Modern Machine-Shop Practice Part 81

To prevent the weight of the work from causing the countersinking being out of true with the hole, the work should be occasionally allowed (by relaxing the grip upon it) to make part of a revolution, as explained with reference to centre-drilling without a work guide. Another and simple form of square centre for countersinking is shown in Fig. 1196.

It consists of a piece of square steel set into a stock or holder.

[Ill.u.s.tration: Fig. 1197.]

Work that is to be hardened and whose centres are, therefore, liable to warp in the hardening, may be countersunk as in Fig. 1197, there being three indentations in the countersink as shown. This insures that there shall be three points of contact, and the work will run steadily and true. Furthermore, the indentations form pa.s.sages for the oil, facilitating the lubrication and preventing wear both to the work and to the lathe centres.

[Ill.u.s.tration: Fig. 1198.]

These indentations are produced after the countersinking by the punch, shown in Fig. 1198. Except when tapers are turned by setting the lathe centres out of line with the lathe shears (as in setting the tailstock over), all the wear falls on the dead centre end of the work, as there is no motion of the work centre on the live centre, hence the work centres will not have worn to a full bearing until the work has been reversed end for end in the lathe.

[Ill.u.s.tration: Fig. 1199.]

[Ill.u.s.tration: Fig. 1200.]

If it be attempted to countersink a piece of work whose end face is not square, the countersinking will not be true with the centre hole, and furthermore the causes producing this want of truth will continue to operate to throw the work out of true while it is being turned. Thus, in Fig. 1199, _a_ represents a piece of work and B the dead centre; if the side C is higher than side D of the work end, the increased bearing area at C will cause the most wear to occur at D, and the countersink in the work will move over towards D, and it follows that the face of a rough piece of work should be faced before being countersunk. Professor Sweet designed the centre-drilling device shown in Fig. 1200, which consists of a stock fitting the holes for the lathe centres, and carrying what may be called a turret head, in which are the centre drills, facing tools, and countersinks. The turret has 6 holes corresponding to the number of tools it carries, and each tool is held in position by a pin, upon a spring, which projects into the necessary hole, the construction being obvious. The facing tool is placed next to the drill and is followed by a countersink, in whatever direction the turret is rotated to bring the next tool into operation. The work should, on account of the power necessary for the facing, be driven in a chuck.

[Ill.u.s.tration: Fig. 1201.]

A similar tool, which may, however, be used for other work besides centring and countersinking, is shown in Fig. 1201. It consists of a stem fitting into the hole of the tail spindle, and carrying a base having a pin D, on which fits a cap having holes _b_, and set-screws C for fastening drills, countersinks, or cutting tools. The cap is pierced with six taper holes, and a pin projects through the base into these holes to lock the cap in position, this pin being operated by the spring lever shown.

[Ill.u.s.tration: Fig. 1202.]

Work that has already been turned, but has had its centres cut off, may be recentred as follows. One end may be held and driven by a chuck, while the other end is held in a steady rest such as was shown in Fig.

802, and the centre may then be formed in the free end by a half-round reamer, such as shown in Fig. 1190, placed in the position of the dead centre, or the square centre may be used in place of the dead centre, being so placed that one of its faces stands vertically, and therefore that two of its edges will operate to cut. The location for the work centre should be centre punched as accurately as possible, and the work is then placed in the lathe with a driver on it, as for turning it up; a crotch, such as shown in Fig. 1202, is then fastened in the lathe tool post, and fed up by the cross-feed screw until it causes the work to run true, and the square centre should then be fed slowly up and into the work, with a liberal supply of oil. If the work runs out of true, the crotch should be fed in again, but care must be taken not to feed it too far. So long as the square centre is altering the position of the centre in the work, it will be found that the feed-wheel of the tailstock will feed by jumps and starts; and after the feeding feels to proceed evenly, the crotch may be withdrawn and the work tried for being true. The crotch, as well as the square centre, should be oiled to prevent its damaging the work surface. It is obvious that in order to prevent the lathe dead centre point from seating at the point or bottom of the work centre, the square centre should be two or three degrees more acute in angle than the lathe dead centre. If the work is tried for truth while running on the square centre, the latter is apt to enlarge the work centre, while the work will not run steadily, hence it is better (and necessary where truth is a requisite) to try the work with the dead centre in place of the square one.

