Some Motions

Abstract

A prime example of straight-line motion is the lathe bed whose straightness, if parallel with the axis of rotation, generates accurate cylinders. It is usually guided by three surfaces arranged in V-flat formation as in figure 31a, giving unique kinematic location so long as the loads are downwards. Where this is not certain, slides are often of dove-tail shape, with three fixed surfaces, the fourth being adjustable, as in figure 31b. If we provide four rigid surfaces, we are over-located as in figure 31c, which shows in exaggerated form the consequences of slight error such as may occur due to distortion, thermal expansion, etc. The three-surface location is free from this trouble; when the V-slide is symmetrical and wear is even on both sides, the wear causes no slackness and no horizontal error. Note that the cutter position is arranged so that wear of the slides or cutter deflection cause only small, second-order errors in the workpiece.