Plug and Ring Gauges.—The principal ones and the originals of all the rest, termed Whitworth gauges after the inventor, are the plug and ring gauges (fig. 29, A and B). The principle on which they depend is that if the two gauges are made to fit with perfect accuracy, without tightness on the one hand or slop on the other, then any work which is measured or turned and bored or ground by them will also fit with equal accuracy. Bored holes are tested by the plug gauge, and spindles are tested by the ring gauge, and such spindles and holes make a close fit if the work is done carefully. Of course, in practice, there is very much vari ation in the character of the work done, and the finest gauges are too fine for a large proportion of engineers' work. It is possible to make these gauges within of an inch. But they are seldom required so fine as that for shop use ; is generally fine enough. For general shop work the gauges are made to within an inch. Standard gauges in which the plug and ring are of the same diameter will only fit by the application of a thin film of oil and by keeping the plug in slight movement within the ring. Without these precautions the two would "seize" so hard that they could not safely be separated.
Plug and Ring v. Horseshoe Gauges.—The horseshoe, snap or caliper gauges (fig. 3o) are often used in preference to the plug and ring types.
Limit Gauges.—Some fits have to be what is termed in the shops "driving fits," that is, so tight that they have to be effected by driving with a hammer or a press, while others have to be "working fits," suitable, say, for the revolution of a loose pulley on its shaft or of an axle in its bearings. The "limit" or "difference gauges" (figs. 29 and 3o) are designed for producing these work ing fits ; that is, the plug and ring gauges differ in dimensions so that the work bored will drive tightly, or slide freely over the work turned. These are variously sub-classified. The system
which is generally accepted embraces force fits, which require the application of a screw or hydraulic press; driving fits, that require less power, as that of a hammer; push fits, in which a spindle can be thrust into its hole by hand ; and running fits, such as that of shafts in bearings. Fixed gauges are made for each of these, but as this involves a heavy outlay there are also adjustable limit gauges (fig. 3o, D) for external dimensions, the standard plug being used for holes. The setting is done by screwed plugs or anvils adjusted by reference bars. In all these gauges the "go on" and "not go on" ends respectively are stamped on the gauge.
Reference Discs and End Measuring Rods.—Shop working gauges become in time so damaged by service that they fail to measure so accurately as when new. To correct these errors reference gauges are provided. These are never used in the shops for actual measurement of work, but are only kept for checking the truth of the working gauges. They include disc, stepped and end measurement gauges. The disc and the stepped are used for testing thy ring gauges, the stepped kind comprising essentially a collection of discs in one piece (fig. 29, D) . The end measure pieces test the external gauges. Some end measure stand ard lengths are so accurate that any sizes taken at random in any numbers from a in. to 4 in., varying by sixteenths of an inch, will, when placed end to end, make up an exact length.
Movable Gauges.—This extensive group may be regarded as compounded of the common caliper and the Whitworth measur ing machine. They are required when precise dimensions have to be ascertained in whole numbers and minute fractional parts. They combine the sense of touch by contact, as in the calipers, with the exact dimensions obtained by inspection of graduated scales, either the vernier dr the micrometer screw. If gauges must not vary by more than --10.0,,0 of an inch, which is the limit micrometer calipers (fig. 35) take several inches in length, the micrometer being reserved for fractional parts of the inch only.