M8 tapping drill

Well spotted Michael.

On the other aspect how sharp and true is the tip of the drill. I'm hoping my Medding drill that I put together last night is really square and true to the table. It will help a lot with reaming in particular. Just need to find a 240v motor now.

I'm surprised that some one hasn't worked out what the actual core size of what a typical 8.1mm dia tap is. Still iffy though.

Yes Mark - I have made use of thread go no go guages. Not at home though. The ring types as well. Using hand ground lathe tools too.

John

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And like all the best debates it involves two equally valid points of view:

• A man working in industry who ignores appropriate Standards is a bloody fool
• Amateurs who adopt industry best practice on principle are just as daft.

For my part, I shall carry on using 7.1 for M8 until I need a better fit. Thanks to this thread I know that's done by tapping a 6.8mm hole.

I'm happy!

Dave

Dave, that is probably the most sensible post so far, putting you in the "I am doing this from an informed point of view" catagory.

Mark

Now we just need 200 posts on each of the other Metric Coarse sizes. then we can move onto fine pitch, BA, BSW, BSF, UNC...

Now John, why did you suggest that with over 200 replies already to tap a humble hole!! but here it is:

Minor dia. or core dia of nut = basic OD -1.082P

Also that 8.1 dia tap has a reason! Above the max OD the nut form is radiused for clearance on ISO Metric and Unified thread forms by 0.072P which would mean on our M8 x 1.25 tap should be around 8.09 dia. plus tolerance.  This of course has nothing to do with the fit of the nut on the screw unless the screw has a sharp V on the crest and is also over the basic dia of the thread. The fit is always controlled by the effective diameters.

And of course this has nothing to do with anything generally other than Toolroom talk but sometimes for some people "it's just nice to know" ! Now back to the shed in OZ.

Edited By D Hanna on 13/10/2016 12:41:54

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MichaelG.

On the assumption that no-one else would have bothered, and I was on the 'bus ... I thought it might be useful to plot this for M8 [8mm dia x 1.25mm pitch]:

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MichaelG.

Edited By Michael Gilligan on 13/10/2016 13:23:49

To throw in another spanner, Tubal Cain claims "Commercial nuts made to British Standards may have a minor diameter (tap drill) diameter such that there is a flat on the crest of the nut thread. Such nuts offer about 65% thread engagement."

I have seen such flats on a larger nuts.

Neil

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.

A spanner is no good, you will be needing a certified and approved/calibrated back to national standards torque wrench

Mark

Neil, we don't have BS standard nuts yet, that will be after the hard brexit

Mark

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?

N.

Well I'm just off to the shed to tap 3 M4x0.7 threads and I'm not telling anyone what size drill I'm using - but I know they will do the job I want.

Steve

In my opinion, hard brexit should be at least 65 Rockwell C.

Tony, sorry about that, it seems Theresa forgot to email me the briefing notes....

Mark

Roy, that is a bit brittle for a high tensile brexit do you not think? should be more like 54 Hrc I should imagine

Mark

As long as we all stay sharp, and tough and un-yealding we will be O.K RoyM

Throwing a towel in may be more appropriate. For some or all maybe.

John

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Excellent idea. Might be another several hundred posts and the need to buy new gear.

I wonder if it;s ISO or DIN ? Maybe BS. BS being pretty appropriate really.

John

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