Inspec Insights

Just How Important is Air Quality and Quantity for CMMs with Air Bearings?

by Tom Snyder, Vice President Service

The importance of maintaining a clean, dry air supply cannot be emphasized enough. Where the Coordinate Measuring Machine’s (CMM's) air supply specifications are not met, the negative effects to the CMM, at a minimum reduce accuracy, and can be very costly when repairs are required.  We attribute 80% of the CMM repairs we perform are directly related to poor or insufficient quantity of air supply.

Air-bearing supplied CMMs, are designed to float on air above its guide ways.  This non-friction system enables the CMM to achieve higher accuracy compared to a ball or roller bearing system.  Friction, wear, contamination, and hysteresis are just a few factors that can 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 guide way surface will be approximately 0.0004 inches (0.010 mm).  This CMM bearing design is very rigid and controls both geometry and repeatability, as long as the air supply specifications are met.  The three factors affecting the CMM’s input air supply which must be maintained at all times are quality (clean and dry), volume and pressure (quantity).

Quality:
It is best to follow your manufacture's recommendations for quality however; a basic guideline of clean, dry air is as follows:

Water Vapor:                        36 degrees F @90 psi Pressure Dewpoint

Solids:                                   No particles > 1.0 µm and max concentration of
                                              1mg/cu.m of air at standard

Hydrocarbons & Oil:          Max concentration 0.1mg/cu.m of air at standard.

If the air bearings or guide ways become dirty from oil, water or solid deposits the CMM may not function or measure properly.  The contaminated air restricts the air flow through the air bearing orifices, reducing air bearing lift, causing them to drag and possibly seizing up the CMM.  Contaminated air will damage both the guide way and the air bearings, not to mention deteriorate the CMM’s accuracy.

Quantity:

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 constant.  However, this does not mean at the compressor end, as there is always a pressure drop between the compressor, and the CMM. Air pressure should be monitored right at the CMM. 

Refer to the manufacture’s recommended air Volume (cfm) and Pressure (psi) setting and scheduled maintenance.  Frequent filter replacements and regular compressor maintenance will maintain a clean, dry air supply to your CMM. 

A clean environment and conducting scheduled CMM maintenance should also be enforced within your quality system and personnel made responsible. These preventative maintenance measures will protect your investment, keep it measuring accurately and help ensure product quality for a long time. 

3 comments

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  • Simon Bleakley | Nov 15, 2017
    Does this mean that when the CMM is not being used it still has to have air pressure?

  • Erika Brady | Oct 31, 2016
    Having a clean, dry air supply is not something that I often put on the top of my to-do list. Reading this was a good reminder. I like that Air-bearing supplies are designed to float on air above it's guide ways. I am sure that helps with the quality of the product. jttp://alioindustries.com/product-systems/ait-bearing-systems/

  • Williams Rolling | Mar 24, 2016
    The cоntаminаtеd аir rеstricts the air flоw thrоugh thе аir bеаring оrifices, rеducing air beаring lift, cаusing them tо drag and pоssibly seizing up the CMM

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