Modern Machine-Shop Practice Part 80

[Ill.u.s.tration: Fig. 1173.]

In Fig. 1174 is shown a device for guiding the centre punch true with the axis of the work, so as to avoid the necessity of finding the same by lines for the centres. It consists of a guide piece B and a parallel cylindrical centre punch A, C representing a piece of work. B is pierced above with a parallel hole fitting and guiding the centre punch, and has a conical hole at the lower end to rest on the work, so that if the device be held upright and pressed down upon the end of the work, and the top of the centre punch is struck with the hammer, the indentation made will be central to the points of contact of the end of the work with the coned hole of B. If then the end of the work has no projecting burrs the centring will be centred true.

[Ill.u.s.tration: Fig. 1174.]

[Ill.u.s.tration: Fig. 1175.]

In the absence of these devices, lines denoting the location for the conical recess or centre may be made, when either of the following methods may be pursued.

[Ill.u.s.tration: Fig. 1176.]

[Ill.u.s.tration: Fig. 1177.]

[Ill.u.s.tration: Fig. 1178.]

In Fig. 1175 is shown what is known as a pair of hermaphrodite calipers, which consists of two legs pivoted at the upper end; the bent leg is placed against the perimeter of the work, as shown, and held steadily, while with the point a line is marked on the work. This operation is performed from four equidistant (or thereabouts) points on the work, which will appear as shown in Fig. 1176, providing the radius to which the point was set be equal to the radius of the work. The point at which the lines meet is in this case the location for the centre. If, however, the radius to which the points are set is less than the radius of the work, the lines will appear as in Fig. 1177, in which case the location is in the centre of the inscribed square, as denoted by the dot; or if the radius be set too great the lines will appear as in Fig. 1178, and the location for the centre will again be as denoted by the dot.

[Ill.u.s.tration: Fig. 1179.]

Another and very old method of marking these lines is to place the work on a pair of parallel pieces and draw the lines across it, as shown in Fig. 1179, in which W represents the work, P, P the parallel pieces of equal thickness, S a stand (termed a scribing block) carrying a needle N, which is held by a thumb screw and bolt at B. The point of the needle is adjusted for the centre of the work, a line is drawn, the work is then rotated, another line drawn, and so on, until the four lines are drawn as in Fig. 1180, when the work may be turned end for end if light, or if heavy the scribing block may be moved to the other end of the work.

[Ill.u.s.tration: Fig. 1180.]

The centre locations are here made true with the part of the work that rests on the parallel pieces, and this is in some cases an essential element in the centring.

Thus, in Fig. 1181, it is required to centre a piece true with the journals A B, and it is obvious that those journals may be rested on parallel pieces P, P, and the centres marked by the scribing block on the faces E, F in the manner before described.

[Ill.u.s.tration: Fig. 1181.]

If there is a spot in the length of a long piece of work where the metal is scant and out of round, so that it is necessary to centre the work true by that part, the surface gauge and parallel pieces may be used with advantage, but for ordinary centring it is a slow process. When a piece of work is not cylindrical, and it is doubtful if it will clean up, the centring requires care, for it must not always be a.s.sumed, that if two diametrically opposite points meet the turning tool at an equal depth of cut, the piece is centred so as to true up to the largest possible diameter.

[Ill.u.s.tration: Fig. 1182.]

This is pointed out in Fig. 1182, which is extracted from an article by Professor Sweet. "In a piece of the irregular form A, the points _a_ and _b_ might be even and still be no indication of the best location for the centre, and in the piece B it is evident that if _c_ and _d_ were even, nothing like the largest cylinder could be got from it. In the case of shape A, the two points _e_ and _f_ should be equidistant from the centre, and in the case of shape B, the three points _g_, _h_, _i_ should be equidistant from the centre."

The depth of the centre drill holes should be such as to leave them in the work after it is cut off to its proper length, and will, therefore, be deeper as the amount to be cut off is greater.

The diameter of the centre drill is larger as the size of the work increases, and may be stated as about 3/64 for work of about 1/2 inch, increasing up to 1/8 inch for work of about an inch, and up to three inches in diameter; for work of a foot or over the centre drill may be 3/16 inch in diameter.

[Ill.u.s.tration: Fig. 1183.]

The centre drilling and countersinking may, when the work is cut to length, be performed at one operation, but when it requires to be cut to length in the lathe, that should be done before the countersinking. A very simple chuck for centre drilling is shown in Fig. 1183, with a twist drill (which is an excellent tool for centre-drilling). If the work is held in the hand and fed to the drill by the lathe dead centre, the weight of the work will cause the hole to be out of straight with the work axis, unless the grip is occasionally relaxed, and the work made to rotate a half or a quarter turn as the drilling proceeds.

After the work is centre-drilled and cut off to length, it must be finally countersunk, so as to provide ample bearing area for the lathe centres.

[Ill.u.s.tration: Fig. 1184.]

The countersinking should be true to the centre hole; and it is sometimes made to exactly fit the lathe centres, and in other cases it is made more acute than the lathe centre, so that the oil may pa.s.s up the countersink, while it is bedding itself to the lathe centres.

