Fig. shows an outside micrometer. It consists of the following main parts.
1. Metallic frame
2. Axial graduated sleeve
3. Circumferential screwed spindle
4. Hardened steel anvil
6. Ratchet stop screw
7. Lock nut
Micrometer works commonly on the principle of nut and bolt assembly. The sleeve carries inside threads at the end, which forms the nut, and the screwed part of the spindle passes through it. The spindle and the thimble are secured to each other such that by rotating the thimble the spindle rotates. With the result, when the thimble is revolved, it advances towards or retards away from the fixed anvil, together with the spindle of the micrometer. The sleeve carries the graduations, which, in conjunction with the beveled and graduated part of the thimble, give the measure of the opening between the end faces of the anvil and the spindle. The ratchet arrangement provided at the end of the thimble prevents the spindle from pressing further against the surface of the piece being measured after the required feel has been attained, thus facilitating a uniform reading and preventing the instrument from being damaged. Lock nut or locking lever is used for locking the micrometer for a desired amount of time after taking or setting the reading. The construction of the outside micrometer is discussed as under.
COMMON PARTS OF OUTSIDE MICROMETER
The U frame of micrometer is made of steel, cast steel, malleable cast iron or light alloy.
(2) Hardened anvil.
It protrudes from the frame for a distance of at least 3 mm for holding and supporting the jobs for measurement.
(3) Screwed spindle.
It does the actual measuring and possesses threads of 0.5 mm pitch.
(4) Barrel or Sleeve.
It has datum or fiducially line and fixed graduations.
This is a tubular cover fastened with the spindle and moves with the spindle. The beveled edge of the thimble is divided into 50 equal parts, every fifth being numbered.
This part is commonly recognized as friction stop of the micrometer, which acts as a precautionary measure also. It is a small extension to the thimble in which the ratchet slips when the pressure on the screw exceeds a certain amount. This produces uniform reading and prevents any damage or distortion of the instrument.
(7) Spindle clamp.
It is used to lock the instrument at any desired setting or at any particular reading .