The CMM’s required level of accuracy will depend on the parts to be measured. A CMM used to measure castings at a forge will not have the accuracy requirements of the same CMM at Oak Ridge National Lab.

In fact, as parts become more expensive to manufacture, the expense for the re-calibration of the CMM becomes less significant. For example, as the CMM is used, over time, its accuracy degrades. This can lead to measuring parts that appear out of tolerance, when in fact they are actually in tolerance. These rejected parts quickly add up and soon the cost of the rejected parts is more expensive then the re-calibration. Of course parts may be measured as in tolerance, when they are not, which can cost a company too.

Every CMM has its own characteristics and will behave differently, depending on its environment, material composition, and age.

The environment is very important. The room temperature should be kept as stable as possible. Today, most CMMs are constructed out of half a dozen or more different materials. These materials expand and contract differently due to temperature changes. Having a high temperature swing throughout the day can increase the need for re-calibration. A temperature chart recorder is helpful in diagnosing this condition.

The CMM’s air supply is also a contributing factor. Avoid turning air pressure on and off if possible since air regulators do not always return back to the same setting, and with air bearings having sixty to eighty millionths air gap one or two PSI can make a difference. Low air pressure is also a problem; it will prevent the axes from floating correctly and creating a strain on the drive system. This will not only have the potential to burn out the motors but will also score the air bearings and require their replacement.

As was addressed by Thom Hoey in the last issue of Tech Notes, dirty air is probably the most common reason for a complete overhaul and should be monitored very closely.

In terms of a CMM’s materials, a well built CMM, with a base made out of solid granite, like our Legend for example, is much more stable than one whose base and frame are built out of a variety of different materials, or less stable materials, such as aluminum.

Also, an older CMM, say five to ten years old, which hasn’t been moved during that time, will have settled and usually will have very dependable behavior patterns, and thus usually will not need to be re-calibrated as often as a new CMM.

Once you have a good understanding of your CMM’s characteristics, the best way to stretch out the time between calibrations is preventative maintenance. The guide ways should be free of any dust, oil, fingerprints, coffee stains, etc. Some machines require the drive band to be cleaned and lightly oiled to prevent rusting. A clean CMM insures trouble free operation and a longer time between calibrations.

If the level of accuracy you want to maintain is relatively close to the CMM accuracy specifications, I recommend a more aggressive calibration schedule, perhaps as often as four months, but no more than every six months until you have a good, reliable history of your CMM. Then, once a year for a recalibration will most likely be sufficient.

Remember, if you want to save money on calibration, all too often you waste money on rejecting good parts or having bad parts accepted -- problems that down the road will come back to haunt you.