BEAM COMPASSES AND TRAMMELS.
In [fig. 188] is shown a set of beam compasses, together with a portion of the wooden rod or beam on which they are used.
Fig. 188.
The latter, as will be seen by the section drawn to one side, A, is in the shape of a T. This form has considerable strength and rigidity. Beam compasses are provided with extra points for pencil or ink work, as shown.
While the general adjustment is effected by means of the clamp against the wood, minute variations are made by the screw, B, shifting one of the points, as shown in the [figure].
This instrument is quite delicate, and, when in good order, is very accurate. It should be used only for fine work on paper, and never for scribing on metal.
Fig. 189.
A coarser instrument, and one especially designed for use upon metal, is shown in [fig. 189], and is called a trammel. There are various forms of this instrument, all being the same in principle. The engraving shows a form in common use. A heavier stick is used with it than with the beam compasses, and no other adjustment is provided than that which is afforded by clamping against the stick.
In the [illustration], a carrier at the side is shown, in which a pencil may be placed. Some trammels are arranged in such a manner, that either of the points may be detached and a pencil substituted.
A trammel, by careful arrangement, can be made to describe very accurate curves, and hence can be used in place of the beam compasses in many instances. For all coarse work it is to be preferred to beam compasses. It is useful for all short sweeps upon sheets of metal, but for curves of a very long radius a strip of sheet iron or a piece of wire will be found of a more practical service than even this tool.
The length of rods for both beam compasses and tramels, up to certain limits, is determined by the nature of the work to be done. The extreme length is determined by the strength and rigidity of the rod itself. It is usually convenient to have two rods for each instrument, one about 11⁄2 or 2 feet in length and the other considerably longer—as long as the strength of the material will admit.