=Different Forms of Centers.=--In some poorly equipped shops it is necessary to form centers by the use of a center-punch only, as there is no better tool. If the end of the punch has a sixty-degree taper, a fair center can be formed in this way, but it is not a method to be recommended, especially when accurate work is required. Sometimes centers are made with punches that are too blunt, producing a shallow center, such as the one shown in the upper left-hand view, Fig. 27. In this case all the bearing is on the point of the lathe center, which is the worst possible place for it. Another way is to simply drill a straight hole as in the upper view to the right; this is also bad practice in more than one respect. The lower view to the right shows a form of center which is often found in the ends of lathe arbors, the mouth of the center being rounded, at _r_, and the arbor end recessed as shown. The rounded corner prevents the point of the lathe center from catching when it is moved rapidly towards work which is not being held quite centrally (as shown by the ill.u.s.tration), and the end is recessed to protect the center against bruises. Stock that is bent should always be straightened before the centers are drilled and reamed. If the work is first centered and then straightened the bearing on the lathe center would be as shown in Fig. 29. The center will then wear unevenly with the result that the surfaces last turned will not be concentric with those which were finished first.

[Ill.u.s.tration: Fig. 30. Tool Steel should be centered Concentric, in order to remove the Decarbonized Outer Surface]

=Precaution When Centering Tool Steel.=--Ordinarily centers are so located that the stock runs approximately true before being turned, but when centering tool steel to be used in making tools, such as reamers, mills, etc., which need to be hardened, particular care should be taken to have the rough surface run fairly true. This is not merely to insure that the piece will "true-up," as there is a more important consideration, the disregard of which often affects the quality of the finished tool. As is well known, the degree of hardness of a piece of tool steel that has been heated and then suddenly cooled depends upon the amount of carbon that it contains, steel that is high in carbon becoming much harder than that which contains less carbon. Furthermore, the amount of carbon found at the surface, and to some little depth below the surface of a bar of steel, is less than the carbon content in the rest of the bar. This is ill.u.s.trated diagrammatically in Fig. 30 by the shaded area in the view to the left. (This decarbonization is probably due to the action of the oxygen of the air on the bar during the process of manufacture.) If stock for a reamer is so centered that the tool removes the decarbonized surface only on one side, as ill.u.s.trated to the right, evidently when the reamer is finished and hardened the teeth on the side _A_ will be harder than those on the opposite side, which would not have been the case if the rough bar had been centered true. To avoid any trouble of this kind, stock that is to be used for hardened tools should be enough larger than the finished diameter and so centered that this decarbonized surface will be entirely removed in turning.

[Ill.u.s.tration: Fig. 31. Three Methods of Facing the Ends Square]

=Facing the Ends of Centered Stock.=--As a piece of work is not properly centered until the ends are faced square, we will consider this operation in connection with centering. Some machinists prefer lathe centers that are cut away as shown at _A_, Fig. 31, so that the point of the side tool can be fed in far enough to face the end right up to the center hole. Others, instead of using a special center, simply loosen the regular one slightly and then, with the tool in a position as at _B_, face the projecting teat by feeding both tool and center inward as shown by the arrow. Whenever this method is employed, care should be taken to remove any chips from the center hole which may have entered. A method which makes it unnecessary to loosen the regular center, or to use a special one, is to provide clearance for the tool-point by grinding it to an angle of approximately forty-five degrees, as shown at _C_. If the tool is not set too high, it can then be fed right up to the lathe center and the end squared without difficulty. As for the special center _A_, the use of special tools and appliances should always be avoided unless they effect a saving in time or their use makes it possible to accomplish the same end with less work.

=Truing Lathe Centers.=--The lathe centers should receive careful attention especially when accurate work must be turned. If the headstock center does not run true as it revolves with the work, a round surface may be turned, but if the position of the driving dog with reference to the faceplate is changed, the turned surface will not run true because the turned surface is not true with the work centers. Furthermore, if it is necessary to reverse the work for finishing the dogged or driving end, the last part turned will be eccentric to the first. Therefore, the lathe centers should be kept true in order to produce turned surfaces that are true or concentric with the centered ends, as it is often necessary to change the part being turned "end for end" for finishing, and any eccentricity between the different surfaces would, in many cases, spoil the work.

