But a set of plug and collar gauges provide within themselves to a certain extent the means of testing them. Thus we may take a collar or female gauge of a certain size and place therein two or three plug gauges whose added diameters equal that of the female or collar gauge.
Fig. 1396.
In [Fig. 1396], for example, the size of the female gauge a being 11⁄2 inches, that of the male b may be one inch, and that of c 1⁄2 an inch, and the two together should just fit the female. On the other hand, were we to use instead of b and c two males, 7⁄8 and 5⁄8 inches respectively, they should fit the female; or a 1⁄2 inch, a 5⁄8 inch and a 3⁄8 inch male gauge together should fit the female. By a series of tests of this description, the accuracy of the whole set may be tested; and by judicious combinations, a defect in the size of any gauge in the set may be detected.
Fig. 1397.
The wear of these gauges is the most at their ends, and the fit may be tested by placing the plug within the collar, as in [Fig. 1397], and testing the same with the plug inserted various distances within the collar, exerting a slight pressure first in the direction of a and then of b, the amount of motion thus induced in the plug denoting the closeness of the fit.
In trying the fit of the plug by passing it well into or through the collar, the axis of the plug should be held true with that of the collar, and the plug while being pressed forward should be slightly rotated, which will cause the plug to enter more true and therefore more easily. The plug should be kept in motion and not allowed to come to rest while in the collar, because in that case the globules of the oil with which the surfaces are lubricated maintain a circular form and induce rolling friction so long as the plug is kept in motion, but flatten out, leaving sliding friction, so soon as the plug is at rest, the result being that the plug will become too tight in the collar to permit of its being removed by hand.