Rule of thumb for determining depth of tapping

[Ussuri]

We are currently looking a tapping into mild steel to accept an M12 grade 8.8 bolt.

The material we would like to tap has a yield strength 25% of the standard nut strength. Are there any rules of thumb, design guides which would allow me to work out how much additional embedment I need to ensure the bolt reaches its tensile capacity?



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[israelkk]

Machinery's handbook has the formulations to calculate the thread length of engagement.

 

[CoryPad]

The force resistance of the internal threads is proportional to the product of the materials strength and the shear area. If you have 1/4 the strength, you need four times the area. Area is proportional to the product of diameter and length. If you need four times the area and have constant diameter, you need four time the length.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dimjim]

If you are not using a nut, the strength of the
nut has nothing to do with calculating the depth
of thread in your part. See the Machinery's Handbook
for calculating the depth of thread which appears in
the index under "Working Strength of Bolts" and appears
in the "Stripping of Internal Thread" section.
Certainly as CoryPad suggest, 4 times would be more
than adequate. I have seen many standard for thread
depth for St 37 Steel requiring engagement depths of
only 1.5 times the thread diameter for 8.8 screws or bolts.
This may be based on yield stress rather than the
ultimate tensile strength however.