About tap & die's

Thread strips along a line (actually cylinder) of equal shear strength in male and female. Male is usually made of stronger material, so the cylinder diameter is bigger than the mean diameter. Increasing the size of the tapping drill within reason has little effect on stripping force but it does increase the interface pressure, so it can't be taken too far. Root diameter of M18*2.5 is 14.7m so 15.5 tapping drill gives 76% engagement. I'd happlily use a bigger drill, 16mm would be 61% which might be a bit low, depends on the job and what load it might see.

However to digress, or even get back to the OP's original post, M18 is a big thread to learn on!

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.

I did a similar experiment at the same time as Andrew and stripped the female thread, think I used M8 Imported hex screw and 10mm black bar,. As I said all depends on what the fixing is expected to do as to how much you can play with sizes.

Edited By JasonB on 06/11/2017 07:29:49

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

Just noticed I had the decimal point in wrong place on 0.072P and ).036P in my post above. Too late to edit it. Sorry about the error.

"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 0.072P if my memory is correct. The height of that radius will be 0.036P. Add this to the 0.541P and we have thread depth of 0.577P in the nut IF THE BORE IS EXACT."

Taps are manufactured to give a particular class of fit, usually 6H unless otherwise stated. New taps should be towards the upper end of the tolerance band, so that as they wear they stay within tolerance for a long as possible.

My Dormer M10 spiral flute tap measures 10.12mm OD. The tap is used, but I don't suppose it has been used more than a couple of dozen times. I'd be surprised it it had worn by 0.08mm.

Andrew

Hi Karl,

For what it,s worth,I made three tap wrenches many moons ago,and am still using them. Might give you some idea of the size you require,since a too small wrench is useless The sizes I made are as below.

A Main part [jaws] 1/2"x1/4" flat with 1/4" handles,total length overall 6".This was to handle taps up to 1/4"

B 1"x1/2" 1/2" 16" 5/8"

C 1X1/2" 5/8" 24" Anything over.

This last one can handle 3/4" GB,and has done,just make sure the hole is the correct size and use cutting fluid.

I simply filed a "V" on one side of each flat [no mill around in those days] and then drilled and tapped the the pair as in toolmakers clamps,nothing fancy.

Hope this helps.

Karl,

That post did not come out as intended ! In each case,the first measurements refer to the size of the jaw material.

The second measurement refers to the dia of the handle material.

The third one refers to the overall length of the wrench

The fourth one gives the size tap they were meant to handle.

PS That,s what happens when you try to take short cuts when Posting !!

Wow this site as to be the most friendliest site i have ever been on so many replies in such a sort time ,

I am very grateful to all reply's however its a little over my head ,

As i said when i start the thread i am new to this and with all the reply's i still find it very confusing to understand the logic behind what i need to know,There are so many of you saying different thing and i am not sure which is the correct one ,i feel that the answer i am looking for is all ready in the post but i all so feel very frustrated to my lack of knowledge .

It would be great if some one could give me in layman terms a simple reply .I know to you guys may think what you have told me, may all ready seem a simple reply in a layman terms kinder of way ,but to me i feel confused sorry about that .

please read my first post and give me the answer before my brain explodes

For M18 x 2.5

'which size drill bit' 18-2.5 = 15.5mm

'what size bar' M18 = 18mm

Dave

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Karl,

Try this chart for a straight answer: **LINK**

http://www.wlfuller.com/html/tap_drill_chart.html

In the fullness of time, come back & read the advice and explanations already given.

... It should all fall into place.

MichaelG.

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Edit: Having just seen Dave's post ... We can debate the 0.3mm difference 'til the cows come home 

Edited By Michael Gilligan on 06/11/2017 18:48:05

Hi Karl it's simple to produce an 18 MM M18 thread on a bar, the bar needs to be 18 MM 18mm diameter. To produce a tapped hole 18 x 2.50 M18 x 2.5 then according to ZEUS Precision tables, which are accepted as an industry norm, then you need to drill the hole 15.50 MM  15.5mm. Hope this clarifies your enquiry after all the confusing answers so far submitted.

