Minimum thread engagement

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

Will be interested to hear more experienced feedback...but...

My old mdf machine used linear bearings made from 1/8" (3.2mm) aluminium angle with skate bearings - I was using M8 bolts & tapping the aluminium...that held very well.
Based on that experience alone I was thinking of using M8 for my new build on 3mm steel & M10 where I have 4mm steel to play with.

Lets see what the elders say...

There are a number of solution. Seeing as you have a welder, then welding a plate or nut inside the box is the easiest if the bolts are near the ends. If the bolts are in the middle, then consider using nut inserts (aka rivet nuts):

How to install a rivet nut (rivnut) and the tools you need - YouTube
4mm-8mm THREADED RIVET NUT INSERT SETTING TOOL RIVNUT - PROFESSIONAL | eBay
Small Head Open Steel Grooved Rivet Nuts - Rivnuts - Nutserts - Blind Nut. (uk) | eBay

Joe if you have the welder out then weld another plate to the box section you will bolt too. No biggy really and will double your depth to tap into.