Fig. 36. Handle for Inside Micrometer
Fig. 37. Wrench used with Inside Micrometer
The point F, which is hardened on its outer end, screws into the frame, and is secured by the taper-headed screw O, which screws into and expands the split and threaded end of the point F. The handle, Fig. 36, clamps over the knurled part of the frame for use in small, deep holes. The rods, six in number, running from 1 to 6 inches inclusive, are made by screwing a sleeve onto a rod with a hardened point and locking it with a taper-headed screw on its threaded and split end, the same as in the point F. The extension pieces, Fig. 38, are adjustable, on their socketed ends, in the same way, and run in lengths of 6, 12, 18 inches, etc.[[19]]
Fig. 38. Adjustable Extension Pieces for Inside Micrometer
Direct Fractional-reading Micrometer
Fig. 39. Direct Fractional-reading Micrometer
The direct fractional-reading micrometer shown in Fig. 39 is the result of talks with many mechanics in which all agreed that such a feature added to a micrometer would, by making it both a fractional and decimal gage, more than double its practical value. While approximate readings in 64ths, etc., may be obtained by the graduations on the barrel B as on an ordinary inch scale, the exact readings of 64th, etc., may be obtained only by reference to graduations on the movable thimble A. There are but eight places on A which coincide with the long graduation line on B when any 64th, 32d, 16th, or 8th is being measured, and each of these eight places is marked with a line, and the 64th, 32d, 16th, or 8th for which that line should be used is marked thereon. (See a and b, Fig. 40.) The line a would be used for 3/32, 7/32, 11/32, etc., and the line b for 1/64, 9/64, 17/64, etc. Now suppose we wish to accurately measure 15/32 inch. We first roughly read it off the inch scale on sleeve B by turning out thimble A. Having secured it closely by drawing edge of A over that graduation, we find that the line a (Fig. 40) on the movable thimble very nearly or exactly coincides with the long graduation line on B. When these lines coincide, we have the exact measurement of 15/32 inch without reference to how many thousandths may be contained in the fraction. Thus all through the scale any fraction may be found instantly. There is no mental arithmetic, use of tables, or memory work in using the tool. The new graduations are independent of the old, and may be used equally well with or without them.