Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 12

To test the truth of a straight-edge having the above measurements, get a clean board 1 ft. longer and about 7 in. or 8 in. wide. Lay the straight strip at about the centre of the board, and with a sharp pencil draw a line on the board along the trued edge of the strip, keeping the side close to the board, and making the line as fine as possible. Now turn the strip over, placing the edge on the other side of the line, and if the trued edge is perfectly straight the line also will appear so. If the line is wavy, the edge must be planed until only one line is made when marked and tested from each side; mark a fresh line for each test, otherwise there will be confusion and inaccuracy. One edge now being perfectly true, proceed with the other edge. Set a sharp gauge to the required width, and mark the second edge lightly on each side of the rule, working the gauge from the true edge; then the wood is planed off to the gauge marks, and the second edge tested as to its being true with the first one, using the pencil line, as before. A still more delicate test than the gauge line for parallelism is by the use of a pair of callipers. The points of the callipers are drawn along the edges, and if they are perfectly parallel there will be no easy or hard places, the presence of which might possibly not be detected by the pencil line. If the edges will stand both these tests, the strip is perfectly straight and parallel.

Fig. 3.—Home-made Striking Knife.

Fig. 4.—Whitworth method of Testing Straight-edges.