How to use a depth micrometer

A depth micrometer is used to measure the depth of holes, slots, counterbores, recesses, and the distance from a surface to some recessed part. This type of micrometer is read exactly opposite to the method used to read an outside micrometer. The zero is located toward the closed end of the thimble. The measurement is read in reverse and increases
in depth amount as the thimble moves toward the base of the instrument.

The micrometer depth gage consists of a flat base attached to the barrel (sleeve) of a micrometer head. It has a range of 0 to 6 inches, depending on the length of the extension rod used. The hollow micrometer screw (the threads on which the thimble rotates) has a range of 1 inch. The flat base is 4 inches and six extension rods are supplied with this gage.

The measurement rods simply snap into the rod-receiving shaft. There are no set screws to mess with.

Calibration is simply made by inserting the 0-1 rod and turning the thimble counterclockwise so the measurement rod tip is recessed completely into the base. Firmly hold the base flush on a flat surface while turning the thimble ratchet clockwise until the measurement rod tip is stopped by the flat surface and the ratchet “clicks” three times. Tighten the thimble lock. The sleeve zero line should line up with the zero on the thimble scale. If they are not aligned, position the tip of the calibration wrench in the calibration wrench hole on the sleeve and rotate the sleeve to achieve alignment.

To make an actual measurement:
First choose the measurement rod suitable for the depth to be measured. Clean the mounting surfaces of the measurement rod and the rod-receiving shaft. Insert and rotate the measurement rod into the rod-receiving shaft to seat it completely. Turn the thimble counterclockwise so that when the base is flush on the top (reference) surface, the measurement rod does not touch the surface to be measured. Holding the base firmly on the reference surface, rotate the thimble ratchet clockwise until it “clicks” three times (as the measurement rod stops on the measurement surface). Read the depth measurement. No need to go into detail on how to actually read the scales – that instruction can be found in the outside micrometer section.

What there is to know about a Depth Micrometer