M8 tapping drill

By the way, this now the 100'th post.....

Oh and also, in-between all these posts I have just managed to get to grips with the Samba Baiao rhythm I was given by my drum instructor. To say I am chuffed is an understatement! 

Edited By Mark C on 12/10/2016 20:56:46

Rod, perhaps you should read my original post again, I did not insist on anything, I just commented on what is stamped on some M8 taps in the hope it may help the OP. Is it your round?

Chris Gunn

Indeed you are correct.

Probably Dormer as they come with 6.8 drills..

I had to go back and see what I was getting aerated about!

1 Several people WERE recommending drills other than 6.8.

2 Claiming the DIN standard as setting the drill size is a bit tenuous, and only makes sense when the tolerance range is given along with the matching thread engagement (a sensible 67% up to silly).

3 Two manufacturers make the case for using thread engagement which would be well below the DIN standard, and one of them positively recommends it (Guhring).

4 Tubal Cain can be criticised but his choice of 65% engagement for threads below 3/8 inch is barely below the DIN figure of 67%. His table (in his handbook) giving figures as low as 50% (and as high as 75%) is from an authority that he does not name, not his own. I haven't got his WPS series book.

Neil, that seems to be a common problem with postings here, people comment without getting the facts. The taping set I show are Guehring spiral flute complete with nicely labelled 6.8 drill.

So back to the last post:

point 1 - several = more than 2 but not many.....

point 2 - does the din standard show a dimension linked to the thread size or not? please advise what it says if not.

point 3 - please see my first comment above.

point 4 - I am not criticising anyone, simply stating what is (apparently rather annoyingly) a fact that the accepted drill size for standard (sometimes described as coarse) M8 threading in standard (read normal steel/aluminium etc) engineering materials is 6.8mm. Walk in any fastener supplier selling thread cutting tools and ask for an M8 tapping drill and you will either be questioned about application and materials (I doubt that very much) or presented with an 6.8mm drill..........

Mark

Given that GUHRING seems to be generally accepted as an 'expert witness' in these matters ... and has gone to the trouble of providing this handy on-line calculator: **LINK**

http://www.guhring.com/Tech/TapDrill/

Might I suggest that people calm down a little; try the calculator; and bookmark the page.

MichaelG.

Sense at last

All that is needed now is a get on with it and if problems do occur realise why they can happen.

The whole area is a complete and utter nonsense when different sized bolts and screw can be fitted in the same tapped hole.

The make of the tap or drill doesn't matter. It's not a good idea to assume it does especially these days. eg Eclipse high speed steel. Dormer drills - not for me any more. Taps have various grades

**LINK**

Nuts, bolts and screws and dies too.

Personally I will stick with minimising the cutting pressure to avoid back cutting and to get the correct pitch on all threads. Really the best way to do that is for me to measure my own taps and do some sums. It's no use saying this is an 8mm tap because in practice it wont be 8mm dia. It's much the same with reamers which is rather annoying. To me anyway.

I understand that the ISO tolerances are looser than what imperial tended to use and tolerances between parts have been shifted.

Anyway the OP has tapped his holes now and hopefully is happy with the results. He did get on with it and as expected in the majority of cases it will be ok and doesn't matter all that much. Small sizes and tough materials can be a bit more difficult.

I bought some serial taps recently. I was glad to see that some of the smaller sizes were very close to size. Probably by accident. Some one also mentioned one way of getting a decent fit was to skip the 3rd tap. Be interesting to see if that is correct.

John

-

Edited By Ajohnw on 12/10/2016 23:21:27

Michael, that is a useful link. Did you take a look at the link I posted in my post of 12/10/2016 17:43:19 ? I have not taken time to check it for accuracy but it looks right for M8. Using the two calculators should give all the information you could possibly require regarding metric thread dimensions to enable an informed decision on any given application.

Mark

PS. what is the red flag all about when I cut and paste?

John W, that is a better link than your last attempt......

Here is a picture of something especially for you, everyone else will be able to manage without them but for someone who has been in engineering design that last statement is mostly just nonsense.

Mark

Michael, thanks for the link.

The link suggests that engagement values of 60-70% are applicable to most situations. Putting a value of 65% for 8x1.25 gives a drill of 6.95mm, rather bigger than the 6.8mm drill they have supplied to Mark which gives 74% engagement. But, Mark's taps are spiral with wide flutes that suggests to me that they will clear swarf rather better than straight flute so will be less prone to breaking than my old stuff.

It's all very confusing.

Rod, try 70% as they suggest if you don't know the engagement required (read, standard or general purpose value) - it works out much closer. I doubt the flute form or point design has much influence on tapping drill but it certainly effects the swarf formation. One point to remember with spiral flute is the swarf comes out the same side as you tap from and is very keen to damage fingers.

Mark

.

And putting 70% for engagement gives

If nothing else; this clearly demonstrates how sensitive 'engagement' is to hole size

... I say hole, rather than drill, for obvious reasons

[we've been exercised enough recently, regarding run-out, etc.]

MichaelG.

Much better for blind holes, which is what I mostly seem to end up tapping.

I'll stick with my 60% or so engagement. I havn't blown the cylinders off any of my engines - yet.

You'd better join Chris and I for our drink - I'll get the first round in.

G'night,

Rod

Although it seems like a long time ago, I think that the original post was set up to eradicate tap breakage through drill size. I would like to suggest that unless you are taping a deep hole,( eg 2.5D) into a blind hole, then a spiral point tap should be your preferred tap, and not a spiral flute tap. Spiral flute taps need to have a smaller dia body by virtue of the fact that the swarf needs to pass backwards and out of the drilled hole, thus making the taps considerably weaker and more prone to breakage unless all the machining conditioners are near perfect. A spiral point tap pushes swarf away from the tap allowing the tap to be more robust, they are also more easily controlled when using as a hand tap enabling a square start without too much difficulty. Roy M.

Oh dear Roy, that should see us well past 200 replies!

Mark

This thread thread certainly has more than 65% engagement

Edited By Daniel on 13/10/2016 06:40:38

Yes, you would not think there was quite so much involved in drilling a hole.

Iain asked for some simple advice and he has received it - along with a lot of confusing and sometime conflicting posts. Surely it is time to conclude this thread.

Down the pub Gopher Geoff asks 'what was the score?'

'3-2' replies Old Bill.

'That second penalty should never have been awarded' adds tapper Tom.

'Sorry Tom', interjects Northants Nobby, 'Geoff has got his answer and we can all go home now'.

'Goodnight then, Guys'

'Same again tomorrow, eh?'

'Bye..."

'Cheers'...