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.
[Illustration: 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.
[Illustration: 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.