The short answer
Tolerance cost is not linear. Going from a loose commercial tolerance to precision is one jump. Going from precision to very high precision is another, much steeper jump.
General tolerance
General tolerance is where the shop can run stable tools, ordinary workholding, and standard inspection without babysitting every feature. Many non-critical machined parts live here and should stay here.
This is the cheapest place to be because process windows are wide and scrap risk is manageable.
Precision tolerance
Precision work usually means better datums, more controlled fixturing, more frequent tool offset updates, and more targeted inspection. The machining itself may not look radically different, but the process discipline definitely does.
This is where the quote starts reflecting not just cutting time but control time.
High-precision tolerance
High-precision features force the supplier to manage thermal growth, insert wear, machine condition, and lot-to-lot variation much more aggressively. Secondary grinding, honing, lapping, or air gaging may enter the plan.
This is also where quantity matters less than many buyers expect. A five-piece lot can still need a production-grade control plan.
The decision rule
Put precision only where fit, sealing, motion, or calibration requires it. Everywhere else, use rational title-block tolerances. Parts get expensive when the whole drawing is written as if every dimension lands on a bearing journal.
Good tolerance strategy separates critical from non-critical. Bad strategy taxes every feature equally.
What an experienced buyer does next
Related reading: GD&T basics for procurement engineers: the 10 most common callouts and Wall thickness minimums for CNC milled and turned parts.
The right move is usually to define the real functional requirement, remove the decorative requirements, and let the supplier build a route around what actually matters.
Comparison table where relevant
| Tolerance level | Typical 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 | Arguments and scrap | Worst case |
How to specify this in your RFQ
Separate critical dimensions from general dimensions on the print. If one feature needs post-process grinding, say that explicitly. If a datum structure controls assembly, build the tolerance stack around that instead of tightening everything by instinct.
A clean RFQ does not just list requirements. It separates must-haves from preferences so the supplier can optimize where it is safe.
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.