Fig. 1406.
Fig. 1407.
For gauging work of very large diameter, as, say, several feet, to minute fractions of an inch, as is necessary, for example, for a shrinkage fit on a locomotive tire, the following method is employed. In [Fig. 1406] let a represent a ring, say, 5 feet bore, and requiring its bore to be gauged to within, say, 1⁄100 inch. Then r represents a rod made, say, 1⁄2 inch shorter than the required diameter of bore, and w, [Fig. 1407], represents a wedge whose upper surface c d is curved, its lower surface being a true plane. The thickness at the end c is made, say, 51⁄100 inch, while that at d is 48⁄100 inch; or in other words, there is 3⁄100 of an inch taper in the length of the wedge. Suppose then that the rod r is placed in the bore of a as in figure, and that the wedge just has contact with the work bore and with the end of the rod when it has entered as far as e in [Fig. 1407], and that point e is one-third of the length of the wedge, then the bore of a will measure the length of the rod r plus 49⁄100 of an inch. But if the wedge passed in to line f, the latter being two-thirds the length of the wedge from d, then the bore would be 50⁄100 larger than the length of the rod r. It is obvious that with this method the work may be measured very minutely, and the amount of error, if there be any, may be measured.
The rod must be applied to the work in the same position in which its measurement was made, otherwise its deflection may vitiate the measurement. Thus, if the rod measures 4 feet 111⁄2 inches when standing vertical, it must be applied to the work standing vertical; but if it was measured lying horizontal, it must be applied to the work lying horizontal, as there will be a difference in its length when measured in the two positions, which occurs on account of variations in its deflection from its own weight.
Fig. 1408.