In thus using a square centre to true work, great care should be taken not to cut the work centres too large, and this may be avoided by making the temporary centre-punch centres small, and feeding the crotch rapidly up to the work, until the latter runs true, while the square centre is fed up only sufficiently to just hold the work steady.

[Ill.u.s.tration: Fig. 1203.]

To test the truth of a piece of rough work, it may, if sufficiently light, be placed between the lathe centres with a light contact, and rotated by drawing the hand across it, a piece of chalk being held in the right hand sufficiently near to just touch the work, and if the chalk mark extends all round the work, the latter is as true as can be tested by so crude a test, and a more correct test may be made by a tool held in the tool rest. If the test made at various positions in the length of the work shows the work to be bent enough to require straightening, such straightening may be done by a straightening lever.

In shops where large quant.i.ties of shafting are produced, there are special straightening tools or devices: thus, Figs. 1203 and 1204 represent two views of a straightening machine. The shaft to be straightened is rotated by the friction caused by its own weight as it lies between rollers, which saves the trouble of placing the shaft upon centres. Furthermore, the belt that is the prime mover of the gears driving these rollers is driven from the line shaft itself without the aid of any belt pulley. The tension of this driving belt is so adjusted that it will just drive the heaviest shaft the machine will straighten; but if the operator grasps the shaft in his hand, the driving mechanism will stop and the belt will slip, the shaft remaining stationary until the operator sets it in motion again with his hand, when the belt ceases to slip and the mechanism again acts to drive the shaft.

Fig. 1203 represents the mechanism for driving the shaft S, to be straightened, which lies upon and between two rollers, R, R'. Upon the shafts of these rollers are the gear-wheels A and B, which are in gear with wheel C, the latter being driven by gear-wheel D. Motion to D is derived from a pair of gears, the pinion of which is driven by the belt from the line shaft. H is a head carrying all these gears (and the rollers) except D. There are two of these heads, one at each end of the machine, the two wheels D being connected by a rod running between the shears, but the motion is communicated at one end only of this rod, the shaft is driven between four rollers, of which two, R R', are shown in the engraving.

[Ill.u.s.tration: Fig. 1204.]

[Ill.u.s.tration: Fig. 1205.]

In Fig. 1204 the straightening device is shown. A frame consisting of two parts, F, F', is gibbed to the edge of the shears at G and H. The upper part of this frame carries a square-threaded screw I, and is capable of sliding across the shears upon the part F'. It rests upon the shears through the medium of four small rollers (which are encased), two of which are at J, K, and two are similarly situated at the back of the frame F'. The motion of F across the machine is provided so that the upper part F may be pushed back out of the way, to permit the shaft being easily put on and taken off the friction rollers R R'. The motion along the shears is provided to enable the straightening device to be moved to the required spot along the shaft S'. The shaft S is laid on two pieces N, P, and a similar piece _r_ is placed above to receive the pressure of the screw I, which is operated by a hand lever to perform the straightening. The pieces N, P rest upon two square taper blocks V, which are provided with circular k.n.o.bs at their outer ends to enable them to be held and pushed in or pulled out so as to cause N, P to meet the shaft before I is operated. This is necessary to accommodate the different diameters of shaft S. The operator simply marks the rotating shaft with chalk in the usual manner to show where it is out of true, and then straightens wherever it is found necessary.

Fig. 1205 represents a similar device for straightening rods or shafts while they are in the lathe. A is a frame or box which is fitted to rest on the [V]s of the lathe shears, the straightening frame resting on the box. Instead, however, of simply adjusting the height of the pieces P to suit different diameters of the shaft, the whole frame is adjusted by means of the wedge W, which is inserted between the frame F and the upper surface of the box A. At H is a hole to admit the operator's hand to move A along the lathe shears.

