Sometimes whole assemblies can be combined into a less-expensive die-cast part.
Excerpt from the April 2003 issue of Machine Design
Gating through the center bore of this horizontal gear drive ensures uniform alloy distribution for consistent fill of the thread and gear tooth forms. The walls of the center hole are held parallel within +/-0.0005 in. The drive gear, used in an automobile seat, is cast ready to use with no finishing or deburring operations required.
Designers of a new automotive seat were in a quandary over the most economical way to build the seat's geometrically complicated, horizontal gear drive. It was proving tough to find a process that met the gear drive's precise tolerancing requirements and do so on a budget. The gear drive incorporated not only an Acme screw thread at one end of a bearing journal but also a cross-axis helical gear on the other.
One potential option was machining. But this was costly and time-consuming. It would have entailed making two components in two separate operations, and then press fitting the assembly together with a spline engagement. Additionally, there were concerns about inconsistent runout and whether gear teeth could be positioned with enough relative accuracy. Powdered-metal processes were another possibility for the gears. But tooling constraints and tolerance-control issues precluded their use. Plastics also were ruled out due to tolerance and strength limitations.
That left die casting. One plus was that it combined the individual components -- the screw thread, helical gear, internal bearing journal, and two thrust faces -- into a single part. Zinc alloy also provides strength and dimensional stability. Moreover, die-casting production costs were 40% below those for machined steel.
Tooling to form the Acme-screw thread cavity incorporated four side cores. Gating through the center bore ensured the alloy would consistently fill in the tooth forms. The resulting gear drive is cast ready to use and needs no finishing or deburring operations.
The option to convert multiple components and operations into a single die-casting operation is a major reason designers consider this process. But, die casting also offers many other cost-cutting opportunities and can improve part quality as well.
Why die casting?
The potential for piece price reduction is the usual motivation for high-volume die casting. Economies of scale start at 50,000 pieces annually. Numerous factors can affect production economics. These include component complexity, alloy properties, die-casting technology used, precision of the die-cast tool, and cycle rate. One reason die casting can be thrifty is that a single cast part often replaces multiple components. And it is frequently possible to incorporate features in the casting that eliminate secondary milling, boring, reaming, and grinding operations.
Flash-free die-cast tooling also eliminates the need for finishing operations. And additional savings come from material reduction, use of less-expensive metals, improved tolerances, and good part-to-part consistency.
As a rule of thumb, designs incorporating complex configurations are well suited for die casting. Good candidates include gears, shafts, cams, ratchets, levers and pinions, and others performing mechanical functions. Containments such as end bells, plates, motor and gear housings, spacers and seats also are frequent choices for die casting.
You can achieve significant cost reduction results from consolidating multiple components and assembly procedures into a single die casting. A screw-machined stud assembled to a stamped plate is cast as a single net-shape component.
Die casting is well known for reducing manufacturing costs in external, internal, face, helical, spur, and worm gears -- casting them to AGMA 6 to 8 specification. Most tooth forms can be cast, including teeth with helix angles as great as 20°. Up to 50 external threads/inch are cast flash-free to Class 2A tolerance without cleaning or chasing, as are multi-start threads.
