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Tolerances

The investment casting process is capable of excellent repeatability. As investment cast process capability of +/- three standard deviations when a single point location is repeatedly measured is typically +/-.003 to +/-.004 per inch. However, tolerance capability is largely influenced by part configuration. Symmetrical shapes with uniform wall sections will solidify with much less variation and distortion than non-symmetrical, non-uniform shapes.

Linear tolerances are controlled by four dominant factors:

• Process Variation (20-40% of linear tolerance)
• Prediction of Part Shrinkage Factors (10-20%)
• Diemaker and Tooling Tolerance (5-10%)
• Part Shape Causing Distortion (25-75%)

Normal Linear Tolerances
Normal linear tolerances for normal dimensions reflect these four sources. In many instances we can achieve tighter tolerances.

Signicast is capable of holding +/- .002 in some features, although +/- .004/inch is a more typical premium investment casting tolerance.

 

 

 

 

 

 

 

Premium Linear Tolerances
All three sources of variation can be reduced by:

1. Part redesign, including addition of tie bars, ribs, and gussets to certain shapes
2. Tuning of wax injection tooling after the first sample to meet the nominal dimension
3. Straightening/Coining
4. Additional Inspection/Gaging
5. Machining

All of these can assist in obtaining tighter than normal tolerances, with additional costs associated with items 2 through 5. Signicast will work with you to meet the design requirements in the most economical manner. Premium tolerances capability can be achieved, but must be considered on a part-by-part, dimension-by-dimension basis.

General Linear Tolerances
Dimensions that are on a drawing or CAD file simply to describe the shape can be most economically produced by applying a broader General Linear Tolerance. In many cases, the 3-D CAD file can contain all necessary geometry and these types of dimensions do not need to be described on a part drawing or inspector drawing.

Download our Design Guide to learn more about tolerance data for specific features such as:

• Tooling
• Flatness and Straightness
• Concentricity
• Angularity/Perpendicularity/Draft
• Surface Texture
• Gate Witness
• Radii
• Through Holes
• Blind Holes
• Wall Thickness
• Splines/Gears/Threads
• Letters, Numbers, Logos
• Gaging

References
Investment Casting Handbook, 1997 – Shows many applications, design data, alloy selection information and quality control information.

Dimensioning and Tolerancing – ANSI Y 14.5 – An excellent engineering drawing standard published by the American Society of Mechanical Engineers. This standard defines symbols, describes tolerance and datum methods, and contains valuable information to assist design of mating components.