M8 tapping drill

(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

Obviously, it only goes 2/3 as far!

Neil

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

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

There was no such thing as Loughborough University in 1934

MichaelG.

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

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

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

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

MichaelG.

humble arborist perhaps?

Mark

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

A classic diversionary tactic

N.

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

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More a case of reference-checking

MichaelG.

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

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

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.

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

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.

The OP admits that he did not have a quality tap, his (and my) technique may not be perfect and tapping fluid costs £25 for a small bottle. Those of us in a home workshop have the opportunity to balance all these factors and determine (if we wish) that by using a larger drill we can mitigate the risk of tap breakage with our sub-standard equipment with little loss in strength of the subsequent fastening. Where's the problem in that? After all, we don't have the opportunity to offset the costs of premium equipment against tax or reclaim the VAT. I might also add that the OP was happy with the resultant thread from a hole made with his 7.0mm drill.

I'm not sure that Hopper's apprentice would get a job in Mark's DIN standard workshop

Just for clarification: Tubal Cain gives the whole range of hole sizes required for various degrees of engagement. He pointed out that he had significantly improved on the amount of tap breakage that he had suffered by reducing the engagement to between 60 and 70%. The table I showed was one that I put together by extracting the appropriate figures from his comprehensive tables.

Cheers,

Rod

Would TCs findings be based on carbon steel taps? maybe with some of the modern HSS and coated ones he would have faired better when using higher engagement percentages.

J

PS Rod you are obviously not bying your tapping fluid from the right places

Probably, that's all the impecunious model engineer could afford. But that's really my point: If you don't have the best equipment then best practice isn't necessarily for you.

Some of the contributers to this thread, looking at it from an industrial perspective, seem to be insisting that there is a "correct" answer. All I'm trying to say (ad nauseam) is that there isn't. There are other options, depending on the application.

Tapping Fluid. Actually, I don't have any - I use a tiny little tub of what the kind fitter who gave it to me decades ago referred to as "Goose S**T". And It's nearly all gone

Rod

I did mention Ambersil. The stuff I use is "Tufcut" and lasts for ages if you use it carefully. It can be used for more than just tapping but I have coolant so only use it for tapping mainly. It is less than 14 quid at Cromwell for 400ml aerosols tin. There is a Rocol product that is also extremely good. They make a HUGE difference when tapping in almost all circumstances.

Mark

Think I said similar earlier on, treat each job on its own and use what suits eg if its a cylinder head then go for more engagement than a couple of 12BA roundheads in soft brass cleading holding a name plate on.

I got my last bottle of Dormer Supercut free but not used it yet as I have a long way to go until the CT85 is empty - several years worth. But you can get teh Dormer one for £11-12.oo +VAt for 400g

Rod,

First bit incorrect (there is definitely a right size for the standard thread), second bit correct (quite right, use whatever floats your boat)!

Mark

The pugnacious one