How accurate can complex sand-cast parts be after post-processing?

Inherent Sand Casting Tolerances and Their Limits

As-cast complex sand-cast parts typically hold dimensional accuracies within ISO 8062 CT7 to CT9 grades, translating to a general tolerance of approximately ±0.5 mm to ±1.5 mm for a 100 mm dimension, depending on the alloy and part geometry. This inherent variation stems from factors like sand mold stability, alloy shrinkage, and cooling rates. For complex parts with internal cores and varying wall thicknesses, maintaining consistent tolerances across the entire component is a significant challenge. This "as-cast" state is suitable for many non-critical applications but is insufficient for features requiring precise fit, sealing, or interface with other components.

Precision Enhancement Through CNC Post-Machining

The primary method for achieving high accuracy in complex sand castings is strategic Post Machining. By applying CNC Machining to critical features, tolerances can be dramatically improved to machining-grade levels, often within ±0.025 mm to ±0.05 mm. This process involves machining datum surfaces first to establish a precise geometric reference, then machining mating faces, bolt holes, threads, and sealing surfaces to their final dimensions. For example, a complex sand-cast pump housing in Leaded Bronze C83600 can be cast to a near-net shape and then have its flange faces, bore, and bolt circles precision-machined to ensure leak-proof operation and perfect assembly fit.

The Role of Alloy Selection and Part Design

The final achievable accuracy is heavily influenced by the selected Casting Material. Alloys with low shrinkage and good machinability, such as A356 Aluminum and ZA-8 Zinc, are more predictable and easier to hold to tight tolerances after machining. Furthermore, part design optimized for manufacturability during the die castings Design service phase is critical. This includes adding machining stock (typically 1-3 mm) to critical surfaces, designing uniform wall thicknesses to minimize distortion, and specifying datum features that simplify fixturing on CNC equipment.

Achieving Functional Accuracy with Surface Treatments

For many components, "accuracy" also encompasses surface integrity and corrosion resistance. Processes like Sand Blasting are used to clean the cast surface and prepare it for further operations without significantly altering critical dimensions. Subsequent treatments like Powder Coating or Anodizing add a thin, functional layer. It is standard practice to complete all precision machining *before* applying these coatings, as their thickness (typically 0.02 mm to 0.1 mm) must be accounted for in the final assembled tolerance stack-up.