The forming of cogs belonging to wheels and racks by the aid of chiselling, is effected in conjunction with drilling, a circular hole being drilled at the bottom of each intended tooth-gap, previous to commencing the chipping.

In order to make a cog-wheel by such means, a circular plate or disk of metal, whether steel, iron, or gun-metal, is so prepared that its two broad sides are flat and parallel to each other, and its rim circular. If a lathe can be used for this purpose, the disk is thereby properly shaped, having its spindle-hole also bored by the same means. But if turning cannot be done, the place for the centre of the required wheel is determined, and is used for scribing circles upon the broad sides, and the rim is made circular by chipping to these lines.

In the case of a wheel which has its spindle-hole bored requiring circular gauge-lines, to which the rim is to be chiselled, it is necessary to tightly fix an ender at each end of the hole so that the centres can be found and accurately marked upon each. A circular plate having its hole thus bored, and containing an ender at each end, is indicated by Fig. 808, in which Figure one of the enders is seen tightly fixed. By means of the centre dot which is marked on each, both broad sides of the plate are delineated, three circles being scribed on each broad side as denoted in the Figure. Of these three, the one first marked is the outer one, and denotes the intended extreme diameter of the wheel. To this line the ragged rim is chipped, and as soon as it is made tolerably circular the plate is ready for the two inner lines. The scribing of these two circles will denote the middles of all the intended teeth, and also their depths; and when both broad sides are thus scribed, it is ready for marking with a springy divider to show the places and shapes of all the required cogs.

If several plates are in progress all are thus scribed, previous to drilling any one of them, that all the drilling may be done at one operation, either at a lathe or at a drilling-machine, according to circumstances. As soon as the places for the teeth are indicated, the circles for showing the places of all the holes at the bottoms of all the teeth-gaps can be scribed; after which the drilling proceeds by entering the drill from both broad sides of the wheel.

When all the holes are drilled, a hack-saw should be used, if the wheel is of iron or brass, in order to make two saw-cuts into each drilled hole, by which means the superfluous gap portion is cut out in one piece. By a series of sawing processes thus effected, the entire number of teeth-gaps and teeth required are roughly formed, and chiselling is avoided. Finishing the teeth to the exact shape required, is finally effected with filing.

A solid piece which is to be made into a rack, is shown by Fig. 809. The lining required for such a piece is done with a divider and scriber-block, the block being used for marking the long lines along the object while it is situated on a lining-table, and the divider being required to properly mark the intended teeth so that they shall be equidistant from each other. The long lines are marked upon two opposite sides of the object at one scribing with the block, which is moved entirely around the work while on the table. These lines denote the bottoms of all the intended teeth-gaps, the middles of the teeth, and also the extreme heights or distances from the bottoms of the gaps. In addition to the long lines, a series of short ones are marked; these are shown in the Figures across the top of the work, being square to the sides, and also equidistant from each other. Each one of these lines is marked along the centre of each intended tooth-summit, consequently one line only is made to represent one tooth, and is not erased until the entire shaping is completed. By exercising care to file each tooth so that this centre line is always in the middle, all the teeth-summits are made exactly at the same distance apart.