For many applications, zinc alloy die cast gears provide an opportunity to dramatically cut manufacturing costs 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. Figure 1
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. Figure 2
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. Figure 3
The ability of die cast tooling technology to produce very complex gear shapes at a high production rate provides opportunities to increase functionality at reduced manufacturing costs. High tolerance helical gears, up to a 28 degree angle, can be cast in a single tool at a slight premium to simple spur gears. For many applications, this provides the opportunity for better transmission performance and reduced noise. The die casting tool for a helical gear is designed to rotate as the cast part is ejected, in much the same way as a screw is backed off. A punch-out center gating technique ensures uniform distribution of the zinc alloy to provide good detail and consistency in tooth forms on the outside circumference, and forms a semi-burnished through hole to tight tolerances.
When the moveable and fixed halves of the tool close, molten zinc alloy is injected into the cavity around the center hole circumference, which can measure from .060″ to .320″. After the molten alloy solidifies, a highly polished core rod, which forms the center hole, advances from the moveable side of the tool. As it advances, it punches out the sprue, or gate. The core rod then retracts, leaving a semi-burnished hole that is cast to a dimensional tolerance of ± .0005″, with walls parallel within ± .0005″ and a mirror-like surface finish of between 0.1 and 0.2 microns.
Depending on application requirements, a worm gear as described above is also an excellent candidate for zinc alloy die casting. The parting lines (where the side cores of the tool meet to form the gear shape) are precisely placed and controlled to a mismatch of less than .002″, but limit tolerance capabilities to AGMA 5.
Die casting of rack gears allows features such as side ribs to be incorporated into the die cast design. To provide superior strength, hardness, and creep resistance to a gear rack for a window security system, ACuZincÆ zinc alloy was selected, but ribs were added on either side of the rack–not only to increase strength, but also to provide a guide to the pinion gear to improve performance. A flatness tolerance of .006″ and a gear pitch tolerances of ± .002″ are held consistently through long production runs.
Zinc Alloy Choices
Strength and wear properties of the die casting alloy required for the application’s function, plus the mating surface, are determining factors in selecting zinc alloy as the material of choice for die cast gears. Zinc alloys, such as those in the ZAMAK, ZA and ACuZinc families, offer a wide range of good casting characteristics*. Zinc is the strongest die cast alloy at room temperature. Hot chamber die cast zinc alloys can be cast flash-free to tight tolerances with complex detail. Dimensional stability ensures part-to-part consistency over long production runs. Net shape manufacturing is one of the main advantages of hot chamber die cast zinc alloys.
Zinc alloys do have limitations for certain operating environments. They are not suitable for high loads and high temperatures. With sustained loads, alloy creep can become an issue. As mentioned previously, the mating surface may exclude use of zinc alloy. A glass-filled nylon or plastic mating part, or one with an abrasive surface, will quickly wear the zinc alloy gear.
The most commonly used zinc alloys in the hot chamber die casting process are ZAMAK 2, ZAMAK 3, ZAMAK 5, ZA-8, and ACuZinc. Overall, their mechanical properties compare favorably with powdered iron, brass, and screw-machined steel.
ZAMAK 3 is the most widely used zinc alloy as it offers the best combination of mechanical properties and economics. ZAMAK 2 and 5, with higher copper content, exhibit better wear resistance, although some dimensional stability is lost. ZAMAK 5 offers a higher creep resistance, but ZAMAK 2 has the highest shear strength.
The newer high aluminum and copper content ZA alloys have greater strength, superior wear and creep resistance, and lower densities. ZA-8 is the only member of this group that can be cast in the high speed, hot chamber die casting process.
ACuZinc, with a copper content of 5 to 6 percent, has the highest structural strength, greater hardness, creep resistance, improved wear and corrosion properties, and lubricity. ZAMAK 2, with its high tensile strength and lubricity, comes a close second to some of ACuZinc’s properties and may be a suitable alternative in particular applications. Consistently close tolerances are characteristic of the hot chamber die casting process. For a typical 1″ component with Cpk = 1.33, linear die cast tolerances, flatness and roundness are as close as .001″, and concentricity to .002″.
ACuZinc has extended zinc alloy’s use to structural applications because of its creep resistance, with a yield strength of 338 MPa. However, ZAMAK alloys can also be suitable for some load applications, as additional creep resistant properties can be designed into the component.
Zinc alloys, in particular ZA-8 and ACuZinc, have significantly greater hardness than other die casting materials, with a Brinell hardness of up to 103 and 118 respectively. ACuZinc’s low coefficient of friction makes it usable for bearing applications, but ZAMAK 2, with a Brinell hardness of 100 and high lubricity, can also be considered for bearing conditions. The properties of zinc alloy give enormous flexibility in selecting a die casting material. Each must be examined on its own merit. While similar in many respects, they exhibit sufficient diversity to meet a wide range of application specifications for small component manufacturing. In addition, their castability opens opportunities for component cost reduction.
Thinking Outside the Die
Design engineers need to think beyond die casting as simply the forming of metal components. At its simplest, that is what it is. But its real benefits are realized when die casting is used as a manufacturing process to reduce production costs. Where an application consists of several parts, the die-cast caster will look for ways to combine them into a single component, consolidating gears with shafts, ratchets, and cams. Applications that require swaging, riveting, screw machining, stamping, press-fitting, welding, or assembly of one or more parts can be die cast in one operation.
