If the wooden strip be made the full length of the work the dog or clamp driving the work will require to be removed every time the wooden gauge is applied, and since the work must be turned end for end in the lathe to be finished, it would be as well to let the length of the wood gauge terminate before reaching the work driver, as, say, midway between e and f.
When a lineal distance is marked by lines, and this distance is to be transferred to another piece of work and marked thereon by lines, the operation may be performed, for short distances or radii, by the common compasses employed to mark circles, but for greater distances where compasses would be cumbersome, the trammels are employed.
Fig. 1446.
[Fig. 1446] represents a pair of trammels made entirely of metal, and therefore suitable for machinists’ use, in which the points require to be pressed to the work to mark the lines. a a represents a bar of square steel; or for very long trammels wood may be used. b represents a head fastened tightly to one end, and through b passes the leg or pointer c, which is thus adjustable as to its projecting distance, as c can be fastened in any position by the thumb-screw d. The head e is made to a good sliding fit upon the bottom and two side faces of a a; but at the top there is sufficient space to admit a spring, which passes through e. f is the leg screwed into e, which is locked in position by the thumb-screw g. The head e is thus adjustable along the whole length of the bar or rod a a. The object of the spring is as follows:—If the head e were made to fit the bar a a closely on all four sides, the burrs raised upon the top side of the rod a a by the end of the thumb-screw g would be likely to impede its easy motion. Then again, when the sliding head e has worn a trifle loose upon the bar a a, and is loosened for adjustment, it would be liable to hang on one side, and only to right itself when the screw g brought it to a proper bearing upon the under side of the bar a a, and thus tightening the head e would alter the adjustment of the point. The spring, however, always keeps the lower face of the square hole through e bearing evenly against the corresponding face of the bar, so that tightening the screw g does not affect the adjustment, and, furthermore, the end of the set-screw, bearing against the spring instead of against the top of the rod, prevents the latter from getting burred.
The flat place at i i is to prevent the burrs raised by the thumb-screw end from preventing the easy sliding of leg c through b.
Fig. 1447.