M8 tapping drill

I'm making a clamp for some milling and want to put am M8 bolt in.

My data books say that the tapping drill is 7.1mm. Needless to say I don't possess such a beast. I have a 7 and a 7.5.

I also have a particularly poor set of carbon taps (slowly being replaced) and I'm a bit concerned about cutting a 1 mm deep thread with them. On the other hand 0.5mm seems a bit shallow...

What should I do?

Iain

I'd use 7.1mm and buy a decent tap. If I didn't have that drill size I'd use 7mm and buy a decent tap.

Andrew

Every home workshop should have a copy of Drills Taps and Dies by Tubal Cain. This has an invaluable table of tapping sizes for all sorts of threads. TC recommends not using high levels of engagement unless thread strength is absolutely critical, for easier tapping and fewer breakages.

Having said that the lowest value of engagement he goes to is 65% for M8 for which the recommended drill size is 7.1 mm. For 70% engagement he gives 7 mm. If the linear relationship continues (it won't because of crest rounding) then 7.5mm would give ~40% which seems much too shallow. As you are making clamps maybe the best approach is to use 7 mm, take it easy and use lots of cutting fluid. Make sure the tap enters the hole straight, turn only a quarter of a turn at a time, reversing the tap so you feel the swarf clear, and take it easy and hope!

I really recommend getting a good set of proper HSS taps and drills - I bought a full metric set of high quality spiral flute taps with corresponding split point drills from Greenwood Tools at the last exhibition I attended and they are excellent.

Don't bother with the cheap carbon steel taps and dies as sold by DIY retailers, they are generally very poorly made and will give you nothing but frustration.

A 7.1mm drill will give you 65% engagement, which is what I would use out of choice. However, 7.0mm is still only 73% engagement which is unlikely to give you too much trouble when tapping. Use plenty of tapping compound (or oil) and clear the thread of swarf occasionally by withdrawing the tap and you should be OK.

HTH,

Rod

Edit:  Cross post with John Haine - sorry to repeat his sound advice

Edited By Roderick Jenkins on 10/10/2016 20:59:01

Thanks for all the advice.

I am gradually replacing the nasty tap set with rather better ones, but can't afford a whole set at once.

I will drill to 7 mm and tap carefully.

Iain

.

Don't.!!! (IMHO)

Buy good ones as singles when you need them.

In a set there will be some you use frequently and some you will never use at all.

Nick

Is this M8 x 1 or standard M8 (8 x 1.25)?

M8 x1.25 needs 6.75 so 6.7 or 6.8 for normal people. Easy way with 60 degree metric is subtract pitch off diameter so 8-1.25=6.75

Mark

How about a 9/32'' drill?

Hi Iain,

For all practical purposes, I would drill 6.5 or 7.0mm for tapping M8.

Unless the project demands any higher precision.

The sound advice here, is to tap carefully and rewind every quarter turn, to clear the chips,

you can always feel if the tap is straining unnaturally.

All the best,

Daniel

Edited By Daniel on 10/10/2016 22:27:39

If you only need to buy one tap, buy a spiral flute tap, it will do through hole and blind holes. Drill Service have a HSS M8 SF tap at, I think, £5.43p, plus VAT of course.

If you drill 7.1mm, as recommended by Tubal Cain, and use a SF tap you don't need to keep backing off, let alone faffing about every quarter turn. A SF tap is intended for machine tapping, but works equally well for hand tapping.

Andrew

The difference betwen 7.1 and 7mm is 0.1mm or 4 thou in Imperial ( yeah yeah .003937, I know) so one is looking at 2 thou a side, is it that critical?

Daniel, I hope your 6.5 mm drill is badly sharpened and drilling oversized - 6.5mm is pretty much the core diameter of a M8 coarse (1.25mm pitch) and would (in theory) give 100% engagement

Makes a valid point though: We might argue about what the best drill in theory is to make a hole to give a certain % of thread engagement but with our home workshop bits and drills it is almost certain that the hole will end up bigger than we intend anyway.

Rod

Andrew is spot on regarding spiral flute machine taps - that pretty much all I use.

I still lack understanding why you want a bigger hole than standard 6.8 tapping drill unless it is metric fine or special material/application?

Mark

I agree Mark,My small set of taps and drills to match them for 8mm is 6.8 tapping drill.I have just finished tapping 8mm with no trouble .

Frank

+1 on Roderick and others' comments: your 7mm drill is going to drill oversize so will be well close to a 7.1mm hole. Drill 7mm and proceed with caution as always with carbon steel "El-Snapo" taps.

Hi Iain,

As others have said, good taps make the job much easier. I use 6.9mm or 7mm tapping drills for M8, but I use serial taps and don't have any problems even in steel.

Thor

Yes Rod,

I would be mightily surprised if one of my 6.5 mm drills came out anywhere near it's actual size.

Enter my el cheapo drill press into the equation and I will probably have an oval hole, anything between 6.7 & 7 mm. Give or take a tad.

After that, as Thor suggests, I usually go the serial tapping route, which is also quite forgiving.

It all depends, as usual, on what degree of precision is actually required for a given situation.

M8 is a rather meaty lump, after all

All the best,

Daniel

After I wrote my last email, I popped out to my shed and checked my-loose-taps-and-dies drawer. It turns out that I'd speculatively bought an HSS M8 tap at the Doncaster show - from Tracey Tools I think.

Whilst not the serial taps as mentioned, I expect this will do nicely.

The tap / die sources are very helpful too and I'm thinking that some of these might make it onto my Christmas list...

Iain