The surface gage is used in laying out work for the bench, lathe, or planer. The ordinary form consists of a heavy base, an upright which is firmly attached to the base, and a scriber or scratch awl. In the universal gage, the upright is pivoted at the base so that it may be used at any angle. In some forms the base is grooved in order that the gage may be used on cylindrical work as well as on flat surfaces, Fig. 1.
To use the gage, the part of the work to be laid out must be prepared so that lines drawn on the surface will show distinctly. A rough or unfinished surface is covered with chalk, a finished or bright surface should be copper-plated by applying a thin coating of copper sulphate 'solution with a brush or a piece of waste. In use, the work and the gage are then placed on a true surface and the scriber adjusted to the desired height. The lines are drawn by moving the surface gage along on the true surface, keeping the point in contact with the work. After scribing the lines, it is well to place light prickpunch marks at frequent intervals along the lines, so that the position may be located if the chalk or copper sulphate becomes effaced.
Fig. 1. Universal Surface Gage Courtesy of the L. S. Starrett Company, Athol, Massachusetts.
The straightedge consists, in its simplest form, of a thin flat piece of steel, often unhardened, with accurately finished straightedges. The very small sizes used in fine work are occasionally made with a hardened knife edge. A non-conducting handle is sometimes used with the small sizes to prevent distortion from the unequal heating due to handling. The short lengths used for ordinary shop purposes have one edge beveled and are thick enough to avoid bending, Fig. 2. The larger sizes, from 3 to 10 feet or more in length, are usually made of cast iron with one finished edge. The metal is so distributed as to combine lightness with great rigidity, the tendency of the ends to drop being resisted by the truss-like form of the casting shown in
Fig. 3. The flat form is used, in connection with the scriber, to draw accurate straight lines on plane surfaces. All styles are used to test the truth of plane surfaces by placing the straightedge on the surface to be tested in not less than the six positions shown in Fig. 4.
Fig. 2. Steel Straightedge.
Fig. 3. Cast-iron Straightedge.
Fig. 4. Diagram Illustrating Use of Straightedge.