Beam Compasses and Trammels - In Fig. 114 is shown a set of beam compasses, together with a portion of the wooden rod or beam on which they are used. 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, while at the same time it is not clumsy or heavy. Beam compasses are provided with extra points for pencil and 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 shifting one of the points, as shown. This instrument is quite delicate and when in good order is very accurate. It should be used only for line work on paper and never for scribing on metal. A coarser instrument, and one especially designed for use upon metal, is shown in Fig. 115 and is called a trammel. It is to be remarked in this connection that the name trammel, by common usage, is applied to this instrument and also to a device for drawing ellipses, which will be found described at another place.

Fig. 114.   Beam Compasses.

Fig. 114. - Beam Compasses.

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.

Fig. 115.   Trammel.

Fig. 115. - Trammel.

A trammel, by careful management, 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 the 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 more practical service than even this tool.

The length of rods for both beam compasses and trammels, up to certain limits, is determined by the nature of the work to be clone. 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 3 1/2 or 4 feet in length and the other considerably longer - as long as the strength of material will admit. In the case of the trammel, by means of a simple clamping device, or, in lieu ci better, by use of common wrapping twine, the rods may be spliced when unusual length is required; but a strip of sheet iron or a piece of fine wire forms a better radius, under such circumstances, than the rod.

The Protractor is an instrument for laying down and measuring angles upon paper. The instrument consists of a semicircle of thin metal or horn, as represented in Fig. 116, the circumference of which is divided into 180 equal parts or degrees. The principles upon which the protractor is constructed and used are clearly explained in the chapter on Terms and Definitions (Def. 68 "Degree"). The methods of employing it in the construction of geometrical figures are shown in Chapter IV (Geometrical Problems) among the problems. For purposes of accuracy, a large protractor is to be preferred to a small size, because in the former fractions of a degree are indicated.

Fig. 116.   Semicircular Protractor.

Fig. 116. - Semicircular Protractor.

While a number of geometrical problems are conveniently solved by the use of this instrument, it is not one that is specially adapted to the pattern cutter's use. All the problems which are solved by it can be worked out by other accurate and expeditious methods, which, in most cases, are preferable. It is one of the instruments, however, included in almost every case of instruments sold, and the student will find it advantageous to become thoroughly familiar with it, whether in practice he employs it or not.

Besides the semicircular form of the protractor shown, corresponding lines and divisions to those upon it are sometimes put upon some of the varieties of scales in use, as shown in Fig. 120.

Scales. - Many of the drawings from which the pattern cutter works - that is, from which he gets dimensions, etc., - are what are called scale drawings, being some specified fraction of the full size of the object represented. Architects' elevations and floor plans are very generally made either 1/8 or 1/4 inch to the foot, or, in other words, 1/96 or 1/48 full size. Scale details are also employed quite extensively by architects, scales in very common use for the purpose being 1 1/2 inches to the foot and 3 inches to the foot, or, in other words, 1/8 and 1/4 full size respectively. It is essential that the pattern cutter should be familiar with the various scales in common use, that he may be able to work from any of them on demand. Several of the scales are easily read by means of the common rule, as, for example, 3 inches to the foot, in which each quarter inch on the rule becomes one inch of the scale; also, 1 1/2 inches to the foot, in which each eighth of an inch on the rule becomes an inch of the scale; and, likewise, inch to the foot, in which each sixteenth of an inch on the rule becomes an inch of the scale. However, other scales besides these are occasionally required, which are not easily read from the common rule, and sometimes special scales are used, which are not shown on the instruments, especially calculated for the purpose. Accordingly, it is sometimes necessary for the pattern cutter to construct his own scale.

The method of constructing a scale of 1 inch to the foot is illustrated in Fig. 117, in which the divisions are made by feet, inches and half inches. In constructing such scales, it is usual to set off the divisions representing feet in one direction (say to the right) from a point marked 0, while the divisions for inches and fractions thereof are set off the opposite way (or to the left from 0) as shown in the illustration. In using the scale, measurements are made by placing one point of the dividers at the number of feet required; the other point can then be moved to the other side of the 0 to the required number of inches, thus embracing the entire number of feet and inches between the points of the dividers.