[Ill.u.s.tration: Fig. 32. Grinder for Truing Lathe Centers]

Some lathes are equipped with hardened centers in both the head-and tailstock and others have only one hardened center which is in the tailstock. The object in having a soft or unhardened headstock center is to permit its being trued by turning, but as a soft center is quite easily bruised and requires truing oftener than one that is hard, it is better to have both centers hardened. Special grinders are used for truing these hardened centers. One type that is very simple and easily applied to a lathe is shown in Fig. 32. This grinder is held in the lathe toolpost and is driven by a wheel _A_ that is held in contact with the cone-pulley. The emery wheel _B_ is moved to a position for grinding by adjusting the carriage and cross-slide, and it is traversed across the conical surface of the center by handle _C_. As the grinding proceeds, the wheel is fed inward slightly by manipulating the cross-slide.

This grinder is set to the proper angle by placing the two centered ends _D_ and _D_{1}_ between the lathe centers, which should be aligned as for straight turning. The grinding spindle will then be 30 degrees from the axis of the lathe spindle. The grinder should be carefully clamped in the toolpost so that it will remain as located by the centered ends.

After the tailstock center is withdrawn, the emery wheel is adjusted for grinding. As the wheel spindle is 30 degrees from the axis of the lathe spindle, the lathe center is not only ground true but to an included angle of 60 degrees, which is the standard angle for lathe centers.

There are many other styles of center grinders on the market, some of which are driven by a small belt from the cone-pulley and others by electric motors which are connected with ordinary lighting circuits. The tailstock center is ground by inserting it in the spindle in place of the headstock center. Before a center is replaced in its spindle, the hole should be perfectly clean as even a small particle of dirt may affect the alignment. The center in the headstock is usually referred to as the "live center" because it turns around when the lathe is in use, and the center in the tailstock as the "dead center," because it remains stationary.

=Universal, Independent and Combination Chucks.=--Many parts that are turned in the lathe are so shaped that they cannot be held between the lathe centers like shafts and other similar pieces and it is often necessary to hold them in a chuck _A_, Fig. 33, which is screwed onto the lathe spindle instead of the faceplate. The work is gripped by the jaws _J_ which can be moved in or out to accommodate various diameters.

There are three cla.s.ses of chucks ordinarily used on the lathe, known as the independent, universal and combination types. The independent chuck is so named because each jaw can be adjusted in or out independently of the others by turning the jaw screws S with a wrench. The jaws of the universal chuck all move together and keep the same distance from the center, and they can be adjusted by turning any one of the screws _S_, whereas with the independent type the chuck wrench must be applied to each jaw screw. The combination chuck, as the name implies, may be changed to operate either as an independent or universal type. The advantage of the universal chuck is that round and other parts of a uniform shape are located in a central position for turning without any adjustment. The independent type is, however, preferable in some respects as it is usually stronger and adapted for holding odd-shaped pieces because each jaw can be set to any required position.

[Ill.u.s.tration: Fig. 33. (A) Lathe Chuck. (B) Faceplate Jaw]

=Application of Chucks.=--As an example of chuck work, we shall a.s.sume that the sides of disk _D_, Fig. 34, are to be turned flat and parallel with each other and that an independent chuck is to be used. First the chuck is screwed onto the lathe spindle after removing the faceplate.

The chuck jaws are then moved out or in, as the case may be, far enough to receive the disk and each jaw is set about the same distance from the center by the aid of concentric circles on the face of the chuck. The jaws are then tightened while the disk is held back against them to bring the rough inner surface in a vertical plane. If the work is quite heavy, it can be held against the chuck, before the jaws are tightened, by inserting a piece of wood between it and the tailstock center; the latter is then run out far enough to force the work back. The outside or periphery of the disk should run nearly true and it may be necessary to move the jaws in on one side and out on the other to bring the disk to a central position. To test its location, the lathe is run at a moderate speed and a piece of chalk is held near the outer surface. If the latter runs out, the "high" side will be marked by the chalk, and this mark can be used as a guide in adjusting the jaws. It should be remembered that the jaws are moved only one-half the amount that the work runs out.

