M8 tapping drill

[Roderick JenkinsParticipant @roderickjenkins93242]

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

Rod

[clogsParticipant @clogs]

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,

[Anonymous]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

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

[AjohnwParticipant @ajohnw51620]

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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[Roderick JenkinsParticipant @roderickjenkins93242]

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

[Mark CParticipant @markc]

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

[Mark CParticipant @markc]

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

[Neil WyattModerator @neilwyatt]

Posted by clogs on 11/10/2016 10:05:20:

for all M8x1.25 I use a 6.5mm drill and always a HSS tap…having had trouble with HSS speed taps

You don't think the two could be related?

Neil

[Neil WyattModerator @neilwyatt]

Posted by Roderick Jenkins on 11/10/2016 11:28:56:

even down at 60% the shear strength of the thread is 2.5 times the tensile strength of the bolt core

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

[AjohnwParticipant @ajohnw51620]

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

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Mark C on 11/10/2016 14:03:11:

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?

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

[MWParticipant @mw27036]

Posted by Roderick Jenkins on 11/10/2016 16:11:37:

Posted by Mark C on 11/10/2016 14:03:11:

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? 

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

[Steve PaveyParticipant @stevepavey65865]

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.

[JasonBModerator @jasonb]

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

[Anonymous]

Posted by JasonB on 11/10/2016 16:44:54:

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

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

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by JasonB on 11/10/2016 16:44:54:

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%)

Good point! Nothings fallen apart yet

cheers,

Rod

[Mark CParticipant @markc]

Michael W, So (having took a few moments to work out who TC was) just because Tubal Cain said something then all the years of engineering wisdom and practice should be thrown out the window? Bye the way, you sound like you are confusing "black" fasteners for high tensile ("black" bolts are often machine cut and soft for fabrication use I think, but stand to be corrected? ) – bolts are marked with the tensile rating on the head so most "bog standard" bolts are 8.8 with cap heads being 10.9 or 12.9 (often considered aircraft grade). The use of stainless bolts/screws without a specific reason is a recipe for problems later depending on the material combinations…..

Rod, your argument just comes over as another good reason not to mess with iffy taps etc. If you have spent hours making something why would you take the risk for the sake of a couple of quid? If you need all the different threads available to hand then you must be into things in a big way. If not there is little point buying sets of taps with a load of sizes you will not use.

Steve, my argument also – anyone getting involved in this game (engineering) ought to consider the little yellow Zeus book the first thing they buy! It was the 7.1 drill that made me respond…

Mark

[Mark CParticipant @markc]

Andrew, what length thread engagement did you have? Was it definatly a tensile break or some other failure mode? I only ask as I have on occasion applied bear technique and ripped threads out of Al parts with 8.8 bolts – let alone 12.9! Was the thread clean or oiled?

Mark

[Mark CParticipant @markc]

John W, that link regarding nut size is very poor. It only describes BS nuts which are not what you will pick up down the shops (generally speaking). Most nuts in the UK are DIN or (ISO sometimes) a lot being grade c.

Thread engagement is a generalisation – different strokes for different folks… At 1.5 x you don't need to take much account of the materials – obviously, less requires an understanding of application and materials used etc.

Mark

[steve de24Participant @stevede2433577]

Well said Rod and good point Jason.

By coincidence I recently had to make a special M8x1.25 steel nut.

Tubal Cain recommends 7.1mm drill but I used 7mm because that's all I had. Used a £3 carbon tap from Tracy Tools (usual disclaimer) and had no problems at all. The thread the nut is fitted to has an OD of 7.98mm and it is a nice smooth fit.

For interest I've just screwed the nut onto a M8 bolt from a national hardware store. The bolt has an OD of 7.78mm and the fit is poor (wobbly) but is about the same, certainly not worse, than the nuts that came with the bolt (I made sure the same number of threads were engaged when I did the comparison).

Makes me wonder what the strength of these 8.8 grade nuts and bolts actually are compared to what they should be.

Not a scientific experiment but perhaps of interest.

Steve

[JasonBModerator @jasonb]

Andrew it must be that builders merchants studding that comes up soo undersize Just measured a bit of M8 and it comes out a 7.67mm , I've also had M12 where the crests were so flat due to it being indersize that it looked more like ACME form but the resin bonding method can be quite forgiving

 

I think a lot of it comes down to expeciance and the job in hand, for example the other day I was making and fitting some bearing caps fort ahit & miss engine. I drilled the tapping hole sin teh engine 4.00mm which is my usual size for 2BA but I also needed a simple jig to screw them to for some machining and I drille dthat 4.2mm as it only had to hold them the once and there was not a lot of load.

I think we have been down the bolt pull out route before, can't remember what the tapping size was for this M8 thread but the 7.8mm overall dia bolt came off better than the steel plate.

EDIT, I just found the old post, it was 10mm thick mild steel drilled 7.1mm rather than my usual 6.8mm.

Another interesting snippet from that post for those using TC tapping sizes is that the die should cut a full thread not an undersize one!! Which suggests his % engagements only apply when the male part is on size so if your male part is small alter your hole to suit.

 

Edited By JasonB on 11/10/2016 18:34:32

[SillyOldDufferModerator @sillyoldduffer]

Posted by Mark C on 11/10/2016 14:03:11:.

…

In industry, we use the standard published tapping drill size…

Mark

Hi Mark,

What is the authoritative standard for this? The web references and Machinery's Handbook don't all agree with your Zeus. I found 6.7, 6.75, 6.8, 6.9, 7, 7.1, 7.4 and J all suggested for M8. The values are recommendations, not absolutes. (7.4 is a red-herring, the value is for cold-form tapping)

The debate may be at cross purposes. For amateur purposes I almost always use the values Rod lists in his table. It's because the tap works a good deal less hard when cutting for lower engagements. Low stress tapping when strength or fit doesn't matter means I'm less likely to break the tap and they last longer. Industry works to different parameters, at least I hope they do!

Cheers,

Dave

[Iain DownsParticipant @iaindowns78295]

Wow! I kind of wanted to see if a 7mm drill would do and I appear to have started World War 3!!!!!

To answer one of the questions posed to me, it's a standard / coarse metric thread so 1.5mm I guess.

My tapping drill information came from, Harold Hall in this instance rather than TC, though I have both books.

I will happily drill to 7mm and tap next time I get out to the shed, which is likely to be less soon than I want ….

Iain

[DanielParticipant @daniel]

Hello,

Okay – you all win

Have been to my supplier today and invested in a load of specific tap sized drill bits.

Having said that, I opted to follow the formula ((dia – pitch) + 0.1), to give a degree of clearance.

If we simply subtract pitch from diameter we will have 100% engagement, which, apparently, is to be avoided.

Unless I've missed something.

Mind you, now I've entered the murky world of increments smaller than a mm.

Best,

Daniel

[Author]

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