As a simple rule of thumb for metric threads you can just subtract the pitch from the nominal dia so 18 – 2.5 = 15.5mm drill. The hole can be drilled a little larger depending on what loadings you are putting on the fixing as that makes tapping easier.

The diameter of the rod is even easier as that is the same as the thread size so on M18 you would need 18mm dia rod.

J

I usually consult 'engineering toobox dot com.

There it quotes 0.614 x pitch.for metric threads, so 1.228 x pitch reduction, but that would not leave anything to be cut at the internal crests, so JB's rule of thumb is good.

As a point of note, 18mm coarse 2.5mm pitch) is not listed as a 'standard' size on that site – only 16 and 20mm. 18mm is quoted for 1.0, 1.5 and 2.0mm pitches. But if you have a tap and die of that size, they obviously exist!

Be aware that this is a substantial thread for manual tap & die work on steel. I did some 3/4" BSW on EN8 not long ago – not far off the size you're doing, and I was pretty knackered by the end of it!

M18 x 2,5 is standard size

Using a tapping compound will help in your endeavour, my favourite is Trefolex for steel. It is important to keep reversing the tap to break the cuttings that are being made if you don't you risk jamming the tap. When getting the tap started keep checking it is square until it is well under way. Make sure you have a nice cold beer waiting for you after you have finished, you will have earned one!

Mike

Do you mean…

Work done will be the same whatever the handle size, but longer ones reduce the force needed. ?

The 15.5 will be the root of the thread. The crest will be around 17.5 when finished according to an 18mm bolt I have in the drawer.

Work done will be the same whatever the handle size, but longer ones reduce the force needed. ?

If you must, but work done will be force multiplied by displacement (mustn't say distance, but practically the same here), but if the handle is twice the length only half the rate of doing work would be required to turn it at the same speed (not angular), albeit at half the rate of displacement. Basically, as a theoretical example, a seven stone weakling may actually be able get the job done (at a lower rate of doing work), with a longer handle while a 14 stone strapping lad/lass could do it with the shorter handle. But it might take twice as long for the weakling, with both working at the same % capacity of their power output (if one was 'twice'as strong as the other). OK?

that will reduce the size to 15.5mm with thread is that right ,Seems so far apart in size ,finding this hard to get my head round ,I will get there i hope .

Sort of, but only a bit over half of the material will be removed, the other half is needed to engage with the other thread.

19mm is likely a more common (so cheaper) size. Virtually the same as 3/4".

Jason has summed it up.

Tapping size for metric is nominal – pitch.

Material for male thread is nominal size of thread.

If you make the tapping slightly larger, there is less engagement, and so reduced strength.

If you make the diameter of the material for the male thread smaller, again, the engagement decreases, and so does the strength.

Obvious if you think about it, but don't go mad a 17mm tapping drill WILL mean a very weak thread.

A 15.7 won't do any real harm, probably, unless you plan to use it to winch Queen Elizabeth 2 up a dry slipway!

But this means that a slight decrease in engagement will not automatically result in a stripped thread, (unless you put a scaffolding pole over your spanners).

Go for it!

Howard

Andrew,

How many threads were engaged for the bolt to break, rather than stripping the threads. Presumably the bolt was not screwed down tight with head to aluminium, or bottomed out? It might not be the same story for a man-sized bolt like 18mm!​

Tubal Cain's book on threads is very illuminating. The reason nuts can be plain steel while the bolt is high tensile is because the threads are in compression on the nut (stronger basically) and in shear on the bolt. The other interesting thing was demonstrating the benefit that only a small increase in tapping drill size (over the Zeuss table figures) can dramatically reduce the torque needed to cut the thread and in turn reduce the chances of breaking the tap. The point was also made about how tapping a hole involves an element of extrusion. This was illustrated by attempting to pass the original tapping drill through the tapped hole and finding that the hole had closed up.

The OP should be right now but looking through some info on percentage of thread engagement it seems a bit out. As many of us started out in our early work with BSW and BSF I'll start there.

If we look at the BSW thread form, we will note that both male and female form is the same which means (disregarding working clearance) that if the nut has the full depth of 0.6403P and likewise the bolt has full form we have 100% engagement. However, if we use the workshop formula of OD-Pitch for drill size, then the depth will be only 0.5/06403 = 78% hence the quoting of around this figure for all thread forms. How does that sound?

Now moving on to Metric and Unified thread form (they have the same thread form), the basic thread form depth and maximum engagement of 100% is 0.541P. Note that the female thread has the root radiused to clear the OD of the male thread (see more on this below) Once again, using our OD-P formula, the engagement here will be 0.5/0.541 = 92%, much more than some are quoting. With a bit of metal flow we are most likely at around 96% causing much more load when hand tapping and cause of some tap breakages Maybe this is brought about by some thinking that the nut has the same thread depth as the male thread of 0.613P. NOTE: this figure is for the max male thread depth. Minimum is given as 0.591P. These figures give clearance between minor dia of nut and root of male thread..

Another thing noticed is the quoting of OD on some fasteners being well undersize. Yes this has an effect on the amount of thread engagement depending on how much under it is but has nothing to do with sloppy thread fits. The fit is controlled by the difference in effective diameters.

As I've mentioned above re the root of the female (nut) thread being radiused to clear the screw OD, has anyone taken the time to measure a tap OD? an M10 x 1.5  will measure around 10.2mm. Some of this is used by the clearance for the male thread. The crest of the tap thread is radiused 0.72P if my memory is correct. The height of that radius will be 0.36P. Add this to the 0.541P and we have thread depth of 0.577P in the nut IF THE BORE IS EXACT.

Please note that my post isn't aimed at teaching anyone to suck eggs, just to point out some facts about the threads and maybe help beginners/hobby workshoppers understand why we do things.

Edited By D Hanna on 06/11/2017 09:48:33