Colin,

 

Jason is quite right that an unspecified metric thread can be assumed to be a coarse series one.

 

However, I strongly disagree with his suggestion to use a 6.8 mm drill. This will give you over 90% thread engagement and you will probably break the tap (or your wrist!) if you try to do it by hand. I just tapped 3 M8x1.25 threads in a 10 mm thick MS plate today, and I used a 7.1 mm tapping drill, which was plenty hard enough. This gives about 65% thread engagement, and the thread flanks will still be about 2.5 times stronger than the core of the screw.

 

Do yourself a big, big favour and buy a copy of “Drills, Taps and Dies” by Tubal Cain, No 12 in the Workshop Practice series. It’s only about six quid and it will tell you almost anything you are likely to want to know on the subject. The tapping drill tables at the back are worth it alone.

 

For some strange reason, almost all other standard reference works give ludicrously small tapping size holes for metric coarse threads; these give 85-100% thread engagement and are probably aimed at industrial users, with sharp taps and torque-controlled machines. Machinery’s Handbook even admits that aiming for these levels of engagement is unnecessary and risks breaking the tap, and that 60-70% is perfectly good, and then goes on to reproduce the same old table which does the exact opposite.

 

One of the reasons for this is that ISO metric taps have a diameter greater than the nominal figure (by about 11% of the thread pitch). Most other thread forms do not have this feature, and the commonly quoted tapping sizes are not quite so ridiculously small, though they are mostly still tighter than they need to be.

 

David

Guess I’ve got strong wrists as I’ve been using dia-pitch for 30odd
years ever since that was what I was tought and seldom break a tap

Ditto. Well, except for the strong wrists bit (Carpal Tunnel decompressions both sides !)

I cannot recall seeing any instances of “oversize” tapping holes being recommended on manufacturing drawings – the “Thread diameter minus pitch, rounded to nearest standard size” rule seems pretty universal.

 

The only good reason for further proliferation of thread standards was the fact that it was easier to remember / calculate tapping sizes in the new metric system than with the BSF, Whit , BA , UNC, ANF etc. but we did have letter drills and number drills in those days!

 

Bob

Well folks, I suggest you actually read the references I gave and then try it yourselves. I have to say “Seldom break a tap” doesn’t really cut it for me. Since using the tables I referred to (since about 20 years ago) I have broken precisely one tap – an M3 tap, doing it by hand, and almost certainly because I bent it. Broke plenty before when using the figures in the Zeus booklet. As for 6.4 mm, that is actually smaller than the core of an M8 male thread – insanity.

 

And no Clive, the bolt definitely does not wobble at even 50% thread engagement. If you look at the form of the nut/bolt threads you will see that is no more likely than if you use the quite unnecessary 90% thread engagement.

 

David

The standards and specs for threads were laid down for serious use in industrial and commercial applications sometimes with life threatening implications.

 

The depth of thread engagement set by the tapping drill size impacts on the cross section of thread available for shear in the axial plane through the bolt, in other words, when the bolt is in tension. It can sometimes be compensated by extra thread length, but non of this affects the fit of the two mating threads. Hence DL’s comments on wobble.

 

In terms of holding strength thread engagement depth has little influence, this being more to do with the root diameter of the bolt, either in shear for holding or tension for an axial load (where the thread depth is also implicated)

 

As modellers we very seldom test any of these threads to even a fraction of their true potential strength and can get away with exaggerated amounts of “disengagement” that would be cause for rejection in some industries. It is certainly lighter on the pocket to “drill big and cut easy”

Bob

I’m afraid I’m 100% behind David on this. Since reading the Tubal Cain book, my record of tap breakage has virtually ceased. The Zeuss tables quote drill sizes for tapping on industrial rigid machinery, very rare in amateur model engineering circles and not hand tapping. As he explains, when you tap a thread, part of thread forming is done by extrusion, particuarly in ductile materials. This is often the cause of tap breakage as it causes the user to over torque the tap as the drilled hole closes up on the tap. It’s easily tested by drilling a hole, tapping it and then trying to poke the same drill through the tapped hole. The core has usually closed up slightly. The graphs displayed in the book show the correlation between reducing the thread engagement (drilling larger) against tap breakage against lost strength in the thread. 65% engagement still gives an internal thread stronger than the core strength of the screw but reduces dramatically the torque being applied to the tap and the saving in tap breakage. That not withstanding, it’s a fascinating read about the principles of what you’re trying to do and I’d argue the best of the Workshop Series books.

Can I ask those who use 65% or whatever how that is calculated? is it base on the male thread actually being the nominal size?

 

Have you measured commercial fixings lately, I’ve just measured half a dozen different M8 fixings and the average major dia is 7.8mm so does start to shake about a bit in a shallow 7.1mm hole And as for those far eastern hold down studs I could see them pluuling out.

 

Andrew you must have different books to me as mine gibe M8 core as 6.376 so clive may just squeeze a tap into a 6.4mm hole, getting it out is another matter

 

I’ll stick to using what I said as its close to the BSI recommended of 6.75 especially if I’m putting up structural steelwork, I’m sure the Structural engineer and BCO would not be happy if I deviated. Funny enough teh BSI alternative is a tighter hole at 6.6mm not looser.

 

J

 

Got to admit that I had no idea that zapping and tapping an 8mm hole could generate so much useful information

Well, I’m reluctant to say anything, after all the technical input. But here goes, for metric taps have always used the dia. minus pitch figure, then use my next biggest drill available. All depends on the material being tapped. I have been known to “wiggle” the drill in the hole to get a tap in ( bigger than usual for ME). Of course in a perfect design all bolts will be in shear only and only need to be stopped from falling out, Depth of dowels will also depend on materials used.