About tap & die's

Hello guys & gals

I am just learning about tap & Die never done it before and would like to ask some questions before i start ,But for me its very confusing even after looking at charts online i still am not sure which size drill bit i would need so i can make M18X2.5 thread .

And what size bar would i need in order to make a thread on that to fit the hole with new thread on from the M18X2.5 ..

Thank you ..

Karl

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

Lots of reference tables for drill sizes on the internet.

This a link to one: **LINK**

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!

The diameter stated, in this case 18mm, is nominally the dia of the shaft to be threaded. The actual diameter is slightly less than nominal due t the rounded top or crest of the thread. The hole diameter to be threaded is the core diameter of the thread, this being the nominal diameter 18mm less the pitch of the thread, roughly. As previous stated this can be used as the hole drill diameter but may result in a tighter hole which can cause breakage of the tap. Drill tap tables are numerous on the web, with reccomendations for softer and tough materials. You will soon get the feel of the hole sizes which suit your work.

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!

Is there not a book called "Zeus" still around these days? If you dont find what you need there,it must be rare indeed ! Well worth buying,although I got mine for nothing !

Great read !

M18 x 2,5 is standard size

Zeus books are still available from Buck and Hickman and probably other sources.

Martin C

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

18mm is not that big. Just need a handle size commensurate with the job. Work done will be the same whatever the handle size, but longer ones reduce the power needed. I think Abom was tapping about an inch and a quarter, handraulically, on a recent video -but I maybe he has enough beef to swing most handles!

Do you mean...

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

Wow so many replies wasn't expecting that :D Thanks all .

So is that what M18 mean 18mm ?

So 15.5mm drill bit for the hole,

And 18mm rod bar to match, Then add thread from M18 2,5 ? 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 .

Oh only have 15.5m rod at the moment will need to order some 18mm then ..

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.

Karl , sent you a PM.

Trevor

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".

But then the OP will need to find a lathe to reduce the dia to 18mm, far easier to buy 18mm from the start which is a stock metric size.

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

Something else to think about - I did an experiment with M4 threads; with a high tensile bolt in aluminium 6082, with 50% thread engagement, the bolt broke first. I normally aim for around 65-70% thread engagement.

As an aside hand taps extrude the material to a small extent so you shouldn't drill for 100% thread anyway.

Andrew

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!​