Metal Turning, Short Course In

lathe techniques

Short coursein metal turningBy W. CLYDE LAMMEY

• METAL TURNING can be as fascinating aswood turning, and while it requires using a wholenew set of rules, learning to operate a metallathe is really pretty simple when you get rightdown to basics.

Learning how to set up work for turning be-tween centers and on a faceplate is lesson num-ber one.

The first step is to rough-cut the workpieceto length with an allowance for facing both ends,when facing is required. For practical purposesthis can be done with a hacksaw, using care tocut the ends square. Or, if the nature of the workand the material available permit, cut the work-

piece over-length so that you can machine it tofinish diameter and length up to the driving dogand then cut off and face the end. When measur-ing the length, be sure to allow for the dog.

The next step is centering. If the work willpass through the hollow, headstock spindle, then

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chuck it in the lathe's three-jaw chuck, letting itproject a minimum amount. Now face off andcenter-drill the ends with a combination centerdrill and countersink as pictured and detailed inFigs. 2, 3 and 10.

If the workpiece is too large to pass through

1. The first step in mounting between centers isfacing the ends of the workpiece square. Indent thecenter so the center drill will start accurately2. When the workpiece will pass through the lathespindle, center-drilling is done in the lathe. Chuckthe work and feed the center drill slowly3. If the work is too large for the lathe spindle,chuck it and center-drill in a drill press.Center-punch the work first and feed slowly4. After center-drilling, attach a driving dog andmount on centers. Make sure tail of the dog clearsand the work turns freely without play5. When turning between centers, use screw orautomatic carriage feed to get smoother work, and toprevent gouging and overloading of the bit6. Use a rule to locate the cutting tool on the axisof the lathe. Here, rule tilts away, indicatingthe point set too high. Recheck and reset the tool7. The vertical rule indicates that the tool is setdead on center. Some machinists prefer the cuttingtool set slightly above center on some work8. When turning ferrous metals, an occasional dropof light oil at the cutting point will give asmoother cut and help to prevent undue heating

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HOW TO GET STARTED IN METALTURNI IMG

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CORRECT BIT PROFILES

the spindle but will drop through the hole in yourdrill-press table, locate and scribe the centers,using either a centering head on a machinist'scombination square or a centering (hermaphro-dite) caliper as detailed (Fig. 9.) Then, usingthe lathe chuck, hold the workpiece and drill thecenter with the combination drill and countersinkin the drill press as pictured in Fig. 3.

If too large in diameter for either operation,lay off and scribe the centers as described, thendrill and countersink in two operations as inFig. 11. The facing cuts can be taken after thework is mounted between centers. When mount-ing the workpiece between centers, be sure thatthe tail of the dog engages the slot in the face-plate properly and that the end clears the head-stock before you start the lathe. Turn the workby hand to make sure everything is in the clear.

Small metal lathes, under 9-in. swing, usuallyare supplied with one large faceplate which canserve both as a work mounting plate and as adriving plate. Such a plate has one open-end slotfor driving the dog and several closed-end slotsto which work of irregular shape may be boltedor clamped for machining operations such asfacing, drilling and boring. See Fig. 12.

types of platesLathes of 9-in. swing and up are usually fur-

nished with a small plate having a single open-end slot for driving the dog. Larger plates withclosed-end slots and spaced holes are suppliedas accessories. Such plates are useful for mount-ing a wide range of workpieces of such shapethat they cannot be chucked. Both types areshown in the details, Fig. 12. A typical, irregu-larly shaped small casting is shown, being of ashape that cannot be chucked in either a three-jaw universal chuck or a four-jaw chuck havingindividually adjustable jaws. Such a piece can bemounted and worked on a driving plate such asthat supplied with small lathes by using two,three, or more L-clamps and bolts (Fig. 13), ifthe work permits, as fastenings. Although similarclamps are available ready-made, you can makethem yourself, using mild steel which is easilycut with a hacksaw, filed and ground as needed.Make them in several sizes to accommodate workof varying proportions and thicknesses. If desir-able, you can improvise a chuck for noncriticalmachining by making L-shaped chucking jawsfor the faceplate.

On large faceplates, machinists sometimes findit convenient to mount the work on an angle plate(Fig. 14), bolted to the faceplate. Like the

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lathe techniques

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L-clamps, angle plates are available ready-made,but you can make one suitable for noncriticalwork from a short length of ordinary steel angle.This should be filed or ground, or both, on threefaces as indicated, with special care taken tomaintain the outside angle at 90 deg. and allsurfaces flat.

When you're mounting work on an angle plate—or, in some instances, clamping work to thefaceplate—it is necessary to provide a counter-balancing weight. These are easily made fromround steel stock by cutting and machining disksof varying diameters and thicknesses and center-drilling to take a small bolt for attachment. Aftermounting the counterbalance, make short testruns to make sure the setup is in close balance.

