\


\\ ./ /


Dimensional variation in an investment casting may be caused by many factors. Most of these factors are closely controlled by the foundry, but minor "lot to lot" variations do occur which result in the tolerance bands defined later in this section. While it is often true that machining tolerance on


a given part may be closer than tolerance for an investment casting ... it is also often true that critical review of a design will allow a slight back- ing off on tolerances, undercuts, blind holes, etc., for translation into higher production yields and lower initial piece costs through investment castings.


Wax or plastic temperature, pressure, die tempera- ture, mold or shell composition back up sand, firing temperature, rate of cool, position of the part on the "tree", and heat treat temperature (if re- quired), all bear directly on tolerances required in the investment casting industry. The amount of tolerance required to cover the various process steps is dependent, basically, on the size and shape of the casting and will vary from foundry to foundry. This is because one foundry may special-


ize in thin walled, highly sophisticated castings, another in mass production requirements, and yet another in high integrity aerospace, or aircraft applications.


One factor, however, is a constant. The cost of any casting will increase in proportion to the precise- ness of the specifications, whether on the chem- istry, nondestructive testing, or tighter tolerance bands.


3


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24