Modern Machine-Shop Practice Part 79

If the live centre does not run true the following difficulties are met with.

[Ill.u.s.tration: Fig. 1156.]

If one end only of the work requires to be turned and it can be completely finished without moving the work driver, the work will be true (a.s.suming the live spindle to run true in its bearings and to fit the same). It will also run true if the work be taken from the lathe and replaced without moving the driver or carrier, providing that the driver be so placed as to receive the driving pressure at the same end as it did when the work was driven; and it is therefore desirable, on this account alone, to always so place the work in the lathe that the driver is driven by its tail end, and not from the screw or screw head. But if the work be turned end for end it will not run true, because the work centre at the unturned end of the work will not be true or central to the turned part of the work.

It is obvious then that lathe centres should run true. But this will not be the case unless the holes into which they fit in the lathe are axially true one with the other and with the lathe spindles. If these holes are true, and the centres are turned true and properly cleaned before insertion, the centres may be put into their places without any adjustment of position. Otherwise, however, a centre punch mark is made on the radial or end face of the live spindle, and another is made on the live centre, so that both for turning up and for subsequent use the centre will run true when these centre punch marks are exactly opposite to each other.

The best way to true lathe centres is with an emery-wheel. In some lathes there are special fixtures for emery grinding, while in others an attachment to go in the tool post is used. Fig. 1156 shows such an attachment.

In the figure A is a frame to be fastened in the slide rest tool post at the stem A'. It affords journal bearing to the hand wheel B, to the shaft of which is attached the gear-wheel C, which drives a pinion D, on a shaft carrying the emery-wheel E, the operation being obviously to rotate wheel B, and drive the emery-wheel E, through the medium of the multiplying gear-wheels C, D.

The emery-wheel is fed to its depth of cut on the lathe centre P, by the cross feed screw of the lathe, and is traversed by pulling or pus.h.i.+ng the k.n.o.b F, the construction of this part of the device being as follows: G and H are two bushes, a sliding fit in the arms of frame A, but having on top flat places I and J, against which touch the ends of the two set-screws _k_, _l_, to prevent them from rotating. The emery-wheel and gear pinion D are fast together, and a pin pa.s.ses through and holds G and H together. Hence the k.n.o.b F pushes or pulls, as the case may be, the bushes through the bearings G, H, in the frame A, the pinion and emery-wheel traversing with them. Hence pinion D is traversed to and fro by hand, and it is to admit of this traverse that it requires its great length. The stem A is at such an angle that, if it be placed true with the line of cross feed, the lathe centre will be ground to the proper angle.

Fig. 1157 represents a centre grinding attachment by Trump Brothers, of Wilmington, Delaware. In this device the emery-wheel is driven by belt power as follows. A driving wheel A is bolted to the lathe face plate, and a stand carries at its top the over-head belt pulleys, and at its base the emery-wheel and spindle. This stand at C sets over the tool post, and is secured by a bar pa.s.sing through C and through the tool post, whose set-screw therefore holds the stand in position. On the end of the emery-wheel spindle is a feed lever, by means of which the emery-wheel may be fed along the lathe centre. Cup piece B is for enabling wheel A to be readily set true on the lathe face plate, one end of B fitting the hub of A, while the other receives the dead centre which is screwed up so that B will hold A in place, while it is bolted to the lathe face plate, and at the same time will hold it true.

In the absence of a centre grinding attachment, lathe centres may be turned true with a cutting tool, and finished with water applied to the tool so as to leave a bright and true surface. They should not, for the finest of work, be finished by filing, even though the file be a dead smooth one, because the file marks cause undue wear both to the lathe centres and the work centres.

The dead centres should be hardened to a straw color, and the live centre to a blue; the former so as to have sufficient strength to resist the strain, and enough hardness to resist abrasion, and the latter to enable it to be trued up without softening it.

[Ill.u.s.tration: Fig. 1157.]

When, after turning them up, the centres are put into their places, the tailstock may be moved up the bed so that the dead centre projects but very little from the tailstock, and is yet close to the live centre, and the lathe should be run at its fastest speed to enable the eye to perceive if the live centre runs true, and whether the dead centre is in line with the live one, and the process repeated so that both centres may be tested.

A more correct test, however, may be made with the centre indicator.

[Ill.u.s.tration: Fig. 1158.]

CENTRE INDICATORS.--On account of the difficulty of ascertaining when a centre runs quite true, or when a very small hole or fine cone as a centre punch mark runs true when chucked in a lathe, the centre indicator is used to make such tests, its object being to magnify any error, and locate its direction. Fig. 1158, from _The American Machinist_, represents a simple form of this tool, designed by Mr. G. B.

