M8 tapping drill

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Martin Connelly on 14/10/2016 08:58:20:

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.

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

[JasonBModerator @jasonb]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by JasonB on 14/10/2016 13:19:38:

Would TCs findings be based on carbon steel taps?

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

[Mark CParticipant @markc]

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

[JasonBModerator @jasonb]

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

[Mark CParticipant @markc]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

Put your dukes up

Rod

The stubborn one

[AjohnwParticipant @ajohnw51620]

Posted by Roderick Jenkins on 14/10/2016 13:40:19:

Posted by JasonB on 14/10/2016 13:19:38:

Would TCs findings be based on carbon steel taps?

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

Your 100% right Rod but pointless arguing with the perfect world people were everything is exact including taps. Fact is they are not exact, neither are drills or the bits that go into the holes. It's also perfectly possible to do adequate jobs with cheap stuff. Eg if a tap isn't going to be used much why buy ground hss when cut thread will do. My most used taps come from Screwfix. HSS and size wise no different to far more expensive brands. The split dies have a very unusual adjustment fitting which doesn't work that well. I recently bought some serials from ArcEuro. As expected there is more size variation. I wouldn't expect any different given the price.

Those that want exact should go for the precision parts too. The price of the screws would shock most people and they are still not exact.

I often use a scraping of hand soap as a lubricant for tapping. I do have some of the "proper" stuff but it's messy and not any better. Hadn't thought of trying goose fat. It is odd stuff as far as fats go and can be bought from supermarkets for superior roast potatoes. I've bought some lubricant type tallow – what I always used at work and gave excellent results. It will get tried eventually. Last time this cropped up I remembered something – sperm whale oil when it was available was the best of the best for a tapping lubricant. The modern equivalent is jojoba oil so I will probably try some of that too.

LOL Tap's back cutting clearly doesn't matter round here.

John

–

[Roderick JenkinsParticipant @roderickjenkins93242]

I think the Goose S**T was actually Trefolex.

I'm not sure there is actually that much disagreement amongst the various contributors either – we're just trading semantics over the meaning of "correct".

Rod

[Anonymous]

Posted by Roderick Jenkins on 14/10/2016 15:40:44:

…………we're just trading semantics over the meaning of "correct".

Boring; let's have two falls, two submissions or a knockout to settle it.

Oh, and take the can openers away from those misusing them.

Andrew

[AjohnwParticipant @ajohnw51620]

You wont have any problem buying more of that Rod. It looks to be pretty popular.

When I use soap I'm probably using lanolin. The hand soap I use is extremely good for removing workshop grime from my hands and an excess of oil of some sort content would explain that.

I have a feeling that Lucas bought in bulk tallow wasn't the usual kind but though it was worth a try. I've also used a smear of cutting oil at times.

I like the chips to come out easily.

John

–

[Howard LewisParticipant @howardlewis46836]

Funny mentioning tallow, since thats what our forefathers used. Actually, to my possible shame, despite having Rocol RTD, I very often use the fat left over from when SWMBO has grilled bacon. Works a treat and seems to cost nothing. And it melts in all the heat generated on this thread (Is that a pun?)

Howard

[Neil WyattModerator @neilwyatt]

> I recently bought some serials … As expected there is more size variation.

Isn't that the point of serial taps?

Neil

[BWParticipant @bw]

Posted by Neil Wyatt on 11/10/2016 14:42:50:

Posted by Roderick Jenkins on 11/10/2016 11:28:56:

even down at 60% the shear strength of the thread is 2.5 times the tensile strength of the bolt core

As soon as you allow for minor deformation under tension spreading the load over two or three turns of the thread there is more strength in the thread than the core. That's why you rarely strip a screw but normally either snap one or pull the thread out of the hole (when its in a softer material like CI or Ally).

Neil

What books should I be reading to be able to understand material strengths & properties and be able to intuitively know things like this in a few years time ?

Bill

[steve de24Participant @stevede2433577]

Bill,

I would recommend :-

Mechanical Engineering Design by Joseph E Shigley.

Get it from your library or buy a second hand copy from amazon (less than £3 at the moment), but make sure you get a metric edition if those are the units you are familiar with (it tells you on the front cover).

Steve

[HopperParticipant @hopper]

Posted by Bill Wood 2 on 15/10/2016 08:27:14:

….

What books should I be reading to be able to understand material strengths & properties and be able to intuitively know things like this in a few years time ?

Bill

Any of the Workshop Practice series. There is one specifically on Taps and Dies, IIRC. Another on workshop materials. And many more topics that are all useful. Plus they are cheap to buy. Highly recommended.

Model Engineer's Handbook by Tubal Cain. A good basic, practical reference book in the workshop.

The Amateur's Workshop by Ian Bradley. More project-oriented but a good basic primer.

That lot will get you started.

Beware of overthinking this stuff, though. Most professional machinists can't quote you chapter and verse of the science behind what they do every day. They just know from training and experience what works.

[CirclipParticipant @circlip]

10 pages for an M8, thank goodness it wasn't an M42.

Regards Ian.

[Chris GunnParticipant @chrisgunn36534]

I have just produced a spreadsheet of the tapping drills for metric coarse taps from M2 to M20, using the data from all the sources I normally refer to, like Zeus, Starrett, Machineries handbook and so on. If anyone wants a copy please PM me. I have tried incorporating it into this text but the formatting messes the table up or is too many characters. It is now in the form of an Excel spreadsheet so you can add your favourite reference sources to it.

Chris Gunn

[Mark CParticipant @markc]

And I have spent a spare 5 min. running an FEA on an ideal M8 thread assembly (well it was more like 5 hours by the time I got the mesh sorted and the restraints and contacts working…. ).

This is what they look like;

[Mark CParticipant @markc]

The last one is a factor of safety plot that has been adjusted to show the areas that would fail (I only applied a 5000 newton load to the stud). You can see that it fails how you would expect for a tough steel stud in mild steel thread. If there was clearance it would probably show a slightly different distribution but not vastly so – I don't think anyway?

Mark

[Mark CParticipant @markc]

PS, six posts to go and we will have reached 200 !!!

[AjohnwParticipant @ajohnw51620]

I'm amazed it's not dead and still alive.

Chart ? Subtract pitch from dia but do measure the dia to check. It may even be undersized where many of us get ours from. Drills too especially at smaller sizes.

Then if needed think.

John

–

[Mark CParticipant @markc]

And for all those that might be interested in what the thread looks like with different tapping drills, here is a comparison drawn exactly to scale with a nominal male thread for comparison. The left hand thread is at nominal size, the middle is what it looks like with a 6.8 hole and the right hand one shows the same things with a 7.1mm hole.

Mark

[Michael GilliganParticipant @michaelgilligan61133]

Now put a nasty bit of commercial quality "allthread" in the right hand one.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Putting it simply, threads fail along a line where the shear strength of the female equals the shear strength of the male. If you're tapping the hole in mild steel, gunmetal, etc the material of a commercial male bolt is a lot stronger than that of the the female. This means that the shear line is bigger than the average of (root of female) and (crest of bolt) diameters, and having a bigger than ideal tapping drill doesn't matter. There is obviously a limit to this argument

[Author]

Posts