piece of work it is best to hold the caliper in a vertical position (101) with the legs at right angles to the axis of the piece, and adjust to a point where the weight of the calipers will just allow the points to pass over the dia - meter of the piece. This slight resistance i s known as the "feel". Never force the calipers over the piece as the legs will spring and inaccuracy will result. If you get the same "feel " on the sample as you get on the piece being turned, the diameters will correspond within close limits. Never t r y to caliper a piece revolving in the lathe where accuracy is required.
For obtaining the approxi - mate diameter this i s permissible, but for accurate dimensions, the lathe should be stopped. "D" when the " mike " is set on a diameter i s the first numeral after the decimal point in your reading. On the bevel edge of the thimble " E " are twenty - five grad - uations, each of these representing one thousandth of an inch. In illustration (103) you have showing on the sleeve seven graduations representing . 025 each, equal - ing 7 x . 025 or .
175, and three graduations on the thimble beyond the zero (0) mark, each representing .001, so that the caliper is set at .175" plus .003", equaling .178", o r for greater convenience read the highest nu m - era1 showing on the sleeve, 1 or one - tenth (. 1 ); beyond this read the number of graduations showing (3), which equals 3 x .025" or .075", then add to this the graduations on the thimble, making .l78". For convenience in using micrometers, tables of fraction and decimal equivalents, also English and metric equivalents, will be found in Tables section. When calipering with micrometers the same " feel " is necessary a s with other calipers, and they should not be forced over the work.
Hold the " mikes " between the forefinger and the thumb (104) and let the weight carry them over the diameter. Do not caliper with the piece revolving a s this will damage the anvil and the end of the spindle. Check the " mikes " occasionally with a reference disc (standard gauge) to see that they register correctly. OUTSIDE MICROMETER CALIPER The micrometer caliper (102) is used for measuring to very close dimensions, its graduations reading to one thousandth of an inch. It consists of five principal parts, the frame " A " , the anvil " B " , the spindle " C " , the sleeve "D" a nd the thimble " E " .
The spindle " C " has a thread cut on it which fits a tapped hole in the sleeve "Dl1 which is not exposed. The threads a r e cut 40 to the inch so that exactly one revolution of the thimble "EM advances the spindle " C " one - fortieth of an inch, or twenty - five thousandths (.025) which is the gap be - tween the anvil "B" and the spindle " C " , the measuring point. The sleeve "D" is graduated with 40 divisions to the inch and numbered 0 - 1 - 2, etc. to 10, a number at every fourth division, so that figure one, o r four divi - sions, represent 4 x .025 of an inch (1/4 0 ) or one - tenth of an inch (. 100). The number last showing on the sleeve