H3, H4 Designation on Taps

[metman]

We have 3 different sizes of tapped holes called out:

7/16-20 UNF-3B
3/4-16 UNF-3B
7/8-14 UNF-3B

On the drawing it calls out the hole geometry for each tapped hole per MS33649.


We have go/no-go thread gages per class 3B and we have the boring tools per Ms33649. The taps do not have a 3B designation rather they have H3 for the 7/16 qnd 3/4 taps and H4 for 7/8 tap.

What is the H3 and H4 referencing and/or what does it mean?

Jesus is THE life,
Leonard

 

[qaqc1]

These are tolerance class for the taps themselves. Class 3H should generaly be suitable for class 3B female thread as per ANSI B1.1 Tap class GH5 for class 2B thread & so on.

If you want I shall post a link which gives some of this information. Maybe you can ask some tap manufacturer to give you a tap selection guide.