[Ill.u.s.tration: Fig. 34. (A) Radial Facing. (B) Boring Pulley Held in Chuck]

A round-nosed tool _t_ of the shape shown can be used for radial facing or turning operations of the kind ill.u.s.trated. This tool is similar to the form used when turning between centers, the princ.i.p.al difference being in the direction of the top slope. The radial facing tool should be ground to slope downward toward _a_ (see Fig. 35) whereas the regular turning tool slopes toward _b_, the inclination in each case being away from that part of the cutting edge which does the work. The cutting edge should be the same height as the lathe centers, and the cut is taken by feeding the tool from the outside in to the center. The cut is started by hand and then the power feed is engaged, except for small surfaces.

The first cut should, if possible, be deep enough to get beneath the scale, especially if turning cast iron, as a tool which just grazes the hard outer surface will be dulled in a comparatively short time.

If it were simply necessary to turn a true flat surface and the thickness of the disk were immaterial, two cuts would be sufficient, unless the surface were very uneven, the first or roughing cut being followed by a light finishing cut. For a finishing cut, the same tool could be used, but if there were a number of disks to be faced, a square-nosed tool _F_, Fig. 35, could probably be used to better advantage. This type has a broad flat cutting edge that is set parallel with the rough-turned surface and this broad edge enables a coa.r.s.e feed to be taken, thus reducing the time required for the finishing cut. If a coa.r.s.e feed were taken with the round tool, the turned surface would have spiral grooves in it, whereas with the broad cutting edge, a smooth surface is obtained even though the feed is coa.r.s.e. The amount of feed per revolution of the work, however, should always be less than the width _w_ of the cutting edge. Very often broad tools cannot be used for finishing cuts, especially when turning steel, because their greater contact causes chattering and results in a rough surface. An old and worn lathe is more liable to chatter than one that is heavy and well-built, and as the diameter of the work also makes a difference, a broad tool cannot always be used for finishing, even though, theoretically, it would be preferable. After one side of the disk is finished, it is reversed in the chuck, the finished surface being placed against the jaws. The remaining rough side is then turned, care being taken when starting the first cut to caliper the width of the disk at several points to make sure that the two sides are parallel.

[Ill.u.s.tration: Fig. 35. Tools Ground so that Top Slopes away from Working Part of Cutting Edge]

=Example of Boring.=--Another example of chuck work is shown at _B_, Fig. 34. In this case a cast-iron pulley is to have a true hole _h_ bored through the hub. (The finishing of internal cylindrical surfaces in a lathe is referred to as boring rather than turning.) The casting should be set true by the rim instead of by the rough-cored hole in the hub; this can be done by the use of chalk as previously explained. Even though a universal type of chuck were used, the jaws of which, as will be recalled, are self-centering, it might be necessary to turn the pulley relative to the chuck as a casting sometimes runs out because of rough spots or lumps which happen to come beneath one or more of the jaws.

[Ill.u.s.tration: Fig. 36. Boring Tool]

The shape of tool _t_ for boring is quite different from one used for outside turning, as shown by Fig. 36. The cutting end of a solid type of tool is forged approximately at right angles to the body or shank, and the top surface is ground to slope away from the working part _w_ of the cutting edge, as with practically all turning tools. The front part or flank, _f_ is also ground away to give the edge clearance. This type of tool is clamped in the toolpost with the body about parallel with the lathe spindle, and ordinarily the cutting edge would be about as high as the center of the hole, or a little below, if anything. When starting a cut, the tool is brought up to the work by moving the carriage and it is then adjusted radially to get the right depth of cut, by shifting the cross-slide. The power feed for the carriage is then used, the tool feeding back through the hole as indicated by the arrow, Fig. 34. In this case, as with all turning operations, the first cut should be deep enough to remove the hard outer scale at every part of the hole. Usually a rough-cored hole is so much smaller than the finished size that several cuts are necessary; in any case, the last or finishing cut should be very light to prevent the tool from springing away from the work, so that the hole will be as true as possible. Boring tools, particularly for small holes, are not as rigid as those used for outside turning, as the tool has to be small enough to enter the hole and for this reason comparatively light cuts have to be taken. When boring a small hole, the largest tool that will enter it without interference should be used to get the greatest rigidity possible.

