Proper calibration of a depth micrometer's starting zero or reference point is vital to the use of the tool. A person only employs a depth micrometer when an accurate measurement from one plane to another is needed, and having a misaligned zero point means the results the tool provides are inherently faulty. Most depth micrometers ship with a set of rods calibrated to the tool at the factory, so minimal adjustments are needed when changing out rods.
Retract the micrometer rod until it reaches the minimum tolerance for that particular rod. Retract a 2 to 3-inch rod to exactly 2-inches, or completely close a 0 to 1-inch measuring rod.
Lay the reference base of the micrometer on a flat surface calibrated to the minimum tolerance of the rod being adjusted. Lay the micrometer's base directly on a flat surface, such as a table or granite block, when calibrating a 0 to 1-inch rod, or use calibrated gauge blocks to raise the reference height of longer rods. For example, when calibrating a 2 to 3-inch rod, lay the reference base on two 2-inch gauge blocks and rest the tip of the rod on the flat surface the blocks are resting on.
Examine the measurement lines on the fixed sleeve and rotating thimble of the depth micrometer. If the line "0" line on the rotating thimble does not line up exactly with the horizontal datum line on the stationary sleeve, an adjustment needs to be made.
Open the micrometer's case and remove the micrometer spanner wrench. Use a micrometer spanner wrench from another micrometer if one is not included with the depth micrometer.
Grasp the depth micrometer's sleeve with the spanner wrench. Rotate the sleeve until the "0" measurement line on the thimble aligns perfectly with the horizontal datum line on the sleeve. Make sure the relationship between the reference base and the contact surface of the rod tip is maintained while you make any adjustments.
If the micrometer develops any looseness or "play," open the thimble until you can see the micrometer's adjusting nut. Use a micrometer spanner wrench to tighten the nut slightly and eliminate any play. Perform this procedure rarely, and avoid over-tightening the nut. In factory settings, supervisors or inspection team members are often the only ones allowed to adjust measurement tooling.
Avoid using interchangeable rods that were not included with the micrometer.