Precision Repair & Calibration, Inc is ISO/IEC 17025 Accredited

Hard Gage Calibration Principles and Methodologies

Calibration is a comparison of a known to an unknown. A standard of known size is used to check a Unit Under Test. Four critical components of calibration are the operator, the gage being used to check, the Unit Under Test & environmental factors, such as temperature and humidity.

The operator is to be familiar with the calibration procedure, the gage being used to check should be of traceable size from its most recent calibration, the Unit Under Test should be cleaned of any foreign object debris and normalized to the environment where the test is to be taken, and the temperature/humidity of the area should be at/or correctable to 20 degree Celsius with no more than 45% humidity.

Methods of Calibration

Pin/Plug gage sizes represent their diameter. Diameter the distance between two points whose connecting line passes through the center. This allows for an infinite number of diameters on every pin/plug gage. By using equipment that measures the distance between these two points, the size of the pin/plug gage is determined. Calipers, micrometers, Universal Linearity Machines & bench micrometers are all examples of two-point measurement equipment described as such:


A standard caliper is a hand tool that consists of a sliding jaw measuring against a fixed jaw. Parallelism of these jaws ensures the consistency of measurement, no matter where on the jaws the measurement is taken. The cylindrical gage is placed between the jaws, which are then closed onto the pin/plug gage, giving a measurement of diameter.


An outside diameter micrometer is a hand tool that works by bringing a movement anvil towards a stationary anvil. Again, parallelism of the anvils is maintained to ensure the entire surface of the anvils may be used. Flatness is another measurement monitored to reduce error is measurement. A concave or convex anvil will not read the same throughout the entire surface of the anvil.

Universal Linearity Machine

A Universal Linearity Machine, or ULM, consists of adjustable ends; one of which has a moveable arm and the other a stationary arm of adjustable length. Anvils are attached to these ends and length standards of known size are used to set the ULM. The pin/plug gage is then placed between the arms and a measurement is taken. Large gages are fixed on a table that has vertical movement. A diameter can then be taken at different heights once the gage is secured. Sine error is minimized by tilting the stationary table to make the anvil point of contact as perpendicular to the gage under test as possible.

Bench Micrometer

A bench micrometer is a lot like a micrometer hand tool, except it’s much larger and has an adjustable stationary anvil. These micrometers have linearity, flatness and parallelism that are checked with traceable standards. A cylindrical gage is then measured between the anvils. Force of the movement anvil is adjustable. Cylindrical gages have recommended forces to which they are to be checked.

For more info on Precision's principles and methodologies contact us.

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