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Tapping drill size

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IanT18/10/2019 15:52:47
1498 forum posts
142 photos

Or of course Martin - you could just use a 7mm drill (which hopefully you already have) - it would still be easier to tap with than a 6.8mm one!

Sometimes we get so close to the problem - it gets very hard to see the blooming obvious!!




Edited By IanT on 18/10/2019 15:54:23

Mick B118/10/2019 16:00:35
1543 forum posts
83 photos
Posted by Vic on 17/10/2019 13:02:33:

I’ve always used 6.8 and have had no problems. Don’t think I’d want to go above 7.0 unless I was experiencing problems with thread quality.

Same here. Before I actually put me 'and in me pocket to actually buy a 6,8 drill, I used a 17/64", rather casually ground so as to cut a titchy bit oversize.

That worked, too. laugh

thaiguzzi18/10/2019 16:17:21
688 forum posts
131 photos

Done hundreds of M8 over the decades, all common materials, hand and machine tapped, Zeus book says 6.8mm, 6.8mm it is. Or imperial equivelant.

JasonB18/10/2019 20:24:00
17826 forum posts
1949 photos
1 articles

You don't say whether you are making the studs to go into your tee nuts or using off the shelf studding but if the later then it is worth measuring that before deciding if you can up the tapping drill size. Some studding is quite a bit under size so the percentage thread depth can drop quite a lot if you use a larger drill.

Some typical M8 studding.


And a M10 stud that looks like it was roll threaded on undersize stock


PS i'm mostly on the 6.8mm side of the fence

Edited By JasonB on 18/10/2019 20:30:21

Robin Graham18/10/2019 22:38:07
691 forum posts
161 photos

Harold Hall gives a brief, but useful (to me at least!) discussion of tapping drill sizes on his website here

He also provides tables giving thread engagement at various drill sizes for BA, Metric, Metric Fine and Model Engineer threads.

His site isn't easy to navigate (not complaining, it's free, thanks Harold if ever you read this!) but there's tons of good stuff there.


Mike Poole18/10/2019 23:23:54
2538 forum posts
60 photos

I have been lucky enough to be gifted quite a lot of tooling from toolmakers and jig fitters who have retired, the taper shank drills seem to have a lot of 10.2, 8.3, 6.8,5.1 drills. I assume that these were the preferred tapping sizes used by chaps with hundreds of years of experience between them, even before I noticed this these were the sizes that I use.


Martin Shaw 120/10/2019 10:35:41
116 forum posts
37 photos

Who knew threads could be so complex. I note from Harold Hall's tables that a 7.00mm drill gives a thread engagement of 65% which I think will be fine for this application and since I have that size I will use it. The thread is to receive a SS cap head screw which I will measure just in case they are seriously undersize, once again my thanks to all the contributors, I know a bit more now.



Michael Gilligan20/10/2019 12:00:28
15450 forum posts
668 photos
Posted by Martin Shaw 1 on 20/10/2019 10:35:41:

Who knew threads could be so complex. […]


Here’s a treat for you, Martin: **LINK**


David Noble20/10/2019 12:07:35
167 forum posts
8 photos
Posted by Martin Shaw 1 on 20/10/2019 10:35:41:

Who knew threads could be so complex.

I have spent a lot of time machining threads and before metrication, I had a book around 1 1/2" thick which had a different thread form on each page. A couple of the more interesting ones ( I just had a wry smile thinking what a non engineer would think about an 'interesting' thread form) were a screw thread for a bottle closure and a steam coupling thread for the hose between rail carriages.


Martin Shaw 131/10/2019 18:18:17
116 forum posts
37 photos

I thought a wee update might be in order. I am making a bigger version of the clamp shown front left, and the piece of steel which started out at 30mm by 20mm has been milled to suit and at the weekend I will access the power bandsaw at the railway to chop off 26mm wide bits allowing a finish to 25mm wide. I bought some blank T nuts, at £1 each it's just not worth making them, to which the drilling and tapping presented no problem at all, Sometimes I think the fear of cocking things up is a too powerful disincentive to progress, and even the power tapping was susprisingly undemanding. Anyway a good result.




Nigel McBurney 131/10/2019 21:15:42
694 forum posts
3 photos

30 years ago I was involved on a large hard drive LM25 cast aluminium base,round the clock 7 days a week and tapped thousands of M8 holes using 6.8 mm drills,with no problems.If I was tapping stainless steel I would probably use 6.9 or 7 mm,

Nigel Graham 201/11/2019 20:18:50
581 forum posts

Not sure if anyone else picked it up. but Martin tells us this is for making T-nuts.

A tip, if these are for the conventional solid-floored T-slot in a machine-table or accessory.

Do not make the thread run right though, or stake the bottom thread, so the stud cannot contact the bottom of the slot. Tightening the assembly with such contact puts an unfair strain on the T-slot flanges, and can break them if cast, or distort them if cut in rolled plate.

The flanges should be gripped in pure compression between the T-nut shoulders and whatever is being clamped down.

not done it yet01/11/2019 20:29:20
4482 forum posts
16 photos
Posted by Nigel Graham 2 on 01/11/2019 20:18:50:

The flanges should be gripped in pure compression between the T-nut shoulders and whatever is being clamped down.

The ideal situation - it is very difficult to damage the flanges that way, but unfortunately not always possible. Certainly, vises and suchlike are bolted this way, but on many occasions the holding studs are pulling on T-nuts which have no support above the table. In these circumstances it is simply a matter of using more support, with less force on each T-nut, if a lot of holding power is required.

Howard Lewis02/11/2019 13:00:15
3132 forum posts
2 photos

I nearly always use the "Nominal - Pitch" size for tapping Metric threads. The only time that I break taps is if I either bend them, or do not back out and clear the swarf. Bending is the quickest route to a broken Tap. Wherever possible, I use my Tapping aid (A drill chuck running in a redundant drill stand. Keeps the tap square to the job, and applies no bending moment, just torque. )

In the Mill, a spring centre supports the Tap while a small Tap Wrench applies the torque.

Tapping lubricant (Such as Trefolex or Rocol RTD ) is always applied.


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