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57. Lathe Milling
Jul 16, 2017

A CENTRE lathe with vertical equipped for many operations that would otherwise require a universal milling machine. For you can make single-point tools perform the same operations as multiple-tooth cutters by reducing the rate of feed so that a dig-in cannot occur. You can make the tools from round and square bits and pieces of silver steel rod, to mount in holders in chucks or to fit in the taper in the lathe spindle. The cost in cash is small. If time presses, and you must keep to bare essentials, there are several easy-to-make holders that can be made from mild steel bar. They can be case-hardened or left soft. For frequent use, you should case-harden them before they become dented and worn.

 

Lathe Milling

 

 

 

According to need, you can set the vertical slide squarely or at angles on the cross-slide. The angle plate you can mount on the vertical silde, flat or sloping. To put on cuts, you have the lead-screw for the saddle, and the screws for the cross and vertical slides. Any of these screws can be used to move work past a rotating cutter; and so, besides making set-ups at compound angles, you can feed work in any of three planes at right-angles. Diagram A shows some typical operations which are performed by running tools in the chuck with the work mounted on the vertical slide. For boring A 1, you can sometimes USC an ordinary boring tool in the independent chuck. If the hole is small, you can make a . round-shanked tool from silver steel rod and mount it in a holder. In each case, set the tool by adjusting it in the chuck jaws. Fix the vertical slide and cross-slide by the screws to their gib pieces; or wedge the slides with packing to prevent movement. Feed the work by the saddle. For facing A2, you can fit a round tool in a reamed hole in a rectangular mild steel holder, fixing it through a grub screw. Adjust the circle swept by the tool by setting the tool and the holder. Apply cut this to the bed or retaining its position with lead-screw nut. Feed the work with the cross-slide or vertical slide. For slotting or grooving A3, you can use an end mill in a holder in the chuck, setting the tool to run concentrically. Apply cut again from the saddle, and feed the work by the cross-slide or vertical slide. The ordinary end mill has two cutting edges so that the end of the tool appears as at Bl. To re-sharpen it, you need an accurate square edged grinding wheel. When this creates a problem, the solution is to make an end mill with a single cutting edge, B2. Then you can re-sharpen it on any flat face on a grinding wheel. The single-edged end-mill will do the same job as a double-edged type when it is run fast with slow feed for the work.

 

 

You can make a holder to mount round-shank tools in the independent chuck as at B3. Chuck two pieces of rectangular mild steel bar; face and centre the end and drill through undersize. Finish the bore with a standard drill or a reamer. The abutting edges of the pieces you can ease with a file for the holder to grip the shanks firmly. To keep the halves of such a holder together, turn the ends circular and machine the grooves for circlips, B4. Make the circlips from coils of springs. A shank pushes into the holder with a friction grip.

Another holder for a chuck is shown at C, upper diagram. In this the tool is clamped by a grooved cotter made from a bolt. Fit the bolt first. Drill and ream the hole for the tool. Then cut the head off the bolt. The holder at C, lower diagram, has a taper shank to fit in the lathe spindle. The rear end can be tapped for a draw-bolt. You slit the other end with a saw to line WX for clamping. A holder for square tools, diagram D, consists of a centre block with side-plates YZ fixed by countersunk screws.  

 

 

56. Dividing in the Lathe
Jun 25, 2017

IT is a great convenience, even on a simple lathe, to be able to mark round components with the commonly-required numbers of divisions, such as four or six, as are necessary when making squares and hexagons; and to make other numbers of equal spacing in a neat and regular manner, such as to provide serrations on the edges of small bosses and knobs by which a finger grip can be obtained on fittings. It is true, square and hexagon mateterial can be obtained in ranges of standard sizes; but these do not cover all requirements-such as an exceptionally large size, or if a fitting is needed with a circular flange larger than the hexagon, or if a square or hexagon is required on the end of a shaft for key or spanner manipulation. To make squares or hexagons to a high degree of precision, milling is, of course, necessary. But for general purposes careful checking by micrometer if desired is quite satisfactory. When the lathe has means of dividing, the material is machined slightly larger than the size over the corners, a pointed tool being mounted sideways in the tool holder and set touching the work.

 

 


Dividing in the Lathe

 

 

 

 

Using Four-jaw Chuck

At each located position the tool is traversed by saddle or top slide, leaving a scribed line. To produce the flats, the material between the lines is filed away, the job being removed from the chuck and held in the vice, still on the bar or in soft jaws to avoid damage.

