Engineering Resource
A quotation is not machine time multiplied by an hourly rate. Several factors move the number far more, and most of them are decided on the drawing.
The main cost drivers
- Material: cost, availability and how difficult it is to cut.
- Machine time: driven by material removal volume, tool life and cutting parameters.
- Set-ups: each one adds time, fixturing and stack-up error.
- Tolerance and finish: tighter values demand extra passes, temperature control and measurement.
- Inspection and documentation: CMM time, reports and certificates are real work.
- Batch size: set-up and programming cost is amortised across the quantity.
Why tolerance and finish step up, not slope
Cost does not rise smoothly as a tolerance tightens; it steps. Crossing from a general tolerance to a tight one may add a semi-finish pass, in-process measurement and a slower cycle. Crossing again to a temperature-critical tolerance may add a controlled environment and a different class of machine time. Each step is discrete.
How batch size changes the picture
Programming, fixturing and first article inspection are largely fixed costs. Over ten parts they dominate; over thousands they almost vanish. This is why the same part can look expensive as a one-off and economical in series — and why it pays to think about the eventual volume before quoting a prototype.
Reducing cost without losing function
- Loosen tolerances that carry no function, and keep the tight ones where they do.
- Design internal radii the tool can reach in one pass.
- Keep wall thickness uniform and pockets shallow where possible.
- Avoid specifying a fine finish across every face.
- Choose the least difficult material that meets the requirement.