A just-right hard turn

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END USER: Axle Alliance Co. LLC, (313) 592-7438, www.daimler-trucksnorthamerica.com. CHALLENGE: Overcome workpiece variations while effectively hard turning large ring gears. SOLUTION: A workpiece measuring process for a vertical turning machine. SOLUTION PROVIDER: EMAG LLC, (248) 477-7440, www.emag.com

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Editor’s Note: For information about EMAG LLC’s new VLC 250 P turning machine and the company’s upcoming 2011 Technology Days, scheduled for May 10-11 at EMAG’s U.S. headquarters in Farmington Hills, Mich., click here. The VLC 250 P will be one of several new EMAG machines featured at the 2011 Technology Days event. Cutting Tool Engineering is the exclusive media partner of the event.

Established less than a decade ago, Axle Alliance Co. LLC is part of Daimler Truck North America LLC and supports many Daimler brands by machining rear-axle ring gears and pinions, as well as supplying multiple finished assemblies. 

Based on its German sister company’s success with the builder’s machines, Axle Alliance uses six machine tools from EMAG LLC, Farmington Hills, Mich. Mike Bagdasarian, senior manufacturing engineer for Axle Alliance, Redford, Mich., noted that the machines—VTC 315 Duo, VTC 315 single spindle, two VLC630s, VSC 400 R single spindle and VSC 400 Duo—were purchased simultaneously and each one is dedicated to a part line.

Courtesy of Axle Alliance

Axle Alliance’s VSC 400 Duo vertical turning machine from EMAG hard turns ring gear prior to gear teeth grinding.

The dual-spindle VSC 400 Duo vertical turning machine, for example, hard turns ring gears prior to grinding the gear teeth. The 390mm-dia., medium-carbon steel ring gear has a hardness of 59 to 62 HRC.

Although Axle Alliance considered purchasing machines from other builders, some builders “no quoted,” Bagdasarian said. He attributes that to the parts’ large sizes, heavy weights and tight tolerances. “When we told them we needed to meet a target cycle time in addition to our tolerance requirements from the first operation to the second operation, I think some suppliers had no choice but to no quote,” he added.

When the VSC 400 Duo was runoff in EMAG’s German factory where it was built, the machine achieved the tolerance requirements, according to Bagdasarian. However, Axle Alliance experienced problems when it conducted on-site runoffs, which also included a subsequent grinding operation. The grinding machine came from a different builder that didn’t participate in the off-site turning runoffs. 

Because of distortion during heat treatment, a stack-up of in-process tolerances or both, there are slight size variations from part to part. The machine was initially set up to remove a specific amount of material in both the first and second turning operations, but a lack of controls to monitor the tooth height variance from part to part led, in some cases, to too little or too much stock remaining for grinding. “We had to have 100 percent control over how much stock was removed in the first turning operation,” Bagdasarian said. “It was an oversight on our part. Not having full control of stock removal in the first operation relative to the tooth height dimension had a huge impact on the success of the subsequent operation.”

To achieve that control, programmers from Axle Alliance and EMAG worked together to develop an algorithm that enables the machine’s in-process touch probe to premeasure part height similar to how a pitch line chuck might be used, Bagdasarian explained. “We measure the part across a set number of teeth and compare those values to a nominal value,” he said.

With the measuring subprogram in place, the machine measures a part, determines how far it is from the nominal value and adjusts the Z-axis direction to achieve the required amount of stock removal. “Every part that we machine in the first turning operation is treated as an independent entity,” Bagdasarian said. “The second turning operation is only taking off the resultant amount of stock that’s left over from the first operation.”

He noted that a capability study of 35 workpieces that measured the stock removal on the gear teeth using a coordinate measuring machine for gears showed a variation of less than 15µm. “It’s as repeatable and consistent as you can ever imagine,” Bagdasarian said.

He noted that Axle Alliance’s EMAG machines continue to produce large quantities of high-quality ring gears and pinions without any need to upgrade or replace equipment. “When the need arises for additional equipment,” Bagdasarian said, “I would, without fail, put EMAG at the top of my list.”