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.
Shannon Wetzel, Senior Editor
(Click here to see the story as it appears in the Sept./Oct. issue of Metal Casting Design & Purchasing.)
One of the benefits of the casting process is the ability to create complex shapes and internal passageways 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 determine the most cost-effective method.
Several factors impact the choice between a cast-in or machined feature, including casting process, alloy, location, size, complexity, quantity, and dimensional and surface finish 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 differing capabilities for achieving certain tolerances.
“As you go from sand casting to permanent mold to diecasting and investment casting, you want more as-cast features because you achieve better tolerances,” 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 finish, which is important in applications when the part is meant to create a seal with another component or assembly. Machining becomes expensive, however, when multiple fixtures and jigs must be designed and produced to machine different 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 first aspects of the casting design metalcasters will check when determining 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 flesh 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 Alloy Inc., Muskegon, Mich. “The 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 adjustments 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 effective, 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 cheaper to CNC machine the hole.”
Machining time, setup and fixturing adds cost, but so does core production, corebox tooling and core removal. If a feature can be machined within the same plane or datum the part is already going to be machined, the additional cost to do so is negligible. In some cases, the metalcasting facility also may determine coring a feature will be too problematic to consistently reproduce and opt to machine it.
“We started looking at every part we made and realized we had created significant problems for the molders and the quality of the part by trying to put so many features into the tool with cores or pull pieces (in permanent mold casting),” Morel said. “Now we look at, what if we eliminate the core? How will that affect scrap and quality?”
Cores can shift in the mold, and certain geometries are tough to remove after the casting has solidified.
“One part we make features a cast-in bolt head feature,” Morel said. “The problem is, the hexagon head is so small, if that core is nicked or damaged, we have to go in with a pencil router and dig the piece of the core out.”
Morel Industries eventually switched to machining the bolt head.
“When we programmed it into the machining center, it was perfect every time,” Morel said.
Still, at the nonferrous sand and permanent mold casting shop, cores often are less expensive than machining.
“If it is a small feature utilizing a shell core made in a corebox with multiple cavities, the core cost is so inexpensive, it does not make sense to machine it,” Morel said.
Generally, the smaller the diameter, the more cost effective it is to machine it. A deeper hole will also lend itself better to machining. Determining the break-even point of drilling or coring a hole will depend on the section thickness of the surrounding part.
Shape plays a factor, too. For instance, some bolt holes are required to be oval so the part has some leeway for adjustment when it is bolted. Oval holes are tough to machine.
“It’s not just drilling those holes. You have to do some machining to the sides, too,” Cook said.
On the flip side, a hole or feature that is more square or rectangular can be more difficult to core.
“A round pin core has a strong, stable geometry,” Cook said. “A square core is not as stable because it has flat surfaces which are inherently weaker. You will have 3,000F steel pushing on the sides of the core. In the casting you may have bellies of metal protruding into what is supposed be a square opening.”
Square or rectangular holes can be cored, but will likely need to be gaged after casting to ensure they meet dimensional specifications. This will add to the cost.
Because so many variables affect the cost of coring and machining, casting engineers often price out a feature on a part-by-part basis.
“We cost it every time. We know exactly what the cost will be to make the core—the weight, the binders, how long it will take,” Morel said. “Then we go to the machine shop for that cost. They are pretty sophisticated and can figure in tool change and run time.”
Ultimately, the decision will be a mutual choice made by the customer and the casting supplier, with the goal of achieving the best quality part at the least cost.
“There are so many variables, you have to have a foundry you can trust,” Cook said.