Designing Die Cast Parts to Reduce Secondary Machining

Short answer: secondary machining exists to hit tolerances as-cast processes can't — the goal isn't to eliminate it entirely, but to plan for it deliberately instead of discovering it's needed after the tool is already cut.

Start by separating "critical" from "reference"

The single biggest driver of unplanned machining cost is treating every dimension as equally important. Identify which features are genuinely functional and tight-tolerance (bores, mounting flatness, sealing surfaces, threaded interfaces) versus which are cosmetic or non-critical. As-cast tolerance can usually hold the non-critical dimensions; machining should be reserved for the ones that actually need it.

Design machining stock into the casting, not as an afterthought

If a feature needs machining, the casting needs extra material there on purpose — a boss or pad with enough stock to clean up to final dimension. Trying to machine a feature that was cast at or near final size risks breaking through to porosity or leaving inconsistent stock across the production run.

Reduce the number of features that need tight tolerance

Sometimes a design can be changed to avoid needing a machined feature at all — for example, using a self-locating boss instead of a precisely bored hole, or moving a fastening interface to a location where as-cast tolerance is acceptable. This is a conversation worth having during DFM review, before tooling is committed, not after.

Think about fixturing during design, not after

How a casting will be held for machining affects achievable tolerance and cost. Parts with a clear, stable fixturing datum (a flat reference surface, a natural locating boss) machine more efficiently and repeatably than parts where the fixture has to compromise around a delicate cosmetic surface or an awkward shape.

Batch machined features where possible

If a part needs several machined features, consider whether they can be positioned to be machined in a single fixturing setup rather than requiring the part to be re-fixtured multiple times. Every re-fixturing step adds cost and a small amount of accumulated tolerance risk.

When machining is simply unavoidable — and that's fine

Some features — precision bores for bearings, sealing surfaces with tight flatness requirements — will always need machining regardless of design cleverness. The goal of DFM isn't to eliminate machining as a matter of principle; it's to make sure machining is used only where it's actually needed, planned for in the casting design, and fixtured efficiently.

See Die Casting DFM and Secondary Machining for Castings for how we handle this during quoting and production.