Doing Double Duty: Machine Both Mills and Measures

Few think of retooling the production machinery and using it for inspection.

Yet that is exactly what came to mind for Martin Thorden, director of operations at the Volvo Aero Connecticut manufacturing facility just outside of Hartford, Conn. He saw that converting a five-axis milling center into a coordinate-measuring machine (CMM) was the most cost-efficient method for measuring the fan cases and other large structures that the machine mills for aircraft engines, gas turbines and compressors.

After the Zimmermann portal-style machine finishes cutting, the operator directs the machine to grab a probe, run through a short calibration routine, and begin the measurement cycle that checks the preprogrammed points. Once launched, the measurement cycle runs untended for one to nine hours, depending upon the part being checked.

Thorden gives credit for the conversion to the machine’s controller, a Sinumerik 840D computer numerical control (CNC) from Siemens Industry Inc. (www.siemens.com/industry) of Elk Grove Village, Ill. “We have been able to use the control to take on additional tasks that were previously handled by our CAD/CAM system,” he says. This includes the ability to run PC-DMIS NC inspection software from Hexagon Metrology Inc., headquartered in North Kingstown, R.I.

The Siemens CNC is also crucial to the accuracy of the machine, which is comparable to that on some CMMs, says Val Biester, business developer at Siemens. “It’s due to the faster processors,” she adds. Not only does faster processing permit finer axis interpolation and therefore better accuracy, but it also helps the controller to perform various kinds of error compensation that enhance the machine’s mechanical accuracy. In the case of the Zimmermann milling machine, the CNC also oversees a high-accuracy package that controls the temperature through a series of cooling ducts and circuits.

Thorden decided to measure the machined parts right where they were made because their thin walls made them rather flimsy. Not only would moving the parts to a CMM add a material handling operation and introduce refixturing errors, but it would also require buying CMMs and investing hundreds of thousands of dollars more in fixtures to keep the parts in the right shape.

The operator also catches errors right away. “If we find an error, we have only one bad part,” says Thorden. “We don’t have 10 expensive parts with the same error in line waiting for the CMM.”

>> Moving Quality Out of the Lab: Click here to read how a new generation of automated test and inspection tools integrates testing into the manufacturing process itself.