The short answer
Tolerance cost is not linear. General commercial tolerances are relatively cheap. Precision tolerances cost more because the process and inspection tighten up. High-precision tolerances cost much more because the supplier has to manage thermal behavior, tool wear, part stability, and sometimes secondary finishing with very little margin for drift.
General tolerance
General tolerance is where many non-critical machined features should live. The supplier can run a stable route, use standard inspection methods, and avoid turning every feature into a process-control exercise. That keeps the quote sane and the route flexible.
For brackets, covers, housings, and non-critical geometry, this is often exactly where the drawing should stay.
Precision tolerance
Precision features usually involve fit, alignment, hole position, sealing, or controlled motion. The supplier still machines the part efficiently, but the fixturing, tool-offset control, and inspection plan become more disciplined. That costs more because the process window gets narrower.
This level is normal and justified for many real functional features. It just should not spread across the whole print.
High-precision tolerance
High-precision work is where the route may need thermal control, more frequent in-process checks, grinding or honing, selective assembly, or more expensive metrology. The cost jump here is real because the supplier is buying down very small process risk.
A five-piece lot can still be expensive if one bore or journal needs high-precision behavior. Quantity does not eliminate the discipline required.
How to use tolerances intelligently
Tighten only the dimensions tied to function. Leave non-critical dimensions on rational title-block tolerances. Related reading: Surface finish guide for machined parts: Ra, Rz, and what to call out and Engineering drawing best practices for machined parts.
A good drawing separates critical from non-critical. A bad drawing taxes every feature equally.
Comparison table where relevant
| Tolerance level | Best use | Cost effect |
|---|---|---|
| General | Non-critical geometry | Baseline |
| Precision | Fit and assembly features | Noticeable increase |
| High precision | Bearing, sealing, calibration features | Steep increase |
| Undefined but tight intent | No clear function | Worst commercial outcome |
How to specify this in your RFQ
Identify the few dimensions that actually drive fit, motion, sealing, or calibration. If one feature needs a special route such as grinding or honing, say so. Buyers save money by being selective, not by being vague.
Have a part that needs quoting? Email your drawings to rfq@precisionmachining.co - we return a competitive quote within 24 hours. Phone: +1 312-579-0808.