by Kip Hanson
Is your coordinate measuring machine up to snuff?
Often abused and generally unappreciated, coordinate measuring machines (CMMs) are the underdog of the machining world. That’s because, unlike their metal-cutting cousins, CMMs don’t actually produce anything. No chips, no shiny machined parts, nothing but measurements and statistics. But remove the lowly CMM from any shop and you’ll be checking parts with height gauges and dial indicators, suffering an inspection backlog sure to make even the most patient shop owner hot under the collar. Simply put, CMMs deserve more respect.
Keep it Clean
A big part of that respect begins with a preventative maintenance program. Yet Gary Rockwell, director of aftermarket products for Michigan-based Nikon Metrology Inc., says this is one thing many shops forget. “There are basically two types of CMM—air bearing machines and those with mechanical bearings. Both require preventative maintenance.” Rockwell sayys most PM work involves cleaning the bearing surfaces, as well as the scales and feedback system. “I actually clean the entire machine if I can; the less dirt on the machine, the less likely it is you’ll contaminate any important surfaces.”
On air bearing machines, which comprise the lion’s share of CMMs today, cleanliness begins with the air supply. “Any kind of oil or vapour coming into the machine will clog the air bearings,” Rockwell says. “And moisture on the bearing ways attracts dirt, compounding the problem. But a supply of clean, dry air coming in provides an air curtain as it exits the bearings, helping to force dirt away.”
It’s also important to be in tune with your equipment. As Rockwell points out, this is no different than driving the family car down to the grocery store. “You hear something different, or feel something different, and you know something has changed.” At this point, you might be ready to call the CMM repairman (or woman), but not so fast: most of their machines today come with built-in calibration utilities, allowing you to easily check squareness, perpendicularity and repeatability, three indicators of CMM health. And even on less sophisticated equipment, verifying against a master gage is a good way to check for problems. “Measure it ten times. It should repeat very close to zero. If not, it might be as simple as a loose stylus. Or it could be something much more serious.”
Who you gonna call?
Cleaning and basic calibration can be done in house, but few shops are equipped for much more than that. Annual certification to ISO 17025 is a must when doing aerospace, medical or defense work, and this means equipment such as ball-bar gauges, lasers and check masters, any of which cost more than the last company picnic. And if your machine is broken, you have no choice but to call a metrology service company to fix it. One of these is C.M.M Services Inc., Kitchener, ON. President Paul Anstett says that he, like others, offer preventive maintenance programs, but the sad truth is that most shops wait until their CMM breaks before asking for help. “It’s only a small percentage of operators that take pride in their machine, keeping it clean and operating well. Most of them just use it. If something goes wrong, that’s when I get called in.”
Anstett has an extensive toolkit. An oscilloscope to check signal strength of the axis drives. A level to make sure the machine sits square to the world. Master balls, step gages…the list goes on. “We run seven different measurements to check 105 maintenance points. This includes the x-y, y-z, and z-x planes, squareness and volumetric accuracy, corner to corner.” And when Anstett finds something wrong, what then? He explains that, once the machine is square and level, many problems can be corrected through the error map, a 3-D virtual representation of the CMM’s measuring space. By checking at regular intervals throughout the machine’s working volume, a metrology technician can identify those areas that need adjustment. For example, say you’re off by 20 microns at a certain part of the x-axis. Plug a correction value into the CMM software and the machine’s computer system will compensate for this discrepancy. SMT
Kip Hanson is a contributing editor. [email protected]
