Holes that are partly formed by forging, are entirely machined by slotting, whether little or much metal is to be removed, because such holes do not furnish any metal for bearings on which a drill-point could rotate. To properly place a boss-end upon a slotting-table, the same means are adopted as for drilling, one boss-face being put upon a parallel ring or packing-pieces to maintain the boss at a suitable height for the clearance of the tool. The downward vertical motion of a slotting-tool is analogous to the vertical motion of a drill; therefore the parallel blocks will cause the hole to be slotted in the desired right-angular position.

A rod or bar which is to have its boss slotted for an angular hole, may be so situated on the table that the rod's length is parallel with the slide-rest traverse which is parallel with the machine-front; by this traverse, the rod will in due course be moved in the direction of its length; and such movement will shape a plane of the hole which is parallel with the rod's length. It will be perceived that the boss should be situated at the middle of the table, because a gradual rotation of the table is necessary. It is presumed that a boss is to have an octangular hole, requiring eight planes to be produced as boundaries of the hole when finished. Such a hole can be shaped by a gradual rotation of the boss, the movement being much like the rotation of a lever-boss which is having its cylindrical outside shaped. But instead of rotating it during the entire process of cutting, it is only shifted each time one of the eight planes is to be commenced, the table and piece at this time being moved an eighth part of a rotation. This gradual movement will cause the hole to have the shape of a regular octagon, and will cause each plane to be of the same width as any other belonging to the hole. If the boss were rotated a sixth part of a rotation each time, the entrances of the hole when complete would have a hexagonal form; or, if moved a third part, the entrances would be triangular. It may therefore be seen that if the boss of a rod is in the middle of the table-face and concentric with the axis of rotation, a regularly formed hole having either three, six, eight, or any desired number of planes, may be accurately formed, and all the planes of the holes will be of the same width.

But angular holes which are of regular hexagonal or octagonal forms, are seldom required for boss-portions of rods and bars; nearly all are oblong, the greatest length of the hole being in the length of the rod or bar to which the hole belongs. Consequently, a regular gradual rotation of the table and boss at only one adjustment of the slide-rest, will not produce the shape desired; and, in addition to fixing the boss in the middle of the table-face, it needs an additional adjustment by the traverse-screws, every time the table is moved the sixth, eighth, or other portion of its rotation. For these adjustments the tool-scriber can be used, the point of which will indicate the exact situation of the object beneath by observing its gauge-line ; and after the boss has been shifted by partly rotating it, in order to commence a plane, the traverse screws of the rest are caused to slowly adjust the boss until the gauge-line is seen to be in the proper place. In this condition the table is now fixed, to prevent further rotation till the plane is produced, and another one to be commenced. Every adjustment of the article for commencing a plane, causes the hidden plane to be placed parallel with the machine-front, and therefore parallel with the traverse which moves parallel to the front, as directed; and to allow room for the backward retreating motion of the tool from the metal during the upward-travel, the plane surface being formed is always between the tool and the machine-front, and not between the tool and the main-standard. Consequently, shifting the boss by rotating the table an eighth of a rotation, puts each one of the planes successively into the same condition of parallelism with the machine-front, and also into very nearly the same place beneath the tool, the small additional adjustment that was said to be needful, being performed with the traverse.

The slotting-tools suitable for planing the boundaries of an angular hole, are corner tools, vee-point tools, groovers, and mortisers. The tool first used is either a mortiser similar to Fig. 791, or a groover with a curved cutting edge resembling Fig. 782. It is specially necessary to first employ a groover where a comparative large quantity of metal is to be cut out. The groover is made to enter the metal at each corner of the hole, which is the junction of each two contiguous planes. At every corner the tool is caused to form a groove which shall extend into the metal as far as the gauge-lines that exist at that corner; so that if three-eighths of metal is to be removed from that corner, the groove will be three-eighths deep. If the edge of the tool is curved the corner will be curved, and this shape is preferable to a sharp angular form, to avoid weakening the boss or whatever article may be in progress. Eight grooves are therefore made for an octangular hole, each one requiring the table to be partly rotated and adjusted. By this grooving, if cai'efully done, all the eight junctions of the planes belonging to an octangular hole, may be finished, because the metal can be removed as far as the gauge-lines which show the specified dimensions. This treatment also forms eight superfluous projections, one on each of the eight hidden planes to be produced. To remove these portions, an ordinary vec-point tool, similar to Fig. 787 or 794, can be used. Such a tool will operate effectually after grooving, because it is not required to cut at any junction, the vee-point being suited for traversing flat surfaces; whereas, if it were used to commence these surfaces previous to grooving the corners, the sides belonging to the thick part of the tool-point would greatly hinder the cutting, through coming into contact with the metal at the corners. A vee-tool may, however, be used for commencing, when only a very small quantity of metal is to be cut out; in which case, the vee-tool is caused to first remove the metal from the plane, without removing any from the corners ; these are left untouched, and after the plane is finished, the small amount of metal at the junctions is removed with a corner tool, or with a narrow groover (Fig. 782).