Thread Engagement

[GTJim]

What is the rule of thumb on thread engagement in through holes?
I only ask because my searching keeps coming up with 1.5 x Dia, so an M5 would require 7.5mm of thread.
This rule would make it impossible to construct a steel frame from SHS and drill, tap and bolt into it.
If my memory recalls correctly I thought it was a min number of threads, approximately 3 full threads or something like that. Please correct me.

Jim

[Jonathan]

It depends on the material you're tapping:

Screw thread Calculations (http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html)

That formula gives 4mm for M5 in steel. Bear in mind though that this calculation is based on making the thread stronger than the bolt, so the bolt fails first. If this is for a CNC router frame then clearly you're not going to approach the forces required to shear M5 bolts, so you can get away with a bit less than 4mm so long as the thread isn't so shallow that it strips when you tighten the bolt. 4mm wall thickness box section would be a good size anyway.