M8 tapping drill

Iain, good to hear that you were successful producing a usable thread.

That is a nice tilting table you have in the pics. How big is it and where did you get it from? I'm looking to get something similar for myself.

Steve

M8 is 1.25mm pitch so drill 6.75mm according to many tables rules no make it 6.8. If the tap is over 8mm which it probably will be add that on and round up.

Zeus reckon 6.8mm but the certain standard I mentioned for BSF etc was Zeus. So on that basis I would drill 6.9 providing the tap was 8mm.

More tricky at small sizes and sooner with metric fine. M2 drill size is 1.6mm from the same source so get out your number drills and use a No52., 1.62mm dia. If the tap is significantly over size up accordingly. One of the big problems with sizes like this is keeping the tap axial to the hole 'cause it will probably break if it isn't.

That's what I do anyway. Use Zeus and up the sizes a touch based on the size and my judgement. I also check the size of drills at smaller sizes in particular as they are often a touch under.

John

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Edited By Ajohnw on 11/10/2016 23:16:55

Steve - this is a 'tilting angleplate' from RDG tools, see here. Current price is 53 quid inc VAT and postage.

I ignore the angle markings on it and set it using a cheap electronic angle gauge from Lidl.

My only complaint is that it is subject to rust even when one tries to keep it oiled up.

Iain

You're right, Dave, because there is no authoritative standard on tapping drill sizes, variation could be infinite within the engagement range of the thread, it isn't the end of the world if you use a 3.4 drill to tap M4 rather than 3.3.

In some circumstances i've had threads in plastics close up due to part shrinkage, even the bog standard rolled bolts are tight, therefore drilling oversize the zeus value could be recommended to compensate for this!

Michael W

I often don' use tapping drill tables at all. Just poke the end of the tap in a hole in my drill gauge, or in the drill rack itself, until I find thehole that allows the taper tap go in up to about one-quarter thread depth, by eye. This means that size of drill will give you about 75 per cent thread depth. It works for me.

Iain,

You have received a lot of advise that may be a bit confusing. The truth is there is no absolutely correct tapping size for any particular thread. This is explained very well by Tubal Cain in excellent book Drills, Taps and Dies ( No 12 in the Workshop Practice series). Also have a look at this site which shows the range of tapping sizes for all normal screw threads in an informative and interesting way: **LINK**

You could do the same with your tilting table as I do with my surface plate, I keep it in the cupboard with the hot water cylinder in the house.

Ian S C

Micahel W and Iain, there is an authoritative guide to metric tapping drills and it is the DIN standard for the thread itself and none of it is confusing. The only time it gets confusing is when you start getting peoples take on the "best" way which is purely their own method that suits them - it may or may not suit your application.

You can all argue 'till the cows come home about your own interpretations but the fact remains that if you want a thread that meets DIN then you will use the correct drill to suit according to the standard sizes. If you want to use bigger or smaller then that is your choice but the fact remains the same.

Did Mr Cain sit on the standards committee? I don't know but I doubt it, please correct me if I am wrong. His books may be very good for model engineering but this was a discussion about the drill to use for an M8 thread..... the answer - emphatic answer that is - 6.8mm.

Mark

Iain, thanks for the tilting table info.

Lambton, thanks for the link on tapping drill size.

Steve

No mr C did not sit on a standard committee but he did lend his ear to a few companies regarding production machinery. I'm not a production worker in my own shop so effectively i'll tap to whatever i find works best, not to DIN.

I just happen to like TC's work and read his books so i follow his advice, it's not the only advice though, thats the point i try to make and different charts give different measurements,

So i suppose if you want to be a captain of industry you can follow the DIN standards authority and tap to that.

When i did set the machines at work, there is absolutely no reference to any chart, we literally just use what works best for the thread in that material and stick to it. 

Michael W

Edited By Michael Walters on 12/10/2016 11:44:49

Not if you're using a roll form tap.............

Andrew

Mark,

Tubal Cain (real name Tom Walshaw) was a professional engineer who designed large diesel engines and then went into academia whilst also being a maker of models and a writer on workshop techniques for model engineers. He did actually convene a discussion group amongst the model engineering fraternity to suggest standards for ISO metric fastenings to replace the BA threads widely used by model engineers - this was accepted as a codicil to the Bristish Standard. I respectfully suggest that he knew what he was talking about.

