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Courtesy: Hitachi Ltd., Katsura, Japan

Automobile clutch pressure plate. High strength, reliability, and ease of machining led to the section of Ductile Iron for the part shown. It is exposed to heavy alternating loads.

PRINCIPLE NO. 5-

SPECIFY NO MORE DIMENSIONAL ACCURACY THAN NEEDED

Due to the nature of the casting process, the dimensional accuracy of raw castings is more difficult to control than, for example, machining to size.

Principal dimensional inaccuracies in Ductile Iron arise from the following sources:

  1. Inherent inaccuracies in the machined dimensions of the pattern and core box.
  2. Deformation of core(s) in liquid iron.
  3. Inaccuracies due to the presence of parting line(s).
  4. Relief of cast-in stresses, if and when castings are heat treated.

With the exception of source a. the inaccuracies are dependent on the physical size of the casting. The larger the casting the more inaccuracy expected. Dimensional inaccuracies from mold deformation during pattern withdrawal are minimized through proper tapering of the pattern (draft). A minimum draft of 1:100 suffices only for very shallow patterns. Deep patterns may need to be drafted as much as 5:100 for maximum accuracy.

One simple qualitative method for evaluating the degree of success toward a uniform solidification pattern is to inscribe circles into selected sections of the design. If the diameters of the circles are all the same, as they would be in a straight wall, optimum conditions prevail. Sudden increases in the circle size (to twice or more of the diameter in an adjoining section) require special attention by the foundry.

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