M8 tapping drill

[DanielParticipant @daniel]

Sorry,

For M8 ; drill 6.8 !

[Roy MParticipant @roym]

Although breaking a tap is often a disaster, it should not sound the death knell for the workpiece. You can always drill out the broken tap.

[Mark CParticipant @markc]

Iain, Don't be sorry! I don't know which data books you have but as has been said a number of times (by a few of us) the 7.1 is correct for M8x1.0 pitch which is why the question came up. I think you meant M8 x 1.25 by the way? That is what you would pick up as a standard M8 bolt/nut (everything else needs to be specifically asked for as a rule). You drill what you like, feel free to drill 10mm and push the bolt in with epoxy putty if you wish – that is your choice, I just gave the accepted size for the standard thread.

Dave, the size comes from the DIN standard (you can see it in the pictures John posted – it is the D1 dimension and is linked to the other dimensions accordingly as the thread is 60 degree (which is why the diameter less pitch works). Strictly, 6.75 is the correct size in theory but we generally have 6.8 drills (what's 50 microns between friends! ).

Daniel, I hope you enjoy your new drills!

Mark

[DanielParticipant @daniel]

Mark,

Can't wait.

Just looking for a thread to do, now . . .

[Anonymous]

Ah well, I must have used a bit more BF and BI, as I broke the bolt when I tried a similar experiment to JasonB:

I don't remember the internal thread stripping with SHCS on the left. I think it was more a case of not being able to apply enough force for something to break.

I'll get me coat now; I can't be an engineer as I have a Zeus booklet somewhere but haven't referred to it in years.

Andrew

[Mark CParticipant @markc]

just for information (and 'cos I am a helpfull person) here are the thread callouts from my cad package (solidworks) working to industry standards.

You might notice they have the drill size listed first (for convenience of machinist) with the depth or otherwise. The second line gives the thread type and pitch followed by the thread class (note for John W, 6H not H6 which is a hole tolerance not a thread class) finally having the depth of the thread listed.

Again, let me stress that I am not trying to tell anyone what to do. I am just providing the correct information as I understand it. It just happens that many here are at variance for their own reasons but what ever floats your boat is good for you.

Mark

[Mark CParticipant @markc]

Hey Andrew, that looks like a stainless steel bolt? That would probably be cheating perhaps

Mark

PS. Never mind having the book, you could not possibly be an engineer – no self respecting engineer would get in an airplane given the lack of safety factor used when they build them infernal flying machines…..

Edited By Mark C on 11/10/2016 19:40:30

[DanielParticipant @daniel]

An after note for Rod J,

Instead of going to work this morning, which I should have done, I went to the workshop and test drilled a piece of steel plate. The drill bit claimed to be a 6.5mm, from what I maintain to be a reputable supplier.

Lo . . . The hole was indeed oval (no joke), varying from 6.4 to 6.49 mm !!

BUT . . . . It wasn't oversize !

Hence my trip down to the supplier to source some more tap specific drills.

Now I'm viewing my digital vernier with some suspicion

Best,

Daniel

[AjohnwParticipant @ajohnw51620]

What I bear in mind when I drill to tap is what I was told to do during training. Drill and tap a hole at core size. Drill and tap according to some standards – for BSF at the time. Then increase a touch to make sure that the valley rads aren't being cut by the tap. The difference in force needed is huge at the core size to the next step up and the decrease down from that when the valley rad isn't being cut at all is dramatic as well. The rads don't contribute to the strength of the thread at all so there is no point in cutting them on either part as they are clear.

Metric taps in theory don't need to have the rads but will in practice. The thing to do is make sure that these aren't being cut what ever form they have and to also account for the actual diameter of the tap.

People don't seem to worry about the effort needed but should bear in mind that if the tap is screwing itself in it will back cut as it goes in so the effort needed from us to provide the force is an important aspect.

To be honest I sometimes wonder how we finished up with thread forms that may have no crest or valley rads. There used to be an argument that it was easier to cut them on a lathe but it doesn't really old water.