[Ill.u.s.tration: Fig. 37 (A) Setting Outside Calipers. (B) Transferring Measurements to Inside Calipers. (C) Micrometer Gage]

=Measuring Bored Holes.=--The diameters of small holes that are being bored are usually measured with inside calipers or standard gages. If the pulley were being bored to fit over some shaft, the diameter of the shaft would first be measured by using outside calipers, as shown at _A_, Fig. 37, the measuring points of the calipers being adjusted until they just made contact with the shaft when pa.s.sed over it. The inside calipers are then set as at _B_ to correspond with the size of the shaft, and the hole is bored just large enough to admit the inside calipers easily. Very accurate measurements can be made with calipers, but to become expert in their use requires experience. Some mechanics never become proficient in the art of calipering because their hands are "heavy" and they lack the sensitiveness and delicacy of touch that is necessary. For large holes, a gage _C_ is often used, the length _l_ being adjusted to the diameter desired. Small holes are often bored to fit hardened steel plug gages (Fig. 38), the cylindrical measuring ends of which are made with great accuracy to standard sizes. This type of gage is particularly useful when a number of holes have to be bored to the same size, all holes being made just large enough to fit the gage without any perceptible play.

[Ill.u.s.tration: Fig. 38. Standard Plug Gage]

_Setting Work in the Chuck._--When setting a part in a chuck, care should be taken to so locate it that every surface to be turned will be true when machined to the finished size. As a simple ill.u.s.tration, let us a.s.sume that the hole through the cast-iron disk, Fig. 39, has been cored considerably out of center, as shown. If the work is set by the outside surface _S_, as it would be ordinarily, the hole is so much out of center that it will not be true when bored to the finished size, as indicated by the dotted lines. On the other hand, if the rough hole is set true, the outside cannot be finished all over, without making the diameter too small, when it is finally turned. In such a case, the casting should be shifted, as shown by the arrow, to divide the error between the two surfaces, both of which can then be turned as shown by the dotted lines in the view to the right. This principle of dividing the error when setting work can often be applied in connection with turning and boring. After a casting or other part has been set true by the most important surface, all other surfaces which require machining should be tested to make sure that they all can be finished to the proper size.

=Inaccuracy from Pressure of Chuck Jaws.=--Work that is held in a chuck is sometimes sprung out of shape by the pressure of the chuck jaws so that when the part is bored or turned, the finished surfaces are untrue after the jaws are released and the work has resumed its normal shape.

This applies more particularly to frail parts, such as rings, thin cylindrical parts, etc. Occasionally the distortion can be prevented by so locating the work with relation to the chuck jaws that the latter bear against a rigid part. When the work cannot be held tightly enough for the roughing cuts without springing it, the jaws should be released somewhat before taking the finishing cut, to permit the part to spring back to its natural shape.

[Ill.u.s.tration: Fig. 39. Diagram Ill.u.s.trating Importance of Setting Work with Reference to Surfaces to be Turned]

[Ill.u.s.tration: Fig. 40. Drilling in the Lathe]

=Drilling and Reaming.=--When a hole is to be bored from the solid, it is necessary to drill a hole before a boring tool can be used. One method of drilling in the lathe is to insert an ordinary twist drill in a holder or socket _S_, Fig. 40, which is inserted in the tailstock spindle in place of the center. The drill is then fed through the work by turning the handle _n_ and feeding the spindle outward as shown by the arrow. Before beginning to drill, it is well to turn a conical spot or center for the drill point so the latter will start true. This is often done by using a special tool having a point like a flat drill.

This tool is clamped in the toolpost with the point at the same height as the lathe centers. It is then fed against the center of the work and a conical center is turned. If the drill were not given this true starting point, it probably would enter the work more or less off center. Drills can also be started without turning a center by bringing the square end or b.u.t.t of a tool-shank held in the toolpost in contact with the drill near the cutting end. If the point starts off center, thus causing the drill to wobble, the stationary tool-shank will gradually force or b.u.mp it over to the center.

