M8 tapping drill

[D HannaParticipant @dhanna35823]

Posted by Ajohnw on 13/10/2016 11:04:49:

Posted by Michael Gilligan on 13/10/2016 00:00:17:

Posted by Roderick Jenkins on 12/10/2016 23:44:17:

The link suggests that engagement values of 60-70% are applicable to most situations. Putting a value of 65% for 8×1.25 gives a drill of 6.95mm, rather bigger than the 6.8mm drill they have supplied to Mark which gives 74% engagement.

.

And putting 70% for engagement gives

Drill Diameter:

6.863mm

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.

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

–

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by D Hanna on 13/10/2016 12:16:16:

… sometimes for some people "it's just nice to know" !

.

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

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]:

.

 

MichaelG.

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

[Neil WyattModerator @neilwyatt]

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

[Michael GilliganParticipant @michaelgilligan61133]

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.

[Mark CParticipant @markc]

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

Mark

[Mark CParticipant @markc]

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

Mark

[Neil WyattModerator @neilwyatt]

Posted by Michael Gilligan on 13/10/2016 13:34:23:

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.

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.

[Tony Pratt 1Participant @tonypratt1]

Posted by Mark C on 13/10/2016 13:46:01:

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

Mark

Hard Brexit is now known as clean Brexit.

Tony

[steve de24Participant @stevede2433577]

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

[Roy MParticipant @roym]

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

[Mark CParticipant @markc]

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

Mark

[Mark CParticipant @markc]

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

[Roy MParticipant @roym]

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

[AjohnwParticipant @ajohnw51620]

Posted by Neil Wyatt on 13/10/2016 14:23:30:

Posted by Michael Gilligan on 13/10/2016 13:34:23:

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.

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.

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

John

–

[AjohnwParticipant @ajohnw51620]

Posted by steve de24 on 13/10/2016 15:11:07:

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

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

–

[MuzzerParticipant @muzzer]

Ah, but if it takes 200 posts to make a hole, how many posts does it take to make half a hole? Eh?

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Mark C on 13/10/2016 15:32:09:

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

After Brexit we shall be reverting to good old British Vickers hardness, none of these imported units (the tariffs will be too high)

Rod

[Tony Pratt 1Participant @tonypratt1]

Posted by Mark C on 13/10/2016 15:31:00:

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

Mark

It's alright I will send you a copy

Tony

[MWParticipant @mw27036]

Posted by Roy M on 13/10/2016 15:27:16:

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

Meanwhile the labour party are scrambled to do whatever they can to ensure it's atleast oil quenched rather than brine, so as to atleast retain a gentle temper.

Michael W

[AjohnwParticipant @ajohnw51620]

Get back on track.

Assuming that a tap follows a male thread form which is very likely h1 as shown on many diagrams is 0.6134 P or 0.76675 on M8 so the core diameter including the valley rad for an 8mm tap is 6.4665mm or 6,5665 if the tap happens to be 8.1mm dia. That diameter seems to be more down to clearance specs to me not tolerance. The end of hole based tolerance which would put the clearance mostly on the male and assume that a tap produced something pretty close to it's actual diameter.

However there will be a rad on the peaks of the tap as well, big H / 6. Big H is 0.866 P so that is 0.18mm and adds in part to the diameter of the tap and will reduce the core diameter further.

So it looks like drilling D – P will clear the core of the tap as in this case that comes out at 6.75mm and I would guess if accurate will give ISO engagement couple with and ISO male thread which will also be undersized.

Actually I probably would use 6.8 on M8 but by the time it got down to M4 and smaller I would start going over sized on harder material and definitely so at M2. I generally do measure my taps as well. One thing decent digital callipers are good for. Reamers too where most seem to have been made deliberately over size for a long time now. Drills I sometime find are under. It's just a case of slowly rotating what ever it is in the callipers in a direction that slides the cutting edges rather than tries to cut with them.

LOL some one should check the sums.

John

–

[JasonBModerator @jasonb]

Posted by Ajohnw on 13/10/2016 18:28:39:

Get back on track.

Assuming that a tap follows a male thread form which is very likely .

John surely it will mirror the FEMALE thrad form as that is what the tap is designed to cut.

Edited By JasonB on 13/10/2016 18:41:53

[steve de24Participant @stevede2433577]

Posted by Ajohnw on 13/10/2016 15:51:36:

Posted by steve de24 on 13/10/2016 15:11:07:

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

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

The work complied with MBS (My Bloody Standard) which governs the work in my shed.

Steve

[MWParticipant @mw27036]

Posted by steve de24 on 13/10/2016 19:13:38:

The work complied with MBS (My Bloody Standard) which governs the work in my shed.

Steve

It's a small world steve, because my work also complies to that standard, i hear it's getting popular.

Michael W

Edited By Michael Walters on 13/10/2016 19:44:00

[Mark CParticipant @markc]

"might be several hundred posts and the need to buy new gear" sounds like a really good reason to keep posting if I can have more stuff without clearing it with the accounts department first!

Mark

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