TC's point was that he and fellow MODEL Engineers have a particular problem in breaking taps and leaving the broken bit in the hole. He suggested that using a larger than recommended hole for tapping greatly reduced the risk of tap breakage without materially affecting the strength of the fixture. I concede that 8mm is at the top end of model engineering size, but having broken an M6 tap which siezed in aluminium I contend that it can still be an issue.

DIN standards may be fine for German car manufacturers but they are not necessarily applicable in every case. Proper design would employ the use of stress analysis on the joint to determine the best type of fixing balanced with ease of manufacture. I don't know, but my guess is that in the aerospace industry they don't necessarily just call out the nearest equivalent from the DIN CAD package library.

I can only agree that if you want to meet the DIN standard then use the DIN tables but I do not agree that the DIN standard is the "proper" way and that any other is a fudge. Horses for Courses.

I'm very happy to agree to differ - it has been an interesting discussion

Best wishes,

Rod

Edited for typos

Edited By Roderick Jenkins on 12/10/2016 12:03:29

Michael, if that was the practice you used then best of luck to you but I will only subcontract work to shops that run a quality system and the idea that operators used what they wanted would have them off my list of suppliers in double quick time... If you took the time and looked at the drawing I posted you can see the manufacturing instructions and I would reject any part that did not conform to those instructions/dimensions without prior agreement. Lets not get confuseing industrial and home workshop practice though.

Your advice "You're right, Dave, because there is no authoritative standard on tapping drill sizes" sounds like statement of fact to me and it is wrong.

Mark

Andrew, is that a large spoon in your hand?

Mark

As status quo say;

W.Y.W

Michael W

Edited By Michael Walters on 12/10/2016 12:16:01

Rod, I have never read any of his works and have no idea about his background etc. I don't know how old his advice is but I suspect that a lot of the problems with taps have been reduced given the advances in material science and coatings used on quality taps - cheap crud is an altogether different situation and as John W mentioned, may not even be the correct size? At that point, perhaps a good helping of clearance would be a good idea but I never said anything about having to use the specified drill.

With regard breaking taps in Al, that is an altogether different issue and far more common. I have also managed to break taps in Al but only when short-cutting and tapping dry (the knack is to understand when the tap is getting tight and get some cutting fluid - WD40 - usually! ). The other material that can be a pain is Stainless and that also requires some understanding and experience to get things right. I always use Ambersil cutting and tapping aerosol for steel (including stainless grades).

I don't think we are differing much at all. Perhaps only in the difference between written intent and read understanding?

Mark

Mark,

We are certainly looking at the subject from different perspectives.

Sadly my collection of taps, garnered over 35 years, haven't kept up with the times

Oh! to be able to afford all the latest kit

Cheers,

Rod

Yes even if the tap is 8.1mm diameter Mark. Personally I wish there was some SAE standards. Far more sensible lot. The other point is will the drill be 6.8mm. Measuring them can prove interesting.

The actual bolt and screws themselves are interesting. I bought several lengths in M6 hex socket screws as I use them for holding work down at times. High tensile precision grade. The cost is prohibitive for a casual user. They do measure 6mm dia though. At one time I used to be given used once dormer drills as the toolmaker who gave them to me was only allowed to use them once on a particular grade of nitriding steel used for plastic moulding tools and other things. Perhaps we should all do that as well.

Where it matters I would also suggest that people stick to 1 to 1 1/2 dia engagement lengths as they are unlikely to have any problems what ever they use or are fixing providing the torque doesn't exceed what the materials can take. Nuts thickness came about on the basis that it's more likely to strip but only just as some people have probably found at times.

Precision. I'd guess that if a typical M8 bolt of some sort is measured it will be circa 7.8mm dia, maybe a touch over. If it's rolled thread the pitch is likely to be more accurate than cut thread but if they can be found old cut thread stuff will usually have a much better fit to nuts made in the same way. Measure a tap and this time it is most likely to be oversized.

So if 0.1mm matters at M8 best account for all of this as well.

Best answer is to get on with it and bear these facts in mind if and when people have problems.

John

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I think you'll find that the DIN standard is silent on the matter but refers back to IS 10952 - 1984 which does give 6.8mm drill size for M8 coarse.

BUT it also states:

So Tubal Cain and the collected minds are acting wholly in accordance with the standards.

Neil

<edit - corrected that reference>

Edited By Neil Wyatt on 12/10/2016 13:51:58

Good grief, so it is, how did that happen? My mistake, I thought I was holding a small stirrer.

Andrew