Die casting takes on machining

Oct. 3, 2012
Net-shape castings that eliminate machining operations can cut costs

Authored by:
Kenneth J. Korane
Managing Editor
[email protected]
Dixon Quick Coupling


Engineers often have an important decision to make when they design and source a metal part: Should it be machined or die cast? In the past, a cursory review of production volumes, tolerances, features, and alloy requirements would clearly favor one manufacturing method over the other.

Today, the choice is anything but clear. Advancements in die-casting technology now make it possible to cast tight-tolerance features that once would have required multiple machining operations. By eliminating machining steps, near-net-shape casting offers a manufacturability advantage that can reduce the costs of many metal components.

The experience of Dixon Quick Coupling, a manufacturer of hose fittings and accessories, offers one such example. The Charlotte, N.C.-based company recently converted a knurled sleeve for a quick-connect coupling from machining to die casting.

Converting a proven machined part to a die casting can be a big step, says Cindy Karriker, the company’s supply-chain manager. “We had to make sure the potential manufacturability benefits of die casting would not come at the expense of durability and quality,” Karriker says. With help from engineers at Dynacast, headquartered in Charlotte, Dixon managed to lower the cost of making its sleeve component while maintaining quality. Here’s some advice they offer others looking to do the same.

Pick the right part. Not all machined parts are good candidates for die casting. Low-production volumes, for example, make it tough to justify the expense of hard tooling. Requirements for exotic metal alloys can also rule out die casting. Looking beyond these obvious deal breakers, many parts are excellent candidates for die casting, particularly when net-shape or near-netshape casting replaces machining.

At Dixon, eliminating several machining operations tipped the scales in favor of die casting. According to Karriker, the sleeve’s original two-piece design required three machining steps — roughing out the part, adding surface detail, and cutting the part in half. The die-cast version maintains the two-piece design but the two halves require no machining prior to assembly, making die casting less expensive. “Multiple machining setups can drive a lot of cost,” Karriker says.

Add surface detail in the tool. One challenge in Dixon’s switch from machining to casting involved the knurling on the connector surface. This ergonomic feature improves “grip-ability” and is easy enough to machine, notes Karriker. But machining the knurling on the cast part would have limited the advantages of net-shape die casting.

To overcome this hurdle, Dynacast’s engineers worked with Dixon to replicate the knurling pattern on the tooling surface. According to Helmut Wolf, Dynacast’s special projects director, this tooling-based approach requires more upfront design effort. “It can be difficult to get the surface detail right when converting machined parts to die castings,” he says. But adding surface texture in the tool eliminated an entire machining step — and the cost that goes with it. The results have been worth the effort, indicates Karriker.

Wolf points out that knurling is just one type of machined surface feature that can be brought into the tool. Dixon also cast its logo into the part, rather than machining it. Other cast-in surface details include external threads, various coupling features, and part numbers. “All these examples require extra attention to the tooling design, manufacturing, and even maintenance,” says Wolf. “But the cost reduction from replacing secondary machining operations can be enormous.”

Prototype and test. Dixon’s original sleeve component had been made from a cold-finished 1200 Series steel. To ensure a zinc diecasting alloy, Zamak 8, would work the same, Dixon’s engineering team machined prototypes from zinc-alloy bar stock. The prototypes were subjected to an extensive battery of mechanical tests, including aggressive drop, crush and drag testing. “We even used a jack hammer on the parts,” Karriker says.

Testing proved that the new material was up to the job, and the move to a zinc alloy did not require any adjustments to the original part’s design. “There were no functional issues at all,” Karriker says. Only the part’s surface features needed some revision to permit the cast-in knurled pattern and facilitate ejection from the tool.

Anticipate production changes. The move from machining to die casting may trigger changes in production flow. Usually, though, a little planning can minimize or even eliminate the manufacturing implications of these changes.

Dixon’s die-cast sleeves have longer lead times than in-house machined versions, resulting in a need for higher in-process inventory levels. “We carry more parts in inventory than we used to, but it wasn’t a deal breaker when we looked at the total cost benefits of die casting,” says Karriker.

Some production changes may even be positive. For example, Dixon’s assembly operations have benefited from the switch to die casting. As Karriker explains, the machined sleeves had greater potential dimensional variations than do castings. “Part-to-part variation on the machined component made it more difficult to assemble the finished couplings,” she says. “Improved dimensional consistency was a big plus for die casting because it made final assembly a lot easier.”

Understand tooling’s true cost. Ask a manufacturer with machining experience about the biggest downside to die casting and you’ll likely get a one-word answer, tooling. Karriker agrees tooling was the biggest hurdle Dixon had to overcome when first considering die casting.

Yet once she ran the numbers related to production volumes and the tooling payback period, Karriker found that upfront tooling costs were small relative to the cost reduction Dixon will see over the product’s life cycle. What’s more, Dynacast guaranteed its proprietary four-slide tools for the life of the program.

According to Wolf, it’s common for those new to die casting to overemphasize tooling costs relative to the overall cost. What’s more, many first-time analyses comparing die casting to machining fail to account for machining’s true costs. For example, many machined parts require expensive jigs or fixtures. There are ongoing labor costs associated with setup and handling of machined parts. And for captive machining operations, there are capital costs and machine hour rates to consider.

“Add up all of machining’s costs, and you start to see die-cast tooling as a relatively minor cost,” says Wolf.

Getting the knurl right
Adding a knurl to a machined metal part isn’t all that di cult. Any machine shop with the right tools can do it. Knurling gets a lot more di cult, however, when you try to cast it as a surface feature.

One concern is that the raised portions of the knurl pattern will interfere with the release and ejection of parts from the tool. The pattern may require slight modifications near the parting line. Typically, pattern changes are barely noticeable on the finished part. Gate vestiges and parting lines can also interrupt the knurled pattern, creating a small  at section near parting lines.

“Casting in a knurling pattern is harder than it looks,” says Dynacast’s Helmut Wolf. The payo… , however, can be significant if casting the pattern eliminates a machining operation. According to Wolf, there are a few design steps that simplify the task of making knurling a net-shape feature.

Consider ejection implications. On the Dixon job, Wolf recommended subtle changes to the geometry of the knurl’s raised sections — rounder edges, fillets, and draft — to ease ejection from the tool. He also slightly changed the pattern near the parting line. These geometry changes cannot be seen or felt by someone using coupling. “They were made only for reasons of manufacturability,” Wolf says.

Consider cosmetics. To eliminate disruptions gating can make on the knurling pattern, Wolf was careful to locate gates on the pads formed by raised portions of the knurling pattern. The pattern geometry around the parting line was also slightly altered for cosmetic reasons.

© 2012 Penton Media, Inc.

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