John

–

[Iain DownsParticipant @iaindowns78295]

Alright, you can all stop now!

I've snuck out to the shed drilled my holes and tapped my thread.

7mm works absolutely fine for a standard M8 thread.

As I mentioned earlier this was for a clamp which is not quite finished. The piece is intended to be a 55 degree prism as part of my ongoing (never ending?) attempts to tune up my micro mill.

First part was to slice off the top of a cast iron bar with a slitting saw.

I seem to have learned a lot about using a slitting saw with this task – the hard way. For a whole variety of reasons the cut is not as precise as I would like so my next step is to face it off. But how to hold it?

Like this! (obviously with something to protect the surface under the bolts).

Iain

[Roderick JenkinsParticipant @roderickjenkins93242]

Rod

[steve de24Participant @stevede2433577]

Iain, good to hear that you were successful producing a usable thread.

That is a nice tilting table you have in the pics. How big is it and where did you get it from? I'm looking to get something similar for myself.

Steve

[AjohnwParticipant @ajohnw51620]

M8 is 1.25mm pitch so drill 6.75mm according to many tables rules no make it 6.8. If the tap is over 8mm which it probably will be add that on and round up.

Zeus reckon 6.8mm but the certain standard I mentioned for BSF etc was Zeus. So on that basis I would drill 6.9 providing the tap was 8mm.

More tricky at small sizes and sooner with metric fine. M2 drill size is 1.6mm from the same source so get out your number drills and use a No52., 1.62mm dia. If the tap is significantly over size up accordingly. One of the big problems with sizes like this is keeping the tap axial to the hole 'cause it will probably break if it isn't.

That's what I do anyway. Use Zeus and up the sizes a touch based on the size and my judgement. I also check the size of drills at smaller sizes in particular as they are often a touch under.

John

–

Edited By Ajohnw on 11/10/2016 23:16:55

[Iain DownsParticipant @iaindowns78295]

Steve – this is a 'tilting angleplate' from RDG tools, see here. Current price is 53 quid inc VAT and postage.

I ignore the angle markings on it and set it using a cheap electronic angle gauge from Lidl.

My only complaint is that it is subject to rust even when one tries to keep it oiled up.

Iain

[MWParticipant @mw27036]

Posted by SillyOldDuffer on 11/10/2016 18:41:02:

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

You're right, Dave, because there is no authoritative standard on tapping drill sizes, variation could be infinite within the engagement range of the thread, it isn't the end of the world if you use a 3.4 drill to tap M4 rather than 3.3.

In some circumstances i've had threads in plastics close up due to part shrinkage, even the bog standard rolled bolts are tight, therefore drilling oversize the zeus value could be recommended to compensate for this!

Michael W

[HopperParticipant @hopper]

I often don' use tapping drill tables at all. Just poke the end of the tap in a hole in my drill gauge, or in the drill rack itself, until I find thehole that allows the taper tap go in up to about one-quarter thread depth, by eye. This means that size of drill will give you about 75 per cent thread depth. It works for me.

[LambtonParticipant @lambton]

Iain,

You have received a lot of advise that may be a bit confusing. The truth is there is no absolutely correct tapping size for any particular thread. This is explained very well by Tubal Cain in excellent book Drills, Taps and Dies ( No 12 in the Workshop Practice series). Also have a look at this site which shows the range of tapping sizes for all normal screw threads in an informative and interesting way: **LINK**

[Ian S CParticipant @iansc]

You could do the same with your tilting table as I do with my surface plate, I keep it in the cupboard with the hot water cylinder in the house.

Ian S C

[Mark CParticipant @markc]

Micahel W and Iain, there is an authoritative guide to metric tapping drills and it is the DIN standard for the thread itself and none of it is confusing. The only time it gets confusing is when you start getting peoples take on the "best" way which is purely their own method that suits them – it may or may not suit your application.

