Cost savings, a streamlined production process, and increased quality are only a few of the reasons why die casting is an excellent alternative for manufacturing small gears.
Excerpt from the October 2004 issue of Gear Solutions by Stuart Burke
For many applications, zinc alloy die cast gears provide an opportunity to cut manufacturing costs dramatically and improve part-to-part consistency. The cost of simple spur gears, as well as complex helical and worm gears, can be cut by up to 80 percent. Die casting can enhance rack gear performance by incorporating features unattainable by other manufacturing methods.
Bold statements? Not really. High pressure, hot chamber die casting machines and high precision tooling debunk the myths that many have considered to be the limitations of the die casting process. Extremely tight tolerances can be held over long production runs, and gears are cast net-shape and flash free, ready for use with no secondary finishing operations required. And precision die casting leaves no room for variations. Part to part consistency is inherent in the tooling.
The complex geometry of a newly designed horizontal gear drive used in an automotive seat adjustment mechanism challenged design engineers in finding a cost-effective manufacturing solution. With a worm gear at one end of a bearing journal and a helical gear at the other, the gear drive required precise tolerancing.
Production options included machining, but this required production of two separate components in two separate operations, plus press fitting with a spline engagement to complete the assembly. This would be a costly and time-consuming process, and the project management team was concerned about inconsistent runout and the accurate positioning of the gear teeth relative to each other. Tooling constraints and tolerance control issues precluded the use of powdered metal. Tolerance and strength concerns ruled out the use of plastics.
Die casting tool technology presented an opportunity to combine the individual components–the worm gear, helical gear, internal bearing journal, and two thrust faces–into a single zinc alloy die casting. The material provided the strength and dimensional stability required, and the process cut production costs by 40 percent compared to machined steel. Tooling design incorporates four side cores which come together to form the worm gear cavity, while gating through the center bore ensures uniform alloy distribution for consistent fill in the tooth forms. The gear drive is cast ready to use, with no finishing or deburring operations required.
The ability to convert multiple components and operations into a single operation is one of the major reasons for considering the die casting process. However, die casting offers many other opportunities to cut costs and increase quality.
Why Die Casting?
The potential for piece price reduction usually drives the move to high volume die casting production. Economics start at 50,000 pieces per annum and relate to a number of factors such as component complexity, alloy properties, die casting technology used, precision of the die cast tool, and cycle rate. In addition to being able to replace multiple components, secondary milling, boring, reaming, and grinding can be eliminated by incorporating features as part of the casting. Flash-free die cast tooling means no finishing. Additional savings come from material reduction, use of less expensive metal, improved tolerances, and part to part consistency.
What Can Be Die Cast?
The greatest process benefits are realized when requirements call for complex configurations, close tolerances, and part to part consistency. Die casting reduces manufacturing costs for 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 degrees. Up to 50 external threads/in. are cast flash-free to Class 2A tolerance without cleaning or chasing, as are multi-start threads. Internal threads can be cast to .001″.
Even a simple spur gear can be die cast at a fraction of the cost of machined gears: approximately 80 percent less. While the savings are not as substantial in comparison to powdered metal, aluminum cast, and stamped gears, much finer tooth forms can be die cast net shape. But die cast zinc alloys in this simple spur gear configuration cannot compete with plastic gears from a cost standpoint.
Double spur gears can be easily cast net shape, eliminating multiple milling and hobbing processes, as well as the undercut required. A gear can also be cast directly onto any vertical face. During the casting process, other features can be incorporated, which replace the need for additional operations. Indicator and timing marks, either embossed or recessed; part name/number; and manufacture date or other identifying information are included in the casting cavity.