The speed and accuracy of CMM inspection addresses several manufacturing challenges, including tight machining tolerances, rapid conversion between jobs, 100 percent inspections and QS9000 certification standards.
To ensure its next run will be as good as the last, Motek Engineering and Manufacturing, Inc. relies on a coordinate measuring machine (CMM) to perform first article inspection on all computer numerical control (CNC) station setups, periodic in-process tooling inspections and final inspections on all prototype models.
In its Cambridge, Minnesota, machining facility, Motek produces transmission parts for all-terrain vehicles, snowmobiles, four-wheel drive trucks, off-road equipment and farm tractors. Machinists use two- and four-axis CNC lathes, vertical and horizontal CNC machining centers, CNC grinders and spline equipment to shape a variety of metals into more than 40,000 transmission parts per month. When technicians configure a CNC machining center for a new job, quality assurance technicians verify the accuracy of the setup with a Zeiss Prismo 7 CMM (Carl Zeiss Inc., Minneapolis, Minnesota).
For Motek, the speed and accuracy of CMM inspection addresses several manufacturing challenges, including tight machining tolerances, rapid conversion between jobs, 100 percent inspections and QS9000 certification standards. Motek must meet machining tolerances as tight as 0.0005 inch consistently to satisfy its customers' expectations. CMM inspection enables technicians to verify machining to these tight tolerances, and it can provide SPC data to demonstrate the consistency of the machining.
Motek uses CNC machining centers to accommodate a quick transition between jobs. Each time a CNC is re-programmed to produce a new part, technicians must verify that the tooling is set up properly. The Zeiss Prismo 7 CMM enables technicians to verify the setup in minutes, where manual CMM and hard gage inspection took several hours. In addition, the high accuracy of the CMM allows operators to determine precisely which adjustments need to be made, saving them time previously spent checking and re-checking the machining setup.
Motek must verify and provide inspection data for every prototype part it produces. With traditional inspection methods, verifying every characteristic on an average prototype lot of 30 parts took two to three days. The speed of CMM inspection enables them to complete prototype verification within hours.
Previous part inspections at Motek were performed with a manual CMM and hard gage. The old CMM required a skilled technician to position the probe head manually, and record the readings on an inspection report. This process was extremely time consuming and did not permit Motek to perform comprehensive statistical process control analysis. Furthermore, hard gages did not provide the accuracy Motek required, and were dependent on operator skill to provide accurate readings.
"Our CNC machines are capable of producing extremely tight tolerances, but we didn't know this until we could check the results with the CMM," said Brent Harcey, quality assurance manager at Motek.
The new CMM enables Motek to locate the center point of a sphere, project a bore centerline through a gearbox and perform tool wear analysis.
As the previous inspection program at Motek was time-consuming, technicians often based tooling alignment decisions on a sample of only a few parts. The speed of the CMM enables technicians to check a much larger sample size, and the ability to process inspection data enables technicians to make vital process corrections based on the results.
According to Mr. Harcey, "Where we were previously checking samples of one or two parts, we're now checking 20, 30 and 50. This gives us the SPC trending data to support corrective actions in the machining process."
In addition, Motek uses the SPC data to showcase the quality of its products. "We are able to supply our customers with concrete data to verify the quality of the parts we produce," said Harcey. "Prior to this, we relied on hard gages, which aside from being expensive, did not allow us to provide data to support the part accuracy."
Rapid conversion between jobs is essential for machine shops like Motek. When it purchased the CMM, Motek wanted to complete CNC setup verifications in an hour; however, the CMM can complete a standard verification with reports in 20 minutes.
In a process that may have taken up to three hours with a manual CMM, an average inspection program now runs between five and seven minutes on the Prismo 7. Also, the accurate, reliable readings reduce the amount of time technicians spend repositioning or "tweaking" the machining tools. For example, Motek can complete more than 13 setup changes in an average ten-hour shift.
"Previous verifications took several hours to complete, and the results weren't always accurate," explained Mr. Harcey. "So the machine operators would adjust their tooling, wait for the results, and adjust again and again until it was right. Sometimes, it could take an entire day to verify a CNC setup. The CMM enables us to perform one check, and use the results to position the machine in minutes," he added.
To further expand its range of capabilities, Motek plans to add off-line programming capabilities and a CMM with Zeiss VAST scanning technology. Off-line programming will enable Motek to import or generate CAD data and use it to prepare a CMM measuring program off-line while the Prismo 7 is busy performing inspections.
The current Zeiss ST probe head enables Motek to check features to pinpoint accuracy. However, to verify contours and minute details, Motek currently relies on an optical comparator. Zeiss VAST scanning technology will enable Motek to trace contoured surfaces and check detailing with the CMM to variable accuracy levels, further increasing efficiency. MMSblog comments powered by Disqus