bored to exact diameter, except at the two ends where the bore is slightly enlarged. These enlarged ends are termed the “counter-bores” and their object is to prevent a ridge being formed at the ends of the cylinder as the piston wears down the metal. The end flanges for the cylinder covers are also faced at the same time by the tools carried in the projecting arms. These flanged faces and the bores and counter bores can be seen in the pair of cylinders shown in Fig. 22. These cylinders have circular piston valves, and the boring of these is also done on a similar machine with smaller boring bars.
The planing of the various sides and flanges, and also of the steam chests port facings, when the latter have ordinary slide valves, may be done either on an ordinary large planing machine or on special machines made for the purpose. The cylinders have to be set on the machine by the bore to plane the top and bottom surfaces.
In those works in which the boring is done before the planing, one method is to use two standards which are splined underneath, the tongue or spline fitting into one of the grooves in the table of the planing machine. The top part of each standard is provided with a circular face plate which just fits into the counterbore of the bored cylinder. The standards are placed so that the face plates enter the counterbore at each end, and they are pulled up to grip the cylinders by means of a long central bar with nuts. The centre line of the bore is thus held
