What level of precision can be achieved with post-machined sand-cast parts?

Inherent Limitations and Post-Process Capabilities

While the Sand Casting process itself typically holds tolerances in the range of ±0.5 mm to ±1.5 mm depending on part size, the integration of Post Machining dramatically elevates the achievable precision. The sand-cast part acts as a "near-net-shape" blank, which is then precisely finished on CNC equipment. This hybrid approach allows sand-cast components to meet precision requirements that are otherwise exclusive to processes like full CNC Machining from solid billet.

Achievable Tolerances on Critical Features

For critical features such as bores, mating surfaces, and threaded holes, post-machined sand castings can consistently achieve standard machining tolerances. This includes: Linear Dimensions: IT grades of IT10 to IT12, translating to tolerances as tight as ±0.05 mm on carefully controlled features. Hole Sizes & Bores: H7 to H9 fits are readily achievable, enabling precise bearing and shaft press-fits or sliding fits. Surface Finish: The typical rough "as-cast" surface of 6.3-25 µm Ra can be refined to 1.6-3.2 µm Ra or better through milling, turning, or grinding, which is essential for sealing surfaces and dynamic components.

Strategic Application in Part Design

The key to high precision lies in the initial die castings Design service. Engineers strategically specify machining stock—extra material added to critical surfaces—on the casting drawing. During die castings Post Machining, this stock is precisely removed, establishing a new, accurate datum structure and achieving the final dimensions. This method is far more cost-effective than machining an entire part from a solid block of metal, especially for large components.

Comparison to Other Processes

This combination makes sand casting competitive for large, complex parts that require isolated high-precision features. While a fully machined billet might guarantee IT7-IT9 tolerances throughout, it does so at a tremendous cost in material and time. Similarly, while Aluminum Die Casting can offer better as-cast tolerances (±0.2 mm), it is constrained by part size and alloy selection. The sand casting and machining hybrid offers a superior balance of design flexibility, material choice, and targeted precision for low-to-medium volumes.