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achining operations can make up a significant portion of a part’s cost, and finding ways to reduce or eliminate machining can lead to a lower unit price. But sometimes machining is necessary to achieve the required dimensional accuracy or it’s less expensive than casting-in a feature. “Several things go into a cost-based decision,” said Tom Prucha, vice president


of technical services for the American Foundry Society, Schaumburg, Ill. “Is the feature part of a singular operation, such as drilling? Or does it require separate operations? Can you create the feature via a casting operation, either with coring or a design change? Does casting eliminate an operation or increase productivity throughput? What are the key tolerance requirements for a feature?” According to Michael Finn, president, Finn Metalworking and Cutting Solu-


tions, Loveland, Ohio, alloy grade, casting process, shape, heat treatment, mechan- ical properties, microstructure, dimensional tolerance, finish, and machinability have a significant influence on the cost of machining a part. “Te production planner needs to know the available machine tools in the selected machining facility before selecting the machining parameters,” Finn said. Careful consideration of what goes into the cost of casting-in or machining your part’s features will lead to a lower total price of a cast part.


Machinable By Design One way to reduce machining cost is to eliminate the need for machining at


all. According to Edward Vinarcik, product engineer for a company that purchases castings, machining processes by their nature create waste. He said that after a design is completed, each machined feature should be evaluated by answering a


series of questions, starting with three basic queries (Fig. 1): • Is the feature required? • Can the feature be cast net-shape? • Can the feature be positioned to consolidate tools and fixturing?


Gary Ruff, CEO and president of Ruff and Associates, Rochester Hills, Mich.,


added that how a feature is cast net-shape affects cost, as well. “If designers want a window in a casting, they usually design it in a way that requires a core,” Ruff said. “Many times you can change the draft or orientation of


a wall and window so you don’t need a core. I’ve been recommending casters and designers look at producing the hole though the mold without a core even if it adds slightly more draft.” Cores add cost. Tey require addi-


tional tooling and setup and can slow production rates. Finding suppliers who can or want to produce cored castings—particularly in iron—is another hurdle, according to Ruff. “With the rationalization of the


supply base over the past few years, the number of iron casters that have the capability to produce complex cored parts has been reduced,” Ruff said. “And in many cases, they don’t have internal coremaking capabili- ties or have not added capacity. So there is time spent acquiring the core, breakage may occur during transport, and coldbox cores have a limited shelf life. Price is going to reflect that. In today’s market, you should design without a core, if at all possible.”


In Some Cases: Leave Part Features to a Machine Shop


In market segments such as auto-


motive, where designs are becoming increasingly complex and requirements for holes and surface finish are more stringent, meeting those needs as-cast is difficult. A cast-in hole or cored cav- ity eliminates machining time, but if the end user requires tight tolerances,


Fig. 1. This flow chart can be used to determine whether to machine or cast a feature. Sept/Oct 2012 | METAL CASTING DESIGN & PURCHASING | 25


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