DEMO — fictional range. Local brief only, NOT SENT. No personal data is collected or transmitted.
DATUM
Name the reference, or the shop will name it for you.
Every dimension on a drawing is measured from something. When the drawing does not say what, the choice passes to whoever makes the part, and it is made for convenience rather than for the assembly.
The four questions worth asking.
They are asked before the drawing is issued, because each one is cheaper to answer then.
- What does the part assemble into
- That mating feature is usually the datum, not the largest face
- What carries the load
- Those dimensions need a tolerance that reflects the load case
- What is only clearance
- A generous tolerance here buys budget for the dimensions that are not
- What will be measured
- A datum nobody can reach with an instrument is a datum nobody will use
Where a datum usually comes from
| Part | Usual datum | What it costs to leave unstated |
|---|---|---|
| Stepped shaft | The turned axis | Seats that do not share a centre |
| Milled housing | The bore, or the pocket floor | A pocket referenced to the wrong face |
| Drilled plate | The boss centre | A pattern that misses the component it carries |
| Any fitted part | The mating component | A fit that is correct on paper and tight in assembly |
The expensive mistake.
Leaving the datum to the shop. The part is made correctly against a reference the drawing never named, passes its own inspection, and fails at assembly. Nothing in the paperwork is wrong.
And the one that looks harmless.
Choosing a datum that cannot be reached with an instrument. It gets measured from somewhere else, and the drawing and the report quietly disagree.