Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 13

Sir J. Whitworth’s famous method of trueing engineers’ straight-edges should interest the woodworker. Three straightedges are prepared singly, and each is brought to a moderate state of accuracy; two of them, A and B (Fig. 4), are compared with each other by placing them edge to edge, and any irregularities found are removed, the process being repeated until A and B fit each other perfectly. The third straight-edge, C, now is compared with both A and B, and when it fits perfectly both these two, then there is no doubt whatever that the three are straight, approximating to the truth in proportion to the labour that has been spent upon them. Why this is so is obvious when it is remembered that though A may be rounded instead of being straight, and B may be hollow sufficiently to make them fit each other perfectly, yet it is impossible for C to fit both the rounded and the hollow straight-edge.

Fig. 5.—Testing Surface with Straight-edges.