Finding a 5-axis solution

Tim St. Martin knew there had to be a better way of fixturing parts on his company’s 5-axis machine tools. “We’d visited some of our suppliers and saw how they were holding parts on their 5-axis machines,” said the senior manager of manufacturing engineering for Carlsbad, Calif.-based orthopedic implant manufacturer Alphatec Spine Inc. “They were mounting the blanks into a ‘picture frame’ and then screwing that frame to a tombstone-mounted fixture. This basically limited access to one side of the workpiece per operation. Many in our company thought we should hold them the same way.”

Management charged St. Martin with finding the most effective way to produce a family of titanium spine plates. Alphatec had purchased a pair of 42,000-rpm Mikron HSM 400U LP 5-axis machining centers to bring machining of its Trestle-Luxe anterior cervical plating system in-house.

In particular, Alphatec needed an optimal way to fixture the parts because it would be making a family of 33 different part numbers with production volumes of more than 1,700 pieces per month. While the company considered making its own fixtures, contacts at GF AgieCharmilles, builder of the Mikron machines, suggested Alphatec look at workholding from 5th Axis Fixtures Inc., San Diego, which specializes in workholders for 5-axis machines.

St. Martin focused on the PY6420D1151S clamping system, which utilizes a dovetail-shaped clamp to grip premachined workpiece blanks from one edge, allowing access to five sides in one machining operation. The operator places the material in the fixture, tightens an Allen screw and the clamping system draws the material down onto the ground face of the fixture, ensuring repeatability in the Z-axis.

St. Martin said: “I told the other team members there are a hundred different ways to present the workpiece to the tool so let’s give this one a shot. If it doesn’t work, we’ll try it with a picture frame. But we hit the ground running with this thing. It fit perfectly with what we were trying to do.”

Before selecting the PY6420D1151S, Alphatec had examined several different workholding methods. The shops that previously made the spine plates for Alphatec located the blanks with a pair of precision-ground holes onto fixture pins, machining one side, flipping them over to hit the back and then clamping them down to cut away the web of material they had used to hold the parts. “Even on the small plates, it was a long cycle time, plus all the handling,” St. Martin said.

Courtesy of Alphatec Spine

Multiaxis profiling on a 24mm-long Trestle-Luxe anterior spine plate at Alphatec Spine using workholders from 5th Axis Fixtures.

Initial results with the 5th Axis workholders were excellent, reducing the number of operations on the Mikrons from three to one. Since then, the process has been further improved. “We’ve taken a lot of machining time off the Mikron spindles by moving the roughing operations to our Haas vertical machining centers,” St. Martin said. This simple step eliminated 25 minutes of cycle time on the most expensive machines in the shop and significantly reduced overall manufacturing time.