Bolt Selection thickness

[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

[mekanik]

As per the Blackrat M6 is a good option reasonably course thread.
To be on the safe side do a test hole and torque a bolt, if it doesn't strip the base material it should be OK, you could keep incrementing the torque til it does strip then you know your safety factor.
Picture of what you are trying to achieve would be helpfull.
http://www.finaldrives.eu/PartsBoltTorque.htm
Regards
Mike

[grain_r]

Hey Guys

Thank you very much for the replies and I am sorry I haven't updated the thread it didn't let me know that anyone had replied for some reason.
The image below is what I am trying to bolt together.
I have used a 4.2mm Cobalt drill and then used a 5mm tap in the pillar drill and started it to keep the tap straight and then finished it off by hand although the threads aren't very good at all on two of the holes it has only got 1 thread fully around and hardly any on half of the hole which I guess must be an oval hole or something. I'm just not getting the result I should be the main issue seems to be the drill wanderd around using the pillar drill. I have used a big centre punch and then even started with a step drill of 4mm and then swapped to the 4.2 I have even turned the drill by hand and drilled the starter hole before turning the drill on and that doesn't seem great either.

I guess it's most likely the pillar drill that's not up to the job the sad truth is I think I will get a better job using my dewalt hand drill. It is a Kinzo pillar drill so a cheapo unit but I would have thought I should be able to get a hole really maybe its the monkey that's using it blaming his tools



I have my Build thread which has more images on too

http://www.mycncuk.com/threads/7936-...-few-questions

Thanks

Rob