The cylindrical hole of a lever-boss, or other boss, can be accurately formed in cases of necessity by means of proper lining, chiselling, and filing. The lines for such a purpose should resemble those shown in Fig. 801. Such lines are delineated upon two opposite sides of the lever, and are marked while it rests on a lining-table, the two faces of each boss having been previously made tolerably flat and parallel to each other. While on the table, the faces of the bosses are situated at right-angles to the surface, and the lever is packed up with plates or wedges until the intended centres of the bosses are placed equidistant from the table. In this condition a scriber-block is employed to mark a periphery entirely around the lever, including its four boss-faces, the periphery being a primary centre-line which distinguishes the lever into upper and lower halves. From this line the intended diameter of the hole to be chipped is scribed upon the boss-faces, and if a taper hole is desired, the two different diameters are indicated by scribing circles of the proper diameters. By thus marking the places of the intended holes the entire quantity of metal to be removed is shown, and the different thicknesses to be removed at different places around the hole are also shown. Two circles for this purpose, marked on two faces of a boss, whether for a parallel hole or a taper one, indicate boundaries of the now hidden curve of the cylindrical hole or taper hole which is to be produced; and if the metal can be removed to form a straight hole which shall be terminated by the two circles scribed on the boss-faces, the entire boundaries of the hole will be in their intended places, and the dimensions of the hole will be those desired, supposing that the circular gauge-lines are situated exactly where they should be.

After lining the boss-faces, the superfluous metal in the hole is to be bevelled to the gauge-lines by filing, or by light chiselling, which prepares the hole for being grooved in the manner shown in the Figure (801). After grooving, the remaining ridges are partly cut off with flat chisels, and completed with chisels whose cutting edges are slightly curved, to suit the concave form of the hole. Chiselling a hole in this manner requires considerable care, especially if it is several inches in length, to prevent the chisel being driven too deep into the metal. To avoid this a straight-edge must be frequently put into contact with the bottom of each groove, previous to chiselling it to the total depth intended.

The hole being carefully chipped is ready for filing with half-round files, to obtain as near an approach as possible to the cylindrical or conical form which is required, the filing being conducted with frequent application of a straight-edge, square, and inside-callipers. These three tools are requisite for a cylindrical hole, to effect three objects; the straight-edge being necessary to make all sides of the hole straight, the square being required for adjusting the hole to a right-angular position with the boss-faces, and the callipers being needful to adjust the hole to a circular form throughout its entire length.

A hole which is conical must be adjusted to shape and dimensions without a square, inside-callipers being used for making the hole circular, and to the exact diameter required, as in the filing of a parallel hole, and an angle-gauge being employed to adjust the sides of the hole to the proper angle with the boss. Such a gauge has an adjustable blade or arm, which is made to subtend the same angle with its pedestal, as the side of the required hole subtends with the boss-face, this face being that adjoining the mouth or large end of the hole, consequently the blade of the gauge is entered at this end whenever it is requisite to ascertain the condition of the work.