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Dial indicators help users measure the distance between two plates, but the readings given by the tool could be faulty. Any measurement tool can fall prey to a myriad of possible factors that can create false measurements. Slipped teeth or incorrect zero settings result in wildly varying and incorrect readings. Calibrating a dial indicator prior to using it is the only way to be sure that the tool is reading correctly and its measurements are accurate.
Wipe the surface plate, the gauge blocks and the tip of the dial indicator's plunger to remove any dirt or debris. Lock the dial indicator in place on its stand, resting the indicator high enough to allow the plunger to travel its full length and still be able to touch off against the surface plate.
Rest the tip of the plunger directly on the surface plate. Rotate the indicator's face until the "0" marking aligns with the dial.
Raise the plunger and slip a 0.020-inch gauge block underneath. Lower the plunger onto the surface of the gauge block. Check that the indicator reads 0.020-inch. Raise and lower the plunger two more times to check that the dial indicator gives the same measurement in the same spot repeatedly.
Repeat the process with 0.025-inch, 0.050-inch, 0.100-inch, 0.250-inch, 0.400-inch and 1-inch gauge blocks. Check each block three times to ensure the dial indicator's repeatability. If the indicator's plunger reaches a 2-inch depth, stack the blocks to check the dial indicator at 1.025-inch, 1.050-inch, 1.100-inch, 1.250-inch, 1.400-inch and 2-inch increments as well.
Reject the dial indicator if its readings are off by more than 1 increment over the first 2-1/3 revolutions of the dial. Allow the dial indicator's readings to be off by 1 additional increment for each full rotation of the dial thereafter. Compensate mentally for the incremental offset when taking readings with the tool.
Calibrate a dial indicator daily, or whenever the tool is used if it is used less frequently. Refer to the facility's measurement tooling procedures if using a dial indicator in the workplace.