Page:Miscellaneous Papers on Mechanical Subjects.djvu/23

 Rh Take three plates of cast iron, of equal size and proportionate strength. The metal should be of a hard quality. The plates should be well ribbed on the back to prevent them from springing, and each of them should have three projecting points on which to rest, placed triangularly in the most favourable positions for bearing. The object of this provision is to insure constant support at the same points. The plate would otherwise be subject to perpetual variation of form, owing to the irregular strain, occasioned by change of bearing. A provision of this kind is equally necessary while the plate is undergoing the operation of correction, and when it is afterwards used as a model.

In fixing the plates on the table of the planing machine, care should be taken to let them bear on the points before mentioned, and to chuck them with as little violence as possible to the natural form, otherwise they will spring on being released, and the labour of filing will be increased in proportion. It is proper also to relax the chucks before taking the last cut. With these precautions, if the machine itself be accurate, and the tool in proper condition, the