There was a time when automotive seat designers were in a dilemma over the most cost-effective way to make geometrically intricate and horizontal gear drive. Earlier, it was proving difficult to find any single process which could meet its tolerancing requirements within planned budget. The gear drive included not just an Acme thread at an end of the bearing journal, but also a crossed-axis helical gear on the other.
Machining was a potential option, but it was expensive and time-consuming. It entailed producing two parts in separate operations, and later press fitting the assembly together using a spline engagement. In addition, there were also concerns about inconsistent run-out and whether the gear’s teeth could be placed with sufficient relative accuracy. Another possible option for the gears was powdered-metal processes, but their use was precluded due to tool constraints as well as tolerance-control issues. Plastics were also ruled out because of tolerance and strength restrictions.
That left the die casting process as the option. A plus point of die casting was that it joined the components – the screw thread, internal bearing journal, helical gear and thrust faces – into one part. The zinc alloy also gave strength as well as dimensional stability. Furthermore, the production costs in die casting were forty percent below those for the machined steel.
The tooling required to form the cavity of an Acme screw thread included side cores, numbering four. Gating through its center bore made sure the alloy would fill in its tooth forms in a consistent manner. The gear drive that results from this process is die cast, and ready to use, and requires no deburring or finishing operation. The option to change many components and processes into one operation is one of the major reasons that automobile designers consider die casting. However, it also offers several other opportunities to cut the cost, and can enhance part quality.
Why Die Casting is Chosen for This Purpose
The usual motivation to choose the high-volume casting process is the potential it holds for the piece-price reduction. The economies of scale usually start at around 50,000 pieces annually. A number of factors can affect manufacturing economics, including component intricacy, alloy properties, the die casting technology involved, the casting tool’s precision, and cycle rate. One of the reasons die casting can be economical is that a die cast component often replaces several parts. In addition, it is often possible to incorporate certain features in the die cast part that eliminate secondary operations, such as milling, boring, reaming, as well as grinding.
Flash-free tooling also eliminates the requirement for any finishing operation. In addition, extra savings come from the use of less-costly metals, material reduction, enhanced tolerances, as well as component-to-component consistency.
Designs that incorporate intricate configurations are suitable for die casting. Fine candidates comprise shafts, cams, gears, ratches, pinions and levers, and others that perform mechanical functions. Containments like end bells, motor housing, gear housing, plates, seats and spacers are also frequent choices for the metallurgical process.
The process is well known for minimizing production costs in internal, external, helical, spur, face and wear gears – casting them to the specification of AGMA 6 to 8. Most gear tooth forms can also be cast, comprising those with helix angles that are as great as twenty degrees. Up to fifty external threads or inch are also cast free of flash to the Class 2A tolerance without chasing or cleaning, and so are multi-start threads.
Tooling Techniques
Intricate parts with complex features are usually cast with tools that are highly sophisticated. Tool tolerance is also vital. Flashing at a tool face can defeat the die casting’s economics, if it requires either secondary finishing or deburring. In traditional die casting, alloy in molten form is injected into the cavity until it flashes out amid the adjoining surfaces. For the zinc casting of small parts, tools are assembled to precise and specific tolerances that ensure a tight seal around the die cavity. This then eliminates flash.
A die casting tool is essentially a six-sided cube, which opens and closes similar to a clamshell, featuring the parting line. The die cavity is the to-be-formed component’s shape. Any part feature that is parallel to the open/close movement is incorporated easily into the die halves using cores. For instance, a core pin fixed in the moveable half of the die casting tool creates a center bore. For features that offset from the line, where the die halves meet, moveable side cores are driven sideways to be retracted prior to the ejection of the die-cast part from the tool.
Tolerances that are consistently close are characteristic of hot chamber die casting. While wall thickness of the component can even be 0.020 inches, surface finish usually runs from sixteen to sixty-four millimeters. Center bores in the part can be die cast to precise dimensional tolerance.