Fig. 102.   Straight Edge.

Fig. 102. - Straight-Edge.

For tinners' use in general jobbing shops, a three-foot straight-edge in many cases, and a four-foot one in a few instances, will be found very convenient. Some mechanics desire their straight-edges graduated, the same as a steel square, into inches and fractions. There is, however, no special advantage in this; it adds considerably to the cost, without rendering the tool more useful.

A hole should be provided in one end of the straight-edge for hanging up. It should always be suspended when not in use, as in that position it is not liable to receive injury.

It is almost superfluous to add that straight-edges must be absolutely accurate, for if inaccurate they would belie their name. A simple and convenient method of testing straight-edges is to place two of them together by their edges, or a single one against the edge of a square, and see if light passes between them. If no space is to be observed between the edges it is satisfactory evidence that they are as nearly straight as they can be made by ordinary appliances. In addition to having the edges straight it is also necessary to have the two sides parallel.

T-Squares. - With this instrument, as with almost all drawing instruments, there is the choice of various qualities, sizes and kinds, and selection must be made with reference to the kind of work that is to be performed. Whatever quality may be chosen, the desirable features of a T-square are strict accuracy in ail respects, and a thin, flat blade that will lie close to the paper. For most purposes a fixed head, as shown in Fig. 103, is preferable. For drawings in which a great number of parallel oblique lines are required, and particularly where a small size T-square can be used, a swivel head, as shown in Fig. 104, is sometimes desirable. The objectionable feature about a swivel head is the difficulty of obtaining positive adjustment.

When made in the ordinary manner, and depending upon the friction of the nut of a small bolt for holding the head in place, it is almost impossible to obtain a bearing that can be depended upon during even a simple operation. In practice it is found to be far less trouble to work from a straight-edge - properly placed across the board and weighted down or otherwise held in place - by means of a triangle or set-square, as greater accuracy is thus assured.

Fig. 103.   T Square with Fixed Head.

Fig. 103. - T-Square with Fixed Head.

In point of materials, probably a T-square having a walnut head and maple blade is as satisfactory as any. This kind is the cheapest and is generally considered the best for practical purposes. A good article, but of higher price, consists of a walnut head with a hardwood blade, edged with some oilier kind of wood. Still another variety has a mahogany blade edged with ebony. T-squares constructed with east-iron head - open work finished by japanning - with nickel-plated steel blade, are also to be bad from dealers. They are also made with a hard rubber blade, of which Fig. 104 is an illustration. The liability to fracture, however, by dropping necessitates the greatest care in use; otherwise hard rubber makes a very desirable article and is the favorite material with many draftsmen.

As to size, T-squares should be selected with reference to the use to be made of them. Generally, the blade should be a very little less in length than the width of the table or board upon which it is to be used. Where a large hoard or a table is used it will be found economical to have two instruments of different sizes.

Fig. 104.   T Square with Fixed and Swivel Head.

Fig. 104. - T-Square with Fixed and Swivel Head.

The Steel Square. - One of the most useful tools in connection with the pattern cutter's outfit is an ordinary steel square. The divisions upon it concern him much less than its accuracy. He seldom requires other divisions than inches and eighths of an inch; therefore in selection the principal point to be considered is that

Drawing Took and Materials of accuracy. The finish, however, is a matter not to be overlooked. Since a nickel-plated square cost but a trifling advance upon the plain article, it is cheaper in the long run to have the plated tool.

A convenient method of testing the correctness of the outside of a square, and one which can be used at the time and place of purchase, is illustrated in Having procured a square winch is accurate upon the outside, the correctness of the inside of another square may be proven, as shown in Fig. 10li. Place one square within the other, as shown. If the edges lit together tightly and uniformly throughout, the square may be considered entirely satisfactory.

Fig. 105.   Testing the Exterior Angle of a Steel Square.

Fig. 105. - Testing the Exterior Angle of a Steel Square.

Fig. L05. Two squares are placed against each other and against a straight-edge, or against the arm of a third square. If the edges touch throughout, the squares may be considered correct.

Fig. 106.   Tes ing the Interior Angle of a Steel Square.

Fig. 106. - Tes ing the Interior Angle of a Steel Square.

An accurate square is especially desirable, as it affords the readiest means of testing the T-square and the drawing table and beard, as elsewhere described. The greatest care should be given, therefore, to the selection of a square. For all ordinary purposes the two-foot size is most desirable. In some cases the one fool size is better suited. Many pattern cutters on cornice work like to have both sizes at their command, making use of them interchangeably, according to the nature of the work to be done.