pattern. A diameter’s degree of out- of-roundness also affects holes and bosses, although the 10% reduction still applies. Generally, a bolt circle diameter with holes and bosses will have the same out-of-roundness as any other diameter. Tus, a 2-in. (5.08-cm) bolt circle diameter will be wound within 0.025-in. (0.004- cm) total indicator reading. Te mass of surrounding metal
affects the roundness of a cast hole. Adjacent uneven mass will pull the hole out-of-round. If the surrounding metal is symmetrical, holes up to 0.5 in. (1.27 cm) in diameter can be held to +/-0.003 in. when checked with a plug gage. Te longer the hole or the greater the mass of the section around it, the greater its effect on hole tolerance. Te openings at the top and bottom of the hole will be approximately the same dimension, while the center will be a large diameter. Trough-holes, which
Fig. 4. A factor of two must be applied to the tolerance on all dimensions controlling a curved hole feature. A diameter tolerance of +/-0.005 in. (0.013 cm) also applies.
require clearance, can be held to fairly close tolerances if the larger diameter in the center is ignored. If the side- walls of the hole are used as bearing surfaces, a simple reaming operation will size the cast opening. Tapered holes should be dimen- sioned at the lesser diameter at an
angle of +/-0.5 degrees. If the hole is tapered relative to another feature, the above applies but the angle should be held to +/-1 degree. Curved holes are formed
by soluble wax or preformed ceramic cores and result in a doubled normal tolerance. A factor of two should be applied to the tolerance on all dimensions controlling such a feature (Fig. 4). Since curved holes cannot be sized, a tolerance of +/-0.005 in. also applies. Angular holes usually are formed by metal cores within
the pattern die, so the tolerance restrictions for curved holes do not apply. If the angle formed by the two centerlines is greater than 120 degrees, the hole can be sized. If the angle is less than 120 degrees, a diameter tolerance of +/-0.005 in./in. must be used. ■
40 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2011
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