Tapping drill size

I have a need to tap some M8 holes. From published information it seems the tapping drill can be anything from 6.8mm to 7.5mm. I appreciate the small drill will produce a tight thread and the large one a loose fit, so is approximately the mid point between the sizes the optimum, given that the application is some T nuts. I would appreciate your thoughts.

Regards

Martin

I personally use a letter "J" drill for general use M8 threads which is 7.05mm. Zeus charts tell you to use a 6.8mm

Phil

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.

Martin

The tapping drill size for metric fixings can be calculated simply by deducting the pitch from the diameter.

The difference in tapping drill sizes you have found may be due to different pitches, an M8x1.25 requires a tapping drill of 6.75mm where an M8x0.75 needs 7.25mm.

Emgee

Given the application is T-nuts which can be expected to be stressed, best to go for a small drill leaving the maximum amount of metal for a strong tight fitting thread - 6.8mm.

When strength doesn't particularly matter, and often it doesn't, I generally use a larger than recommended tapping drill because bigger holes reduce the work the tap does. Taps in bigger holes last longer and are much less likely to break, especially in small sizes. But a loose fit also means low strength: don't deliberately weaken load bearing fastenings.

Dave

+1... my method of getting the appropriate drill size, I generally aim for 60% thread form.

George.

I normally use the diameter minus the pitch for tapping drill sizes in metric threads i.e. 8mm - 1.25mm = 6.75 or round up to the nearest preferred size 6.8.

In ordinary steel I'd drill 7 to 7.1mm. In brass or aluminium 6.9 to 7mm and in stainless steel 7.3mm. In percentage terms I aim for 60% to 70% in most materials, may be 75% in softer materials and 50% in tough materials. The foregoing applies to coarse threads, for fine pitch i'll go down a tad in size.

The drill size has little or no effect on the fit of the thread. That's largely determined by the tolerances of the tap and mating screw.

While not universal the bolt usually fails in tension long before the internal thread fails in shear. Many years ago I did some experiments tapping 6082 and using high tensile (12.9) bolts. On the first test the bolt broke before the thread stripped and that hole was drilled for 50% thread engagement.

Andrew

6.8 mm. Allways use diameter minus pitch. If drill cuts big you have room to manoeuvre, if it’s over tight you can open up. Smaller sizes often give trouble when being clever and drilling bigger to avoid tap breakage. I have a few heli- coil sets for just that reason.

Must remember not to be a smarty pants.

I will admit to breaking an 8BA tap in gauge plate recently. But that was because I was clumsy, nowt to do with the tapping drill size. The tap broke well above the hole, and the broken piece was loose and unscrewed with fingers. The hole was the last of 8 and I got careless and applied a bending force, equals a snapped tap.

Andrew

Many thanks chaps, the concensus appears to be 6.8mm so far be it from me to disagree. Tracy tools can supply reasonably inexpensively so I'll go there.

Martin

Umm, no. That will give a 50% thread depth (actually 47% for a 60° thread), but one might not want that amount of engagement. It'd make a bloody poor BSP or NPT thread seal...

The question was asked regarding M8 coarse, ie standard thread, and my reply was regarding metric threads. Imperial particularly BSP and American pipe threads with their taper options are a different kettle of threads entirely.

Martin,

For my bread & butter metric tapping in mild steel, I use an inexpensive set of taps (purchased from Lidls) and they seem to work very well. They also came with a matching set of "tapping" drills & the one provided for the 8mm tap is 6.8mm. However, I found these tapping sizes to be hard work in mild steel - so I don't use them.

Instead I use the 65% flank engagement recommended by Tubal Cain in his ME Handbook (my copy is the 1993, 8th version - page 63) which for an 8mm (1.25mm pitch) thread is given as 7.1mm. These sizes work very well and makes tapping a far more relaxed process - and I've never been concerned about the strength of anything I've tapped using this thread engagement.

To save me reaching for the book every time - I've stuck a reminder from Tubals book on the box. Just my experience...

Regards,

IanT

Oodles of good advice in Tubal Cain's Model Engineer's Handbook. I should read it more often!

In Section 4, he explains that most tap drill tables are designed for tapping machines fitted with slipping clutches and automatic reversers. He says: 'The tables are designed to accept the higher thread engagements found in production workshops are not suitable for hand tapping.'

Therefore his tables of tap drill sizes in Model Engineer's Handbook have been designed to give at least 65% engagement in small sizes (below 3/8", and up to 75% for larger sizes. Thus he recommends 7.1mm for M8 rather than the usual 6.8mm.

There's also a table of recommended engagements by material. This ranges from 50% for Stainless Steel (the nasty tough type, not the free-cutting variety) up to 70% in Brass and 75% in Cast Aluminium. He also points out higher engagements are needed when tapping sheet metal - at least 85%.

Dave

+1 on Tubal Cain's recommendations. (7.1mm in this instance) He says 65% thread engagement gives no significant reduction in strength, yet the tap requires only one third of the torque of an 85% thread, Less torque = less work and less chance of broken tap. More critical on smaller sizes of course. 8mm is stout enough not to worry overly much.

The second chart lower down this web page gives some very useful data including thread depth percentages.

http://theflyingxs.blogspot.com/2012/02/metric-drill-tap-chart.html

I stand by my "general" M8 (1.25) tapping size of 7.05mm

I cannot remember the last time I broke a tap or stripped a thread, but each to their own, you will soon discover when you get down to much smaller sizes that the usual tapping size charts are no good because the thread engagement percentage is too high.

For that reason I spent a lot of time researching and created my own tapping drill chart many years ago it has served me very well and I use nothing else now.

Phil

Further interesting thoughts from posters today, for which I thank you. Since I've yet to order bits it seems 7.1mm might be a better choice, and whilst I have a full 0.1 increment set up to 6mm I'm reluctant to shell out for the 6.1 upwards set as most of it would remain in pristine condition.

Regards

Martin

It can be a good idea to keep a tapping drill in the same place as you store your taps, and only use it for that purpose, and keep your drill sets for general use.

That way you dont even need to look it up on your tapping size chart every time.

Phil

Martin,

I'm fortunate enough to have acquired various 'sets' of drill bits over the years, including the 1mm-6mm & 6.1-10mm (in 0.1mm increments). I keep these sets for "Best" and for common holes sizes have cheapie sets that I (ab)use for most tasks. I would agree that most of the drill bits in these full sets may never be used in anger - but it's nice to have them when needed - and (certainly for me at least) they will last a lifetime.

However, before you go out and purchase any new drill bits - have a look at your Imperial bits (if you have any). Most folk have accumulated a few and perhaps never think of using them in "metric" mode.

For instance, if you happen to have a 9/32" bit (not so common I will admit) then you might find it will also drill a metric sized hole of 7.14(375) mm - which I think will probably be near enough for your needs!

Regards,

IanT