Metrology is the study of measurement. Accurate measurement is necessary to determine whether a part falls within the tolerance specified on a shop drawing. Accurate measurement requires measuring tools capable of a greater degree of accuracy than the measurement that you need.
Calibrating a measuring tool involves setting the zero point and checking its measurement against a variety of gauge blocks across the range of the measuring tool.
Gauges are metal or ceramic shapes manufactured to extremely tight tolerances so that they can be used as measuring devices (i.e. they can be used to gauge the size of an opening).
Gauge Blocks are cube shaped gauges.
Pin Gauges are cylindrical gauges.
If a set of gauges is marked as a plus set this means that it is slightly oversized (by 0.0002").
If a set of gauges is marked as a minus set this means that it is slightly undersized (by 0.0002").
Learn about the calibration and operation of the tools below so that you can make accurate measurement
A caliper has two sets of jaws. The larger jaws are used to measure outside dimensions (OD).
The smaller jaws are used to measure inside dimensions (ID).
Notice how I'm turning the outer most knob to tighten the micrometer down onto the part. This outermost knob will make an audible click when enough force has been applied to give a reliable measurement. Keep in mind, though, that turning this knob too quickly as it tightens on the part can overtighten the micrometer which will give a slightly smaller measurement.
Also, it's worth checking a part dimension several times so as to get a feel for when you have successfully held the part in the center of the vise without any twisting, which will cause the measurement to be longer than the actual dimension of the part.
Using Mitutoyo Micrometers#
Check out this short video and then practice calibrating a Mitutoyo micrometer.
Job Shop Measuring and Metrology Tips#
Check out this more in-depth video of NYC CNC discussing using Mitutoyo micrometers on the shop floor.