Geometrically complex designs, like this aluminum electronics housing, will influence a casting’s final cost.
• Inherent Cost—Tis depends on the baseline cost of the metal and the alloying elements.
• Purity—Alloys with fewer impurities are more expensive. Lower impurity levels mean fewer gating and risering returns are allowed in the melt charge, which can increase waste.
increase the costs and lengthen de- livery schedules. Tolerances that are too loose lead to more extensive ma- chining and usually have a heavier section size than required. Here are five considerations when determin-
• Castability—Of these three considerations, castability has the greatest effect on total cost. In steel and aluminum alloys, different compositions have dif- fering levels of fluidity, weldabil- ity and castability. With reduced castability, additional metal must be poured into the mold to ensure a sound casting.
have a working knowledge of the major factors influencing dimensional allowances and tolerances. Tolerances that are more rigid than necessary
3
Dimensional Allowances and Tolerances
Buyers and designers must
ing allowances and tolerances: • Shrinkage Allowance—Metal contracts as it solidifies and cools. To obtain a casting of desired size, the pattern is made slightly larger. This addition to the dimensions is called a patternmaker’s “shrinkage allowance.”
• Draft Allowance—This term describes taper on the vertical faces of a pattern, which al- lows it to be removed with-
“
out tearing the mold. Draft normally varies between one and three degrees of taper. Small or machine-drawn pat- terns require minimal draft. Large patterns, inside pockets on patterns and hand-drawn patterns generally require greater draft. Maximum draft should be provided where it does not interfere with a cast- ing’s desired geometry. The metalcaster should be consulted to minimize or eliminate draft allowance on surfaces that must be machined. For a no-draft condition, a loose piece in the pattern or corebox can be used. This will increase the cost of
Purchasing agents must evaluate the many important factors that directly influence a casting’s total cost.”
Jul/Aug 2014 | METAL CASTING DESIGN & PURCHASING | 27
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