Micrometers are used to measure thickness, diameters, and depth of slots with small distances. The rotations of the screw and thread forms are read from a scale and determine the accuracy. A micrometer is composed of a frame, anvil, barrel, lock nut, screw, spindle, thimble, and ratchet top. The frame is the thick C-shaped body that holds the anvil and barrel. The thickness minimizes contraction and expansion to maintain measurement accuracy. The sample rests against the anvil while the spindle moves towards it. The barrel is the stationary component with the scale on it. The lock nut is used to hold the spindle stationary. The screw is inside the barrel. The spindle is a cylindrical component that moves toward the sample and anvil. The thimble has graduated markings on it and is turned to move the spindle. Lastly the ratchet stop is at the end of the handle and limits applied pressure.
When calibrating a micrometer, the standard accuracy must be greater than a 4:1 ratio over the accuracy of the gage being calibrated. The gage block must have a certificate of NIST traceability to have an accurate standard.
Here are some guidelines for a five-point calibration on micrometers:
- Look at the frame for any signs of damage
- Make sure the spindle and anvil faces are flat, free of pinholes, and are clean
- Check to see if the spindle feels smooth while moving the micrometer from 0 – 25ml. If it is hard to turn, there could be some damage. The micrometer should turn smoothly throughout the range
- Make any repairs that are needed to be done before proceeding
- Check five points with traceable gage blocks. For an example, you could take the entire range, split it in half, split it in half, split it in half again and split it for the last time. The zero point is repeatedly checked but not reported; and if you have a measurement point that is checked daily, then you should include that number in the calibration procedure.
To make sure your micrometers are calibrated accurately, you should have them done by an accredited calibration lab.