Investment Casting Technical Notes

Linear Tolerance

Normal tolerances can be expected for production repeatability of all casting dimensions.

Premium tolerances require additional operations at extra cost and achieve closer tolerances on selected dimensions only, even tighter tolerance than those on the following chart can be obtained. The tolerance achieved will depend on the alloy and configuration, and should be determined during consultation with a Consolidated Casting Corporation engineer.

An exception to these general tolerance rules exists on thin wall thickness where the minimum tolerance must be ± .020".

Geometric Tolerance

Straightness

Tolerance covering the straightness of an axis is the diameter or width within which the axis must lie. As a general guide, a constant section will usually have an axial bow of .005" per inch. Ribs, gussets and tie bars can inhibit warpage, and mechanical straightening can often improve the straightness of the casting.

Flatness

Flatness is the total deviation permitted from a plane and consists of the distance between two parallel planes, within which the entire surface of the part must lie. The shape of the part and the wall section thickness will influence the cooling rate of the wax pattern as well as the casting and may affect the flatness of the as-cast part. As a general guide, for areas under six (6) square inches the expected dishing or flatness is approximately .005" per inch, in addition to the basic linear tolerance.

Concentricity

When the length of a bar or tube is not more than two (2) times its component diameters, the diameters will generally be concentric within .005" per inch of separation. When the length is more than two (2) times the diameters, out of straightness should be added to the eccentricity.

Roundness & Hole Positioning

"Out of round" is defined as the radial difference between a true circle and a given circumference. It is the total indicator reading when a part is rotated 360°, half the difference between the maximum and the minimum condition. The actual inspection method should be specified by the purchaser. The standard is +/-.005" per inch.

Angularity

As cast angular tolerances are a function of the part design, and a minimum tolerance of ±1/2° should be permitted.

Holes & Slots

Holes and slots generally can be cast to standard tolerances, depending on the depth in relation to the diameter or width. Smaller cores (<.25") may be limited to a 1/1 ratio whereas larger cores (>1.0") may provide up to a 5/1 depth to width ratio. Blind holes and slots are much more difficult to cast and may require large fillet radii at the base with only half of the depth to width ratio as that of a through hole.

Wall Thickness

The standard minimum wall thickness for conventional investment casting is .060-.090" depending on surface area, and as low as .030" has been achieved. The wall thickness also depends on the size and shape of the part and the alloy required.

Surface Texture

Industry uses RMS (Root Mean Square) as measured by electronic profilometer or visual comparator. The industry standard is 150 RMS average, but it may vary from 60-200 RMS depending on the size and shape of the part and the alloy required.