A mold shop using this process originally machined the grooves with a four-flute, high-speed, steel-grooving tool, just after completing the bushing counterbore. Cutting the slot was faster than opening the screw hole, but still left plenty of room for improvement. Although the spindle and mold block remained in position in the X-Y plane for both operations, changing from the boring tool to the groove slotter did take time.

Sometimes it also involved raising the spindle or lowering the table to make room for the exchange, which could create an error source. Making the actual cut was slow going to avoid overloading and deflecting the high-speed steel cutter due to lateral forces

Switching to an interchangeable tungsten carbide cutter on a high-speed steel shank sped up the process considerably. The moldmaker can bore the bushing hole and counterbore with a boring tool, then simply change to a grooving tip for the snap ring groove—all with the spindle, table and toolshank left in position. Then cutting the groove is a matter of making an orbiting cut. Since everything is left in place and the interchangeable tips hold axial and radial positional repeatability to datum within +/- 0.0005 inches, location accuracy of the snap ring groove is ensured.

At one mold shop using the carbide slotter, tool changes averaged 10 to 15 seconds and actual machining of the slot took 12 seconds, half the time as before. They mill the slot with depths-of-cut up to 1/8″ in a 1¾” bore.

When the cutters go dull, moldmakers simply replace them rather than regrinding. In the long run it’s cheaper to replace a slot-cutting tip than to get into regrinding. Given the four-week turnaround with regrinders and the shipping times, a mold shop would need to own four cutters to keep just one ready for service at any time.

Switching from boring to snap-ring grooving cuts machining and assembly time for bushing retention by four to one in plastic molds. Left: carbide boring tip creates the counterbore to accommodate the bushing’s top lip. Center: switching tips, a 10 to 15 second job with +/-0.0005-inch positional repeatability. Right: slotting tip after completing the snap ring groove.

The old and new design for retaining pin bushings in plastic molds. The old way used a socket screw to pin the bushing in place with one point contact, required three machining operations and changing the setup. The new way uses a snap ring in a groove.