Case Study One

Troubleshooting Guide
CMM Repair Due to Poor Air Quality

At Inspec, Inc. we have found that about 70% of the CMM repairs we encounter are attributed to poor quality of air.

When our customers call in, continually encountering machine down time, due to poor quality of air, we schedule a meeting with their quality management team to determine a course of action.

First, the source of contamination will be identified. Secondly, a cost-effective course of action will be implemented, including installing additional filters and an Oil Mist Eliminator (see the following picture of an example of a Kaeser version). Lastly, we then complete a re-tube and clean the bearings.

In the event you feel that your air is contaminated, you will need to rectify the situation immediately. Based on the level of contamination, the best course of action would be to have the CMM's air tubing replaced, the air bearings removed and cleaned, and then a proper calibration performed using a laser system.

There are several low cost solutions available for improving the quality of your compressed air. Please feel free to contact us for additional information.

The Importance of Air Supply for Coordinate Measuring Machine (CMM) Air Bearing Systems

If the air supply specification is not met, the effects, not to mention repairs, to the CMM will be detrimental.

CMM's with air bearings are designed to float on air above the granite guideways. This non-friction air bearing system enables the CMM to achieve high accuracy's, whereas a ball or roller bearing cannot. Friction wear, contamination, and historesis are just a few factors that cause roller bearing systems to be less accurate.

Depending on the type of air bearing, (size, round or square, number of air orifices) the gap between the air bearing and granite surface will be approximately .0006 inches. This design is very rigid and controls both geometry and repeatability, as long as the air supply specifications are maintained.

The three factors regarding the CMM's input air supply, (*must be maintained at all times*) are quality (clean and dry), volume, and pressure.

Quality

If the air bearings or granite guideways become dirty, (oil, water, solid deposits) the CMM will not function properly. The contaminated air restricts the air flow through the air bearing orifices, thus seizing up the CMM. Contaminated air will damage both granite guideways and air bearings, not to mention deteriorate the CMM's accuracy.

The definition of clean, dry air is as follows:

Water Vapor:    36 degrees F @90 psi Pressure Dewpoint
Solids:    .12 parts/million by Weight Maximum
Hydrocarbons & Oil:    .01 parts/million by Weight Maximum Conc.

Volume

The CMM's air bearings orifices allow air to flow continuously. This continuous flow of air allows the air bearings to float freely and blow away any small (lightweight) particles of dirt or dust in its path while the CMM is moving along its axes.

A suitable volume of air is required to maintain pressure for optimum CMM efficiency. The volume of air will be dependent upon the number of air bearings, orifice size, and bearing size.

Pressure

The CMM's input air pressure has to be a constant 90 psi. This does not mean 90 psi at the compressor end. There is always a pressure drop between the CMM's input and air bearings. This is due to the CMM's pneumatic system and will vary depending on air tubing length and inside diameter. The pneumatic pressure drop usually will not exceed 8 psi, thus a minimum of 80 psi should be maintained for all opposing air bearings.

A clean environment, daily CMM and frequent filter/compressor maintenance will also protect the customer investment. This is the customer's responsibility and should be enforced.