View Full Version : Bolt Selection thickness

03-09-2015, 10:17 PM
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



04-09-2015, 09:23 AM
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.

04-09-2015, 10:07 AM
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.



04-09-2015, 09:55 PM
I'm thinking about using riv-nuts for the m8 bolts in my build (when I start it !).


05-09-2015, 09:20 AM
I'm thinking about using riv-nuts for the m8 bolts in my build (when I start it !).


05-09-2015, 01:21 PM
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

05-09-2015, 01:39 PM
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.

03-01-2016, 09:34 PM
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 :peaceful:

I have my Build thread which has more images on too




04-01-2016, 01:29 AM
Hi Rob
Sorry to hear you are having problems.
Are your Drills ground finish and do they run true in the drill chuck?
I have had drills that were totally useless because of being bent.
Drill the holes @ one sitting ie just go for the 4.2mm.
Please don't start drilling your frame until you have perfected your marking out/drilling & tapping technique.
Also you have made a nice job of tacking your plates onto the tube, if i was going to finish weld i would only weld the top & bottom of the plates, if you weld down both sides you will get weld shrinkage that will cause them to bend.

04-01-2016, 10:42 AM
If you look closely at the tip of your drill, you will see that it comes to a so-called chisel edge; the very tip is actually a short sharp line and not a point. If you are going to pick up a centre punch mark with a drill like this, you need a dimple from the punch that is at least as big as that chisel edge so that the "point" of the drill sits right inside it. One way to do this is to use a small sharp punch to put the first mark in and then go to a bigger punch to enlarge the dimple. The other way that works for me is to use small centre punch mark and then go through with a smaller drill (say, 2.5mm or so). These are a bit more fragile so need a bit of care but these smaller drills have a much finer point and they will follow the mark and go through the steel much more easily. Then follow up with the 4.2mm which should follow the pilot hole. Advice to use centre drills is great and probably the proper way to do the job, but only if you can clamp the work firmly on the drill table. The points on these things are pretty fragile and while they'll take quite a lot of downforce for cutting, they will snap at the slightest sideways movement.

I don't know if it's encouragement or not, but I drilled all the holes for my linear rails (total of about 100 M5 tapped holes) freehand using a cordless drill. However, I clamped the rail in place and then used a little drill jig to locate in each hole in turn to guide the drill so that it was central in the hole and close enough to square. I am using 3mm steel, but I have strengthened it with 5mm strip on the inside so I was effectively drilling 8mm of steel with a hand-held cordless drill.

For tapping, forget the usual hand taps. Get hold of some spiral point taps if you are tapping through holes, and spiral flute taps for blind holes. They are ground very differently to hand taps so that they can be used in tapping machines to tap a hole in one pass - no backing off every half-turn. I've used a spiral point tap in my cordless drill for tapping some of those M5 holes and (using a good tapping compound to lubricate the tap) they are magic. They are pretty cheap although you'll have to wait for delivery from China. You'll find them on eBay.

04-01-2016, 05:43 PM
Hey guys thank you very much for the help. I have ordered some smaller drill bits and a centre drill to see if that gets me more accuracy I think I must have been off a little on square and but then tapped the hole square which has then given me a problem I will try the top rails for the other side next at least they are accessible for the nuts if needed.

Neale I think maybe the double skin is a rather good idea. I can get to the ends and weld them but I was wondering does it flex a lot when drilling the centre ? or did you go from the outer edge and bold along to keep the strength at all.



04-01-2016, 05:54 PM
I actually welded the extra strip in place. I drilled the box section at intervals between the places where I was going to drill for rail hold-down bolts, used a wooden wedge arrangement to slide down the box section to hold the strip against the inside of the box, and then welded to the strip through the holes. Bit of grinding to clean up the outer surface of the weld and job done. Other people have used Gorilla Glue to fix the strip in place - it only needs to hold while you are drilling and tapping and then until you bolt in the final item. Your idea of welding at the ends and then working along, bolting as you go, would work as long as the strip does not sag too far when you take the bolts out to fix the final item in place. In the case of my long rail supports, these are 1800mm and would have needed fixing at more than just the ends.

04-01-2016, 10:17 PM
A split point drill bit should help you - e.g. http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Twist-Drills/Drill-Bits---Cobalt

06-01-2016, 09:55 AM
Hey neale

Thank you very much for the reply. I have ordered some 40x5mm to put in and will weld the ends up and I might Bolt the middle and the a small tack weld on the bolts to stop me forgetting to leave them alone in the future. Good Idea with the wood I will do that thanks very much.

Thanks Jonathan I have had a look on the site which seems a really good site and I will look at getting one of those drill bits I have the centre drill on order and I'm going to give that a go too.