This instrument, a type of which is shown in Fig. 170, is used for measuring thicknesses and external diameters. It is the instrument of the greatest degree of precision used in machine-shop measuring.
Most micrometer calipers are made to show, on a graduated stem or barrel, readings of measurements to 1/1000 of an inch or to 1/100 of a millimeter if graduated in the metric system. Some of these instruments are supplied with verniers for showing readings varying by 1/10000 of an inch, but measurements of less than 1/1000 of an inch are seldom used.
Material is measured between the points B of hardened steel, one of which is fixed in the half-round frame. The enclosed end of the spindle C is screwed into a fixed sleeve A, and when the spindle is turned, its threads cause it to move in the direction of its length. These threads have a pitch of 40 to an inch, hence one turn of the spindle moves it 1/40 of an inch. The sleeve A is about as long as the distance between the ends of the frame. A hollow thimble D fits neatly over the sleeve, and the right-hand end of the spindle is fastened to the bottom of the thimble so that the thimble is used to turn the spindle and to gage its movements.
Fig. 170. - Micrometer Caliper.
The tapered edge of the thimble is divided around its circumference into 25 equal parts, and a line along the sleeve A is graduated into divisions of fa of an inch. The first of these graduations is marked zero.
When the space between the measuring points is closed, the zero line on the edge of the thimble falls on the line along the sleeve, and as the thimble is turned 1/25 of a revolution (or one of the graduations on its edge) it separates the points 1/25 of 1/40 = 1/1000 of an inch. In this way the readings of 1/1000 of an inch are observed.
Micrometer gages similar to Fig. 164 are made for refined measurement of internal diameters.