The original design of a gear and shaft assembly consisting of a plastic cam, steel pinion, steel shaft, and fine blanked steel gear required one staking and two press fitting operations. The assembly was expensive to produce, and there were continuing problems with the shaft to gear and the shaft to cam runout. Combining die casting technology with assembly techniques, production becomes a single operation, and pre-manufactured components are eliminated.
The gear and shaft are loaded into a specially designed assembly (fixturing) tool which positions the components in their correct relationship. Molten zinc alloy is injected under pressure into a cavity at the intersection which is in the shape of the cam and pinion. The molten metal solidifies in milliseconds, shrinking toward its theoretical center and mechanically locking the gear and shaft together with the zinc alloy cast cam and pinion. The precision of the tooling casts the configuration net shape and flash-free, so secondary finishing operations are not needed. For this application, a diamond knurl on the shaft provides additional strength to the assembly. In strength tests, the gear and shaft components fail before the zinc alloy hub.
The OEM reduced production costs by replacing the pre-manufactured plastic cam and steel pinion with inexpensive zinc alloy, reducing labor requirements. Product quality is also improved. Shaft to gear runout is consistently held to .008″, and shaft to cast hub runout is .003″ maximum.
Multiple operations in the production of motor drive assemblies for food mixers are reduced to one die casting operation. Rather than press fitting a 2 1é2″ die cast gear and a 1 3/8′ sintered iron bevel gear separately to a 1é2″ diameter and 4″ long steel shaft, then pinning through and across them, the three components are assembled and locked into position with an injected zinc alloy joint in one operation. This eliminated costs and quality problems associated with drilling and pinning, as well as cleaning. The die casting process ensures part to part consistency over long runs, maintaining a tolerance on the shaft position to within .004″ T.I.R., and concentricity of the gear OD and the shaft OD to .002″ T.I.R.
Choosing a Process
The choice of alloy dictates the die casting process, as does the size of the component. Pressure die casting is the most common method of producing small zinc alloy components, using either the hot chamber or cold chamber process. Small zinc alloy die cast gears are produced in the hot chamber process, which allows the very precise tolerancing and flash-free, net shape required for high performance, intricate shapes. Zinc alloys with a high aluminum content (ZA-12, ZA-27 and ACuZinc-10) are not suitable for hot chamber die casting and must be cast using the cold chamber process.
The die cast methods for each metal have merits and limitations, depending on component size and complexity, tolerance specifications, production volume, and tooling cost. Overall, hot chamber pressure die casting is the process of choice for production of small zinc alloy components (up to 6 cubic inches). For larger components, other casting processes need to be evaluated. While initial tool costs are high for the hot chamber process, large volume production and part to part consistency reduce piece price. Zero parts per million scrap is common to pressure die casting. Tooling techniques allow production of intricate and complex external and internal shapes to very close tolerances (.001″.) and net shape. The flash-free capability alone can offset tool costs because secondary finishing or machining are not required.
It’s a smart idea to have a die casting supplier on board as part of your initial project planning team to ensure the design incorporates the necessary die casting requirements and specifications. Most suppliers have technical specialists whose sole job is to review component designs for die casting. Value engineering can translate into major cost savings in both design and production by maximizing tooling and die casting techniques. Before the design can be finalized, the die casting supplier must calculate process factors such as flow vectors, gate and runner design, fillets, radii, draft, metal velocity, and fill time. Die casters can employ a number of techniques to maximize tool and component design. Even minor changes can improve performance and reduce costs. Elements such as ribs may be suggested to give the component increased strength, stability, and density, while at the same time wall thickness can be cut to as little as .020″. Where weight is a concern in gears used in precision measuring instruments or gauges, a cross-section can be reduced and/or recesses designed into the component to remove material. The addition of extra threads in a bolt connection will reduce creep and ensure that the load is retained over a long period of time.
Gating is a critical issue, particularly with die cast gears. With most component configurations the molten alloy is injected into the cavity along a parting line on a non-critical feature. With spur, helical, bevel, and worm gears, the alloy flow to fill the teeth forms is augmented by a center gating technique which introduces the alloy into the cavity around the circumference of a through-hole which can be .400″ or less in diameter.
Very complex component configurations with intricate features are common to pressure die casting through sophisticated tool building. The tolerance of the tool itself is critical, as flashing at the tool faces can defeat the economics of die casting when deburring or secondary finishing has to be performed. In conventional die casting tools, molten alloy is forced into the cavity until it flashes out between adjoining surfaces. In the small component zinc alloy die casting process, tools are assembled to tolerances of ± .0001″, closing to form a very tight seal around the cavity that eliminates flash. This precision accounts for the ability to hold part to part consistency over large production runs.
A die casting tool is basically a six-sided cube that opens and closes like a clam shell, with a parting line where the two halves meet. Within that cube is the cavity in the shape of the component to be formed. Any component feature parallel to that open/close motion is easily incorporated into the two halves with the use of cores. For example, a fixed core pin in the moveable half of the tool forms a center hole. For features offset from the parting line, moveable side cores are driven in a sideways motion to be retracted before the die cast component is ejected from the tool. These cores can be at any angle. For a wheel that requires features on the outside diameter corresponding to each month of the year, 12 cores are used, one every 30 degrees.
Cylindrical cores form holes with a .001″ tolerance, which can be tapped to 60 – 75 percent full thread without drilling. Side cores enable the production of holes and undercut features that are parallel to the major parting line of the tool. A movable core can form a hole or slot of virtually any shape to tolerances of .002″. Center bores can be cast to a dimensional tolerance of ± .0005″.
* ACuZinc is a registered trademark of General Motors Corporation. ZAMAK and ZA are trademarks of Eastern Alloys, Inc.