The Part That Was Too Expensive (Machined, Should Have Been Cast)
We designed a housing (a complex shape) for a machine. We machined it (from a solid block of aluminum). On the floor, the part was expensive (a lot of material was cut away, waste). The housing was a complex shape (thin walls, pockets). Machining it (from a solid block) was wasteful. We switched to a casting (the housing is cast, near-net-shape, then machined only the critical surfaces). The part was cheaper (less material, and faster). The mistake was machining a complex shape (should have been cast).
Casting vs machining — choose the process. This article covers the comparison.
Machining (From Solid)
Machining cuts the part from a solid block (a billet). It’s precise (tight tolerances, article 203). But it’s wasteful (a lot of material is cut away, chips). For a simple shape (a plate, a bracket), machining is fine (not much waste). For a complex shape (thin walls, pockets), machining is wasteful (and expensive).
Casting (Near-Net-Shape)
Casting pours metal (aluminum, iron) into a mold. The part is near-net-shape (close to the final form). Then machine the critical surfaces (the mounting faces, the bores). For a complex shape (a housing), casting is efficient (the shape is cast, not machined from solid). The casting tool (the mold) is expensive (upfront), but per-part (for volume) is cheap.
| Process | Best For | Cost |
|---|---|---|
| Machining (solid) | Simple shape, low volume (1–100) | Expensive (waste) |
| Casting (near-net) | Complex shape, high volume (1000+) | Cheap per part (tool upfront) |
Step 1: The Volume (How Many?)
For a low volume (1–100 parts), machining is fine (no tool cost). For a high volume (1000+), casting is cheaper (the tool cost is spread over the volume). The crossover (where casting is cheaper) is around 500–1000 parts.
The process rule: Low volume (100) → machine (no tool). High volume (1000) → cast (cheap per part). The housing that was expensive was machined (complex shape, high volume). Cast it (near-net). Machine only the critical surfaces.
Step 2: The Shape (Complex?)
A simple shape (a plate, a bracket) machines fine (not much waste). A complex shape (thin walls, pockets, undercuts) is hard to machine (a lot of waste). Casting forms the complex shape (the mold has it). For a complex shape, casting.
Step 3: The Tolerance (Critical Surfaces)
Casting has a looser tolerance (the mold shrinks, ±0.5 mm). The critical surfaces (the mounting faces, the bores) are machined (tight, article 203). The non-critical (the outside) is as-cast (loose). Hybrid: cast + machine (the best of both).
Step 4: The Material
- Aluminum casting: Light, good for a housing. Die casting (high volume, fine). Sand casting (low volume, rough).
- Iron casting: Heavy, good for a machine base (vibration damping, article 63). Sand casting.
A Process Checklist
- What is the volume? (Parts?)
- Is the shape simple? (Or complex?)
- Is the tolerance critical? (Which surfaces?)
- Is it low volume? (Machine?)
- Is it high volume? (Cast?)
- Is the casting tool cost OK? (Upfront?)
- Is the material right? (Aluminum? Iron?)
- Is the machining (critical) done?
- Is the cost right? (Per part?)
- Is the lead time OK? (Casting?)
The Bottom Line
Casting vs machining matches the process to the volume. The housing that was expensive was machined (complex, high volume). Cast it (near-net). Machine the critical surfaces. For low volume, machine. The part that was cheap (and fit) wasn’t the solid-block part — it was cast.
