Hello all.
I have ordered the steel for the frame of my machine. I will be welding it into sections which will then be bolted together. The box section that forms the body of the frame is 80x80 3mm walled and the plates that will form the connections 6mm. My question is what thread to use... Just a bit concerned that obviously the larger the bolt the larger the pitch and I don't have much depth to play with - tapping into the 3mm steel. I've read somewhere that in mild steel the minimum recommended thread engagement is 1.5x diameter and I clearly wont get that. I was thinking m6 or m8 but will only get 3 or 2.4 threads engaged respectively, is that enough? Any help on this would be really appreciated

Joe