Getting Back to CMM Basics

November 2007 Vol. 139 No 5

Quality Scan Getting back to CMM Basics

If you make a product, it’s reasonable to assume that you constantly strive to improve the quality of your product. What do you do first? Instinctively, you review designs for improvements, seek better materials, optimize processes and packaging, even improve the customer experience. There are many paths to improved quality, and you should consider better testing among the most important.

When we focus on dimensional integrity, the Coordinate Measuring Machine is very often the “grand filter” through which products must pass, becoming pivotal to the quality function. Yet, when bad habits emerge, the CMM falls into neglect more than we realize. Recovery can be a simple matter of getting back to basics.

Consider the environment your CMM lives in. There is no point to measuring dirt. Cleanliness is germane to precision measurement. While every CMM does not have to be in a clean room, its location must be clean. Allowing the CMM and its surroundings to become dingy contributes to the feeling that its purpose is less than important, resulting in carelessness, increased inspection time, and rejection of good parts. It’s also useful to point out that the CMM is not a workbench or storage shelf.

Parts being inspected should be clean of chips, burrs, dirt and oils, for the same reasons. Dirt and oil can accumulate on a stylus very quickly, introducing errors and causing stylus wear.

Provide enough lighting so operators can clearly see the features being measured. The importance of a bright environment is heightened when creating and testing programs for inspection, to ensure parts are probed where intended. Lights also reveal dirt and chips.

Temperature must be controlled in a range that relates to the material and tolerances measured. In practice, ±5°F (±2.8°C) is OK for a tolerance of ±0.010″ (0.25 mm) over 12″ (305 mm) of cast iron, but it is not good enough for a tolerance of ±0.001″ (0.025 mm) over 12″ of aluminum. You have to do the math—wishful thinking sinks ships.

Energy in the floor will find its way into CMM measurements unless it is isolated. Fork trucks driving by, stamping presses pounding away, even a rail siding outside, are sources of vibration that can cause trouble. Sources or processes can be isolated, or the CMM can be placed on vibration-isolation devices.

Good practice is important. Make sure CMM operators are properly trained. Often the original trainees are gone, and subsequent users are left to pick up what they can. Send operators for CMM and GD&T retraining with some frequency, and always train new users. Maintain a software service contract, if it represents good value, for updates and application support.

Develop procedures or guidelines for CMM use and CMM programming to prevent a free-for-all. A good CMM operator will plan an approach to a new part in advance, whereas many just start randomly measuring. Efficiency is lost, and results become suspect. For example, one operator might measure an ID with four points when the next uses twelve points. While more data are generally better, guidelines should be specific for uniformity and repeatability.

Have your CMM professionally serviced and calibrated at required intervals, including annual preventive maintenance. Even though you calibrate the styli with the calibration ball, you should confirm the machine calibration on a more frequent basis as well. Some measure a “golden part” that may have been certified, or simply ring or plug gages or gage blocks, placed in various locations and positions, to ensure the CMM still measures accurately. Ball bars or step gages are excellent devices for interim checks of the Coordinate Measuring Machine.

Electrical power should, as a minimum, have surge suppression. An uninterruptible power supply (UPS) is better. Compressed air, if required, must be clean, dry, and free of oils, and may require a chiller/dehumidifier and a two-stage filter. Avoid loading unrelated applications on the CMM PC, as conflicts can arise.

A casting clamped on a machining fixture with 3000 lbf (13.3 kN) may well have a different shape when it “rests” on the CMM for inspection without clamping. Softening clay by hand to hold a part on the CMM can transfer heat to thinner sections of the part, and distort its shape. Be realistic in implementing metrology scenarios lest they come back to bite you.

If you accept that the CMM is crucial to your quality efforts, then using it correctly and trusting the results is mandatory. Adhering to these basics can improve your CMM utilization, reliability, and (ultimately) your product quality.