A headstock is a species of double plummer-block, and consists of two plummer-blocks which are cast solid together, having an opening or hole between the two. Such castings are used to support crank-axles belonging to marine-engines of several classes; the intermediate opening being that in which the crank-levers revolve. By some headstocks are termed, entablatures.

The shaping of a headstock should commence with planing; and the lining which is necessary, previous to planing, resembles that described in page 236 for a single plummer-block. The position in which a headstock remains on a planing-table during the planing of the gaps and top surfaces, is shown in Fig. 760; and for this planing, a primary centre line, or some other mark, is requisite, in order that the top surfaces which will adjoin the intended caps, and also the under surfaces for contact with the columns, may be produced at the desired distance from the centre of the axle-gaps. Another primary centre line also is required, previous to commencing the planing of the gaps; this line is denoted by C in the Figure, and is marked exactly at right-angles to the upper planed surfaces termed the top of the headstock ; the line is also properly situated at the desired place between the centres of the intended column-holes, in order that the line shall correctly indicate the centre of the axle-gap when planed. At each side of this line, and parallel with it, another line is scribed to show the intended width of the gap. This lining is performed also upon the other gap-portions of the headstock, and is the means of adjusting it on the planing-table, for the purpose of producing the gaps in the desired relative positions when planed. Through the lines extending downwards to, or nearly to, the planing-table, and also .across the wood enders to the top of the headstock, adjustment is easily effected with a tall el-square by wedging up until the lines are seen to be square to the table, and are also seen to be parallel to the length of the table by observing one of its long gauge-lines. The gradual shifting for adjustment, is effected after the hold-fast plates and wedges are attached, but not finally tightened ; and poppets of proper strength for the weight of the object, are used, to impart the small movement necessary.

Poppets which are specially intended for planing-tables, are shown by those in the Figure (760) and also by Figs. 757, and 758. Such poppets are provided with broad bases, that they may be held on the table with holdfast plates and bolts, instead of providing stems which will extend through the table, to receive nuts underneath. By means of such a broad base or foot, the poppet can be fixed at any place of the table's surface, whether in the middle or at one edge. The poppet shown by Fig. 758, is a forged one, and can be fastened to a planing-table with a plate situated in the space between the stay and the poppet proper. The pin extending from the bottom of this poppet, is put into one of the table's slots, to prevent the possibility of shifting backwards while tightening the poppet-screw. Fig. 759 represents a cranked middle plate, such being used for the purpose of placing the bolt's nut entirely below the gripping portions.

To facilitate the adjustment of a headstock by its lines it is proper, if the shape of the casting will permit, to first place it upon the planing-table with the bottom upwards, in order that the column surfaces may be those first planed. The quantity to be planed off these portions, is ascertained by a species of preliminary lining; and when these column bearings are planed, they are put downwards into contact with the table, or upon parallel blocks, which dispenses with all further adjustment of the headstock with wedges. Lining for the gaps can now be accurately done with a scriber-block and straight-edge, while the work remains on its planed surfaces; and in this condition, the poppet-screws will easily and quickly shift it to its desired position.

After the bottom, top, and gap-surfaces, are planed, the headstock is shifted to place the length of the axle-gaps exactly across the table. This is the position of the one shown in Fig. 761, and is thus fixed in order to plane the two ledges which adjoin each gap. These four ledges are those that will be in immediate contact with the flanges of the axle-brasses, and are planed right-angular to the gaps by means of the primary lines previously used. Each of these lines is now made to coincide with, or stand on one of the table's short gauge-lines, instead of on a long line as before; and after being adjusted and fixed the planing is effected with the vertical traverse screw or screws of the machine.