Contours, Radii And Cams


A designer who plans to dimension a radius on a casting should understand that during the cooling process volumetric


shrinkage occurs which has a disturbing effect on external radii and contours. In a flat casting, concavity is easily


illustrated and understood, (refer to section on Flatness). The same concavity effects are encountered with castings that have contours, but with more dramatic results.


In concave radius applications, with the greatest shrinkage occurring in the center and the outer extremities fixed by the dimensions of


the casting, the cast radius tends to decrease. In dimensioning a drawing for a concave radius, (see Figure 1), the designer should use a reference radius, using dimensions on the casting radius, to control the basic


physical size. The fit to mating configuration to be controlled by using a tolerance band on the radius itself.


Envelope Tolerance I L of Dish


~---+--f ___,_r,, <t


Ref Rad.


Cast Rad. (Increases)


Amount


Figure 2


Note Convex Radius <t of Envelope Tolerance would normally coincide with high point of l" Dim (1.010).


In convex radius casting applications, with the greatest shrinkage again occurring in the center, the cast radius tends to increase. (See Figure 2). The drawing should be dimensioned with these considerations in mind.


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