Straight-edges are sometimes made of cast steel and trued on both edges. These will answer well enough for small work, but if made of a length to exceed about four feet their deflection from their own weight seriously affects their reliability. The author made an experiment upon this point with a very rigid surface plate six feet long, and three cast steel straight-edges 6 feet long, 41⁄2 inches wide, and 1⁄2 inch thick. Both edges of the straight-edges were trued to the surface plate until the light was excluded from between them, while the bearing surface appeared perfect; thin tissue paper was placed between the straight-edges and the plate, and on being pulled showed an equal degree of tension. The straight-edges were tried one with the other in the same way and interchanged without any apparent error, but on measuring them it was found that each was about 1⁄50 inch wider in the middle of its length than at the ends, the cause being the deflection. They were finished by filing them parallel to calipers, using the bearing marks produced by rubbing them together and also upon the plate; but, save by the caliper test, the improvement was not discernible.
In rubbing them together no pressure was used, but they were caused to slide under their own weight only.
A separate and distinct class of gauge is used in practice to copy the form of one piece and transfer it to another, so that the one may conform to or fit the other. To accomplish this end, what are termed male and female templates or gauges are employed. These are usually termed templates, but their application to the work is termed gauging it.
Fig. 1489.
Suppose, for example, that a piece is to be fitted to the rounded corner of a piece f, [Fig. 1489], and the maker takes a piece of sheet metal a, and cuts it out to the line b c d, leaving a female gauge e, which will fit to the work f. We then make a male gauge g, and apply this to the work, thus gauging the round corner.
Fig. 1490.