You can all argue 'till the cows come home about your own interpretations but the fact remains that if you want a thread that meets DIN then you will use the correct drill to suit according to the standard sizes. If you want to use bigger or smaller then that is your choice but the fact remains the same.

Did Mr Cain sit on the standards committee? I don't know but I doubt it, please correct me if I am wrong. His books may be very good for model engineering but this was a discussion about the drill to use for an M8 thread….. the answer – emphatic answer that is – 6.8mm.

Mark

[steve de24Participant @stevede2433577]

Iain, thanks for the tilting table info.

Lambton, thanks for the link on tapping drill size.

Steve

[MWParticipant @mw27036]

Posted by Mark C on 12/10/2016 11:21:14:

Micahel W and Iain,

Did Mr Cain sit on the standards committee? I don't know but I doubt it, please correct me if I am wrong. His books may be very good for model engineering but this was a discussion about the drill to use for an M8 thread….. the answer – emphatic answer that is – 6.8mm.

Mark

 

No mr C did not sit on a standard committee but he did lend his ear to a few companies regarding production machinery. I'm not a production worker in my own shop so effectively i'll tap to whatever i find works best, not to DIN.

I just happen to like TC's work and read his books so i follow his advice, it's not the only advice though, thats the point i try to make and different charts give different measurements,

So i suppose if you want to be a captain of industry you can follow the DIN standards authority and tap to that.

When i did set the machines at work, there is absolutely no reference to any chart, we literally just use what works best for the thread in that material and stick to it. 

Michael W

Edited By Michael Walters on 12/10/2016 11:44:49

[Anonymous]

Posted by Mark C on 12/10/2016 11:21:14:

His books may be very good for model engineering but this was a discussion about the drill to use for an M8 thread….. the answer – emphatic answer that is – 6.8mm.

Not if you're using a roll form tap………….

Andrew

[Roderick JenkinsParticipant @roderickjenkins93242]

Mark,

Tubal Cain (real name Tom Walshaw) was a professional engineer who designed large diesel engines and then went into academia whilst also being a maker of models and a writer on workshop techniques for model engineers. He did actually convene a discussion group amongst the model engineering fraternity to suggest standards for ISO metric fastenings to replace the BA threads widely used by model engineers – this was accepted as a codicil to the Bristish Standard. I respectfully suggest that he knew what he was talking about.

TC's point was that he and fellow MODEL Engineers have a particular problem in breaking taps and leaving the broken bit in the hole. He suggested that using a larger than recommended hole for tapping greatly reduced the risk of tap breakage without materially affecting the strength of the fixture. I concede that 8mm is at the top end of model engineering size, but having broken an M6 tap which siezed in aluminium I contend that it can still be an issue.

DIN standards may be fine for German car manufacturers but they are not necessarily applicable in every case. Proper design would employ the use of stress analysis on the joint to determine the best type of fixing balanced with ease of manufacture. I don't know, but my guess is that in the aerospace industry they don't necessarily just call out the nearest equivalent from the DIN CAD package library.

I can only agree that if you want to meet the DIN standard then use the DIN tables but I do not agree that the DIN standard is the "proper" way and that any other is a fudge. Horses for Courses.

I'm very happy to agree to differ – it has been an interesting discussion

Best wishes,

Rod

Edited for typos

 

 

Edited By Roderick Jenkins on 12/10/2016 12:03:29

[Mark CParticipant @markc]

Michael, if that was the practice you used then best of luck to you but I will only subcontract work to shops that run a quality system and the idea that operators used what they wanted would have them off my list of suppliers in double quick time… If you took the time and looked at the drawing I posted you can see the manufacturing instructions and I would reject any part that did not conform to those instructions/dimensions without prior agreement. Lets not get confuseing industrial and home workshop practice though.

Your advice "You're right, Dave, because there is no authoritative standard on tapping drill sizes" sounds like statement of fact to me and it is wrong.

Mark

[Mark CParticipant @markc]

Andrew, is that a large spoon in your hand?

Mark

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