複雑な砂型鋳造部品は後加工後どの程度の精度が出せますか？

Inherent Sand Casting Tolerances and Their Limits

As-cast complex sand-cast parts generally achieve dimensional accuracies within ISO 8062 CT7 to CT9 grades, corresponding to approximately ±0.5 mm to ±1.5 mm for a 100 mm feature, depending on alloy and geometry. Variability arises from factors such as sand mold stability, alloy shrinkage, and cooling rates. Components with internal cores and variable wall thicknesses face greater challenges in maintaining consistent tolerances. While suitable for non-critical applications, the as-cast state alone is insufficient for features requiring precise fits, seals, or interfaces with mating components.

Precision Enhancement Through CNC Post-Machining

Achieving high accuracy in complex sand castings relies on strategic Post Machining. Using CNC Machining on critical features can tighten tolerances to machining-grade levels, typically within ±0.025 mm to ±0.05 mm. The workflow begins by machining datum surfaces to establish precise references, followed by finishing bolt holes, mating faces, threads, and sealing surfaces. For example, a complex pump housing cast in Leaded Bronze C83600 can be brought to near-net shape during casting and then CNC-finished to ensure leak-proof operation and exact assembly fit.

The Role of Alloy Selection and Part Design

Final achievable accuracy depends heavily on the chosen Casting Material. Alloys with low shrinkage and good machinability, such as A356 Aluminum and ZA-8 Zinc, allow more predictable post-machining tolerances. Part design optimized for manufacturability through our die castings Design service is equally critical. This includes adding machining stock (typically 1–3 mm) to critical surfaces, maintaining uniform wall thicknesses to reduce distortion, and specifying datum features to simplify CNC fixturing.

Achieving Functional Accuracy with Surface Treatments

Functional accuracy also involves surface integrity and corrosion resistance. Processes such as Sand Blasting clean the cast surface and prepare it for further operations without altering critical dimensions. Subsequent surface treatments like Powder Coating or Anodizing add thin functional layers. It is standard to complete all precision machining before applying coatings, as layer thickness (typically 0.02–0.1 mm) must be accounted for in final assembled tolerances.