some simple casting design rules are followed. Te locators can be square cast lugs that will remain on the final product or removed later, gener- ously filleted and drafted holes or depressions, conical or spherical divots, spherical bosses, or even
specifically shaped shoulders (Fig. 1). As long as communication begins early in the design process with your casting and machining suppliers, cast-on locators can be so organic to your design only a person with the blueprint would know they exist.
Rules for Adding Locators Te four main rules when adding
locators are: 1
Never design the locator so the fixture point is on the parting line of the casting. Te parting line is the area of the mold with the low-
est repeatability due to mold swell and potential shift, which can nullify the purpose of the locating point. A good general rule in green sand castings is to keep your hard locating surface a minimum of 0.25 in. off the parting line in the drag side of the mold (Fig. 2).
sibility of sand tears during the molding process which can cause the part to mis- locate on the machining fixture (Fig. 4).
2 3
42 | METAL CASTING DESIGN & PURCHASING | May/Jun 2016
subsequent processing (Fig. 5). Cast-on machining locators can
4
be a great addition to any complex machined casting design. With a little creativity, the end user will never know of their existence, despite their impor- tance to the product. Te addition of cast-on machining
locators may require extra time in the beginning stages of the project, but those costs and more can be recouped easily from a reduced scrap rate and improved consistency in the final parts that are manufactured.
When possible, add bosses or divots as loca- tors that either blend into the design of the part or can be easily removed in
Always use at least three locating surfaces and fixture points to cre- ate a uniform set of datum planes. Tis can
be through the addition of three independent machining locators or a combination of locators and non- drafted/non-ground surfaces (Fig. 3).
When adding bosses or divots for locating points, use generous draft (5 degrees) and fillets (0.125 in.) on the feature to reduce the pos-
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