When equipment includes a four jaw chuck and the jaws overlap the flat surface of a bed, squares can be marked by holding each jaw to a support bar, as at A. Such a bar can be from round mild steel, say about 5/8 in. dia., its length having been obtained by checking with inside calipers from the bed to the under-sides of a pair of jaws when these are horizontal: The bar should be reasonably to length, and it can be used either side of the bed, but on one side only for one job. Another method, applicable in the absence of a four-jaw chuck and also to work between centers is to clamp a straight bar to the work, as at B . Distances X-Xl can be equalized with a surface gauge or scribing block and a mark made on the work with the too!; then the bar can be set horizontal again, after rotating half a turn, for making the second mark. Quarter markings are made with the bar set vertically with a square from the bed. Pressure can be kept on the work from the tailstock to prevent movement.

 

 

 

Quicker Method

Much more speedy and wider in scope, however, are drilled back plates, as at C, in conjunction with a simple plunger device for holding. Indexing for drilling can be done from a change gear which can be mounted on a mandrel in the chuck, key-pinned and held by a setscrew. A locating bar in the tool holder fits between the gear teeth, as at D. When there are two chucks, 12 spacing on one back plate and 40 on the other will give the following divisions most commonly needed: on the first, 2, 3, 4, 6, 12; on the second, 2, 4, 5, 8, 10, 20, 40. The first can be obtained from a 48 or 60 tooth gear; the second from a 40 or 80 tooth gears. The guide for a drill about 1/8 in. dia. should be from silver steel, and hardened. A countersunk screw fixes it to a mild-steel bar which is mounted from the back on a block or wall, or from the bench or stand at the front of the bed, as at E. For a bench lathe, indexing can be as at F, a cross-bracket fixed behind the lathe carrying a pivoted bar with a silver steel pin. Normally, the bar holds by its own weight, or a wing nut and screw can be fitted at Y for tightening.  

 

 

There are numerous ways of measuring inside diameters, depending on the accuracy necessary and whether the check is to verify an identifiable size, test size exactly, or compare diameters for dimensions or general truth. For simple checks of identifiable sizes a rule can be used, or a rule and calipers, or for more precise work calipers and a slide gauge or outside micrometer. Thus, to check the bore of a piece of tubmg a rule can be used direct, or the size can be taken on friction grip calipers and these checked on a rule.

Where a rule cannot be used, as for the bore of a ball race which has radiused ends? Calipers are essential, with verification made on a rule, since the size will be standard and not subject to variation of a few thou. On the same principle, calipers and outside micrometers can be used to higher accuracy. For example, a . Check on the diameter of a cylinder which could be standard, or plus 0.010 in., would need to be made with some care, and with a micrometer to verify the caliper setting. The same is true also if there is a possibility of confusion between millimeter and inch dimensions, when checking on a rule could leave the issue in doubt.  

 

 

Measuring Internal Diameters

 

 

Friction grip calipers are subject to limitations in setting, but the screw type, as at A, will check a diameter to within about 0.002 in. when used correctly with a light touch. To obtain the size of a bore, one leg is positioned at X, and the calipers are rocked to carry the other leg to and fro across the shortest distance and on a diameter, Y-Y 1. Adjustments are made until there is just-detectable friction across the shortest distance, and then the calipers are verified in a micrometer, adjusting this until similar friction obtains on the calipers, when the size can be read in the normal way on the micrometer. The principle is applicable when an internal diameter is being bored in a lathe; but the micrometer should be initially set several thou. Undersize, and care exercised in trial cuts so as not to overrun the dimension. As the bore nears size, all cuts should be run right through, since this avoids variations in depth of cut and spring of the tool, which could result in a bore being bell-mouthed. If a very large diameter is to be tested, the tailstock centre can be rhn up, a rule laid on the point, and a diameter marked on the face of the work with pencil or chalk so that the calipers can be kept across the diameter, as at Y-Yl. For checking machined bores, alternatives to calipers are telescopic gauges and adjustable ball gauges, B and c, the latter for very small dimensions. A telescopic gauge is entered in the bore, expanded, then its sliding plunger locked from the handle; a ball gauge is expanded in the bore from the handle. Both are checked for size (or set) with an outside micrometer.

 

A gauge with a slight taper, as at D. affords means of checking a bore being finished in a lathe. Mild steel can be used for the gauge, the top diameter (0.750 in.) turned to finished size and the taper carried down several thou. Less (0.740in.) in any convenient length, Z. A flat is filed and divided according to the difference in thou. Between the ends, so that for each mark that enters, the bore increases by 0.001 in. End gauges employed for bores must be provided with radii, as at E (right), not with flat ends (left), the comers of which would prevent an accurate check. This applies to the outside ends of gauges with sliding plungers, as at F, the inner ends being flat for feeler gauges to be placed between them, checking over a range. Bodies of such gauges can be mild steel; plungers hardened silver steel.

