The edges of the plates should be planed first, care being taken to make them square and flat. The surfaces should then be planed, the plates being secured to the planer by the edges, which will prevent as far as possible the pressure necessary to hold them against the planing tool cut from springing, warping, or bending the plates. Before the finishing cut is taken, the plates or screws holding the surface plate should be slackened back a little so as to hold them as lightly as may be, the finishing cut being a very light one, and under these circumstances the plates may be planed sufficiently true that one will lift the other from the partial vacuum between them.
After the plates are planed, and before any hand work is done on them, they should be heated to a temperature of at least 200° Fahr., so that any local tension in the casting may be as far as possible removed.
Fig. 1487.
Surface plates for long and narrow surfaces are themselves formed long and narrow, as shown in [Fig. 1487], which represents the straight-edge surface plate made at Cornell University.
Fig. 1488.
The Whitworth surfacing straight-edge, or long narrow surface plate, is ribbed as in [Fig. 1488], so as to give it increased strength in proportion to its weight, and diminish its deflection from its own weight. The lugs d are simply feet to rest it on.