[Ill.u.s.tration: Fig. 41. Flat Drill and Holder]

Small holes are often finished in the lathe by drilling and reaming without the use of a boring tool. The form of drill that is used quite extensively for drilling cored holes in castings is shown in Fig. 41, at _A_. This drill is flat and the right end has a large center hole for receiving the center of the tailstock. To prevent the drill from turning, a holder _B_, having a slot _s_ in its end through which the drill pa.s.ses, is clamped in the toolpost, as at _C_. This slot should be set central with the lathe centers, and the drill, when being started, should be held tightly in the slot by turning or twisting it with a wrench as indicated in the end view at _D_; this steadies the drill and causes it to start fairly true even though the cored hole runs out considerably.

Another style of tool for enlarging cored holes is shown in Fig. 42, at _A_. This is a rose chucking reamer, having beveled cutting edges on the end and a cylindrical body, which fits closely in the reamed hole, thus supporting and guiding the cutting end. The reamer shown at _B_ is a fluted type with cutting edges that extend from _a_ to _b_; it is used for finishing holes and the drill or rose reamer preceding it should leave the hole very close to the required size. These reamers are held while in use in a socket inserted in the tailstock spindle, as when using a twist drill.

[Ill.u.s.tration: Fig. 42. Rose and Fluted Reamers]

=Holding Work on Faceplate.=--Some castings or forgings are so shaped that they cannot be held in a chuck very well, or perhaps not at all, and work of this kind is often clamped to a faceplate which is usually larger than the faceplate used for driving parts that are turned between the centers. An example of faceplate work is shown in Fig. 43. This is a rectangular-shaped casting having a round boss or projection, the end _e_ of which is to be turned parallel with the back face of the casting previously finished on a planer. A rough cored hole through the center of the boss also needs to be bored true.

The best way to perform this operation in the lathe would be to clamp the finished surface of the casting directly against the faceplate by bolts and clamps _a_, _b_, _c_, and _d_, as shown; the work would then be turned just as though it were held in a chuck. By holding the casting in this way, face _e_ will be finished parallel with the back surface because the latter is clamped directly against the true-running surface of the faceplate. If a casting of this shape were small enough it could also be held in the jaws of an independent chuck, but if the surface e needs to be exactly parallel with the back face, it is better to clamp the work to the faceplate. Most lathes have two faceplates: One of small diameter used princ.i.p.ally for driving work turned between centers, and a large one for holding heavy or irregularly shaped pieces; either of these can be screwed onto the spindle, and the large faceplate has a number of slots through which clamping bolts can be inserted.

[Ill.u.s.tration: Fig. 43. Casting Clamped to Faceplate for Turning and Boring]

The proper way to clamp a piece to the faceplate depends, of course, largely on its shape and the location of the surface to be machined, but in any case it is necessary to hold it securely to prevent any shifting after a cut is started. Sometimes castings can be held by inserting bolts through previously drilled holes, but when clamps are used in connection with the bolts, their outer ends are supported by hardwood or metal blocks which should be just high enough to make the clamp bear evenly on the work. When deep roughing cuts have to be taken, especially on large diameters, it is well to bolt a piece to the faceplate and against one side of the casting, as at _D_, to act as a driver and prevent the work from shifting; but a driver would not be needed in this particular case. Of course a faceplate driver is always placed to the rear, as determined by the direction of rotation, because the work tends to shift backward when a cut is being taken. If the surface which is clamped against the faceplate is finished as in this case, the work will be less likely to shift if a piece of paper is placed between it and the faceplate.

[Ill.u.s.tration: Fig. 44. Cast Elbow held on Angle-plate attached to Faceplate]

Work mounted on the faceplate is generally set true by some surface before turning. As the hole in this casting should be true with the round boss, the casting is shifted on the faceplate until the rough outer surface of the boss runs true; the clamps which were previously set up lightly are then tightened. The face e is first turned by using a round-nosed tool. This tool is then replaced by a boring tool and the hole is finished to the required diameter. If the hole being bored is larger than the central hole in the faceplate, the casting should be clamped against parallel pieces, and not directly against the faceplate, to provide clearance for the tool when it reaches the inner end of the hole and prevent it from cutting the faceplate. The parallel pieces should be of the same thickness and be located near the clamps to prevent springing the casting.

