Gages are used in machine shops to make one part of a machine, apparatus, or tool correspond with some other part, so that when the whole is assembled, every part will go in its place with little or no fitting.
In shops where work is made on the interchangeable plan - that is, where a piece of work made today will exactly duplicate a similar piece made at some time in the past - a very thorough system of inspection is necessary. In order that the inspection may accomplish the desired result, gages are made that show any variation of the pieces from a given standard. There are several forms of gages designed for various classes of work, but only those in common use in the general machine shop will be considered here.
Gages are generally made of tool steel; but hardened steel has a tendency to change its size or shape for a considerable time after the hardening has occurred. This change is ascribed by acknowledged authorities to a rearrangement of the minute particles or molecules of the steel, whose original arrangement had been changed by the process of hardening. While this change of size or shape is small, so small, indeed, that it need not be considered, except in the case of gages where great accuracy is required, yet it has led some manufacturers to use machine steel.
If tool steel is used, the tendency to change shape may be overcome to some extent by grinding the gage to within a few thousandths of an inch of finish size, and allowing it to "season" as it is termed among mechanics; that is, it is laid aside for a few months or a year, before being finished to size. This method is, of course, open to serious objection if the gage is needed for immediate use.
To save time, it is customary in many shops to draw the temper to a straw color, allowing the gage to cool slowly and repeating the operation several times. It is necessary to brighten the steel each time before drawing the temper in order that the colors may be readily seen; as this has a softening effect, the gage will not last so long as if left hard.
When making gages the workman should observe the points emphasized with regard to "approximate and precise measurements" in the first pages of this book. While gage-making is generally considered very accurate work, unnecessary accuracy should not be used. If a gage is intended for work where a variation of .005 inch is permissible, it is folly and a waste of time to attempt to make it within a limit of variation of .0001 inch. On the other hand, if the gage is to be used as a test gage on work requiring great exactness, it is necessary to use every possible effort to attain that end.
If a gage is to be made of tool steel, it is necessary first to remove all the outside portion (skin) of the stock, and block the gage out somewhere near to shape; it should then be thoroughly annealed. If the gage is flat and should spring while annealing, it should not be straightened cold, as it would be almost sure to spring when hardened.
It is necessary to stamp the name of the part to be gaged and the sizes of the different parts of the gage. The workman should bear in mind that the effect of driving stamps, letters, or figures into a piece of steel will be to stretch it; consequently, it is advisable to stamp the gage before finishing any of the gaging portions to size, even if there is an allowance for grinding.