The standard internal and external cylindrical gauges, fig. 10, and the internal and external limit gauges, fig. 11, are invaluable where accurate production of duplicate parts of machinery or tools are required. The value of these gauges are well understood in all first-class workshops, though they have not been used to anything like the extent that they should be in the average works requiring to duplicate machine parts. The standard cylindrical gauges are hardened, ground, and lapped accurately to the size stamped upon them.

Fig. 8.

Where work has to be made interchangeable, for success it is obviously necessary to lay down the limits of accuracy within which the work must be done. Thus, suppose the case of a piston rod for a steam engine as an example of this method of working be taken, the rod must in the cylindrical part be made a certain size to ensure its working, without heating the gland, and all the glands must be similarly bored within certain limits, or they will be either tight or slack on a rod. Now, for the glands, a gauge would be prepared like 3wp the internal gauge shown in fig. 11, and every gland before being passed into the manufactured stores would have to be such a size that whilst one end would go in the other end would not enter. Thus it is obvious that the size of the hole must be greater than 1.249 in., and less than 1.251 in.-that is, the bore is l 1/4in. within one-thousandth of an inch. Closer limits, if required, might be specially arranged, although for all ordinary cases this limit would be amply sufficient. In a like manner gauges would have to be made for the piston rod, so that whilst one ring would slip on and be equally tight any where, the smaller ring would not go on.

Fig. 9.

Fig. 10.-Internal.

Fig. 10.-External.

In fig. 11 the external gauge is shown with two parallel jaws, so that any part of the rod can be tested if either a tight or slack place be found when passing the ring gauge along it. By making every rod and gland to gauges in this way, it is absolutely certain that any rod and gland that may chance to be found in the stores may be relied upon to work satisfactorily together. The whole business of the engineer is to produce parts that will work together satisfactorily, and to do this he is constantly making working fits of his rods and glands; but instead of determining this allowance once for all in the form of limit gauges, he prefers to do it, as a rule, in pairs; thus at any time there is an opportunity for some one or other to alter the "difference" allowed, and to make it either too tight or slack.