M8 tapping drill

See what size hole your drill actually cuts. You could grind one lip a little longer to get a larger hole?
Depending how critical the work is I use a 7mm for M8.

A well equipped workshop will have a 9/32" drill which is probably a better size to use than 7.1mm

Neil

So many answers, and yet the OP still hasn't told us whether his M8 is coarse, fine or some other pitch, and without that information any speculation on the correct tapping drill will remain just that.

If he got a 8 mm bolt straight off the shelf at the hard ware store, it would be Metric Coarse, even when I buy at a specialist bolt, and fastener Warehouse I would have to specifie if I anted any thing other than M coarse.

Ian S C

Ian, it' s the same over here in the UK. It has become customary that "Metric" means metric coarse unless specified otherwise.

Rod

Hi all,

it's all very well using the correct size drill or near enough BUT it's only as good as the guy who reground it....

for all M8x1.25 I use a 6.5mm drill and always a HSS tap...having had trouble with HSS speed taps from suppliers like Axminster, they probably source them from ASIA..... I now only use UK, German or USA HSS tap's and dies....

they do cost more but I only buy them for the commonly used size's.........

good luck Clogs,

Just out of idle curiosity I drilled a 7.1mm hole in some 1/4" low carbon steel. The hole was drilled straight off, no centre punch marking or pilot drill. I measured the resulting hole as 7.14mm. I then tapped the hole M8 with a HSS taper hand tap. It went straight though in one go, no need to back off. Tapping was done dry, just as well as I did the tapping in the dining room.

Drill was a Dormer 4-facet and the tap was by OSG.

Andrew

People can make their own minds up about how much engagement they need in a screw thread. The last tap I broke was a good quality branded HSS M6 that snapped off when tapping ally having drilled a hole according to a standard table. I was doing quite a few so had probably got a bit blase. Reference to Tubal Cain's "Drills, Taps and Dies" shows that there is very little extra strength to be gained from a high percentage of engagement, even down at 60% the shear strength of the thread is 2.5 times the tensile strength of the bolt core which means that the bolt will break long before the thread strips. I've therefore put my own thread tables together for the threads I use with about 65% engagement to the nearest 0.1 mm. This, of course, is for fastenings. Adjustment screws, where wobble free good engagement is essential, are a different matter. So far, it works for me.

Rod

I think that tap dia minus the pitch and plus a bit more is the safest option. When it gets up to M8 though they are pretty strong taps so the usual minus pitch should be ok but there is usually some variation in actual tap diameter these days. Or I can't buy taps as good as the ones I used at work which were smack on size.

The standards I can find are odd in some ways. May be root rads etc may not be.

ISO Internal

ISO External

Rad's according to DIN

Cutting tools usually have rads - Whitworth wasn't mad. There may be standards for rolled threads somewhere. Hard to see them having any sharp corners.

John

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This is the table I've put together based (mostly) on 60-70% engagement using my set of 0.1mm spaced drills

For clearance, I take a view depending on the application, based on the OD which, apart from BA, is the nominal OD not the actual thread OD. E&OE

Rod

I really cant see why everyone is making tables or working out engagement for standard tapping drills - especially for metric.... why are you all using custom drill sizes?

As stated earlier, metric M8 is M8x1.25 (I design/draw for a living and we never put the pitch on unless it is non standard - m8x1.0 etc). This is what prompted me to ask what pitch was being used.

In industry, we use the standard published tapping drill size unless there is a special reason otherwise and use a minimum thread engagement of 1.5 times the thread diameter - 12mm in the case of M8.

Mark

John, you can buy taps in whatever precision you want and then they will be the size you expect.

The only time I can imagine problems with taps will be cheap stuff. Given the price of decent taps in standard 6H tolerance, it is not worth having cheap crap... they just break or go blunt or tap over size or.....whatever - for the sake of a couple of quid. Decent taps will tap many times the distance of crud anyway and in any material you are likely to have in a basic workshop. As Andrew mentioned, bite the bullet and get some quality spiral point taps - no need to have taper, 2nd and plug then - one tap does all.... and no problem sticking them in a drill for speed.

Mark

Meant to type spiral flute but spiral point are good also.....

Edited By Mark C on 11/10/2016 14:12:16

You don't think the two could be related?

Neil

As soon as you allow for minor deformation under tension spreading the load over two or three turns of the thread there is more strength in the thread than the core. That's why you rarely strip a screw but normally either snap one or pull the thread out of the hole (when its in a softer material like CI or Ally).

Neil

Strange limit on thread engagement to me Mark. I grew up in a TDO and the limit was min 1 dia and if over 1.5 they were trimmed down.

The reason for this is pitch tolerance. No gain past this max engagement limit and that is on precision screws too.

This is essentially how nut thickness's are derived. For example see

**LINK**

I have seen thicker nuts but only in unusual circumstance - One in particular springs to mind. Big end bolts on a car I used to own. Nuts and bolts had to be replaced once undone. No stock so had to use them and one sheared bending a massive crankshaft and breaking the block. They can be seen in other places too but much over 1 dia is rather unusual.

On diameters when ever I measure a tap I have bought it's over size probably within H6 tolerance. At work they were all ground thread and on size. My impression is that they are generally now made mid tolerance like many other precision hole makers.

John

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Edited By Ajohnw on 11/10/2016 15:02:20

Because many of us here on the MODEL-engineer forum are mere amateurs, have dodgy technique, an assortment of taps of dubious provenance and age, are inclined to use the small rather fragile sizes and for whom, if a tap breaks in a job we have already spent 50 hours on, it is a disaster. We need all the help we can get to avoid tap breakage.

Cheers,

Rod

I quite agree, TC himself even encouraged people to try what worked for them best rather than treating graphs as strict rules, the only governing idea being how good the thread engagement will be.

The strength of your screw will only be as shear-proof as what it's made of, hence why i tend to use the black high tensile varieties rather than stainless, as theyve been designed for this purpose.  

If you want to use a graph for custom sizes you've found to be good with your tools, why shouldn't you do that?

Michael W

Edited By Michael Walters on 11/10/2016 16:28:44

My question re the actual pitch the OP was using was based on him stating that his data book recommended a tapping drill of 7.1mm. If it is in fact a normal metric coarse thread, this contradicts my Zeus tables, and the general rule of thumb of subtracting the pitch from the nominal dia, which points to a tapping drill 6.8mm dia (or as near as dammit). To me 7.1mm points to a metric fine thread, m8x1.

Can I ask those who like to work out sizes to give 65 or 60% engagement if they measure their screws & bolts first?

It quite common for mass produced fixings to be undersize particularly if they are cheap and cearful ones so lets say the far eastern bolt shop also made their bolts with 60% thread form would that then only give you a 36% engagment? ( 60% x 60%)

J

Not being a model engineer I don't buy cheap and cheerful.

I've just measured a couple of commercial M6 screws. Rolled thread measures 5.9mm, cut thread measures 5.85mm. I've never stripped a female thread, but I have broken screws by over-tightening; both deliberately and accidentally. As part of a test setup I broke a high tensile bolt (grade 12.9) in 6082 aluminium drilled and tapped for 50% engagement.

Andrew