DEMO — fictional range. Local brief only, NOT SENT. No personal data is collected or transmitted.
THREAD
A thread is a fit, not a dimension.
A thread is specified by its class rather than by a measurement, and the class decides how the two parts behave together. It is also the feature most often left to the supplier.
The four questions worth asking.
They are asked before the drawing is issued, because each one is cheaper to answer then.
- What does it thread onto
- That mating part decides the class, not the nominal size
- How often is it assembled
- A thread taken apart daily is a different specification from one fitted once
- What is the plating
- Coating thickness changes the fit, and it is applied after gauging
- How is it checked
- A go/no-go gauge is a different claim from a measured pitch diameter
Where a thread class comes from
| Use | Usual class | What it costs to get wrong |
|---|---|---|
| Fitted once, plated | A looser class | A thread that will not start after plating |
| Assembled often | A closer class with a harder material | Threads that wear out before the part does |
| Threaded to a shoulder | Class plus a runout length | A part that bottoms out before it seats |
| Thread into plastic | A coarser thread or an insert | Stripped threads on first assembly |
The expensive mistake.
Specifying the nominal size and leaving the class to the shop. The part is made correctly to a class the drawing never named, gauges fine, and will not assemble after plating.
And the one that looks harmless.
Not stating the thread runout on a part that threads to a shoulder. It bottoms out a millimetre early and the assembly is loose.