The investment casting process is capable of excellent, repeatable as-cast tolerances. With as cast capabilities of +/- three standard deviations when a single point is repeatedly measured—typically +/- .003 to +/- .004 per inch. Since tolerance capability is largely influenced by part configuration, this can vary depending on your part design. However, symmetrical shapes with uniform wall sections solidify with significantly less variation than non-uniform and non-symmetrical wall sections.
Normal linear tolerances for normal dimensions reflect these three sources of variation:
- Prediction of part and shrinkage factors (20%)
- Diemaker and tooling, tolerance (10%)
- Process variation (70% of linear tolerance)
The variation in tolerance is a combination of part configuration effects that result in non-uniform shrinkage and all other process variation in producing a wax pattern, ceramic mold, and the casting.
Premium Linear Tolerances
All three sources of variation can be reduced by:
- Part redesign, including addition of tie bars, ribs, and gussets
- Tuning of wax injection tooling after the first sample to meet nominal dimensions
- Additional inspection/gaging
Premium tolerance capability can be achieved but must be considered on a part-by-part, dimension-by-dimension basis. Signicast is capable of holding +/- .002” in some features, although +/- .004” per inch is a more typical premium cast tolerance. It is important to designate tight tolerances that are necessary to part function and leave the rest open to normal linear tolerances.
General Linear Tolerances
Customers often ask us “What about dimensions that are on a drawing or CAD file simply to describe the shape?” These dimensions can be most economically produced by applying a broader general linear tolerance.
Another strategy that works well is to incorporate these types of features into a 3D CAD file for the part, but not dimension them for inspection purposes. The feature is then tooled per the design, but valuable inspection and evaluation time is used only for features important to the customer.
The Signicast Difference
Investment casting can produce parts of virtually any size, to almost any level of complexity, and can hold tolerances simply impossible using other methods. With our state-of-the-art equipment and automation, we can deliver the best performance and the lowest total cost for every component you develop. Contact our team today to experience the Signicast difference.