If the countersinking is done before the end of the work is squared, it will not be true with the centre-drilled hole.

In order that the countersinking may wear true with the centre-drilled hole, it may be made of a more obtuse angle (as, say, one degree) than the lathe centre, as in Fig. 1184, so that the hole may form a guide to cause the lathe centre to wear the countersinking true to the hole, and thus correct any error that may exist.

[Ill.u.s.tration: Fig. 1185.]

If the countersink is made more acute than the lathe centre, as shown in Fig. 1185, the wear of its mouth will act as a guide, causing the centre to be true with the countersinking; and when the bearing area extends to the centre-drilled hole, there will be introduced, if that hole does not run true, an element tending to cause the work to run out of true again, because the countersinking will have more bearing area on one side than on the other.

It is to be observed, however, that if the difference between the countersink angle and that of the lathe centre be not more than about one degree, the work centre will bed itself fully to the lathe centre very rapidly, and usually before the first cut is carried over the work, unless the work centres have been made to have unduly large countersinks.

[Ill.u.s.tration: Fig. 1186.]

Fig. 1186 represents a half-round countersink, in which the cutting edge is produced by cutting away the coned point slightly below the dotted axial line. This secures two advantages: first, it gives the cutting edge clearance without requiring the grinding or filing such clearance; and, secondly, the cone being the same angle as the lathe centres, filing away more than half of it causes it to give the lathe centre at first a bearing at the small end of the countersink, as in Fig. 1184, and this secures the advantage mentioned with reference to that figure.

It is obvious that such a reamer, however, does not produce strictly a cone countersink, as is shown in Fig. 1187, where the cutting away of the cone is carried to excess simply to explain the principle, and the cone becomes an hyperbolic curve.

The small amount, however, that it is necessary to carry the face below the line of centres, practically serves to make the cone somewhat less acute, and is not therefore undesirable.

[Ill.u.s.tration: Fig. 1187.]

Another method of forming the half-round countersink is shown in Fig.

1188, in which the cone is of the same angle as the lathe centres; the back A is ground away to avoid its contact with the work and give clearance, while clearance to the cutting edge is obtained by filing or grinding a flat surface B at the necessary angle to the upper face of the cone. In this case it is a.s.sumed that the centre-drilling and countersinking are true one with the other. Yet another form of countersink is shown in Fig. 1189, consisting of a cone having three or four teeth. It may be provided with a t.i.t, which will serve as a guide to keep the countersink true with the hole, and this t.i.t may be made a trifle larger in diameter than the hole, and given teeth like a reamer, so as to ream the hole out while the countersinking is proceeding.

[Ill.u.s.tration: Fig. 1188.]

[Ill.u.s.tration: Fig. 1189.]

Unless one side of a half-round reamer is filed away so as to give the cutting edge alone contact with the bore of the hole, an improper strain is produced both upon the work and the countersink.

In Fig. 1190, for example, is shown, enlarged for clearness of ill.u.s.tration, a hole, and a half-round countersink in section, and it is evident that if the countersink is set central to the hole, it will have contact at A and at B, and A cannot enter the metal to cut without springing towards C.

[Ill.u.s.tration: Fig. 1190.]

[Ill.u.s.tration: Fig. 1191.]

But when the lathe has made rather more than one-half a revolution, the forcible contact at B will be relieved, and either the work or the countersink will move back towards D. This may be remedied by setting the countersink to one side, as in Fig. 1191, or by cutting it away on one side, as in Fig. 1192, when the half-round reamer will, if the work be rigidly held while being countersunk, act as a cutting tool. But it is more troublesome to hold the work rigidly while countersinking it than it is to simply hold it in the hands, and for these reasons the square centre is an excellent tool to produce true countersinking.

[Ill.u.s.tration: Fig. 1192.]

[Ill.u.s.tration: Fig. 1193.]

[Ill.u.s.tration: Fig. 1194.]

Fig. 1193 represents a square centre, the conical end being provided with four flat sides, two of which appear at A B, or it may have three flat sides which will give it keener cutting edges, and will serve equally well to keep it true with the drilled hole. But it is questionable whether it is not an advantage not to have the cutting edges so keen as is given by the three flat faces, because the less keen the cutting edges are, the more true the countersinking will be with the hole, the extra pressure required to feed the square centre tending to cause it to remain true with the hole notwithstanding any unequal density of the metal on different sides of the hole. An objection to the square centre is that it involves more labor in the grinding to resharpen it, and is not so easy to grind true, but for fine work this is more than compensated for in the better quality of its work.

[Ill.u.s.tration: Fig. 1195.]

[Ill.u.s.tration: Fig. 1196.]

This labor, however, may be lessened in two ways: first, the faces may be fluted, as in Fig. 1194, at A and at B, or its diameter may be turned down, as in Fig. 1195. In using the square centre it is placed in the position of the live centre and revolved at high speed, all the cutting edges operating simultaneously; the work is fed up by the dead centre and held in the hand.