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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 fl ip side, a hole or feature that is more square or rectangular can be more diffi cult to core. “A round pin core has a strong, stable


Cores are necessary when making complex internal passageways that are inaccessible with a machine tool.


cheaper to CNC machine the hole.” Machining time, setup and


fi xturing 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 negli- gible. 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 signifi cant 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 aff ect scrap and quality?” Cores can shift in the mold, and


certain geometries are tough to remove after the casting has solidifi ed. “One part we make features a


cast-in bolt head feature,” Morel said. “T e 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.”


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Read more about machining cost in the September/October 2012 Metal Casting Design & Purchasing article “Weighing Machining Cost” at www.metalcastingdesign.com


If a part is going to be machined post-casting anyway, it might be more cost effective to machine the holes on that side, as well.


32 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2014 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 of- ten 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 eff ective 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 thick- ness 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


geometry,” Cook said. “A square core is not as stable because it has fl at 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 di- mensional specifi cations. T is will add to the cost. Because so many variables aff ect the cost of coring and machining, cast- ing 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. “T en we go to the machine shop for that cost. T ey are pretty sophisticated and can fi gure 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. “T ere are so many variables, you


have to have a foundry you can trust,” Cook said. 


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