M8 tapping drill

[Mark CParticipant @markc]

Back to drilling tapping holes, we really should consider the material carefully as tapping soft materials (soft plastics and such) would probably require slow spiral 6.8 drills?

Mark

(because I refuse to use the silly little icon things, I have resisted the temptation to insert the one with little horns, but it was an effort… )

[JasonBModerator @jasonb]

Posted by Mark C on 13/10/2016 19:49:15:

Back to drilling tapping holes, we really should consider the material carefully as tapping soft materials (soft plastics and such) would probably require slow spiral 6.8 drills?

Do you keep your slow spiral drills next to your material specific taps? and are they colour coded to match the coloured rings on the taps?

[MWParticipant @mw27036]

Posted by JasonB on 13/10/2016 20:02:26:

Posted by Mark C on 13/10/2016 19:49:15:

Do you keep your slow spiral drills next to your material specific taps? and are they colour coded to match the coloured rings on the taps?

I think it's next to the material specific measuring set, i wouldn't want any sooty cast iron marks from the anvil face of the wrong micrometer on my plastic.

[AjohnwParticipant @ajohnw51620]

Posted by JasonB on 13/10/2016 18:39:09:

Posted by Ajohnw on 13/10/2016 18:28:39:

Get back on track.

Assuming that a tap follows a male thread form which is very likely .

John surely it will mirror the FEMALE thrad form as that is what the tap is designed to cut.

Edited By JasonB on 13/10/2016 18:41:53

The drawings always show zero clearance and both the nut and bolt. I was lucky. I found one that specified the rads. It will work out if clearance is added to either part.I think that is split between the 2

Conclusion – the usual M8 tap drill will clear the core even allowing for the ambiguities. Using 6.9 isn't going to make a significant difference. 7mm may be a step too far.

You have a calculator and there is the whole wide misleading web out there so …………………….

Really I think this aspect only really needs more thought on both rather small and rather large taps. They can get big enough for the effort needed to use them is important even when just cleaning up a pre screw cut thread to most of it's depth.

I do have a couple of spiral taps. I don't find that they help much for hand tapping so stick with straight flute and break the chip. Actually I found them worse. I don't machine tap,

John

–

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 13/10/2016 14:23:30:

Posted by Michael Gilligan on 13/10/2016 13:34:23:

I can't really see why that "throws another spanner in", Neil

… apart from the fact that you probably need one for the nut.

MichaelG.

Because it means the old BS for nut minor diameter was outside the DIN tolerance.

Were British metric nuts different from Continental ones until the standards were harmonised?

.

Perhaps more pertinent :

(a) is/was Tubal Cain a definitive source ?

(b) was he perhaps talking about Whitworth nuts ?

(c) have you looked at the relvant British Standards ?

MichaelG.

[JasonBModerator @jasonb]

Maybe if we all went over to these it would shorten the length of this thread a bit.

J

PS Mark will be happy with the size of the pilot

[AjohnwParticipant @ajohnw51620]

Posted by JasonB on 13/10/2016 20:34:44:

Maybe if we all went over to these it would shorten the length of this thread a bit.

J

PS Mark will be happy with the size of the pilot

People would have a problem driving that in by hand Jason.

John

–

[JasonBModerator @jasonb]

Posted by Ajohnw on 13/10/2016 20:38:05:

Posted by JasonB on 13/10/2016 20:34:44:

Maybe if we all went over to these it would shorten the length of this thread a bit.

J

PS Mark will be happy with the size of the pilot

People would have a problem driving that in by hand Jason.

John

–

No more than they would with any other 6.8mm drill

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 13/10/2016 20:34:44:

Maybe if we all went over to these it would shorten the length of this thread a bit.

J

PS Mark will be happy with the size of the pilot

.

Please, Sir … Do they do a 'plug/third/bottoming' version

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

For info.

I went to the University Library this afternoon, and checked BS 1157 : 1975

For M8 [1.25mm pitch] this specifies a Drill Diameter of 6.80mm, to give 78.5% 'Theoretical Radial Engagement'.

… Nothing too unexpected there.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 13/10/2016 13:30:33:

… I have seen such flats on a larger nuts.

.

< sigh >

If you look closely, Neil … you will also see such flats on most small nuts.

MichaelG.

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/10/2016 20:24:15:

Perhaps more pertinent :

(a) is/was Tubal Cain a definitive source ?

(b) was he perhaps talking about Whitworth nuts ?

(c) have you looked at the relvant British Standards ?

MichaelG.

(a) He taught Engineering at Loughborough since the days when Georg Schlesinger was Professor there. His field of expertise (authority some would say) was diesel engines.

"He graduated in mechanical engineering at Loughborough University in 1934, and eventually, after a career in mechanical design, went back to that university in 1943, becoming, after some years, senior lecturer in mechanical engineering. He went on in 1948 to teach at University of Liverpool. In 1952 he was appointed Head of Department of Mechanical, Civil and Electrical Engineering at Darlington College of Technology. His final academic post was Head of Mechanical, Production and Civil Engineering at Lancashire Polytechnic. "

Not definitive, but possible authoritative.

(b) Only he could answer that.

(c) No I nearly choked myself to death on the DIN documents (which only give data in terms of tolerances for size ranges of thread that you then have to plug into formulae… no nice tabulation of the actual measurements).