  

AS in the case of diameters, there is not always the need for precision on width and length measurements-but when it is demanded accuracy is no less important on the one than the other. Given a micrometer, diameters can easily be checked as work proceeds; but if width and length measurements have habitually been made with a rule-with all the variations that that implies -the need for greater precision may find one unprepared. Yet accuracy when machining widths and lengths is relatively easy to achieve. On a lathe having a topslide feed with graduated collar readings can be taken from this for positioning tools for facing cuts, and in some instances longer lengths can be obtained using the leading screw. Again, very accurate work can be done using simple gauges for setting tools, and this is often practised as occasion demands even on lathes fitted with feed collars. For old-type lathes without feed collars, or on which screws are worn, the use of gauges is virtually imperative to ensure precision and speed up production. The principle is as illustrated at A, where a piece of material is turned with two shoulders.

 

Accurate Length Machining

 

 

The lengths could be measured with a rule,; but a much more precise and speedier way is to machine the lengths slightly oversize then employ two gauges Xl, X2, to set the tool for finishing cuts. With the lathe stopped, a gauge is held to its respective shoulder and the tool set just to touch; then the gauge is removed and the cut taken at the precise position. In most workshops there are numerous objects of reasonable precision, such as drill shanks, silver steel rod, pieces of ground tool steel, etc., in standard sizes which can be used as gauges. If a micrometer is available material may also be turned or filed to size, and then it is possible to add to or delete from nominal dimensions for particular fits. For example, if a flange is nominally 1/8in. wide, but for a clearance fit 0.002 in. endplay is desirable, then the gauge could be made 1/8 in. minus 0.002 in. Again, if necessary, widths and lengths can be obtained falling between inch fractions, which would demand estimation on a rule. The chuck face or a jaw can be used as a datum for gauges, but one extra is required since the two gauges, Yl, Y2, only locate the end face and one shoulder; another would be needed for the second shoulder. Besides this, the gauges are in general longer and would require to be made specially.

 

On second operations, however, measurements can be made from the chuck face or jaws, though when a holder is used it is best to work from its face which, as at B, should be flat and large enough to take the gauges squarely. As at C, a recess can be machined to depth by finishing the interior over-length, then using a gauge to set the tool for the outside facing cut. Where a groove must be accurately located from the end face, either of two methods can be employed. The tool edge may be set flush with the end face, then taken in by feed collar reading, or a step gauge can be used, C. The principle is applicable to backfacing flanges, as at D, where the gauge is a simple adjustable type set by using a block of suitable thickness in conjunction with a straight-edge. A variation of such a step gauge -snipped or sawn, then filed from sheet metal-is as at E for obtaining the over-all dimension of a pair of flanges on a shaft machined between centres. Beyond the flanges, the shorter lengths can be obtained with other gauges. Locating from the saddle, as at F, an adjustable stop can be fitted to the headstock to take gauges in the space.

 

 

ALL turners who have served their time on a light lathe the kind used by most amateurs up to a generation ago-know that a decisive factor in its successful use is unremitting care in grinding and setting tools. This enables you to get round many of the problems caused by the light construction and lack of refinements. Contrariwise, without unrelaxing attention to tools, you are soon in trouble with rough finishes or wavy surfaces on the work. A production centre lathe has a broad bed, a spindle with widely spaced bearings, and a range of feeds for the saddle. Straightforward work can be done on it with much less skill and attention than is needed for the same job on a smaller and lighter lathe of the sort that many of us normally use.

 

 

On the smaller machine, spindle and slides must be in good adjustment; and you may have to feed the saddle by hand from the leadscrew, there being no fine self-acting feed to give an overlapping cut with almost any round-nosed tool. To get a fine finish, you may have to hone a small flat on the cutting edge of the tool, and then watch the depth of cut and the rate of feed to avoid chatter. A good general-purpose turning tool is shown at diagram A, with the essential angles and clearances. These comprise two rake angles, top rake V and side rake W, and three clearance angles, top clearance Y, front clearance X and side clearance Z. The front rake V and the front clearance X permit the tool to cut freely when it is pushed straight to the work by the cross-slide feed. The side-rake W and the side-clearance Z do the same when the tool is fed sideways by e th topslide feed or the leadscrew. The top-clearance Y reduces the length of cutting edge which is in contact with the work during a sideways cut. If this were not done, the wide contact would induce chatter-particularly on a light lathe.

