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Teaming Up for Ford's CVT

 

Three-machine cell grinds key transmission component


By Jim Destefani
Senior Editor

            
                                

"If this were a job anyone could do, then we wouldn't have taken it on," says Rudi Willems, reflecting on the parts he and his team are producing for Ford Motor Company.

"We're not known for volume or production work, but we knew Ford had been looking for a suppler for a number of years and we could see the high accuracy required, and we stepped up and said we'd do it."

Willems is operations manager of the Grinding Division of Metalex Mfg. Inc. (Cincinnati). The part is a complex, micron-close tolerance shaft for the continuously variable transmission used in Ford's new Five Hundred and Freestyle vehicles. Designed by German company ZF Friedrichshafen AG, the CVTs are built at Ford's Batavia (OH) Transmission Plant.

In Ford's search for a supplier, the company probably did not expect to find one in its own backyard. But the company was looking for a supplier that could not only produce the close-tolerance CVT shafts to spec, but one that could provide finished parts within 120 days.

Metalex met Ford's schedule and accuracy demands by installing a new, three-machine grinding cell consisting of a Studer S31, a Studer S33, and a Jung C-740-D, all supplied and installed by United Grinding Technologies (UGT; Miamisburg, OH). Willems says the short time between Metalex winning the business and delivering product to Ford was challenging, to say the least.

"The machines arrived in December of 2004, and we began shipping parts to Ford in February of 2005," he recalls. "That's pretty remarkable. There were several weeks when we all worked pretty much around the clock.

"Keep in mind, the building housing the grinding cell and inspection room was an empty warehouse," he continues. "We didn't have adequate power, air, water, and other services running in there to support cellular grinding. We had to make it a climate-controlled environment. We had to acquire the machines, install the cell, do the training, acquire and install the inspection equipment."

Teamwork and a self-directed workforce are pillars of the manufacturing strategy at Metalex. "It is our culture to give everyone an opportunity to be the best they can be," says cell leader Artie Artrip. "It's our responsibility to follow the Metalex commitment toward continuous improvement and eliminating waste wherever we find it."

Metalex and its customer, Ford Batavia, selected UGT based on three major factors. First, UGT was nearby, in Miamisburg, OH. The company's facility there includes test and R&D capabilities valuable for a start-up effort. Second, UGT's product lines include both cylindrical and surface and profile grinders (among others). Finally, Metalex and Ford felt UGT possessed the CNC grinding and application expertise to get the cell up and running in the required time frame.

And, with finished parts needed by Ford in 120 days, turnaround time was critical. UGT engineers developed the cell using machines already in stock, which were tooled up for the CVT shaft on the floor at Metalex. "There were many issues and disconnects, but United Grinding's technical staff eliminated one issue at a time with the only goal being to ship the customer good parts on time," Willems recalls.

The CVT shaft is produced in three operations in the grinding cell. OP 130 is done on the Studer S31 universal cylindrical grinder. The operation includes grinding on two large diameters, an undercut, and a critical facing operation.

The two diameters are held to 10 µm tolerance and ground to a surface finish of Rz 4. The Rz surface finish parameter differs significantly from the Ra (average roughness) parameter commonly used in North America. Ra is the average distance between microscopic surface peaks and valleys over the sampling length.                  

Based on a German (DIN) standard, Rz (mean roughness depth) is calculated by measuring the vertical distance between the highest peak and the lowest valley in five sampling lengths, then averaging these distances. Although there's not a direct conversion from Rz to Ra, a ratio between 4:1 and 7:1 can be used. In this application, 1 Rz equals approximately 0.21 Ra, for a ratio of close to 5:1.

The cone face finish requirement is Rz 2 - 4, with an acceptable angle on the face of 11º. The dimension from the back plate to the datum point on the cone face is 20 µm.

Performed on the Studer S33 universal cylindrical grinder, OP 140 consists of grinding four small diameters, a critical bearing diameter, and a chamfer on the shaft end. The four small diameters and the bearing diameter are held to 6 µm tolerance. Finishes are again critical--Rz 4 on the small diameters and Rz 2.2 on the bearing diameter.

Following the two cylindrical grinding operations, shafts are placed in a dynamic testing fixture to confirm tolerance and surface finish accuracy. The test apparatus is supplied by Marposs Corp. (Auburn Hills, MI). Now shafts are ready for OP 150, grinding of the ball tracks on the Jung C-740-D surface and profile grinder.

"This is a very critical operation, even though the length of the ball track doesn't look too demanding," Willems explains. "What we're actually grinding here is kind of a Gothic arch in the ball track. There are really two radii with a kind of smooth flat at the bottom, and this has been designed so that the ball bearings run on the same face all the time without touching the bottom.                   

"The profile of the ball track has to be 22 µm, and this needs to be held over the entire length of the groove," he continues. "If you have as little as a 5-µm taper, you're already out of tolerance on the profile. At this point, we're able to hold 5 µm consistently."

                                    

 

About the Five Hundred and CVTs 

Built in Chicago, Ford Five Hundred/Mercury Montego sedans are available in several trim levels and in all-wheel drive or two-wheel drive versions. A continuously variable transmission is standard on all-wheel-drive and some front-drive versions; other cars get a six-speed automatic transmission.               

Unlike a conventional transmission, a CVT doesn't have gears or a clutch. Instead, it uses electronics to choose from an infinite number of gear ratios within a given ratio "spread," which is usually wider than the spread of a conventional automatic transmission.

Ford's CVT uses two variable-size pulleys--one for input from the engine, the other for output to the drive wheels--connected by a steel link belt. The transmission varies the effective sizes of the two pulleys to meet driving demands using a sensor on the car's electronic throttle to constantly monitor the demand for power.

The CVT's absence of fixed gear ratios allows it to keep the engine operating at its optimum power/torque ranges in relation to the driver's load demand. For example, when more power is needed for acceleration or maintaining speed up a steep hill, the transmission seamlessly responds by creating the right drive ratio from an infinite number of possibilities, allowing the Five Hundred's 200-hp (150-kW) Duratec 3.0-L engine to operate in the heart of its power band regardless of driving conditions.

Ford maintains acceleration is considerably smoother with a CVT than with a conventional automatic transmission. The setup also provides reduced noise and emissions and better fuel economy. According to Ford, mileage is 8 - 9% better than a four-speed automatic and 4 - 5% better than a five-speed automatic.



This article was first published in the September 2005 edition of Manufacturing Engineering magazine.   

 


Published Date : 9/1/2005

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