Fixturing a big ring: Turning Performance

In challenging economic times, part manufacturers typically are loath to outsource a job to a competitor when there’s a way to do it in-house. Akron (Ohio) Gear & Engineering Inc. faced this situation a year ago when COVID-19 significantly reduced the workload at the company. It specializes in machining large parts, including industrial gears, and didn’t want to pass on a project to make a 0.9 metric ton (1 ton) ring made of 4340 steel even though the most appropriate machine for the job was being rebuilt, said Akron Gear & Engineering’s Dennis Miller.

“The expense of the whole job was more than if we would have farmed it out,” he said, noting the high cost for required tooling. “But if we weren’t able to do (it), we may have lost their business. We didn’t want to take a gamble.”

With its largest vertical turret lathe, which has a 3,098.8 mm (122″) capacity, unavailable at the time, Akron Gear & Engineering needed to machine the ring on its Juaristi horizontal CNC boring mill but didn’t have the necessary fixturing. Using the boring mill instead of the VTL required the ring to be held vertically rather than horizontally. The boring mill with 3+2 capability has capacities of 2,997.2 mm (118″) on the x-axis, 2,413 mm (95″) on the y-axis and 711.2 mm (28″) on the spindle.

Miller contacted a familiar workholding supplier for a solution but never heard back. Unwilling to wait any longer, he called industrial distributor Atwood Industries Inc. in Cleveland, which recommended workholding from Mitee-Bite Products LLC in Center Ossipee, New Hampshire. After discussing the job with Michael Witzgall, senior applications manager at Mitee-Bite Products, a modular arrangement mounted on T-slot subplates was recommended. The workholding devices included Mitee-Bite Products’ Heavy Duty T-slot clamps with blunt-edge Pitbull clamps and large multifixture stops for the first operation, then modular XYZ Xpansion pins for ID, OD and facing operations.

“The modular design was vital with this application to avoid distortion from clamping pressure,” said David Bishop, general manager of Mitee-Bite Products. “Large, one-piece runs leave very little room for error. Preventing distortion with this heavy, ring-shaped part required solid communication with the customer, an extensive brainstorming session and CAD simulation software to recommend the best options. Mounting the T-slot plates centered on the ring’s diameter allows placement of all clamps and hard stops into a static position and then applying contact pressure in small increments equally in all directions until sufficient holding force was achieved.”

A 0.9 metric ton ring is bored at Akron Gear & Engineering. Image courtesy of Mitee-Bite Products

Lacking experience with goods from Mitee-Bite Products, Miller worried whether its workholders would be up to the task of holding the massive part.

“If the clamps or pins fail,” he said, “it’s coming down on me. You have to put your faith and trust into the salesman who is selling them to you, hoping he is not exaggerating on the quality of his parts.”

Bishop estimated that it would take more than 4,536 kg (10,000 lbs.) of force to shear an XYZ Xpansion pin, which is made of 17-4 PH stainless steel.

“We were not concerned about any lack of strength in the x, y and z directions,” he said.

Miller said the OD of the ring measures 2,247.9 mm (88.5″), the ID is 1,993.9 mm (78.5″) and the thickness is 107.95 mm (4.25″). As it turned out, the part was taller than Akron Gear & Engineering’s angle plates, preventing the top section from being clamped.