 

 

Grinding and Setting Tools

 

 

The angles are shown sharp to the square shank of the tool bit to emphasize them, although the shape of an actual tool is to the dotted lines. The tool bit is intended for mounting horizontally on a top-slide, or in a turret. When it is mounted at an angle in a tool holder, the grinding must allow for this. To begin grinding a tool bit, you can hold it as at Bl at an angle and above the centre on the periphery of a grinding wheel. This produces a curved surface on the end of the bit which you can flatten later on the side of the wheel. In this way, you reduce wear on the side which cannot be rectified so easily as the periphery by use of a dresser. Some of the other grinding can be done on this principle. The first grinding leaves the bit with the front clearance angle X and the top clearance angle Y. You grind the side clearance angle Z as at B2, on the side of the wheel with a twist in the direction of the arrow. The next step is to grind the two rake angles V and W. This you do as at Cl, holding the tool to the other side of the grinding wheel, at an angle, and again with a twist. Lastly, you grind the radius by swinging the tool as at C2. You can cool it in water between grindings, and finally polish its faces with an oilstone. Diagram Dl illustrates how the tool should be mounted with its cutting tip at centre height. Use packing as required-and if the tip is above centre, you can pack under the back end of the bit, although by so doing you alter the cutting angles. With an above-centre setting D2, contact may be below the tool edge; with a below-centre setting D3 , you lose top rake, and the work may tend to climb over the tool. Diagram El shows the normal tool; E2 shows a tool for sharp corners; E3 shows the flat that you can hope for a fine finish. A gauge for setting tools appears at F. You make it from sheet and angle metal, screwing on a strip for setting turning tools, and drilling holes for boring tools.  

 

 

LAST week I described how a 4-jaw independent chuck can be set up in a split clamp on the slotted cross-slide of a lathe, so that work can be held for centering, drilling and machining. Various settings were mentioned to show the scope of the arrangement which, for some things, exceeds that of the vertical slide and machine vice. Points in its favor are that the four jaws of the chuck are usually more accommodating than the two of the vice, and the chuck itself can be adjusted at angles in its mounting, or on occasion made to rotate. Even so, I do not suggest that the arrangement supersedes the vertical slide and machine vice. This week we examine in greater detail some typical settings. The basic one, as I mentioned a week ago, is with the axis of the chuck at centre height and its face to the headstock. Looking down on it, you see it as A. If the clamp is accurate, it should provide a precise horizontal mounting for the chuck, gripping by the boss of the back-plate, so that the face of the chuck is vertical.

 

 

If you are doubtful about this there are several ways of checking alignment; and if you find an error, you can correct it by packing under the clamp. One way of checking is to use a steel square with its stock on the bed of the lathe, and its blade up the face of the chuck. Alternatively, you can mount a test indicator in another chuck on the spindle , or on a driving plate, and let the plunger touch on the face of the four-jaw chuck. You should see the same reading top and bottom. Again, you can use the end of a cranked bar in the same way as an indicator. By the same principle, the crosswise setting of the work or of the chuck can be checked. With the test indicator, you should get the same reading at points P and Q on true work, or the same clearance here when using a cranked bar. When you test on the face of the chuck, you should get the same reading or the same clearance at points R and S. The chuck mounting can be adjusted on the cross-slide for this.

 

 

Chuck Mounting

 

 

The square mounting for the split clamp and the chuck is made by bolts in the T-slots of th creo ssslide, the holes and the slots having the same spacing. As a result, only small divergencies from the basic setting are possible, unless one side of the mounting is clamped to the cross-slide. Large angles can be taken from an adjustable angle gauge, or work a shop protractor, as a Bt, placing one side of this to the face of the chuck, or to the work, and testing points T an d U on the other side. Diagra Bm together with C shows how the base of the mounting is clamped. You tap the bolt hole in the base for a special stud with a larger thread at the bottom. Then a clamp or strap can be fitted to the base, with a short bolt in the T-slot and a block to take the reaction. The cap of the mounting you can fit by a nut at the top of the stud. With the mounting square or at an angle, you can adjust the chuck in it so that the jaws are square or at an angle to the bed of the lathe, as at D. For a square setting, checks are made at V and W with a surface gauge or indicator; or a steel square can be used for angle X. For an angle setting, an angle gauge can be used for either of the complementary angles Y, Z. These methods and their variations are, of course, applicable to work which is part-machined, or on which lines have been scribed in marking off. You can obtain further angles, or quick angular settings, after graduating the edge of the chuck back-plate and fitting a pointer to the cap with a short setscrew, as at E. Divide the back-plate as at F. Centre and machine a mild-steel mandrel for a lathe change wheel. Hold one end in the three-jaw chuck, with the change wheel keyed on, grip the 4-jaw chuck on the length, and support the other end by the tailstock. Use a detent to the change wheel and a pointed tool to the back-plate.


 

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