A key-slot is a key-way that consists of an oblong hole which is formed entirely through the thickness of a rod, bar, or other article. Consequently, a key-slot requires to be made by processes which are different to those adopted for making key-grooves which extend to only a short distance in the direction of the rod's thickness.

The lining for a key-slot of a piston-rod end, or an end of a crank-pin, is the scriber-marks which are made upon the end while it is in its place in the boss of the lever or crosshead; in which boss the key-slot has been previously made, and constitutes a sort of template for scribino-the piece within. It is therefore requisite for the slot in the boss to be in line with the diameter of the circular hole, in order that the key-way to be made in the end of the rod or pin may be exactly in the centre; or rather, in line with the diameter. It will be seen that the lining of the bosses must be given in conjunction with the lining of the rods, pins, spindles, or other portions.

The key-slot of any boss is not shaped till the boss has been accurately bored and turned in a lathe, and is also, in most cases, the last machine-process to which it is subjected. The lining of such an article can be easily executed, because all its surfaces are smooth and uniform. If the boss of a small lever or other article is to be lined, it can be readily handled in any position on a lining-table; and a boss of a large article can be lined on any machine-table, with but very little moving about, because of the several plane surfaces and parallel surfaces which it possesses.

A small lever can be taken to a lining-table and have its boss lined while the levers length is vertical, extending upwards from the table and at right-angles to it. A lever in this position is shown in Fig. 904. It should be held in position with packing-blocks, or by gently bolting it to an el-chuck on the table. The centre length of the lever is adjusted exactly square to the table by means of an el-square, the blade of which is put to the centre or primary line seen in the Figure, the object being gradually shifted until seen to be in the proper position. The bottom cylindrical surface of the boss may, or may not, be in immediate contact with the table, because it is parallel with the centre length of the hole, whether the hole is taper or parallel. And supposing that the boss should not be exactly parallel with the hole, through a defect in shaping, the length of the lever will be correctly adjusted by being in contact with the el-chuck.

While the article is held upright on the table, the place for the key-way can be marked upon both sides of the boss, by means of a scriber block, the point of which is adjusted to the desired height at the time the block rests on the table. This height is the same as that of the centre of the boss, and is easily shown on an ender that fits the mouth of the hole. The scriber-point is therefore put to this centre, and a centre-line is marked across the boss-face ; the line is also continued along the cylindrical surfaces of the boss at opposite sides of the hole. These marks are exactly right-angular to the lever's length, as usually required, and accurately indicate the centre of the intended key-slot, because the lever is right-angular to the table on which the block moves while marking. From this centre line in two directions, upwards and downwards, the specified thickness of the key-slot can now be marked, by using a compasses or divider; and to these marks the scriber-point is next adjusted to scribe two more lines upon the boss, parallel to the centre one and equidistant from it. These outer lines now show the thickness as intended, and the width is next marked to cause the key-way to be mid-way from either face of the boss; after which it is ready for the cutting out.

Lining a boss of a large lever for a key-way, is performed while it remains in any place, no moving about being necessary. It is, however, advisable to put the article into a proper position ; and this consists in shifting it until the boss-faces are about horizontal. In this condition a straight-edge, scriber, and compasses, can be easily used, to mark the required lines upon the lever, the principal one first marked being a straight centre line across the upper face of that boss which is to have the key-way. This line is exactly analogous to one which is made with a scriber-block on a lining-table, and therefore indicates the centre of the key-way required. For this purpose an ender is caused to show the centre of the hole's mouth, which is, of course, a point in some part of the centre line along the lever's side. The centre of the mouth and boss-face being shown, it is next needful to scribe the line referred to square to the lever's length, and also in the desired place to pass through the centre dot on the ender at the hole's mouth. To do this, short arcs are scribed to intersect each other on the boss-face, a compasses or radius-gauge being used. The centres from which the arcs are scribed may be any points in the centre line along the lever; but the convenient points to be selected are those two which constitute intersections of the straight centre line with a circular one on the boss-face. This circular line is shown in Fig. 905, near the edge of the boss, and is of no special diameter; but merely concentric with the boss-face; therefore, from the points of intersection four arcs are marked, and a straight-edge is next put to the two intersections of the arcs, to scribe the line required. In the Figure (905) this lining is shown, the centres from which the arcs are marked, being denoted by the letters C. Fig. 910 is a larger sketch, in which similar lines and letters are seen.

It is now necessary to mark two straight lines upon the boss, one at each side of it. so that they shall be right-angular to the boss-face, and shall also join the line already scribed across the centre of the face. To do this a bisector, having pins of proper length, may be used; and if such is employed, its pedestal is put upon the face near the edge while its blade extends downwards, as shown in Fig. 906; the implement is now gently moved until one edge of the blade, or the straight line on the pedestal which is continued from the edge of the blade, is seen to exactly coincide with one extremity of the centre line previously marked upon the face, which extremity is shown by E in Fig. 905. While the bisector is in position, it appears as in Fig. 906, and a line is now scribed upon the side of the boss at the edge of the blade, which is one of the two lines required, and will show the centre of one entrance for the intended key-way. The bisector is now removed to the opposite edge of the boss-face, and its blade adjusted as before to the same line, but now at its opposite extremity, which also is shown by E in the Figure. Another line is now scribed, which is the second one, and indicates the centre of the other mouth or entrance for the key-way. In Fig. 907, one of these lines is seen, extending from E to E.