Correct grinding, honing and mounting thetool in the toolholder and on the toolpost comenext. To cut properly in ferrous metals, such asordinary steels and certain machining aluminums,

the tool should be ground with a top (back)rake, side rake and relief angle as detailed (Fig.15). The angles given are used in general prac-tice, but some machinists vary these slightly forcertain purposes.

For common nonferrous metals, such as brassand bronze, the cutting tool should generally beground with a negative, or zero, top rake andlittle, if any, side rake. In all cases, except forthreading, the nose should be ground with a veryslight radius. Use a medium-fine grinding wheel(preferably a vitreous wheel) and be careful notto overheat the cutting edge. Carbide-tipped toolsmust be ground on a specially made wheel.

For finishing cuts, tools may be ground with around-bottom chip groove as detailed (Fig. 16).Properly ground (this is, of course, rather diffi-cult on tools less than 1/4-in. square), such a toolleaves a smooth finish, as it curls away a continu-ous chip, generates somewhat less heat whencutting and requires less power. Although shownwith negative top rake, some machinists prefer itground with a slight top rake.

Lathe tools generally will cut more smoothlyif honed on a fine oilstone, such as a hard Arkan-sas stone. Be careful not to alter the rake angleswhen honing. Tool settings are those commonfor ordinary work. Tools ground with a chipgroove should generally be set to cut a squareshoulder. Don't permit any tool to undercut; itmay tend to dig in without warning and damagethe work.

Use of geared chucks is the next step in learn-ing to operate a metal lathe.

Work that cannot be mounted between centersand driven with a dog usually is chucked for

To avoid dropping the chuck during mounting, clamp it on roundstock that will pass through the spindle and provide a "handle"

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lathe techniques

short course in metal turning, continued

machining. Chucks for everyday work are ofseveral types, all operated manually with a spe-cial wrench.

Perhaps the most common is the four-jawchuck on which each jaw is operated individually.These usually come with a set of jaws that canbe reversed to hold inside or outside work ("in-side" and "outside" refer to the way in whichthe jaws engage the workpiece). Concentricrings, equally spaced, are machined on the faceof the chuck to help center the work.

Next in common usage is the universal, orscroll, chuck on which the three jaws are self-

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centering. Turning the T-wrench in the singleactuating socket on the chuck body moves allthree jaws simultaneously. When they close onthe workpiece it is automatically centered. Suchchucks are accurate within .003 or less and comewith two sets of jaws—for inside and outsidework. Some miniature scroll chucks have onereversible set.

In addition to these two common types thereare spring collet and step chucks; both are actu-ated by a spindle drawbar and also a gearedchuck (similar to a drill chuck) that turns ontothe spindle nose and is ordinarily used for "live"

1. The tailstock center is used as an indicator forcentering in a four-jaw chuck. The workpiece iscenter-punched or center-drilled. Not for close work

2. Grip the inside of the workpiece with jaws of auniversal chuck by backing them out rather thanrunning in, to permit both edging and facing of work

3. When you must come close, use a mike, especiallyon small-diameter work where it is more difficultto "feel" a caliper. Be sure of the reading

4. This shows the chalk or pencil-mark method ofcentering in a four-jaw chuck. Here the line is notcontinuous, showing that the work is off center

5. Grip the outside of the workpiece for a finalbackfacing operation by reversing the jaws. On mostuniversal chucks, the jaws should be interchanged

6. Make a mandrel for average work when you don'thave a ready-made one handy by filing a taper on theround of a drill rod'that has been center-drilled

7. Here is the machining of a chucking lug on model-locomotive drive wheel. The work is held in theoutside of three-jaw chuck with reversible jaws

8. On ordinary work not requiring close tolerances,diameters can be checked quite accurately with cali-pers. Finger sensitivity largely determines accuracy

9. Finish the taper by smoothing with a loop of fineabrasive cloth, checking the diameter frequentlyuntil you have the correct taper of .004 to .006

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lathe techniques


drilling operations, with the work supported by apad held in the tailstock.

The collet chuck also is widely used in themachining of small parts requiring close toler-ances. Spring collets range from about 1/8 in. up,the sizes usually increasing by 16ths.

The step chuck is really a collet-chuck acces-sory. It is threaded to fit the collet-chuck draw-bar and each unit is designed to hold two or moresizes of rounds, such as disks or small gearblanks, for machining. Both collet and stepchucks are especially useful for repetitive work;the workpiece can be locked quickly in placein the collet, where it is automatically centered,then machined and released by a short turn of thehandwheel. Modelmakers, experimenters andmetal-working hobbyists find these chucks usefulfor machining small parts.

rough-center the workpieceWhen working with the independent four-jaw

chuck, you rough-center the workpiece, withthe concentric rings on the face of the chuckaiding in placement. For ordinary work not re-quiring close tolerances you can use the tailstockcenter as an indicator when the workpiece hasbeen center-drilled or center-punched. The usualprocedure is to set two adjacent jaws and thenwork the third and finally the fourth to bring theworkpiece to dead center. With care, this can bedone quite accurately.

