Mill-Turn vs. Separate Operations
One setup on a mill-turn vs lathe + mill with a fixture transfer. The tradeoffs are real and the right answer depends on your part.
Tolerance Comparison
| Tolerance Type | Mill-Turn (Single Setup) | Separate Ops (Lathe + Mill) |
|---|---|---|
| Turned diameter | ±0.0002" | ±0.0002" |
| Milled feature | ±0.0005" | ±0.0003" |
| True position (turned to milled) | ±0.001" | ±0.002"–0.005" (fixture dependent) |
| Runout (milled to turning axis) | 0.0005" TIR | 0.001"–0.003" TIR |
| Concentricity | 0.0003" TIR | 0.001"+ TIR |
Mill-turn wins on every cross-feature tolerance. Separate machines win on individual feature tolerances — a dedicated VMC's milling spindle is more rigid than a mill-turn's. The right choice depends on which tolerances matter for your application.
Cost Comparison
| Cost Factor | Mill-Turn | Separate Ops |
|---|---|---|
| Machine rate | $150–$250/hr | $85–$125/hr per machine |
| Number of setups | 1 | 2–3 |
| Fixture cost | Minimal (standard chuck) | $200–$2,000 for milling fixture |
| Handling between ops | None | 15–30 min per batch |
| First article inspection | 1 inspection | 1 per setup (2–3 total) |
Lead Time Comparison
Mill-turn is typically 30–50% faster on total lead time because the part doesn't queue for a second machine. On separate operations, the turned part goes back in the queue for the mill — which may have its own backlog. One machine, one queue, one delivery date.
The hidden cost of separate operations isn't the machine time — it's the risk. Every time a part moves between machines, there's a chance of damage, a chance of fixture error, and a chance of delay. On a $5,000 aerospace fitting, scrapping one part due to a fixture error costs more than the mill-turn premium for the entire batch. Factor scrap risk into the comparison, not just hourly rates.
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