Fig. 2. In this example, with a 3-in. (7.62-cm) outside diameter, 2-in. (5.08-cm) inside diameter and 4-in. (10.16-cm) length, the outside and inside diameters will be concentric within 0.005 in. total indicator reading.
ment castings should not be less than +/-0.5 degrees (Fig. 3). Internal Radii and Fillets—T ese should be given as wide a tolerance as possible. T ey are diffi cult to control and can be checked approximately by radius gages or at a premium by an optical comparator. Contours, Radii and Cams—When
dimensioning an investment cast- ing radius, remember that volumetric shrinkage occurs during cooling and aff ects external radii and contours. In a concave radius, the greater shrinkage occurs in the center and outer extremi-
Fig. 3. The angles in sketch A cannot be sized, but in certain cases after suffi cient data review, the tooling can be reworked to bring the part closer to nominal dimensions. Sketches B and C can be reworked to +/-0.5 degrees and +/-1 degree.
ties. T is tends to decrease the radius, and a reference radius should be used. Dimensions on the casting radius are then used to control physical size. T e fi t to mating confi guration then is controlled by using a tolerance band on the radius. In a convex radius, the greatest
shrinkage also occurs in the center. T is tends to increase the case radius. Surface Finish—T e investment
casting industry uses the root mean square (RMS) or arithmetic aver- age (AA) to measure surface fi nish. Surface texture standards are 125
RMS and 115 AA maximum. Second- ary operations will improve surface fi nish, giving results comparable to the equivalent wrought alloy.
6
Dimension Holes Properly
Hole and boss tolerance depends on
the parent casting confi guration. Linear tolerances aff ect the position of holes or bosses on a fl at plane. Holes and bosses alter volumetric shrinkage, and toler- ance bands can be reduced by 10% for a confi guration that alters the shrinkage
Nov/Dec 2011 | METAL CASTING DESIGN & PURCHASING | 39
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