The operations just given are suitable for the correct formation of any angular hole of a rod's boss-end, or other boss-part, if both entrances to the hole are to be of the same dimensions; or, in other words, if the hole is to be parallel. But it is proper to make all holes in such portions rather taper, for the convenience of an easy fitting of the bearer-brasses, and also that the brasses may be easily entered into and removed from their respective holes.

A small angular hole of only about an inch in diameter can be tapered with filing, after it has been first regularly formed parallel with slotting. But to taper a large hole of several inches in width and length, a proper adjustment for the purpose must be performed while the object is on the slotting-table. By this means, all, or nearly all, subsequent filing is avoided. To correctly place a boss to be tapered, it must be raised at one edge, so that the truly formed boss-faces are prevented from occupying a position of parallelism with the table-face. To support the boss in a pr6per inclined position with the table, and therefore also in a proper inclined position to the vertical motion of the slotting-tool, suitable packing is placed between the lower boss-face and the table. This packing consists of either a taper ring, or taper packing blocks. Such pieces must be permanently fixed during all the operation of tapering the entire hole ; and will incline one edge of the upper face of the boss towards the tool during the shaping of any one of the planes belonging to the hole. The thickest part of the ring or other packing, is situate between the machine-front and the front side of the boss; therefore the upper end of the hole is inclined towards the tool and must be enlarged by the process of cutting. Each time the tapering is to be commenced the boss requires to be partly rotated on the packing, in order to place every plane into one and the same relative position with the table; therefore, by the time the boss has been entirely rotated, and all the planes produced, the upper end of the hole is enlarged to a greater diameter than that of the lower end, and the intended tapering is executed.

When only two opposite sides of an angular hole belonging to a boss require taperingi it can be conveniently done by means of the author's slottil or slotted holdfast represented by Fig. 558. This instrument can be bolted to the slotting-table at some convenient part, and the vee-grips can be made to grip some portion of the rod and hold the boss at any desired angle with the table, after being properly adjusted.

The final shaping of angular holes is conducted with regard to sheet gauges, which are provided with taper stems, if the holes in progress are to be tapered. Inside-callipers also are used; and when a hole is being slotted to fit a hard steel block, or other object that cannot be easily reduced to fit the hole, the callipers must be adjusted with a gentle hammering so that the points of the feet shall only very lightly touch the surfaces in contact, as described in page 208.