Casting Study: Helical Gear

A worm/helical gear set operates the linkage attached to the roof window frame. Originally, the helical gears were machined from brass, reinforced with a formed steel insert and installed with a plastic washer/spacer. The assembled gear’s cost became a factor when product development engineers began looking for ways to trim it. Other materials and processes were considered, but die cast zinc proved to be the answer. Precision tooling and moving slides permit the helical gear to be cast and ejected from the die. In fact, the net-shape gear and tooling design was a merit prize winner in North American Die Casting Association (NADCA) 1995 International Die Casting Competition. While zinc alloy is less costly than the brass it replaces, the casting is designed with ribbed cavities, cutting material cost even further. This resulted in an additional 30 percent material savings. When the castings are ejected from the die, the sprue and gate are punched out and the small hole where the linkage connects is reamed to size–all on the same fixture. The gears are tumbled and ready for assembly.