Foote, for testing lathe centres. A is a piece of iron about 8 inches long to fit the lathe tool post, B is a leather disk secured to A by a plate C, and serving to act as a holding fulcrum to the indicator needle, which has freedom of movement on account of the elasticity of the leather washer, and on account of the hole shown to pa.s.s through A.

It is obvious that if the countersunk end of the needle does not run true, the pointed end will magnify the error by as many times as the distance from the needle point to the leather washer is greater than that from the leather washer to the countersunk end of the needle. It is necessary to make several tests with the indicator, rotating the lathe centre a quarter turn in its socket for each test, so as to prove that the centre runs true in any position in the lathe spindle. If it does not run true the error should be corrected, or the centre and the lathe spindle end may be marked by a centre punch done to show in what position the centre must stand to run true.

[Ill.u.s.tration: Fig. 1159.]

The tension of the leather washer serves to keep the countersunk against the lathe centre without a very minute end adjustment. Or the same end may be attained by the means shown in Fig. 1159, which is a design communicated by Mr. C. E. Simonds to _The American Machinist_. The holder is cupped on one side to receive a ball as shown, and has a countersink on the other to permit a free vibration of the needle. The ball is fitted to slide easily upon the needle, and between the ball and a fixed collar is a spiral spring that keeps the ball in contact with its seat in the holder.

[Ill.u.s.tration: Fig. 1160.]

One end of the needle is pointed for very small holes or conical recesses, while the other is countersunk for pointed work, as lathe centres. The countersink of the needle may be made less acute than the lathe centre, so that the contact will be at the very point of the lathe centre, the needle not being centre-drilled. The end of the needle that is placed against the work should be as near to the ball or fulcrum as convenient, so as to multiply the errors of work truth as much as possible.

In some forms of centre indicators the ball is pivoted, so that the needle only needs to be removed to reverse it end for end, or for adjusting its distance, it being made a close sliding fit through the ball. Thus, in Fig. 1160 the ball E is held in a bearing cut half in the holder A, and half in cap B, which is screwed to A by screws C D.

Or the ball may be held in a universal joint, and thus work more frictionless. Thus, in Fig. 1161 it is held by the conical points of two screws diametrically opposite in a ring which is held by the conical points of two screws threading through an outer ring, these latter screws being at a right angle to those in the inner ring. The outer ring is held to the holder by the conical points of two screws, all the conical points seating in conical recesses.

[Ill.u.s.tration: Fig. 1161.]

It is obvious that the contact of the point of the needle and the work may be more delicately made when there is some elasticity provided, as is the case with the spiral spring in Fig. 1159.

Indicators of this cla.s.s may be used to test the truth of cylindrical work: thus, in Fig. 1162 is an application to a piece of work between the lathe centres, there being fitted to one end of the needle a fork _a_ that may be removed at pleasure.

[Ill.u.s.tration: Fig. 1162.]

One of the difficulties in turning up a lathe centre to run true arises from the difference in cutting speed at the point and at the full diameter of the cone, the speed necessary to produce true smooth work at the point being too fast for the full diameter. This may be remedied on centres for small work, as, say, three inches and less in diameter, by cutting away the back part of the cone, leaving but a short part to be turned up to true the centre.

To permit the cutting off or squaring tool to pa.s.s close up to the centre, and thus prevent leaving a burr or projection on the work end, the centre may be thus relieved at the back and have a small parallel relief, as in Fig. 1164 at A, the coned point being left as large as possible, but still small enough to pa.s.s within the countersink.

In centres for large and heavy work it is not unusual to provide some kind of an oil way to afford means of lubrication, and an excellent method of accomplis.h.i.+ng this object is to drill a hole A, Fig. 1163, to the axis of the centre and let it pa.s.s thence to the point as denoted by the dotted line; there may also be a small groove at B in the figure to distribute the oil along the centre, but grooves of this kind make the returning of the centre more difficult and are apt to cause the work centres to enlarge more from wear, especially in turning tapers with the tailstock set over the lathe centre, these being out of line with the work centre.

To enable a broad tool such as a chaser to meet work of smaller diameter than the lathe centre, the latter is cut away on one side as in Fig.

1164. It is obvious also that the flat place being turned uppermost, will facilitate the use of the file on work of smaller diameter than the lathe centre, and that placed in the position shown in the cut, it will permit a squaring tool to pa.s.s clear down to the centre and avoid leaving the projecting burr which is left when the tool cannot pa.s.s clear down the face to the edge of the countersink of the work centre.

