The two principal sorts of crank-axles are, middle axles having one crank each, used for single engines, and also for pairs of paddle-engines, and the two-crank axles which are used principally for screw-engines. All crank-shafts, small and large, require to be carefully-straightened at the conclusion of forging, especially if they happen to be forged somewhat near the finished sizes. By proper straightening the subsequent lining and centring is facilitated, and the possible necessity for again heating the shafts after they have been tried in the turning-lathes, is avoided.

Large crank-axles are forged so that plenty of metal exists to be removed from the crank or cranks, and also from the axle-portions; the article is therefore first lined and centred with regard to the axle-portion, because it is easier than lining with regard to the crank or cranks.

After the gap-piece of a crank has been roughly cut out with drilling and slotting, or sawing, the first paring given consists in truly turning the axle; after which the article is lined with regard to the turning.

A small axle can be easily lined for the centring by employing a table with a scriber-block and vee-blocks. Fig. 1128 represents an axle thus treated. The piece is placed in the position shown, and a mark is made upon each end with the scriber-block, and the piece is then rotated in the vee-gaps for making two or three more marks, the operation resembling the centring of a straight piece with the exception that tall vee-blocks are used, to allow room for the crank. When an axle with two cranks is lined in this manner, one vee-block should be situate between the two cranks, as denoted in Fig. 1131.

To centre an axle with regard to its crank it would be necessary to place its crank upon parallel blocks, or upon a single one, as in Fig. 1129 ; and after scribing a line upon each end to put it upside-down that the opposite broad side of the crank might be in contact with the block during the scribing of another line. The middle between these two lines is the centre required, and to complete the centring a couple more marks are to be made with the axle in vee-blocks.

When the centre recesses and the extremities of an axle have been properly prepared as directed for turning other objects, the piece is ready to have its axle-portions rough-turned, leaving about an eighth of an inch in diameter to be turned off for finishing, after the crank-pin has been turned, if the axle is only an inch or two in diameter; but leaving at least a quarter of an inch for axles of seven or eight inches. This is necessary because, during the rough-turning, the article will become distorted, both while the axle is being reduced and during the turning of the crank-pin.

"When the axle has been turned so that a part of each axle-end is of the same diameter, the piece can be prepared for turning its crank-pin. The lining to show the places for the crank-pin centres is executed on a lining-table, or planing-table. The object is placed, with the two portions of the axle of the same diameter in contact with a pair of vee-block gaps, and with the crank standing at right-angles to the table, as denoted in Fig. 1130. An el-square is to be used, and put to both sides of the crank, so that if the two broad sides are not parallel with each other, both of them will be equally inclined to the table. The crank is now in position for the marking of the centres, and a holdfast plate is fixed upon the axle to prevent its shifting. The intended length of the crank's throw is to be next shown from the centre length of the axle, because this is indicated by the centre recesses on which the axle is turned. A scriber-block is therefore adjusted until its point is at exactly the same height from the table as the centres of the conical recesses, and a straight line is marked across the crank's two levers, which is denoted by A and A in the Figure (1130). At the adjustment of the scriber-poifit for this marking it will be found that if the point is at the height of one centre-recess, it is also at the height of the other one, because the axis of the axle is parallel with the table on account of the two axle-portions in the pair of vee-gaps being of one diameter. But if it occurs that one portion is larger than the other, the two centre-recesses are not equidistant from the table, but can easily be put equidistant by packing-pieces in contact with the vee-blocks.

The axis of the axle being now shown by the line A and A, a compasses is adjusted to place their points as far apart as the length of the throw; one point is then put into two points in the line, each at about the middle of the lever, and two arcs are scribed upon the upper part of the crank, shown in the Figure by R and R. These are at the same height from the table as the centres for the crank-pin, supposing that the marked side of the crank is exactly square to the table, and the scriber-point is next raised to the highest portions of the arcs, and a couple of lines are marked across the intended centres for the crank-pin. One of these lines is seen and indicated by P.

Having shown the length of the throw, the centring is next completed with the crank lying down on a block, as seen in Fig. 1132. While in this position the point of the block's scriber is placed somewhat near to midway between the crank's broad sides, and a short line marked across each of the two lines P scribed at the previous scribing. The axle is next half rotated, and the same packing-block is put beneath the opposite side of the crank now near the table, and the scriber-block again applied to mark another line across each end, while the point is at the same height as before. The three lines now existing on each end of the crank-pin are seen in Fig. 1133. The point situate midway between is the centre required, and is dotted accordingly. A circle is also scribed around, to guide the operator during drilling.

In order to prevent the centre-recesses being put rather out of place during the drilling, a small fiddling drill is first used, and observed at its entering-in, to see that it is in the centre; after which the drill-holes are enlarged and coned as before described.

A crank thus centred is to be turned with its crank-pin ends in immediate contact with the lathe-pivots ; consequently, the turning is performed with a lathe having a mandril-pivot and a poppet-pivot of suitable length and thickness for the crank in progress. Any large lathe will thus easily turn a comparative small crank. When on the pivots the axle-portion near the chuck constitutes a means of rotating the work. A crank in this situation is represented by Fig. 1134. The axle can be held with a poppet and short driver, as shown; or with a poppet and long thick driver - one which is long enough to reach to, and bear against the crank.

Turning should commence by widening the gap to very near its ultimate width, by means of right and left corner-tools. This will allow room for the turning-tools to turn the crank-pin portion, which must be effected with the ends of the slide-rest tools projecting as far from the slide-rest as the depth of the crank-gap.