Fig. 1419.
Inside and outside calipers are capable of adjustment for very fine measurements; indeed, from some tests made by the Pratt and Whitney Company among their workmen it was found that the average good workman could take a measurement with them to within the twenty-five thousandth part of an inch. But the workman of the general machine shop who has no experience in measuring by thousandths has no idea of the accuracy with which he sets two calipers in his ordinary practice. The great difference that the one-thousandth of an inch makes in the fit of two pieces may be shown as in [Fig. 1419], which represents a collar gauge of 5⁄8 inch in diameter, and a plug 1⁄1000 inch less in diameter, and it was found that with the plug inserted 1⁄8 inch in the collar it could be moved from a to b, a distance of about 5⁄16 inch, which an ordinary workman would at once recognise as a very loose fit.
Fig. 1420.
Fig. 1421.