=Application of Angle-plate to Faceplate.=--Another example of faceplate work is shown in Fig. 44. This is a cast-iron elbow _E_, the two f.l.a.n.g.es of which are to be faced true and square with each other. The shape of this casting is such that it would be very difficult to clamp it directly to the faceplate, but it is easily held on an angle-plate _P_, which is bolted to the faceplate. The two surfaces of this angle-plate are square with each other so that when one f.l.a.n.g.e of the elbow is finished and bolted against the angle-plate, the other will be faced square. When setting up an angle-plate for work of this kind, the distance from its work-holding side to the center of the faceplate is made equal to the distance _d_ between the center of one f.l.a.n.g.e and the face of the other, so that the f.l.a.n.g.e to be faced will run about true when bolted in place. As the angle-plate and work are almost entirely on one side of the faceplate, a weight _W_ is attached to the opposite side for counterbalancing. Very often weights are also needed to counterbalance offset parts that are bolted directly to the faceplate.

The necessity of counterbalancing depends somewhat upon the speed to be used for turning. If the surface to be machined is small in diameter so that the lathe can be run quite rapidly, any unbalanced part should always be counterbalanced.

Sometimes it is rather difficult to hold heavy pieces against the vertical surface of the faceplate while applying the clamps, and occasionally the faceplate is removed and placed in a horizontal position on the bench; the work can then be located about right, and after it is clamped, the faceplate is placed on the lathe spindle by the a.s.sistance of a crane.

Special faceplate jaws, such as the one shown to the right in Fig. 33, can often be used to advantage for holding work on large faceplates.

Three or four of these jaws are bolted to the faceplate which is converted into a kind of independent chuck. These faceplate jaws are especially useful for holding irregularly shaped parts, as the different jaws can be located in any position.

=Supporting Outer End of Chucked Work.=--Fig. 45 shows how the tailstock center is sometimes used for supporting the outer end of a long casting, the opposite end of which is held in a chuck. This particular casting is to be turned and bored to make a lining for the cylinder of a locomotive in order to reduce the diameter of the cylinder which has been considerably enlarged by re-boring a number of times. These bushings are rough-turned on the outside while the outer end is supported by the cross-shaped piece or "spider" which forms a center-bearing for the tailstock. This spider has set screws in the f.l.a.n.g.ed ends of the arms, which are tightened against the inner surface of the casting and are adjusted one way or the other in order to locate it in a concentric position. After roughing the outside, the inside is bored to the finish size; then centered disks, which fit into the bore, are placed in the ends of the bushing and the latter is finish-turned.

The object in rough turning the outside prior to boring is to avoid the distortion which might occur if this hard outer surface were removed last.

[Ill.u.s.tration: Fig. 45. Rough Turning a Cylinder Lining--Note Method of Supporting Outer End]

=Boring Large Castings in the Lathe.=--An ordinary engine lathe is sometimes used for boring engine or pump cylinders, linings, etc., which are too large to be held in the chuck or on a faceplate, and must be attached to the lathe carriage. As a rule, work of this cla.s.s is done in a special boring machine (see "Horizontal Boring Machines"), but if such a machine is not available, it may be necessary to use a lathe.

There are two general methods of boring.

Fig. 46 shows how the lining ill.u.s.trated in Fig. 45 is bored in a large engine lathe. The casting is held in special fixtures which are attached to the lathe carriage, and the boring-bar is rotated by the lathe spindle. The tool-head of this boring-bar carries two tools located 180 degrees apart and it is fed along the bar by a star-feed mechanism shown attached to the bar and the tailstock spindle. Each time the bar revolves, the star wheel strikes a stationary pin and turns the feed-screw which, as the ill.u.s.tration shows, extends along a groove cut in one side of the bar. This feed-screw pa.s.ses through a nut attached to the tool-head so that the latter is slowly fed through the bore. When using a bar of this type, the carriage, of course, remains stationary.