Casting a Better


Part for Machining


Metalcasters can reduce their machining costs through improved production methods and proper part and tooling design.


Shannon Wetzel, Senior Editor


friendlier to machine than others hold an advantage over their competitors. Machinability generally is based on up to five main factors. “For some, it is the ease of machining (meaning lower horsepower to get the job done), for others it is having excellent tool life and low tool failure rates (from breaking),” said Mark Fields, manager of metallurgy and melting at Cast Fab Tech- nologies, Cincinnati. “For others, it is the achievement of an excellent finish, and for still others it can be the overall amount of metal removed. And for some, it may have to do with the cleanliness of the removed material and it’s shape. For most, it is a combination of all of the above.” Depending what machining factor


M


your metalcasting facility wants to im- prove, you can take steps to clean up your melt, tighten your molding processes or adjust part design.


Machinable By Design


Metalcasters can work with their customers to improve a part’s machin- ability through design, which often means finding ways 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


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achining operations can make up a significant portion of a part’s cost, and metalcasters that produce castings that are


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 con- solidate tools and fixturing? Todd Farrar, vice president of iron metalcaster Farrar Corporation, Manhat- tan, Kan., said his company, which also performs machining in-house, makes an effort to discuss with its customers up front the different production or design options that may affect machining and, ultimately, part cost. A cast-in hole or cored cavity, for instance, can reduce machining time, but if the end-user requires tight tolerances, it can be best to machine it. “If we can hold tolerance consistently


across multiple runs, generally speak- ing, we prefer to do it in the casting,” Farrar said. “We tell our customer up front there are different ways to get from A to Z in their part.” Some features may be added to a


casting to facilitate machining when it cannot be avoided. For instance, pat- ternmakers can design-in pads, which are projections of excess metal on the casting to facilitate machining or other secondary operations, or bosses, which are short protrusions on the surface of a casting intended for drilling or tapping. The location and proportion of pads and bosses are critical in order to avoid hot


spots that may lead to shrinkage cracks. According to AFS Aluminum Casting Technology, pads and bosses located on walls not perpendicular to the parting line should be tailed to eliminate the need for sand cores, loose pieces, split metal cores and multiple mold partings (Fig. 2). The height of bosses and pads should be less than the thickness of the casting section but high enough to permit machining without touching the surface of the section (Fig. 3). When sev- eral bosses are on one surface, joining them will facilitate machining (Fig. 4). Designing sloping bosses for a straight parting of the mold or die will save money in tooling construction (Fig. 5). In permanent molds, ribs on the mold parting line can serve as feeders. A casting’s parting line should be


located away from where it will be clamped into a machine tool. “End-users of castings are looking for


tight dimensions in the areas they are calling for machining,” Farrar said. “It can be tough to clamp onto a parting line because it is not always as consis- tent [as the rest of the casting surface].”


Cut Machining Time Since machining cost is closely related


to machining time, modifications to the part to increase machining efficiency is a bonus. Casting a few locators (flat pads on which the machining equipment can clamp) onto the part can cut down the time needed to machine. The pads then


MODERN CASTING / September 2010


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