Maintenance of CMM Machines
The coordinate measuring machine mainly runs trouble-free year after year, interspersed with an annual calibration and preventative maintenance visit. When it doesn’t, there are generally good reasons for it. Maintenance costs will be higher if the machine is placed on the shop floor, but the convenience might often be worth it. Some machines are better suited to shop floor use than others. Hard bearing machines are particularly suitable for shop floor work. Therefore, expect higher maintenance costs for a shop floor machine.
20% of breakdowns and maintenance costs on CMM Machines are caused by poor air quality. A refrigerant air dryer should be considered on all air bearing machines, regardless of the machine's location but especially if the machine is a shop floor machine. In most factories, airlines run just under the plant ceiling. Condensation often occurs within the airlines, and water permeates the CMM, which damages the bearings and short-circuits the electronics. Unfortunately, many factories have a habit of putting oil in the air supply, which works well on machine tools, but it destroys coordinate measuring machines. Moisture and oil in the airlines causes degeneration of the internal piping and blockage of the air bearings. Collapsed clogged air bearings cause significant damage to the bearing ways.
Most machines that have chronic service problems can have the cause traced back to power supply problems. All coordinate measuring machines should have a power conditioner/battery backup system installed, as it’s not just current fluctuation that causes problems but noise transmitted through the machine tools operating on the same circuit.
Keeping the CMM machine clean is the best insurance for continued reliability. The bearing ways on many of the newer brands of CMMs are generally exposed. Therefore, it’s important these ways are wiped clean daily, especially on the shop floor-based systems.
On bridge machines the slave leg runs on the surface plate, and when dirt gets on this portion of the plate, it tends to embed under the bridge bearings. Ultimately, this causes a drop in precision and motor drive problems ensue. When scales are exposed, it’s also important to gently wipe them down with isopropyl or methyl hydrate.
The computer and controller filters should also be inspected weekly, and replaced if necessary, in dirty shop floor environments. This is especially critical, because overheating of electronic components can cause serious and expensive breakdowns.
Preventative maintenance visits once or twice a year, depending on the CMM environment, are highly recommended. The following is a checklist of things that should be inspected:
- Check all filters (PC and controller)
- Visual inspection of all bearing conditions, X, Y and Z.
- Verification and resetting, if necessary, of encoder gaps
- A good cleaning of the machine, including bearing ways and scales
A few minutes spent daily on maintaining your CMM can save you thousands of dollars down the road. If you are unsure what applies to your machine, I recommend you ask your service provider to come over and explain basic maintenance to you.
Calibration of CMM machines. A coordinate measuring machine calibration is essentially a comparison of deviations or errors on your coordinate measuring machine against known international standards. A full CMM machine calibration is not only the verification of errors, but also the correction of those errors, bringing the machine back to the original manufacturer's specifications or, under certain environmental conditions, close to them. Most companies calibrate the coordinate measuring machine every 12 months, or less frequently, depending on the quality program involved.
Many manufacturers check the CMM performance monthly, or as needed; for example, after a collision or a bump. One of the best intermediate checks is the Renishaw machine checking gauge (MCG). This gauge allows the CMM technician to verify the dimensional performance of his machine, a measurement, which can be done within 30 minutes and provides peace of mind for the most important piece of measuring equipment in the plant. It can also alert the user on when to call the service provider for a full calibration.
Calibration versus certification: what is the difference? Certification is the verification of the performance accuracy of a coordinate measuring machine at a given point in time. Should the machine be found to be out of manufacturer's tolerance, then calibration will be necessary. Calibration or recalibration is when the service provider adjusts a CMM machine to bring the accuracy and performance back to the manufacturer's specifications.
When considering a calibrator for your coordinate measuring machine an ISO/IEC 17025 certified provider should be unequivocally the first choice. This standard offers a completely independent assurance of the calibrator’s capabilities and training, irrespective of whether it is an OEM or an independent calibrator. As a further assurance of the provider's capability, an annual audit is held at the calibrator’s facility that ensures all artifacts have been calibrated and are traceable and all procedures are followed to the letter.
The international standard for coordinate measuring machine calibration is ISO.10 360. (Currently part 2:2009). It replaces the old ASME B89 standard in North America. The standard mainly consists of two separate tests. Other tests have been added, but the main two are as follows:
- Length measuring performance designated as E. The test procedure defines a series of measurements of either a step gauge or a laser in the machine's volume. Measurements are taken both in parallel positions to the machine's axes and three-dimensional positions within the machine's volume.
- Probing performance testing designated as R. This is conducted on a precision sphere, wherein random and systematic errors with the probe can be isolated and monitored.
One of the newer tests in the latest specifications is the scanning test. As contact scanning becomes more popular, the new test is proving very useful for users of machines equipped with full contact scanning systems, such as Renishaw SP 25 and Zeiss active scanning probes.