Another fairly accurate method is to rough-center the work and then hold a piece of coloredchalk or a china-marking pencil on the tool-holder. The pencil point should just touch thehigh point of the workpiece when the lathespindle is rotated by hand. Move jaws slightlyuntil the pencil scribes a continuous line.

When a machining job requires working withinclose limits, you must use more accurate center-ing methods. A center indicator, or "wiggler,"gives good results when set up as shown on page1586. Center-punch the workpiece, rough-centerit, and move the chuck jaws slightly until theindicator arm steadies as the work is rotatedand you have it very close.

The dial indicator is more sensitive and faster,and it can be used on a greater range of workshapes. The workpiece need not be center-punched, since the actuating arm of the instru-ment can be located to ride on any smooth sur-face, inside or outside or on the face when it isnecessary to test for out-of-true axially or radi-

ally. When the needle steadies you arequite surethe work is dead-centered.

A note of caution: Use a protective sleevewhen chucking or rechucking workpieces thatare fragile or partially machined. To protect themachined surface from the chuck jaws, cut ashort sleeve from tubing with an inside diameterthat will slip-fit the workpiece (or machine asleeve to fit). Slot it as shown at the top of page1586 and slip on the part of the workpiece to begripped in the chuck. The sleeve protects frommarring and gives support to fragile work thatmight be crushed out of round in the chuck.Avoid overtightening the jaws, or both chuckand work may be damaged.

All-over machining sometimes presents aproblem in holding the workpiece, such as abushing. The first step is to bore, ream and faceto specifications, with the bushing held in thechuck. Then remove the bushing and mount iton a mandrel that will take the bore of the bush-ing in a close press fit.

how to make a mandrelReady-made mandrels come in many sizes,

usually tapered from .004 to .006 per ft. Forwork not requiring extreme precision, you can .make a mandrel as shown in the two bottomphotos on page 1589, tapering it to take thebushing. The latter should be a fairly tight pressfit after the mandrel has cooled to normal tem-perature. Of course, the mandrel must be center-drilled and a flat filed or ground at the big endto assure that the driving dog does not slip whenthe work is mounted between centers for finalall-over machining.

The many extra jobs your metal lathe can do,in addition to all the standard metal-turningoperations are taken up in this part of our story.Even the small machines can be set up to do suchwork as drilling, reaming, milling, thread-cutting,knurling and turning tapers. And, you can turnthe harder woods by replacing the 60-deg. centerswith the spur and cup centers and substituting ashort metal bar for the toolholder. You just swingthe toolpost so that the bar is parallel with theworkpiece.

Drilling is perhaps the most common "side"operation done on a metal lathe. On a small latheyou can use drill bits from needle size to 1/2 in.or more as the tailstock, fitted with a precisiondrill chuck, gives an extremely sensitive feed.

When drilling the larger diameters, it's advis-able to run in a pilot hole first, using a small-diameter bit that you are sure is perfectly straight

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and correctly ground. The hole will serve as aguide for the larger bit. Ordinarily the pilot holeshould not be more than one fourth the diameterof the larger hole—sometimes, less is preferable.Before drilling either hole, make sure the tail-stock quill (or sleeve) taper is clean, with nochips or grime. Also wipe the tapered drill shankand place a drop of very light oil on it beforeinserting it.

You won't need a lubricant to drill most non-ferrous metals and cast iron, but when drillingsteel an occasional drop of light oil will result ina smoother job and prevent undue heating. Whenthe finish hole is to be of large diameter, it's oftenbest to drill undersize and then finish to the re-quired diameter with a boring tool.

reaming to final sizeWork specs frequently call for reaming to final

size. In any case, if you are working to Closelimits it's well to follow through with this step.Drill the hole slightly undersize—about 1/64 in. inthe larger sizes—then go through with a straightreamer of the final diameter desired. This willgive you a smooth, true hole of exact diameteras long as you don't remove the work from thechuck between the two operations.

Accurate screw threads can be cut on a latheequipped with a lead screw. The cutting toolmust be ground with a 60-deg. point and the com-pound turned 29 deg. to the right for cutting thecommon external threads. The tool must be setat right angles to the axis of the workpiece. Ifyou have a center gauge, use it to obtain anaccurate setting as detailed in the third sketchon page 1594. Also, the cutting tool must be setexactly on the center line, or axis, of the work-piece to produce the best work. Somewhere onyour lathe—usually on the inside of the gearcover or on the quick-change gearbox—you'llsee an index chart listing the correct changegears to use in the train to cut a given number ofthreads per inch.

