Brass plate special: General Industry Coverage

Protomatic machines prototype automotive throttle plates in diameters from 60mm to 75mm and thicknesses from 1.1mm to 3.1mm.

The approximately 5 “-tall fixture has a wedge-shaped aluminum base that tilts the part at the desired 6° angle. Bolted to the base is a low-carbon steel flange with a central hole. In the center of the flange is one pole of a 20w electromagnet. The pole has pins that match the holes on a throttle plate. In use, the throttle plate stock is placed over the pins, and a 0.200 “-thick, 2 “-dia. steel cap, also with holes matching the pins, is placed over the plate. The outer rim of the flange forms the magnet’s other pole, and the cap closes the magnetic path through the nonferrous plate.

An M code in the machine tool control activates the electromagnet. Wetzel said it is a problem to hold the thin throttle plates flat, and with the magnet “we probably have 300 lbs. of force holding that plate down in a nice, uniform fashion.” He noted that the magnet coil generates heat, which can go into the part and is a concern because of the ±0.0005 ” tolerance. However, flood coolant and the operation’s short cycle times minimize the opportunity for heat to change part dimensions.

Before milling the plates, the square stock is trimmed to an octagon to minimize interrupted cuts. The custom milling tool, produced by Rojo Industrial, Novi, Mich., machines the plate edge at 3,500 rpm and a 15-ipm feed rate. “Accuracy is more important than speed,” Wetzel said.

Next, two 8mm-long flats are machined on opposite edges of the plate at the point where the shaft that holds the plate pivots in the throttle body bore. A 3⁄8 “-dia. carbide endmill mills the flats at 3,500 rpm and 15 ipm.

Machining the OD angle and flats generally leaves a burred edge. Protomatic deburrs the features with a custom tool featuring a V-shaped cutting edge, which chamfers the top and bottom of the plate simultaneously.

“We don’t want to hand deburr the part,” Wetzel said, adding that the shop’s philosophy is to automate whenever possible. “A lot depends on the volume; if we are doing more than 500 parts, we automate the process.”

Protomatic has converted much of its machining equipment from 3- and 4-axis to 5-axis operation. Wetzel said: “We use multiple axes even in a prototyping environment to reduce handling and ensure consistency. We have very talented people, but even talented people occasionally get distracted. The goal is to mistake-proof the operation when doing critical components.”

After the operations on the mill, the plates go through a coining or stamping operation in a special press. “We make a small dimple, 0.020 ” to 0.030 ” deep on a 0.060 “-thick plate, just enough so people know how to orient it in assembly,” Wetzel said. The manufacturing operations may conclude with abrasive tumbling to impart a matte finish while removing less than 0.0001 “.

Protomatic was founded in 1971, largely to serve automotive customers. Then, amidst the massive upheaval in the U.S. automotive industry, the company converted from 80 percent automotive to 80 percent medical and also serves the aerospace industry. “Since we had a highly talented staff and we were used to doing higher precision parts, the medical and aerospace parts were a perfect blend for us,” Wetzel said. “They are very similar because they are mission-critical types of parts. Even though they are quite different, serving both of those industries makes logical sense.”

—Bill Kennedy