Die-Cast Zinc Solves Assembly Woes

Dec. 9, 2004
Combining die-casting and assembly technologies simplifies production, improves quality, and cuts costs.

Die-cast zinc solves assembly woes

Karel Vycpalek
Peterborough, Ont., Canada

A pin is encapsulated in a zinc-alloy configuration on a tube, forming an internal body shape to which magnetic material is overmolded. Multiple processes — drilling a hole, press fitting the pin and preforming of the shape — are replaced with a single operation. The process resolves tolerance issues of drilling and press fitting relative to the cross hole.

Zinc-alloy die-casting assembly economically replaces machining operations. Instead of turning a hex steel bar to create a shaft with a nut shape in place, a zinc-alloy nut is cast around a shaft, cutting material waste.

Using zinc alloy as a binding agent, a shaft is joined to a squirrel-cage rotor with a hub, eliminating distortion problems caused by press fitting. Close tolerance is consistently held, and spacer bushings are no longer required.

Cable-termination production speeds to 650/hr when two equallength cables are simultaneously cut, then joined with a die-cast zinc-alloy bar (right). Next the cables have barrel fittings cast on their other ends (left). These small (0.22 and 1.23 in. ±0.002 in.) diameter, flash-free terminations have pull-off strengths greater than 112-lb force.

Swaging, press fitting, crimping, and adhesive bonding are all well-known joining techniques. Zinc-alloy die casting has also earned a place on this list. Zinc alloys can join most materials including metals, plastics, ceramics, glass, paper, fibers, and elastomers. Zinc die casting can help reduce costs by eliminating individual

components from subassembliesand is well suited for both joining parts and forming components in a singlestep process called Injected Metal Assembly (IMA).

IMA simplifies component assembly by letting designers join components inside an assembly fixture tool that also serves as the casting die or mold. Components are aligned in the fixture then molten zinc alloy is injected into die cavities where it solidifies and shrinks, locking around the parts forming a strong, permanent joint. IMA produces a finished, ready-touse assembly straight from the mold. Delicate components or those susceptible to distortion are good candidates for IMA, as no force is applied during the assembly process.

Components joined by traditional processes can likely be assembled more efficiently with diecasting technology. For example, in an automotive transmission shift assembly a fabricated eyelet crimped to a steel shaft is a simple mechanical operation. However, by die casting the eyelet shape in zinc alloy directly onto the shaft in one step, assembly becomes part of the manufacturing process.

Likewise, a two-piece cam and steel shaft assebly can be redesigned so that instead of press fitting the prefabricated cam to the steel shaft, the zinc alloy is cast in the shape of the cam directly onto the steel shaft. The alloy's predicable shrinkage mechanically locks the die-cast cam onto the steel shaft. Another example is a recent redesign of a window regulator. Here a stamped steel cup, swaged to a spindle, is eliminated by die casting the cup onto the spindle. In both cases, the cast cam and cup do double duty — serving as an assembly component as well as a joint.

With IMA, part-to-part consistency of the diecast joint over long production runs is also good and tolerances can be maintained to ±0.002 in. For example, maintaining concentricity on cups and shafts, such as those in cooling fans, can prove troublesome with staking or press-fitting operations. But with IMA, the end of the shaft is accurately positioned in relation to the cup as a castzinc hub locks the two together. The molten metal compensates for any inaccuracies in the cup's center hole, ensuring accurate position and fit. The cup is held to within ±0.002 in. and the circular runout between the shaft OD and the cup ID is within 0.003-in. TIR. Another benefit of IMA is that functional features that typically are machined can be formed directly in the assembly mold.

An industrial hydraulic joystick-control assembly serves as another example where IMA improved tolerances over a conventional joining method. The process steps needed to build the original assembly included drilling a hole, press fitting a pin, and preforming an internal body shape around the pin and tube so that a magnetic material could be overmolded onto it.