Dave

Edited to save confusing Karl even more.

Edited By JasonB on 06/11/2017 18:59:47

Jason thanks for the edit I should have thought more before posting.

Dave

Hi guys

Chart showing 15.8 another saying 15.5 :D

so i am going to be assuming that its is 15.5mm for the hole and thread it with the m18 2,5 and go with that with 18mm bar & thread also with same m18 2,5 not received the bar yet so cant try until i get it . I am trying to fix something that i received from USA for wood working ,self centering vise for dowels which is very poorly made and all joins are very loose and inaccurate so i was thinking its time to learn something new to help me fix other things.And try to make this work as intended with a much more accurate fitting sliding joins here is an image

of the thing not my one but one from the internet ,Because my one is in bits lool .

All so the center bar is threaded with two threads in reverse so that it can open close each side at the same time to make a self centering vise but in this case is not doing a great job so my mission is to fix that :D

By using stainless still bars 10mm for each sliding bar and 18mm for center with 15mm Aluminium flat bar ,Width 80mm x 155mm x 2500mm cut in two parts ,to match the vise setup .only one thing i am going to keep out of the one from US is the middle part .If i had more skill and new much more which i dont and is very little at this time i would make it all ,

Thanks guys

if you would like to see it in bit i can post np

Edited By Karl Fuller on 06/11/2017 20:11:50

Edited By Karl Fuller on 06/11/2017 20:12:22

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Karl,

If you read carefully through this forum thread, you should understand ^^^

15.8 gives a little less 'engagement'

MichaelG.

I take it you're aware that you'll need both right hand and left hand threads?

You'll need to be on good terms with your wallet when buying M18x2.5 taps and dies, assuming you can find them.

Given the original equipment is from the US I would have expected the threads to be to the US unified standard?

Andrew

Karl, do you know what grade of stainless steel you have ordered as some can be yery difficult to work and you will have a job cutting that thread. A free cutting mild steel would make your life easier.

Taps £8 from Tracey Tools, £12 dies

Edited By JasonB on 06/11/2017 20:27:11

@Michael Gilligan I would like to get a nice snugfit not lose like it is ,All ready order some from ebay £5 pound each I know there not going to be the best grade that money can buy but ill just be careful using them and if they break there not a lot to replace so long as i can get the job done ,I went and got a Clarke 32 peace set but never had what i wanted and order the M18 2,5

@Andrew No i was not aware of that i am just a novice ,Just looked there is no tread in the middle block after all thats just stays in by a spring loaded ball bearing with a flat head allen key bolts

@JasonB 316 Stainless Steel Round Bar Rod Marine Grade (1.4404) Thank you for the link to http://www.tracytools.com/ ..yeah was thinking mild steel,but the bars that were in the self centering vise where bent and this is why i was thinking something stronger like stainless , Maybe i was jumping ahead of my self as all ways .

Edited By Karl Fuller on 06/11/2017 20:51:29

Edited By Karl Fuller on 06/11/2017 20:55:01

Edited By Karl Fuller on 06/11/2017 20:56:35

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Karl,

This is important ... please think it through, so that you understand the geometry.

The snugness of the fit is dependent upon the mating of the angled flanks of the male and female threads ... it should not be any less snug just because the extremites of the thread form are truncated.

MichaelG.

.

This is a good place to start:

https://en.m.wikipedia.org/wiki/ISO_metric_screw_thread

Especially this picture:

https://en.m.wikipedia.org/wiki/ISO_metric_screw_thread#/media/File%3AISO_and_UTS_Thread_Dimensions.svg

Edited By Michael Gilligan on 06/11/2017 21:04:55

Karl: One thing you'll learn, either the easy or hard way, is that there is no point in buying cheap tools. In some cases it's not that they do the job more slowly or don't last as long, but that they simply can't do the job at all.

I've always found 316 stainless steel to machine well, but it is not easy to thread, as it is quite tough. I know it's technical but I drop the thread engagement to about 50% for stainless steel.

Cutting M18 threads in 316 stainless steel by hand is pretty ambitious, even with quality tooling. All I can say is good luck.

Andrew