[Ill.u.s.tration: Fig. 1206.]

A method of straightening wire or small rods that are too rigid to be straightened by hand, and on which it is inadvisable to use hammer blows, is shown in Fig. 1206. It consists of a head revolved in a suitable machine, and having a hole pa.s.sing endways through it. In the middle is a slot and through the body pa.s.s the pins A, being so located that their perimeters just press the rod or wire when it is straight, and in line with the axis of the bore through the head, each successive pin A touching an opposite side of the wire or rod. It is obvious that these pins in revolving force out any crooks or bent places in the wire or rod, and that as the work may be pulled somewhat rapidly through the head or frame, the operation is a rapid one.

When pieces of lathe work are to be made from rod or bar iron, they should be cut off to the proper length in a cutting-off machine, such as described in special forms of the lathe, and for the reasons set forth in describing that machine.

An excellent tool, however, for cutting up rods of not more than 1/2 inch in diameter, is Elliott's cutting-off tool shown in Fig. 1207. It consists of a jaw carrying steadying pieces for the rod to be cut up, these pieces being adjusted to fit the rod by the screw and nut shown.

On the same jaw is pivoted a tool-holder, carrying a cutting-off tool, which is fed to its cut by the upper handle being pressed towards the lower one.

An adjustable stop or gauge is attached, by means of a small rod, to the swinging arm which carries the cutting tool, and can be removed when its use is not desirable.

[Ill.u.s.tration: Fig. 1207.]

The operation of this tool is as follows:--The rod to be cut up is held in the lathe chuck, projecting beyond any desired distance, and arranged to revolve at the same speed as for turning. The tool is placed upon the rod, and the movable jaw of the rest adjusted to a bearing. If several pieces are to be cut to a length, the gauge is adjusted, the tool moved along the rod till the gauge-stop comes in contact with the end, the handles pressed together, which moves the cutting tool up to the work in such a way that it will come exactly to the centre, thus cutting the piece entirely off, no adjustment of the tool ever being necessary to provide for its cutting to the centre, except keeping the cutting edge (which is not in this respect changed by grinding) at a distance specified in the directions from the part in which it is clamped. As the tool is moved up to cut, by the same operation the gauge is moved back out of contact with the end. When the pressure on the handles is removed, a spring returns the cutting tool to its original position, and also brings the gauge in position for determining the length of the next piece to be cut. The operation is repeated by simply moving the tool along the rod, the cutting up being done with great rapidity and accuracy. It will be noticed that all the appliances for cutting, gauging, &c., being a part of the tool itself, if the rod runs out of truth--in other words, wabbles--it will have no effect on the cutting, the rod to be cut forming the gauge for all the operations required; also that comparatively no time is lost in adjustment between the several pieces to be cut from a rod.

The cutting tool is a piece of steel of the proper thickness, cleared on the sides by concave grinding. It is held in place by a clamp and two small screws, and requires grinding on the end only.

When the work is centred, it should, for reasons already explained, have its end faces trued up.

In doing this, however, it is desirable in some cases to cut off the work to its exact finished length. This possesses the advantage, that when the work is finished, the work centres will be left intact, and the work may be put into the lathe at any time, and it will run true to the original centres. But this is not always the best plan; suppose, for example, that there are a number of collars or f.l.a.n.g.es on the work, then it is better to leave a little extra length to the work when truing up the ends, so that if any of the collars are scant of metal, or if it be desirable to turn off more on one side of a collar than on another, as may be necessary to turn out a faulty place in the material, the end measurements on the work may be conformed to accommodate this requirement, and not confined to an exact measurement from the end of the work.

Again, in the case of work having a taper part to be fitted, it is very difficult to obtain the exact proper fit and entrance of taper to an exact distance, hence it is best to leave the work a little too long, with its collars too thick, and to then fit the taper properly and adjust all other end measurements to suit the taper after it is fitted.

