M8 tapping drill

Good point! Nothings fallen apart yet

cheers,

Rod

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

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

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

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

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

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

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

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

Sorry,

For M8 ; drill 6.8 !

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.

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

Mark,

Can't wait.

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

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

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

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

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

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

-

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

Rod