Brass plate special: General Industry Coverage

Protomatic Inc., Dexter, Mich., specializes in efficiently machining small volumes of precision metal parts, whether prototypes or production runs. One example is a series of prototype automotive throttle plates.

A throttle plate controls airflow into an engine and thereby determines its output. In production, the plates are stamped in volumes of thousands per day. Protomatic typically makes prototype and limited-run plates in lots of 100 to 500. The prototype plates are used for testing. “The fuel system, brakes or anything that is safety critical goes through a battery of tests,” said Doug Wetzel, general manager. “It is not uncommon that one of the big automotive companies will do 200 to 500 plates in different dimensions, varying by 0.0005 ” or 0.001 “. They don’t want to have throttle-sticking problems.”

Protomatic manufactures the plates out of CA260 brass or 6061T6 aluminum in diameters from 60mm to 75mm and thicknesses from 1.1mm to 3.1mm. For brass plates, squares of stock are cut from a 36 “×96 ” sheet with a shear. For a plate with a final diameter of 75mm, the squares are about 75mm on a side. Then two mounting holes are drilled through the plate on a Haas or Hardinge CNC mill. “We like to buy American,” Wetzel said, “so we use domestic machine tools.”

The holes, typically 0.130 ” in diameter and reamed to ±0.001 “, are drilled 1 ” apart. Protomatic holds the stock with clamps or in a vise. “For the volumes that we are doing, we hold them individually,” Wetzel said. “We don’t stack them because we want to control the hole diameter very tightly.”

The next step is milling a 6° angle of a varying width around the edge of the plate. The slanted, compound elliptical shape that results ensures the plate edges are parallel to the bore walls when the plate is tilted 6° in the closed position in the throttle body bore.

Initially, the shop machined the plate edges on a lathe, using a milling head. The square piece of stock was screwed onto an angled mandrel that had locating pins. “On a lathe it was effectively a one-at-a-time operation,” Wetzel said. “Cycle time might be a couple minutes, then you have another minute unscrewing one plate and putting on the next one.”

The operator was almost continually engaged in mounting and unmounting plates. As plate volume increased, Protomatic sought to free up operator time and make the process faster and more repeatable.

The answer was a custom magnetic fixture to hold a piece of stock at an angle on the bed of a CNC mill and a special cutter programmed to perform circular interpolation around the plate edge. The shop puts eight fixtures on the mill table. The longer cycle time allows the operator to run another machine or perform another task, Wetzel noted.

Courtesy of Protomatic

This electromagnetic fixture, featuring a steel top plate that enables it to hold nonferrous parts, was engineered by Protomatic to improve process speed and repeatability when machining prototype automotive throttle plates.

Courtesy of Protomatic