Crossheads should be turned previous to boring the holes in the bosses, because it is easy to adjust a crosshead correctly for boring, by means of its uniform surfaces, which are produced by turning. A crosshead having but one piston-rod, is bored while fixed on a lathe-chuck, and a crosshead for two piston-rods, is bored while on a vertical driller ; but by whichever mode the boring is to be performed, it will be found advantageous to execute the turning, and, in most cases, the planing also, previous to boring.

At the commencement of the rough turning of a crosshead, it is necessary for the turner to measure from the centre of the boss in two directions, when marking the places for the shoulders, bearings, and junctions. For this purpose, pointed wire gauges having bent ends of proper length are used; such gauges being requisite when the objects to be measured possess surfaces which are not level with each other.

Previous to measuring with gauges the crosshead is truly turned along its entire length ; and when proceeding to mark the shoulders, a centre-dot is put about midway between the two faces of the middle boss, supposing that the hole is not yet bored. This dot is shown in the middle of the boss in Fig. 1117, and its place is found with a calliper, with which the four arcs seen on the boss are made by placing one calliper-leg to each of the four corners or junctions shown by the letter J. But when it happens that the hole in the boss has been accurately finished previous to turning the ends, the turner measures from the centre at the end of the hole, which is denoted on an ender fitted therein. In any case it will be easier to first turn the ends, and afterwards bore the middle hole square to the turned parts, than to first bore the hole, and next turn the ends so that their axes shall be square with the hole. The centring operations required for this latter purpose are described in another place.

The narrow sides of a crosshead's arms require to be turned with a slide-rest having a circular movement, in order that the requisite curved form may be obtained in an easy manner. But if the lathe employed does not possess such a rest, the curved form for the arms must be produced with the ordinary top slide of the rest, which is used in about the same way as for turning straight cones. By this mode, it is necessary to first scribe curved lines along one broad side of the arms, to accurately show the intended shape and dimensions ; after which the metal is cut off to the lines by means of a series of turnings with the slide-rest, but with the top slide inclined at different angles with the lathe-spindle ; only a part of the arm being turned with one adjustment of the slide.

The turning required for a two-piston-rod crosshead, is that by which its middle boss is made cylindrical and to fit the bearer-brasses belonging to it; also to properly form the collars and shoulders of the bearing to suitable dimensions, and to the required distance from the end-bosses. The middle boss is first reduced until its diameter is rather greater than the finished diameter of the two collars, or flanges, when it can be marked to show the exact places for the two required shoulders. If the crosshead has not been shaped on a slotting-machine, and therefore its end-bosses are not yet reduced to the finished dimensions, the exact situations of the two shoulders of the large bearing are not of great consequence, because the distance from a shoulder to the centre of the hole in either end-boss can be easily shown after the shoulders are accurately turned by placing a straight-edge thereto. But if the end-bosses are finished, or if they have but very little metal to be cut off, the crosshead should have been accurately lined with regard to the centres of these bosses in order that the shoulders of the mid-bearing shall be made in the proper places.