Geometric Tolerances

There has been a lot of discussion over the years between various suppliers of investment castings regarding tolerance that is appropriate for the process. While there continues to be some debate, what follows is meant to be a general guideline. An investment casting is meant to be a "near net-shape" part that offers the most value. If the buyer and producer maintain a high level of communication, the design requirements and cost efficiencies will be met by processing capabilities.

Straightness, Roundness, & Positioning (holes or bosses)
Axial straightness, roundness (bar or tube shapes) and positioning of holes or bosses is generally tolerance as +/-.005 per inch.

Concentricity
The center of a bore to the outside diameter of a part will be concentric within .003" for each 1/2" of wall thickness. Note, this assumes measurements are taken in the same plane and does not include out of roundness tolerance.

Angularity
As is the case with other dimensional tolerances for castings, the as-cast angle tolerance is largely a function of the part design. In general, a tolerance of +/-1/2 degree is considered minimum.

Blind Holes and Slots
These features are generally cast provided the depth is equal to the width or diameter for small characteristics less than 3/8". As the size increases above 3/8", the depth can move incrementally to double the size of the opening.

Flatness
This feature of casting is measured as the distance between two parallel planes, within which, the entire surface of the part must lie. Once again, the actual shape of the part will largely influence the cooling behavior of the wax pattern and the part as it solidifies; and this, in turn, tends to govern process capability. This requirement is frequently expressed in terms of six (6) square inches of surface area.

Wall Thickness
Although wall thicknesses down to .020 may be achievable in a small area, the normal minimal wall thickness zone lies between .040 and .080 depending upon alloy and square inches of surface area.

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