Diagrams on the change-gear lathe will showposition of the change gears for cutting anythread within the lathe's capacity. With the geartrain arranged to cut the thread desired, center-drill the workpiece and mount it between centers.Drive with a dog as in the left center threadingphoto on page 1593. Usually it is desirable, whenthe lathe carriage has no threading stop, to runin a shallow groove with a cut-off tool at thefinish end of the thread to get clearance toback off the threading tool at the end of the cut.

On the simple threading dial you'll see four

(some have eight) equally spaced marks. Allpass a single witness mark as the dial slowlyrotates. To start any even-numbered thread, suchas 12 or 24 per inch, with the lathe running,engage the half nuts just as the rotating zeromark coincides with the witness mark.

After the first scoring cut, back off the tool,disengage the half nuts, return the carriage to thestarting point by hand and re-engage the halfnuts at the same mark used previously on therotating dial.

Normally the common threads—even finerones—should be cut in several passes, feedingthe tool in from 1/64 to 1/32 in. at each pass, thedepth depending on the thread size.

Cutting an internal thread requires much thesame procedure as does the external thread, ex-cept that the feed is toward you, and you use adifferent type of threading tool. Also, there mustbe clearance in the bore for backing off the tool,and the compound is set 29 deg. to the left in-stead of the right.

Milling, tapping, knurling, taper turning andcutting off are less frequently encountered, yetall can be done on a metal-turning lathe withavailable accessories. The main problem in handtapping is getting the tap started accurately. Thelower left photo on page 1593 shows how; youuse the lathe as a support for the work and tapwrench.

knurling and taper turningA considerable range of accurate milling oper-

ations can be done with accessories shown in thephotos and sketches. Knurling, which providesthat neat gripping surface on small screws andother hand-turned parts, is done with the specialtool sketched, page 1594. To knurl any smallpart, you first turn it to diameter, chuck it (ormount it between centers), set the knurling toolon center in the toolpost and run up to the work.Apply enough pressure to score the worksurface and make the first pass, using the lead-screw feed. Then, just before the knurling"wheels" clear the work surface, reverse the leadscrew, turn up the cross-feed screw slightly toapply more pressure and allow knurls to feedback in a reverse pass. Continue until the knurlis cut in to the desired depth.

On small lathes without a taper-turning attach-ment, tapers may be formed by setting over thetailstock. A machinist's handbook (check yourlibrary) will give formulas for taper per foot toany degree.

Long workpieces to be turned to small dia-

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til11

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meters between centers tend to spring away fromthe cutting tool. To prevent this you'll need afollower rest, one type of which is sketched onpage 1594. The cutting-off tool is useful mainlywhen a number of pieces are to be duplicated. Itmust be set exactly at right angles to the work-piece and precisely on center. Don't try this onwork supported at both ends.

You'll get a great deal of satisfaction from

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1. Sensitive drilling can be' done on a small lathewith the headstock clamped to the verticalcolumn that is an accessory with some makes

2. The first step in thread cutting is to engagethe dial with the lead screw. The dial saves return-ing the carriage with the lead screw on each pass

To drill with a 5/8-in. twist drill in a small lathe, grip workin a three-jawed universal chuck. A small-diameter pilothole serves as guide for the larger bit. Use light, uniform feed

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3. Finish boring of an off-center job shows that itis often best to drill large-sized holes undersized,then finish to the required diameter with a boring tool

4. At the end of the pass disengage half nuts fromlead screw and return carriage by hand to start,with point of cutting tool clearing threaded section

5. In hand lapping, the main problem is to get tapstarted straight and true. This setup utilizes latheas an accurate support for tap wrench and workpiece

6. To drill and ream you first drill an undersizehole, then go through with a reamer—backing off a bit at intervals to clear the chips

7. For the next pass, in-feed the tool a given amounton the cross-feed dial and re-engage half nuts onlead screw as index marks on thread dial coincide

8. Convert the lathe to a vertical mill with accessoryvise, column and table. Here both the cross andthe longitudinal feeds are utilized

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lathe techniques

learning how to make turnings on a metal lathe.If the terminology is strange at first, study thedrawings on pages 1584, 1586 and 1594 for awhile and you'll soon become familiar with thecorrect names of the lathe parts.

As with anything else, familiarity is achievedthrough steady practice. Work slowly and pa-tiently at a number of projects and you'll findyour speed increasing and the quality of yourwork improving. Perhaps the most important partof a self-training course is to perform each stepin an operation with care until the whole routineof machining a part becomes second nature.Eventually you'll be able to achieve professionalresults with little effort.

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