Straight middle-shafts, paddle-shafts, piston-rods, and other heavy forgings of similar shapes, are all centred in about the same manner. The centring of such a piece is performed while it hangs suspended with a portable crane, which is placed over the lathe for the purpose, and is the means of conveying the shaft to the lathe. To avoid unnecessary placing of the object to and from the lathe-pivots, it is centred so that the centre-recesses shall be exactly in the desired places to cause the shaft to rotate truly the first time it is put into the lathe.

The tools required for centring a shaft are callipers, a rectol, a scriber, a monto, and a fiddling drill and coner to form the recesses. A chipping-chisel also is usually requisite, to flatten a small surface at the middle of each shaft extremity, and make it tolerably square to the length. If a shaft is tolerably straight, and has an abundance of metal to be turned off, the place for each centre-recess can be found with a calliper only, the legs of which are separated to a proper distance for scribing four lines similar to those seen in Fig. 1087. Previous to centring with a calliper, the condition of the article must be ascertained with a long straight-edge, and the hollow or concave parts discovered. These being shown, the operator can, after he has found the centre of each extremity with a calliper, alter the dot to suit the concave part or parts discovered with the straight-edge.

Centring a shaft with a rectol is a rapid and accurate mode of showing the places for the recesses, and is suited to any roughly forged shaft, or one which may be crooked. The length of the rectol used for this purpose is suited to the length of the shaft, and is furnished with a gap, or two gaps, of ample size, into which the bearing-flanges or collars of shafts can extend while the instrument is in contact with the objects to be centred, supposing that they are forged with the collars or flanges referred to. One such gap is usually sufficient for two flanges, if of proper length, and one long gap in a rectol is more useful for general work than two shorter gaps. The rectol is placed upon the shaft so that the outer portion is in contact with the shaft extremity, as denoted in Fig. 1119, four marks being made with a scriber in the same manner as for the centring of a bolt, described in page 348.

There is also a mode of centring with a straight-edge and parallel blocks. This method is analogous to the centring with a rectol, but is not so convenient, because of the blocks being distinct from the straight-edge, and therefore liable to fall about while in use. Blocks are available when a gap straight-edge of sufficient length is not accessible, and it is thought unnecessary to make one. In Fig. 1121 the implements for this centring are seen in use, the two parallel blocks being first put upon the shaft, and the straight-edge next put upon the blocks. One end of the straight-edge is situate over one end of the shaft, and while held in this condition by an assistant or two, a calliper is adjusted to place its points about as far apart as the distance between the top edge of the straight-edge and the centre of the shaft-end, which distance is denoted in the Figure by the dotted line which connects the two dotted lines T and M. One point of the calliper-foot is now put upon the top of the straight-edge shown by T, and held there in about the same manner as a compass-point would be held in a centre-dot, and the other point of the calliper is caused to make a short arc upon the shaft-end. This effects the lining in one position ; and the straight-edge and blocks are next shifted to about a quarter of a revolution around the shaft, and again held in position for another mark. After this two more marks are made by similar means; and all the four are made while the calliper-legs remain at the same distance apart.

The four marks thus scribed indicate the centres of those portions of the shaft upon which the blocks were placed for scribing; consequently, to centre any stated portions of the shaft it is only necessary to put the blocks into contact with those portions.

Long shafts can be centred also with a monto. This is an instrument devised by the author, and is applicable to a number of purposes, including centring of shafts. The tool is represented by Fig. 1118, and consists of a straight-edge having a pair of vee-blocks attached. The stem of each block is furnished with an opening having smooth parallel sides, which admit the straightedge and allow the blocks to be slid along and fastened at any desired place. It is necessary for the edges of the straight-edge to be parallel with the lower edges of the blocks and also the entire surfaces of the vee-gaps; and after the blocks have been shifted to the intended places, they require to be fastened either with taper keys or with a couple of fixing screws.

A monto is used in the same manner as described for a pair of parallel blocks and a straightedge; but it is much more convenient because its blocks cannot fall from the implement; it can, therefore, be easily held either at the top, side, or bottom of a shaft while it hangs suspended over the lathe, or while it remains on the lathe-bed or floor. The marks which are scribed upon ends of shafts by means of a straight-edge, a rectol, or a monto, are similar to those marked with callipers, and are seen in Fig. 1120.