to find that the ' steel'rolls vanished!

Testing at our Research Laboratory proved thatit is not desirable to use the biggest extractor whenremoving a broken stud, but to endeavour to haveenough metal left to prevent the five lands of theextractor displacing a shell of metal and jammingthe stud still further. An example test was upon

£" Whitworth bolts, using a No. 4 Extractor, 35 to40 foot lb. being the torque required to remove thebolt, but using the larger No. 5 Extractor, 65 to 70foot lb. was required.

THREAD ROLLINGWe have obtained up to a quarter of a million

pieces from some of our thread generating rolls,which were still capable of further work.

For components with a slight tolerance in thepitch, one can very easily make replacement rollsfrom one master by the following method. It willquickly be found that rolls to produce a right-handthread will make rolls for left-hand threads, thereforethe approved master must be obtained. The sizeof the blank is determined, machined, and put onone arbor and the master roll on the other arbor.This is forced by hydraulic pressure and generatesits form which, after the usual heat treatment andgrinding, will in turn reproduce itself. The pressurerequired to generate 12 t.p.i. Whitworth form intoa roll is 1000 lb.; for 20 t.p.i. Whitworth it is 500 lb.For generating the threads on carbon steel for makingtaps, 600 lb. pressure is used for £" - 12" Whitworthand for V - 20 t.p.i., 300 lb.

Straight and diamond knurls of any number oflines per inch are just as easily produced on rolls.Our own rolls for this work are made from oilhardening steel and the life is still too far off toestimate.

On our machines we have diverted part of thelubricant flow from the rolls to each of the ball racearbor supports, and no further difficulty has beenexperienced, although previously flats were constantlywearing causing sizing trouble. The rate of rollingcan be varied to suit various sizes and a wide range ofmaterials including copper, carbon and high speedtool steels are easily rolled, but it is also claimedthat Monel, high alloy steel, and Nimonic normallyconsidered unsuitable for thread rolling can now berolled.

Fig. 7.

589

TAPSIn the manufacture of taps the most interesting

method of producing the threads is by the form roll-ing process, the required quality and accuracy beingeasily achieved with the added advantage of highspeed production. The resulting forms are strongbecause the material is caused to flow into its newform and the grain structure follows the threadcontour and the finish is good.

The tap blank having had the shank reduced tosize, squared and stamped, the portion to be threadedis centreless ground to a predetermined size just afew thousandths over the final effective diameterwhich divides the crest and root so that they are ofequal area. The volume of metal is then movedfrom the root of the thread to form the crest byfilling the form on the thread rolls completing thecycle in a few seconds. We roll threads from 10 B.A.(0.0669") up to f Whitworth and 1" Gas.

Taps of larger diameters, taper gas and coarserthreads are thread milled. We have thread millinghobs with a width just clear of the tap thread endand on an automatic cycle the hob feeds in and cutsthe thread, moving along by a cam- action for adistance of one pitch of thread only; any requiredrelief can be given per land, this being most importantfor the taper threads.

DIESThe thread milling machine, but with internal

threading hobs, is used for threading pipe dies, themilling operation giving the clearance that testing hasshown is most suitable. These pipe threading diesare now being supplied for 11 threads per inchwith a two start thread lead previously onlyobtainable from the U.S.A.

In connection with dies, an operation of interestis the sawing-off of die blanks. The black bar ofoil hardening steel is already cut into 20" lengthsand turned before the sawing operation. Suitableclamps have been designed, two of which are boltedto the milling machine table, one each end facingeach other, and whilst one clamp is being loaded orunloaded the saws are cutting the bar on the otherside. There is no backlash elimination on thismachine, but the centre of the saws is near enoughthe centre of the work so we conventionally saw oneside and climb cut the other. We use 8" dia. sawswith alternate teeth angled both sides at 45°, in agang of 9 for 2" metal and 13 for bars of 1|" dia.,with only the standard drive on the machines. Thearbor speed is 32 r.p.m. with a feed rate of 0.26" permin.

In the milling section we have aimed for multipleoperations using two jigs on a table wherever possible,enabling a load and unload whilst the second jig is inuse. For example, drill tanging is two per cut andwe flute four taps per operation.

On round or hexagon dies we face, chamfer, drill,and ream the core hole using a 5" B.S.A. automaticmachine on a continuous cycle. The loading of theair-operated chuck is timed on one of the capstanhead advance-strokes, the chuck opens, and a spring

forces the completed blank out. A magazine is carriedforward on the tool slide, and the arrangement issuch that a plunger on the capstan head pushes aloading plunger in the base of the magazine and ablank is forced into the chuck which now closes;the magazine withdraws and the machining cyclecontinues.

The drilling of various chip clearance holes indies is also done in multiples. We found that withthe clearance holes in dies being so close together,little spindle strength would be left if attemptingmultiple hole drilling per die; it paid, with thestrength of spindles and driving gears to withstandheavy speed and feed rates, to place four dies witha reasonably strong width of division, and by measur-ing that distance between the work, we obtained thenecessary distance for spindle centre distances toenable one hole in each of four components at a timeto be drilled, giving multiple hole drilling of fourholes per cycle, or whatever number of holes wasdecided upon.

Parallel threading of the dies is done on a standarddrilling machine using a long straight shank tap afterthe manner of a vertical tapping machine. We useneatsfoot oil as a lubricant and the addition ofpowdered sulphur prevents any inclination to ' pick-up ' in the threading. A burring operation gives aclean cutting face to the lands in the dies and thecutting leads are milled at the correct angle for longand short leads. The previous tapping operationleaves in 0.002", which material is now removed witha master tap, to give the required size. Stampingthe necessary details, size, type of thread, numberof threads per inch and the maker's name is the lastoperation before heat treatment.

For the heat treatment of carbon steels, there areoil-fired salt bath furnaces, designed to operate attemperatures up to 860°C, that being the requiredtemperature for hardening a 1% carbon, 1.5%chromium oil hardening steel, from which we makea big percentage of our dies and taps.

After immersion in the liquid salt for a predeter-mined period, the components are quenched intoan oil quench bath which is connected by an inletand outlet to an underground 1,000 gallon tank somedistance away. Constant circulation of the oilguarantees uniform hardening.

Tempering in air circulating furnaces comes next,after which components are tested on a RockwellHardness testing machine to see that the correcthardness has been attained.

In the case of button dies, the die is split whererequired with an abrasive wheel followed by discingand buffing to clean and polish. Taps are buffed onthe shanks, ends, flutes and leads ground.

InspectionEvery component we make goes through our

Metrology Department for inspection and checkingto the appropriate standard required.

Drills are also visually examined for any errorsin any of the operations, likewise taps, dies and ourother items we manufacture are checked, beforemoving to packing and despatch.

590