- February 19, 2014
Clamp the part to the surface plate. Enter a few parameters. Grab the joystick and go. When checking a hole location, or measuring the distance between two machined surfaces, that's about all you need to know in terms of coordinate measuring machine (CMM) operation. Unfortunately, most inspection procedures are more complex, and good metrology requires a lot more than mastery of a few screen commands. Programming automated measuring equipment needs robust software, a sound program to drive the machine, and some old-fashioned inspection know-how.
Back to School
Like hand-cranked engine lathes and series I knee mills, manual CMMs are quickly going the way of Gray-Dort Motors. "We still sell a fair number of basic CMMs for general shop use, where people need to do a simple layout or take a few quick measurements for a machine setup, but probably 75 per cent of the CMMs we sell today are automated," says Peter Detmers, vice president of sales at Mitutoyo Canada, Mississauga, ON. That's good news for those quality control people who suffer tennis elbow from shoving a probe around all day, but the move to automated CMMs presents a frightful burden: learning how to program.
It's not as scary as it might sound. CMMs do not require complex G and M code programs to drive them through their elaborate dance. Most if not all CMM software packages offer predefined macros—bolt hole patterns, slots and surfaces, even screw threads can be automatically measured by answering a few basic questions on what you want the machine to do. And automated CMMs can be "taught" how to measure a workpiece by putting the machine into recording mode and manually driving the probe through the first measuring routine. Subsequent workpieces are then measured with the resultant program—take a few manual data points to tell the CMM where the part sits and how it's oriented, then execute the recording. The CMM software remembers what you did last and drives the machine axes through the same steps.
Detmers explains this is not text oriented programming, but rather "object" oriented. "We use a very simple language to the operator. So a circle is a circle, a plane is a plane, a line is a line. It's not five lines of code for every feature on the part, it's one line for everything you've just measured." If that's not simple enough, rest assured: CMM's are getting smarter every day. "I think everyone in the industry is working towards more intelligence in the software," says Detmers. "A big part of this is automatic feature recognition. This lets you start probing workpiece features and the software, based on the types of hits you're taking, will recognize what you're trying to measure. Take a series of points that are all in the same Z direction, for example, and the software's going to say 'Hey, you're measuring a plane,' and will do the calculation based on that."
CAD to CMM
Despite the ease of programming available through teach mode and canned macro routines, high volume or complex part inspection requires something more elaborate. This is why Mitutoyo and others offer a CAD interface that accepts 3D solid or wireframe models as a starting point for CMM program creation.
Todd Wojtoviets, technical sales engineer at the Michigan office of Carl Zeiss Industrial Metrology, LLC, says importing from industry standard packages such as Pro-E, Catia, Unigraphics, and even Solidworks is possible.
"We program directly off the CAD model. There are also options for using the PMI data [product and manufacturing information] embedded in the CAD file. This data is verified by the software, and if the operator chooses to use it, then oftentimes we can actually build the entire program based on that."
Regardless of how you program the CMM, you've invested a fair amount of time doing so and will likely want to save that program for future use. Storing data on the computer that drives the CMM is possible but risky—without proper backups, losing a hard drive on any machine makes for a really bad day.
That's why smart shops opt for server-based storage. In Zeiss' case, this is its MCC product, or Master Control Center. Not only does the MCC support secure and centralized program storage, but management of those files across multiple CMMs is much easier. Says Wojtoviets, "what happens is, shops will program a part on machine one. Two months later, they're running that part again but machine one is busy so they'll copy that program to a thumb drive and move it to machine two. For whatever reason, that operator may have to make a change to the program. If so, you'd then have two versions floating around, leading to discrepancies in part measurement."
Multiply that scenario by hundreds of programs across half a dozen CMMs and you could have a real mess on your hands. A master control center assures proper version control of part programs while also providing a view of CMM utilization.
Another consideration is the output of all those programs, the inspection data itself—where are you going to store it? Each time you check a part, the CMM software generates a file that must be traceable to the job that made that part, and quite possibly to a serial number, inspector ID and so on. For shops with multiple CMMs, metrology experts suggest server-based database storage as the best solution for secure, traceable data. Better yet, storing dimensional data in a central database allows for simplified trend analyses, statistical functions, cost of quality reporting, and possible integration to the company's ERP system (Enterprise Resource Planning). SMT