DEMO — fictional range. Local brief only, NOT SENT. No personal data is collected or transmitted.
TOLERANCE
A tolerance is a decision, not a default.
Every dimension on a drawing carries a tolerance, and most of them were copied from an earlier drawing rather than chosen. This page is about deciding which ones actually matter.
The four questions worth asking.
They are asked before the drawing is issued, because each one is cheaper to answer then.
- What carries load
- Those dimensions need a tolerance that reflects the load case, not the machine
- What locates something else
- Position matters more than size, and it is positioned from a datum
- What is only clearance
- A generous tolerance here buys budget for the dimensions that are not
- What nobody measures
- A tolerance on a feature nobody inspects is a cost with no consequence
Where tolerances come from
| Feature | Usually decided by | What it costs to tighten |
|---|---|---|
| Bore diameter | The fit it has to make | Reaming or boring rather than drilling |
| Position between bores | The datum they share | One setup instead of two |
| Face flatness | Whether it seats against something | Facing rather than sawing |
| Surface finish | What slides or seals against it | An extra finishing pass |
The expensive mistake.
Writing a tolerance tighter than the part needs. It does not improve the part; it moves the work to a slower process and adds an inspection step for a dimension that was never going to matter.
And the other one.
Leaving a tolerance off a dimension that locates something else. That dimension then gets whatever the process happens to produce, and the failure appears during assembly rather than at inspection.