burnishing rollers. These are revolving steel rollers secured in the tool posts, or carried by attachments at the back of the lathe (as in Fig. 24); they are pressed against the revolving journals, and impart to the latter a perfectly even and polished surface. Grinding machinery is now frequently used for axles, more especially for finishing the wheel seats.
Crank axles are turned in heavy lathes of the ordinary pattern, as the position of the crank webs does not allow of centre driving. The inside faces of the jaws of the crank webs are slotted out, or more usually milled out by a large revolving disc, into the periphery of which a number of cutters are inserted. A machine of this type is shown in Fig. 25. This has two centring headstocks at the back (one of which is hidden from view), in which the crank axle is held in special chucks, and there is also the revolving disc provided with cutters. The latter is slowly fed into the crank between the crank arms, and cuts these out to the dimensions required. This part of the work completed, the headstocks are set in motion to revolve the crank axle, which is set so that the crank pin is rounded by a cutting tool to a circular form. The axle is then turned round end to end in the centring headstocks and the other crank arms are milled out and the pin turned.
Many crank axles of large modern engines, instead of being forged in one piece, are now built up of nine pieces, two ends and one middle