OK I have now, the 1981 standard for Metric threads is completely impenetrable, and looks the same as the current DIN one to me :-/

Neil

[Neil WyattModerator @neilwyatt]

Posted by JasonB on 13/10/2016 20:34:44:

Maybe if we all went over to these it would shorten the length of this thread a bit.

Obviously, it only goes 2/3 as far!

Neil

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/10/2016 21:25:32:

Posted by Neil Wyatt on 13/10/2016 13:30:33:

… I have seen such flats on a larger nuts.

.

< sigh >

If you look closely, Neil … you will also see such flats on most small nuts.

MichaelG.

I don't make a habit of staring into the depths of small nuts

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 13/10/2016 21:33:25:

"He graduated in mechanical engineering at Loughborough University in 1934"

.

Don't believe everything you find on the web, Neil

There was no such thing as Loughborough University in 1934

MichaelG.

.

Here's a concise history: http://www.lboro.ac.uk/about/history/

Edited By Michael Gilligan on 13/10/2016 21:48:02

[MWParticipant @mw27036]

In that case, what would've been loughborough university,

my old 2ndry school has probably gone through a dozen names over it's lifetime but it's been the same institution since 1630 something, Another case would be the university of hertfordshire art department, was once the St.albans school of art, but the same institution.

Michael W

Edited By Michael Walters on 13/10/2016 21:47:23

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 13/10/2016 21:36:21:

I don't make a habit of staring into the depths of small nuts

.

There might be a 'humble botanist' joke in there; but I can't be bothered.

MichaelG.

[Mark CParticipant @markc]

humble arborist perhaps?

Mark

[D HannaParticipant @dhanna35823]

It's nearly a book now but I can't remember in the many pages seeing anything regarding the true profile of the nut/bolt. The ISO profile and Unified profile is the same base 60 deg profile as shown below. If we look we will understand that at full 100% contact depth there will be a core flat of p/4. Some seem to think that having a flat on the minor dia of the nut thread means that contact percentage and tapping load is reduced. . The full contact depth can only ever be 0.541P. Hope that doesn't look like we are trying to teach some how to suck eggs but I've found many in the trade do not understand these facts.

BSW profile of course is a totally different animal and any flat on the core is a reduced contact % ………. but we all knew that didn't we

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/10/2016 21:42:50:

Posted by Neil Wyatt on 13/10/2016 21:33:25:

"He graduated in mechanical engineering at Loughborough University in 1934"

.

Don't believe everything you find on the web, Neil

There was no such thing as Loughborough University in 1934

A classic diversionary tactic

N.

Edited By Neil Wyatt on 14/10/2016 07:42:43

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 14/10/2016 07:42:03:

A classic diversionary tactic

.

More a case of reference-checking

MichaelG.

.

Your response to (b) is rather more worrying though.

[Martin ConnellyParticipant @martinconnelly55370]

Neil and D Hanna, if you look at the figure above it has the same dimension for height of the thread of 5/8H as in the 1981 metric standard (fig 1). Paragraph 3.3 of this standard defines 5/8H as 0.541265877 P (approximation due to rounding assumed). Since this occurs at both sides of the thread the value to be subtracted from the diameter for the tapping size is twice 5/8H = 1.0825 pitch (more approximation from rounding). For a M8 x 1.25 pitch thread this gives a tapping drill size of 8-1.3531 = 6.65mm and for M8x1 of 6.92mm (more rounding seems sensible). If we round these up to the next single decimal point size drill we get 6.7mm and 7mm. The usual guide of subtract the pitch from the diameter clearly works for these sizes. Using a quality M8 tap in holes drilled to these sizes with correct technique and suitable tapping fluid will work every time. Where is the problem with applying this simple rule for M8 which is what the original post was all about.

Martin

[D HannaParticipant @dhanna35823]

Thanks for adding that to my bit Martin. Until early 2015 I had spent 35 years teaching this to Apprentices and tradesmen and I can still see the glazed eyes when we started into the nitty gritty of the exact maths of screw threads. Few would end up using any of it to that extent unless they were in a tool room with a thread grinder but it was something which was nice to know.

[Mark CParticipant @markc]

I thought I would "improve" a design on a machine by changing from a coarse to fine thread on a jacking screw. I applied the simplistic view that a fine thread would be better due to the lower helix angle giving more force per turn – how wrong I was! It resulted in having to go red faced to the machine shop and have a new set of parts made with the old thread form as the operators could not turn the things when they were loaded up – not even with a 6 ft pole on the end…. Ever since then I have given considerable more thought when making "simple" improvements – especially when threads are concerned.

Mark

[HopperParticipant @hopper]

Posted by D Hanna on 14/10/2016 10:15:37:

…Until early 2015 I had spent 35 years teaching this to Apprentices and tradesmen and I can still see the glazed eyes when we started into the nitty gritty of the exact maths of screw threads.

And rightly so, apprentices being eminently practical lads (and lasses these days) who haven't yet become welded to an arm chair and still have sufficient gumption to drill the recommended sized hole from some chart or other hanging on the wall behind the drill press, and if the tap feels dangerously tight going in, to drill again slightly bigger.

[Author]

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