[Ill.u.s.tration: Fig. 1163.]

[Ill.u.s.tration: Fig. 1164.]

The method to be employed for centring work depends upon its diameter, and upon whether its ends are square or not. When the pieces are cut from a rod or bar in a cutting-off machine, the ends are square, and they may be utilized to set the work by in centring it. Thus, in Fig.

1165 is a top, and in Fig. 1166 is an end view of a simple device, or lathe attachment for centre drilling. S is a stand bolted to the lathe shears and carrying two pins P, which act as guides to the cup chuck or work guide G; between the heads of pins P and the hubs of G are spiral springs, forcing it forward, but permitting it to advance over the drill chuck; the work W is fed forward to the drill. At the dead centre end the work is supported by a female cone centre D in the tail spindle T.

The work rests in mouths of G and D, and as the pieces are cut from the rod they are sufficiently straight, and being cut off in a cutting-off machine the ends are presumably square; hence the coned chucks will hold them sufficiently true with the ends, and the alignment of the centre drilled holes will not be impaired by any subsequent straightening processes; for it is to be observed, that if work is centre-drilled and straightened afterwards, the straightening throws the centre holes out of line one with the other, and the work will be more liable to gradually run out of true as its centres wear.

[Ill.u.s.tration: Fig. 1165.]

Thus, in Fig. 1167, let W represent a bent piece of work centre-drilled, and the axis of the holes will be in line as denoted by the dotted line, but after the piece is straightened the holes will lie in the planes denoted by the dotted line in Fig. 1168, and there will be a tendency for the work centres to move over towards the sides C D as the wear proceeds.

[Ill.u.s.tration: Fig. 1166.]

In Fig. 1169 is shown a centre-drilling machine, which consists of a live spindle carrying the centre-drilling tool, and capable of end motion for the drill feed. The work is held in a universal chuck, and if long is supported by a stay as shown in the figure. The axis of the work being in line with that of the chuck, the work requires no setting.

[Ill.u.s.tration: Fig. 1167.]

In this case the centre hole will be drilled true with that part of the work that is held in the chuck, and the alignment of the centre hole will depend upon the length of the rod being supported with its axis in line with the live spindle. If the work is not straightened after drilling, the results produced are sufficiently correct for the requirements; but it follows from what has been said, that work which requires to be straightened and tried for straightness in the lathe should be centred temporarily and not centre-drilled until after the straightening has been done.

[Ill.u.s.tration: Fig. 1168.]

In Fig. 1170 is shown a combined centre-drill and countersink not unfrequently used in centring machines. The objection to it is, that the cutting edges of the drill get dull quicker than those of the countersink, and in regrinding them the drill gets shorter. Of course the drill may be made longer than necessary so as to admit of successive grindings, but this entails drilling the centre holes deeper than necessary, until such time as the drill has worn to its proper length.

To overcome this difficulty the countersink may be pierced to receive a drill as in Fig. 1171, the drill being secured by a set-screw S.

[Ill.u.s.tration: Fig. 1169.]

Among the devices for centring work by hand, or of p.r.i.c.king the centre preparatory for centre-drilling, are the following:--

[Ill.u.s.tration: Fig. 1170.]

[Ill.u.s.tration: Fig. 1171.]

In Fig. 1172 is a centre-marking square. A B C D represents the back and E the blade of the square. Suppose then that the dotted circle F represents the end of a piece of work, and we apply the square as shown in the cut and mark a line on the end of the work, and then moving the square a quarter turn around the work, draw another line, the point of contact of these two lines (as at G in the cut) will be the centre of the work, or if the work is of large diameter as denoted by the circle H H, by a similar process we obtain the centre E. In this case, however, the ends A B of the square back must be of equal lengths, so that the end faces at A B will form a right angle to the edge of the blade, and this enables the use of the square for ordinary purposes as well as for marking centres.

[Ill.u.s.tration: Fig. 1172.]

The point _a_ of the centre punch shown in Fig. 1173 is then placed at the intersection of the two lines thus marked, and a hammer blow produces the required indentation. The centre punch must be held upright or it will move laterally while entering the metal. The part _b_ of the centre punch is tapered so as to obstruct the vision as little as possible, while it is made hexagon or octagon at the upper end to afford a better grip. By increasing the diameter at C, the tool is stiffened and is much less liable to fly out of the fingers when the hammer blow does not fall quite fair.