internal and external thread undercut

It's not ISO and it's not specifically for threads, but there is DIN 509, which is generally for external and internal undercuts on turned parts.

Here you go. I use these for threads too, you can always take inspiration.

It does not appear that DIN 509 provides sufficient relief for thread runout which ISO 4755 does do.

Not sure about universally accepted standards, probably more like company standards.

Try this for example:

"For every expert there is an equal and opposite expert"
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On external threads there is often concern about surface finish and diameter of turned-down area and fillet radii to ensure that no stress risers are created. On internal threads those won't matter.

If understanding the stress and fatigue results of this is important I would not depend on a standard that doesn't mention the effects on those areas.

Oh, the typical geometry allowances are to provide mechanical clearances for typical taps and dies, so they include the distance from a wall that the threads must terminate on an external thread and the distance from the bottom full minor diameter drilled hole for an internal thread.

Like 3DDave states the typical geometry allowances are to provide mechanical clearances for typical taps and dies. Often it is defaulted to 1.5-2 x P or 2-3 x P. Now there are of course taps that have shorter runouts, like 0.5-1.0 x P, typically called (chamfer) form E or form F for an even shorter runout, but it varies.

Like CheckerHater says it is often a company standard; with variations based on the DIN 509 or ISO 4755 (at least by a number of European manufacturers)

3DDave said:

It does not appear that DIN 509 provides sufficient relief for thread runout

Click to expand...

Yes, you're right. 509 isn't for threads, but I said, you can take inspiration from it.

When I have to design undercuts I mostly take hint from DIN 509 Type E undercuts and I change my geometry according to the requirement.