The Part That Was Too Expensive: Machined vs Cast, Design for Manufacturing
A machined aluminum bracket. It cost $120 to make. The customer wanted 500 units. The part was a simple L-bracket with 4 holes. It was machined from a solid block. This is about manufacturing process selection, not about the machine shop overcharging.
The process comparison
| Process | Cost per part (500 units) | Lead time | Use for |
|---|---|---|---|
| CNC machined from solid | $120 | 2 weeks | Prototypes, low volume (1-50) |
| Cast (aluminum die casting) | $15 | 8 weeks (tooling) | High volume (500+) |
| Forged (steel) | $25 | 10 weeks | High strength, high volume |
| Sheet metal bend | $8 | 2 weeks | Flat brackets, covers |
| 3D printed (SLA) | $40 | 1 week | Prototypes, small batches |
What we did
For 500 units, the machined bracket was $60,000. A die-cast bracket (with tooling of $8,000) was $15 per part = $7,500. Total: $15,500. We saved $44,500. The die-cast part is as strong as the machined one (aluminum alloy). The dimensional tolerance is ±0.2 mm (fine for a bracket). The lead time was 8 weeks (we started the tooling early).
The design change
Die casting requires draft angles (1-2°) on vertical walls. The original machined part had straight walls. We added a 2° draft. The holes were cored (cast in place), not drilled. The part comes out of the die almost finished. Only the critical surfaces are machined.
The design rule
For parts over 100 units, evaluate casting vs machining. For over 500 units, casting is almost always cheaper. The tooling cost is amortized over the volume. A $120 machined part at 500 units costs $60,000. A $15 cast part costs $7,500 plus $8,000 tooling. The part that was too expensive wasn’t overpriced — it was the wrong process for the volume.
The process rule: Match the process to the volume. Machining for prototypes (1-50). Casting for production (500+). For 500 units, a die-cast bracket is 8x cheaper than a machined one. The tooling pays for itself. The part that was too expensive wasn’t overpriced — it was machined when it should have been cast.