Before any one part of a piece of work turned between the lathe centres is finished to diameter, all the parts to be turned should be roughed out, and for the following reasons, which apply with additional force to work chucked instead of being turned between the lathe centres.

It is found, that all iron work changes its form if the surface metal be removed from it. Thus, though the lathe centres be true, and a piece of work be turned for half its length in the lathe, after it has been turned end for end in the lathe to turn the other half of its length, the part already turned will run out of true after the second half is turned up. This occurs from the tension and unequal internal strains which exist in the metal from its being forged or rolled at a constantly diminis.h.i.+ng temperature, and from the fact that the surface of the metal receives the greatest amount of compression during the forging.

In castings it is caused by the unequal and internal strains set up by the unequal cooling of the casting in the mould, because of one part being thicker than another.

When the whole of the work surfaces have been cut down to nearly the finished size, this alteration will have taken place, and the finis.h.i.+ng may be proceeded with, leaving the work as true as possible. In chucked work, or the most of it rather, it is impracticable (from being too troublesome) to rough out all over before finis.h.i.+ng; hence at each chucking all the work to be done at that chucking is finished.

The roughing cuts on a piece of work should always be taken with as coa.r.s.e a feed as possible, because the object is to remove the ma.s.s of the metal to be cut away rather than to produce a finish, and this may be most quickly done by a deep cut and coa.r.s.e feed. Theoretically also the finis.h.i.+ng should be done with a coa.r.s.e feed, since the coa.r.s.er the feed, the less the length of time the cutting edge is in action. But the length of cutting edge in action, with a given tool and under a given depth of cut, increases as that edge is made longer to carry the coa.r.s.e feed, and the long cutting edge produces a strain that tends to spring or bend the work, and that causes the tool to dip into seams or soft spots, or into spongy or other places, where the cutting strain is reduced, and also to spring away from hard spots or seams, where the cutting strain is increased. The most desirable rate of feed, therefore, is that which is as coa.r.s.e as can be used without springing either the work or the tool, and this will depend upon the rigidity of the work of the lathe, and of the cutting tool. Short or slight work may be turned very true by a light cut fine feed and quick cutting speed, but the speed must obviously be slower in proportion as the length of the work increases, because the finis.h.i.+ng cut should be taken without taking the tool out to resharpen it, since it is very difficult to set the tool to the exact proper depth a second time.

Since the cutting edge will, at any given rate of cutting speed, retain its keenness better for a given surface of work in proportion as the time it is under duty is diminished, it follows, therefore, that the coa.r.s.er the feed the better (so long as both the work and the tool are sufficiently rigid to withstand the rate of feed without springing).

Under conditions of rigidity that are sufficiently favorable a tool, such as in Fig. 948, may be used on wrought or cast iron, at a feed of 1/2 or even 3/4 inch of traverse per lathe revolution, producing true and smooth work, providing that the tool be given a very slight degree of clearance, that its cutting edge is ground quite straight, that it is set parallel to the line of feed, or what is the same thing, to the work axis, and that the length of cutting edge is greater than the amount of tool traverse per lathe revolution, as is shown in the figure, the amount of tool traverse per lathe revolution obviously being from A to B. It may also be observed that the leading corner of the tool may with advantage be very slightly rounded as shown, so that there shall be no pointed corner to dull rapidly.

In proportion as the work is light and the pressure of the cut may spring it, the feed must be lessened, so that on very slender work a feed of 100 lathe revolutions per inch of tool travel may be used. On cast-iron work the feeds may be coa.r.s.er than for wrought-iron, the other conditions being equal, because cast iron cuts easier and therefore springs the work less for a given depth of cut. But since cast iron is apt to break out, exposing the pores of the metal, and thus leaving small holes plainly visible on the work surface, the finis.h.i.+ng cut should be of very small depth, indeed a mere sc.r.a.pe; and if the surface is to be polished, a fine feed and a quick speed will leave a cleaner cut surface, and one that will require the least polis.h.i.+ng operations to produce a clean and spotless surface. Bra.s.s work also is best finished with a fine feed and a quick speed.