Due to the small size of the components (0.097-in.-diameter steel pin and 0.37-in.-diameter stainless-steel tube) it was difficult to align the parts and maintain tight tolerances. The original design required careful positioning of the predrilled hole in the tube relative to the cross hole at the end of the shaft, a difficult and often time consuming operation. Another tricky step was press fitting the pin to ±1.5° relative to the cross hole. With IMA, however, the pin is encapsulated in a zinc alloy, forming the internal body shape as the pin is joined to the tube. Consistent tolerance position specifications are held, and secondary preforming of the internal body shape eliminated.

Components susceptible to breakage or distortion are not suitable for press fitting. Ceramic magnets, for example, can easily shatter under pressure. But with IMA, the zinc alloy flows around the magnet, cooling quickly with predicable shrinkage, to form a strong mechanical joint between it and a secondary member, such as a shaft.

In another case, a 1.0-in.-diameter squirrelcage rotor press fitted onto a 0.10-in.-diameter steel shaft was often damaged during assembly resulting in costly waste and lost production time. With IMA the shaft is attached to the squirrel cage rotor with zinc alloy rings and spacer bushings are no longer needed. No pressure is applied during the process, so the rotor is no longer damaged and tolerances are more consistent.

Gears and pinions are other common candidates for IMA as well. Substantial material savings result from eliminating these prefabricated components from shaft assemblies. In replacing a staking operation, a 0.51-in.-diameter helical gear and a 0.55-in.-diameter pinion are cast in zinc alloy onto a 0.04-in. aluminum shaft to a tight concentricity tolerance of 0.004-in. TIR.

Zinc alloy efficiently forms fittings, fasteners, and terminations on wires and cables. For example, a ski-boot manufacturer was crimping prefabricated bullet-shaped steel fittings to multistrand, plastic-coated cables. Casting the bullet-shaped parts in zinc alloy directly onto the cables not only dropped part count but also let designers hold tighter tolerances (±0.002 in. on the 0.35-in. length and 0.13-in. diameter) over crimping. To help ensure that the cast bullet would not slip off the cable, designers use cables with upsets in the plastic coating that let the molten alloy impregnate and completely surround the cable strands. In addition, the tool cavity design incorporates a 0.004-in. corner break at the lower end of the cast bullet. This redesign provides a smooth surface to snap the bullet into the ski boot's retaining clip. The diecast bullets eject net shape and flash-free so no secondary finishing operations are needed. Piece price also dropped as one IMA operator replaced three personnel on the crimping line, yet production almost doubled.

An automotive push/pull cable application from Dura Automotive Systems, Gehren, Thüingen (Dura Deutschland GmbH) also reduced costs and boosted productivity by switching to IMA. The company phased out swaging components and began using IMA to form die-cast zinc-alloy terminations directly onto the ends of cables. To speed the process, Dura further automated production by incorporating a system to measure and cut each cable and feed it directly into the IMA system where the terminations were cast.

Accelerator pedal (push/pull) linkages consist of two cables joined to a bar termination. Barrel terminations are attached to the other end of each cable. Assembling these cables is, however, manually intensive requiring separate casting operations for the bar and barrel terminations.

To speed up the process Dura Deutschland worked with FisherTech to develop an IMA system that could terminate two cables at once. The system pulls, measures, and cuts two cables to 11.42-in. lengths, strips the plastic insulation, upsets the cable ends, and die casts the zinc-alloybar termination, encapsulating both cable ends. The termination is 1.06-in. (±0.020-in.) long with a 0.2-in. (±0.002-in.) diameter. The process takes only seconds.

The terminated cables next transfer to a semiautomatic cable-termination system where an operator slides coiled springs onto the other cable ends. Next, zinc-alloy barrels 0.22 in. in diameter and 1.23 in. long are simultaneously die cast on each cable end. The terminations are burr and flash-free, with no blemish to the plastic insulation and no flats.

With two different length cables, the double-cable system produces 500 zinc die-cast terminations/hr. When the cables are the same length, the number of terminations increases to 650/hr. Production operations, such as cable cutting and insulation removal, are completely eliminated.

(705) 748-9522, www.fishercast.com.

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