Angular holes are those of which the across sections and entrances possess angular boundaries. The boundary of a hole's entrance may be either rectangular, hexangular, octangular, or of some other angular form, the precise form of the boundary depending on the number and shapes of the hole's sides. If a straight hole in one end of a connecting-rod is hexagonal or hexangular along the entire length of the hole, it is said to be a six-sided hole, and possesses six sides or planes. Previous to a correct filing or other paring process these six planes are hidden by the rough exterior metal; and the methods for producing and exhibiting these planes are among the processes here given.

The portions of engines and machines which require angular holes are the ends of slide-rods, coupling-rods, connecting-bars, connecting-rods, and side-rods. The mid parts of a few classes of slide-rods also require angular holes, and the holes in these pieces are usually of a rectangular section, some being square and others oblong. A middle portion of this class is shown by Fig. 890. All angular holes in boss-portions are required for nearly the same purposes, being employed to contain bearer-brasses or friction-blocks, and prevent their rotation by reason of the angular surfaces of the brasses and blocks being made to fit the angular boundaries of the holes.

The paring processes which are adopted for producing angular holes include drilling, shaping, planing, and slotting. If drilling is employed, it serves as a species of preliminary process for commencing a hole, or originating a hole where none exists ; consequently, drilling is the means of roughly making holes into solid ends of rods and bars, which were forged in this condition instead of having their square or six-sided holes partly shaped by punching and drifting while on the anvil, according to the processes for drifting mentioned in page 80. The shaping, planing, and slotting of angular holes always serve as processes for finishing them after they have been roughly made, either by casting, forging, drilling, or by forging and drilling combined.

The lining which is performed previous to the formation of angular holes, is required to indicate the boundaries of their entrances, and is about the same, and is executed by about the same means, whether the pieces lined have holes or are without them. An end of a rod or bar is prepared for lining by means of planing, or by lathe-turning, according to convenience ; and both sides or surfaces of the part in which the hole is to terminate are planed, and also made parallel to each other. Substitutes for these planing processes may be mentioned, which consist of grinding the two sides with a grindstone, and of filing them while the article is held in a vice. When an end of a connecting-rod or coupling-rod has been flattened by one of these preparatory processes, the piece is in a condition to be easily lined, and also to be accurately fixed after lining, in order to be drilled or slotted.

Angular holes in the boss-ends of rods and bars are slotted after the other portions of the pieces have been smoothly reduced to the finished dimensions and shapes intended, by means of lathe-turning and planing; and the flat surfaces of the boss-portions, which are lined to show the desired shapes of the holes, are produced exactly in their proper places and positions with regard to the other portions of the objects. These flat surfaces of any rod or bar must be accurately formed parallel with the rod or bar's centre length; because, to these, considered as bases or primary planes, the planes which will constitute the boundaries of the hole must be made right-angular. In an end-boss of a connecting-rod the centre line extending along the length of the hole through the boss must be right-angular to the centre length of the entire rod, because it is intended to be connected with its pivot-pin or joint-pin at one end, and with the crank-pin at the other end; of which two portions the axes of rotation are right-angular to the rod's length, and parallel with each other. This being the ordinary arrangement also for a great number of bars, levers, joint-rods, and other portions, it is necessary to plane and line the boss-faces of all such articles with regard to one general method, whether the holes are to be square, hexangular, octangular, circular, or oval; and whether they are to be parallel along the lengths of the holes,' or rather taper. The taper form is preferable for the greater number, which will be shown as we proceed.

In order that the hole in any end-boss of a rod may be centrally located, it is only necessary to shape the hole with regard to the gauge-lines on the flat surfaces referred to, which are the boss-faces, and are the top surfaces in the Figs. 890, 891, and 892. The lines on these faces are marked from the primary centre lines which extend, or would extend if continued, along the entire lengths of the respective rods. Two such centre lines are used for every boss, one line being on each face, consequently, across each of the two entrances, or intended entrances, belonging to the hole. A primary line of this character is seen in Fig. 893, extending along an ender to the end of the boss. The ender is required to show the centre dot, whenever a boss is to be lined in which the hole is already forged; but a boss without a hole is lined from the centre dot, which is situated on the superfluous metal to be drilled out. From the dot circles are scribed to indicate the size of hole intended, after which the exact angular form for the hole is marked with a straight-edge and scriber. This marking will denote the shape, and also show the quantity of superfluous metal to be cut out, whether equally or unequally around the hole. It is not quite necessary to mark both ends of the hole, although it is sometimes done; the marking on one boss-face is quite sufficient for all the drilling and slotting that may be required, the lines on both faces being used only during a final filing of the hole to the exact form. It may be perceived that by reason of the two boss-faces being parallel with the machine-table, the one adjustment of the boss by the gauge-lines on the upper face must at the same time adjust the lower face, and therefore cause the hole to be drilled and slotted in the exact right-angular position desired.