It is obvious that the top face of the tool should be given more rake for wrought iron than for cast iron or steel, and that in the case of the very fine feeds, the form of tool shown in figure is the best for finis.h.i.+ng these metals.

In turning a number of pieces requiring to be of the same diameter, it is to be borne in mind that a great part of the time is consumed in accurately setting the tool for the finis.h.i.+ng cut, and that if one piece is finished at a time, this operation will require to be done separately for each piece.

It is more expeditious, therefore, to rough all the pieces out, leaving enough metal for a fine finis.h.i.+ng cut to be taken, and then finish these pieces without moving the tool; which may be done, after the tool is once set, by letting the tool stand still at the end of the first finis.h.i.+ng cut, and taking the work out of the lathe. The carriage is then traversed back to the dead centre, and another piece of work is put in, and it is obvious that as the cross-feed screw is not operated after the tool is once set, the work will all be turned to the same diameter without any further measuring than that necessary for the first piece.

If the tool is traversed back to the dead centre before the lathe is stopped or before the work is removed from the lathe, one of two results is liable to follow. If the lathe is left running, the tool will probably cut a spiral groove on the work, during its back traverse; or if the lathe be stopped, the tool point will mark a line along the work, and the contact of the tool point with the work will dull the cutting edge of the tool. The reason of this is as follows: When the slide rest and carriage are traversing in one direction, the resistance between the tool and the cut causes all the play in the carriage and rest, and all the spring or deflection of those parts, to be in an opposite direction.

Now if the play and spring were precisely equal for both directions, the tool should cut to an equal diameter with the carriage traversed in either direction, but the carriage in feeding is fed by the feed nut or friction feed device, while when being traversed back the traversing handle is used; thus the power is applied to the carriage in the two cases at two different points, hence the spring of the parts, whether from lost motion, or play from wear, or from deflection, is variable.

Again, even with the tool fed both to its cut and on the back traverse with the hand feed handle, the play is, from the altered direction of resistance of the cut, reversed in direction, and the depth of cut is therefore altered.

Thus, in Fig. 1208, let S S represent the cross slide on the carriage and R R the cross slide of the tool rest shown in section, and suppose the tool to be traversing towards the live centre, then to whatever amount there may be play or spring between the slide and the slide way, the slide will from the pressure of the cut twist over, bearing against the slide way at A and B, and being clear of it at G and H. On reversing the direction of traverse of the rest, so as to feed the tool towards the dead centre, the exactly opposite condition will set in, that is, the pressure of the cut will force the slides in the opposite direction, or in other words, the contact will be as in Fig. 1209, at C, D, and the play at E, F. During the change of location of bearing between the slides and the way, there will have been a certain amount of tool motion altering the distance of the tool point from the line of centres, and therefore the diameter to which it will cut. The angle at which the body of the tool stands will influence the effect: thus, if when traversing towards the live centre the tool stands at an angle pointing towards the live centre, it would recede and cause the tool to clear the cut, if removed on the back traverse without being moved to or from the line of centres. Conversely, if the body of the tool was at an angle, so that it pointed towards the dead centre, and a cut was taken towards the live centre, and the tool was traversed back without being moved in or out, it would take another cut while being moved back.

The conditions, however, are so uncertain, that it is always advisable to be on the safe side, and either wind the tool out from its cut before winding the rest and carriage back (thus destroying its set for diameter), or else to stop the lathe and remove the work before traversing the carriage back as already directed. If the latter plan is followed the trouble of setting the tool is avoided and much time is saved, while greater accuracy of work diameter is obtained. It is obvious that this plan may be adopted for roughing cuts in cases where two cuts only are to be taken, so as to leave finis.h.i.+ng cuts of equal depths; or if three cuts are to be taken, it may advantageously be followed for the second and last cuts, the depth of the first cut being of less importance in this case.

The following rules apply to all tools and metals:

[Ill.u.s.tration: Fig. 1208.]