Final Comments
The proper place for a non-covered, air bearing CMM is a clean environment with a well maintained air line. In our opinion they should not be used on the shop floor. Yet, this is a turf the air bearing CMM manufacturers are invading by making false claims.
See a typical example below.
“cost effective metrology frame, delivering accuracy for
shop floor or lab at an affordable cost”
– Sheffield
Here they claim, “accuracy for shop floor”. Note the absence of any protective covers on the bridge or on the surface plate where the leg rides. (See “Interim Testing of CMM Systems” article). Statements like these are unfair to companies who build REAL shop floor machines. To us a “real world condition” is a place where a dozen machining centers spit oil and chips into the air which then settles onto every horizontal surface in the room. This particular CMM would not last much longer than 2-3 weeks before running into trouble.
Sometimes enclosures are being used to overcome this shortcoming, see example at right. Although they most likely are effective in keeping contaminants away from the CMM, they still remind us of incubators used for infants who are not ready to face the real world. Enclosures restrict access to machines and work pieces making it awkward to load and unload heavy parts and still leave the machines exposed every time the doors open. Enclosures are not suitable at all for manual units, quick spot checks or low runs.
To add insult to injury: These enclosures often cost more than the CMM inside.
Since it is impossible to work inside the enclosure, the CMM has to be removed for service and every minor adjustment, adding to the lifetime cost of the system.
The effectiveness of air conditioners in these enclosures is questionable. Every time the door is opened for loading or unloading the CMM gets a jolt of hot air; which affects the measuring accuracy. In non-automatic applications doors are often left open by the operator.
With a properly covered mechanical bearing CMM all the above becomes a non-issue. Some buyers have already smartened up to this fact.
Below is a copy of an RFQ which was sent to us by the Delphi Division of General Motors where it specifically states: No air bearings… No enclosure!
The prospective buyer must educate himself. Misconceptions about CMMs are widespread, not only about bearings and reliability but about software, programming efficiency, user interfaces, accuracy and speed.
In conclusion we may state that the service + maintenance costs are certain to be lower with mechanical bearing machines. This fact and the legendary longevity of Helmel CMM’s assures you that the true cost of owning a Helmel machine, in time, is always less than an air bearing machine.