WHEN TO CAST, WHEN TO MACHINE
A number of casting and machining variables determine the most cost-effective method for incorporating features into your cast part.
O SHANNON WETZEL, SENIOR EDITOR
ne of the benefi ts of the casting process is the ability to create complex shapes and internal pas- sageways within the mold that are not possible with machining. But in many cases, shapes and features can be either cast or machined, and the customer and supplier must collaborate to deter- mine the most cost-eff ective method. Several factors impact the choice between a
cast-in or machined feature, including casting process, alloy, location, size, com- plexity, quantity, and dimensional and surface fi nish requirements. One of the main advantages of machining is its precision. If the tolerances for
dimension and size are too tight for the casting process, the feature or hole must be machined. Various casting processes feature diff ering capabilities for achieving certain tolerances. “As you go from sand casting to permanent mold to diecasting and invest-
ment casting, you want more as-cast features because you achieve better toler- ances,” said Jiten Shah, president of Product Development Analysis, Naperville, Ill. “But the dimensional tolerances are not as great as machining, no matter what process you use.” Machining also provides a nicer surface fi nish, which is important in applica-
tions when the part is meant to create a seal with another component or assembly. Machining becomes expensive, however, when multiple fi xtures and jigs must be designed and produced to machine diff erent planes of a part. Additional time also is needed to change the machine setup for each plane to be machined. Casting in the feature eliminates the secondary machining cost. One of the fi rst aspects of the casting design metalcasters will check when de-
termining whether to cast or machine it is its location. If the feature will be made with a core, its location with regard to the parting line is important. A core is a shaped body, usually made of sand, placed into the mold to form an interior part of the casting, like the cavity the pit makes in the fl esh of a peach. “So much of what I look for when I quote a job is going to be how the mold will
be parted and how it is going to be fed,” said Jeff Cook, sales manager for Eagle Al- loy Inc., Muskegon, Mich. “T e core has to sit somewhere in the mold. If that hole
is up off the parting line, or parallel to the parting line, then you will have an issue [positioning the core].” A core isn’t the only way to cast-in
a feature, however. Metalcasters and design engineers can make adjust- ments to the part to utilize the mold to produce intricate features. “If the hole is perpendicular to the
parting line, I might be able to give that to the customer without a core,” Cook said. “As long as the customer can handle the draft angle required and the feature is not too deep, I may be able to make it with the mold.” If the part will be machined after
casting, regardless of how the hole will be produced, machining might be more cost eff ective, particularly when working with an alloy with good machinability, such as aluminum or iron. Mark Morel, vice president of sales and marketing, Morel Industries, Seattle, said his aluminum and copper- base casting company changed its viewpoint on cast-in versus machined features after it added a machine shop in the mid-1990s. “Our original goals were to try and
cast-in as many of the features as we could,” Morel said. “But what we found was, if the part had to be machined post-casting anyway, it was often
Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 29
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