Interim testing strategies for Coordinate Measuring Machines

Coordinate Measuring Machines (CMMs) typically undergo rigorous performance testing and calibration on a yearly basis. This annual calibration is a necessary part ensuring traceability of measurements, but there are many situations that can occur over the course of a year that result in measurement errors. Certain events, such as a 'crash' of the CMM, will result in a permanent change in the state of the CMM with the subsequent measurement errors. Other sources of error may be more subtle, such as the effects of the CMM cooling or heating over a weekend if the environmental control system is not active. The absence of interim testing data poses several problems. First, in the event that the 'as found' data for the annual calibration is well out of specification, there is no way of telling when the CMM ceased to be capable of performing measurements within the manufacturer's specifications. Second, if there are periodic errors due to temperature swings or other environmental conditions, there is no way of determining if these errors were present when a particular measurement was performed. This paper will discuss some of the common sources of CMM errors, and the types of tests that can reveal these errors. Different interim testing strategies will be evaluated with respect to the trade-off between the errors revealed and the time required to run the tests.