Outside Micrometer

A micrometer is a measuring instrument that can make extremely accurate measurements. Most micrometers are designed to measure to within one-thousandth of an inch! Accurate measurements like this are necessary when even the smallest space between objects can cause problems or difficulties. Several types of micrometers are designed to measure different types of objects or spaces. The outside micrometer, in other words the outer diameter (OD) micrometer, is intended to measure the outer surface of objects. By spinning an internal screw, they look and shift just like a C-clamp that opens and shuts. The workpiece you intend to weigh is clamped between the anvil and the spindle within the micrometer. Using the counting scheme on the center after adjusting the object in the clamp to locate the measurement.

Outside Micrometer Components


It is a C-shaped frame as shown in the picture, it is a rigid part that has both attachment points to the measuring task or object. Its size depends on the measuring range of the micrometer, so the size of the c-frame increases with increasing range. Its main function is to provide the basic structure of the micrometer in which a stationary anvil is placed at one end and a movable spindle moves in or out through the other end of the c-frame.

Zero adjustment screw

It is a screw located on the back of the anvil shown in the picture. As the name suggests, it is used to correct or correct a micrometer zero error if an error is found during the test before measuring.


As discussed above, this is a small stationary cylindrical portion of the micrometer located at the distal end of the c-frame and acts as a single holding point for measuring objects. So we can say that it is a rigid measuring and holding point of a micrometer.


A cylindrical long part that is assembled by the sleeve of all other parts, locknut, and drum. It is a movable part and has a connection with a ratchet, when we turn the ratchet clockwise or counterclockwise, the spindle extends or inwards to adjust it with the size of the measured object.

Anvil surface and spindle surface

The surfaces of the anvil and spindle that are opposite to each other are the micrometer measuring points and hold the workpiece together.

Maintenance of an Outside Micrometer

Clean the measuring surfaces with a clean cloth before and after measuring. It is also good practice to periodically clean the spindle to prevent dirt from entering the bushing. Use the holder on the thimble when you need a long stroke, but when you get close to the object to be measured, use a ratchet lock to avoid tightening the thimble too much and give an erroneous reading. Never leave the micrometer exposed to the hot sun and then take a measurement. It would also lead to misreading. When the micrometer reaches the minimum reading, the horizontal line on the sleeve should line up with the "0" on the cartridge. If this is not the case, it will be necessary to calibrate the micrometer by turning the sleeve.

How to read an Outside Micrometer?

On a regular spindle, the screw pitch is 40 threads per inch. One thimble rotation moves the spindle face precisely towards or away from the anvil face, which is the difference between two pitches on the sleeve. The reading line on the sleeve is divided by vertical lines which correspond to the number of threads on the shank into forty equal sections. Each vertical line, then, represents 0.025 inches. The slanted edge of the thimble is divided into 25 equal parts, each line representing 0.001 inches. The thimble's slanted edge is broken into 25 equal parts, each line representing 0.001, and each line may or may not be numbered, but every fifth line is numbered sequentially. Multiply the number of vertical divisions on the sleeve by 0.025 inches to read a micrometer in thousandths and apply the number of thousandths shown by the line on the thimble that better fits the long middle line on the sleeve.

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