Holding firm: Turning Performance

John Prosock Machine Inc. is a Quakertown, Pa., job shop that handles prototype machining as well as production and assembly jobs. Founded in 1982, the 30-employee shop has 10 mills and 13 lathes. Production runs range from 100 to 2,000 pieces for a range of customers. “We pretty much do anything,” said Claude Farrington, plant manager. “Medical work, driveline components, heavy equipment, parts for remote-control cars, you name it.”

Describing the machining of a prototype aluminum trunnion housing, Farrington said the actual machining of the complex-looking part was not too difficult, but determining how to fixture it was a challenge. The roughly 11½ “-long × 5½ “-wide housing was designed to mount on a boat’s transom and house an electronic linear actuator. It is part of a system that provides instant steering response when activated by controls at the helm, eliminating the slow reactions of a cable system.

Prosock Machine received a DXF file from its customer and loaded it into the shop’s Mastercam CAM software to program milling operations. Turning work was programmed at the lathe.

The housing was machined from a 12 “×6 “×3½ ” 6061-T6 aluminum block. It was clamped with the long dimension standing vertical in a Kurt vise with aluminum soft jaws on an Excel 810 vertical machining center. One end of the finished housing has a single 1.850 “-dia., 3.850 “-long boss, but, to start, two identical bosses were machined side-by-side. “We machined two so when we flipped the part we could use them to align it in the vise. Later we cut the one off that we didn’t need,” Farrington said.

The twin bosses were machined with a 1½ “-dia. HSS endmill, run at 3,000 rpm and a 30-ipm feed rate with a 4 ” length of cut. Farrington described the toolpath as “a figure 8 around the bosses,” stepping down 0.200 ” on each pass.

Courtesy of John Prosock Machine

John Prosock Machine produced this aluminum trunnion housing as a prototype for part of a powerboat steering system.

The housing was then flipped in the vise and one of the twin bosses was located against a stop. On the other end of the finished housing would be two bosses that are not identical, having different diameters and offset 70° from each other. One boss, in line with a boss machined earlier, measures 2.100 ” in diameter. The other boss is 1.514 ” in diameter. Because this second set of bosses was closer together than the first pair, smaller endmills were used. The bosses were roughed with a ⅞ “-dia. HSS hogmill and finished with a ¾ “-dia. HSS endmill, both run at 1,200 rpm and 10 ipm with a 4 ” length of cut. The two bosses are 1.360 ” long, but one is set back 0.600 ” deeper in the part than the other.

Water-soluble coolant was applied throughout the machining process. Farrington described the HSS tools as “generic,” adding that the shop’s solid-carbide tools are from Mill Monster and its inserted milling and turning tools are from Kennametal.