This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.
For drawing lines and laying off distances on curved surfaces, such as shafts, a combination of two straightedges, or a straightedge and a rule, is used. This is often called a keyseat rule because its chief use is laying out keysways on shafts.
However, many machinists call it a box rule. It is usually made in one piece, although some manufacturers provide clamps by which the two separate pieces are held at right angles to one another.
Fig. 5. Keyseat Rule Courtesy of L. S. Starrelt Company, Athol, Massachusetts.
A more simple combination is shown in Fig. 5, the second scale being represented by two special clamps.
The simplest form of square, called the flat square, Fig. 6, is a combination of two straightedges at right angles. This is a useful form where the square is laid on the work. One blade is usually graduated on the inner edge, and the other on the outer edge.
The try square, Fig. 7, consists of a beam and a blade at right angles. The beam is much thicker than the blade and somewhat shorter. Try squares are made both unhardened and hardened. The unhardened form has graduations on one edge and is termed a graduated try square. The hardened type always has a hardened blade, sometimes a hardened beam as well, and is not graduated.
Fig. 6. Thin Steel Squares Courtesy of L. S. Starrett Company, Athol, Massachusetts.
The try square is used as a guide to draw lines at right angles to each other and to given surfaces; to erect and test perpendiculars to plane surfaces; to test the truth of a given surface at right angles to another surface; in short, wherever an accurate layout or test of 90 degrees is required. When used for testing the relation of two surfaces, the beam is pressed closely against the correct surface, and the blade is brought carefully down to the surface under consideration. This does not prove more than that a line at the particular point tested is or is not at right angles to the true surface. By using the blade as a straightedge parallel to the true surface, errors in that direction may be corrected and the surface be made plane.
Fig. 7. Steel Try Square.
In many cases it is necessary to test the relation of lines and surfaces which are not at right angles to each other. For this purpose a bevel is used in which what corresponds to the blade of the square is made adjustable. Its construction is seen in Fig. 8; its use is similar to that of the square.
Fig. 8. Universal Bevel Courtesy of L. S. Starrett Company, Athol, Massachusetts.