Bolt Selection thickness

[grain_r]

Hello Everyone
I am working through the final parts of my machine design and the more I go into things the less I seem to know.
I am working on bolt selection for my 50x50x3mm box section It looks like I can tap this for a secure thread for M5 bolts but if I try to use M8 i wont have enough threads really.
I only thought of M8 as an initial that should look right and M5 sounds a bit small.

Where i'm now getting stuck is:
Is one 8mm bolt as strong as a 5mm bolt in cnc applications when being screwed into 3mm thick steel ?
If not then is 2x 5mm bolts stronger on each corner than 1x 8mm bolt in the centre of a bracket.

I have looked on the web at bolt strength calculators but the numbers they give don't really tell me in a real world situation e.g. 11345nm Strength on a 8mm bolt but I don't have a clue what that means because I have no idea what that really equates to.

I hope this all makes sense

Thanks

Rob

[komatias]

Nothing wrong with M5's

Realistically you need 2x the nominal diameter in threaded length to ensure you have at lease 3 complete threads engaged. This is realistically the best you can get.

M5 coarse threads come with a 0.8mm pitch, M8s come with 1.25mm pitch. You are thus correct that in 3mm steel, even though you can thread it, you will have bugger all thread.

Now, I would suggest that you should space your screw holes out such that you can clamp the bits you want nice and square. One M8 will not do the same job as 2 M5's in that case.

Also think of how the loads will be transmitted. That is what should govern how many screws you need.

[grain_r]

Good Morning Komatias

Thanks very much for that I will go with 5's and spread the load out to keep things square. and then thread the 3mm.

Thanks

Rob

[cropwell]

I'm thinking about using riv-nuts for the m8 bolts in my build (when I start it !).

Rob

[mekanik]

I'm thinking about using riv-nuts for the m8 bolts in my build (when I start it !).

Rob

[Blackrat]

rob, id go with m6 , m5 are a bugger to tap

they say to utilize the full strength of a high tensile bolt you need 1 1/2x its diameter into steel , or 2x into aluminum

however , you gotta work with what you got ... spreading the load is always going to be stronger, but more work

maybe a pic or two would help :D