metric taps

[Hugh GilhespieParticipant @hughgilhespie56163]

For metric and American threads with a 60 degree thread form the calculation of the percentage engagement is straightforward as the thread form is an isosceles triangle.

 

The full ‘sharp’ height of the thread is Pitch x SQRT(1 – (0.5^2)). Then multiply by 2 for thread height on both sides of the diameter. This gives

 

‘Sharp’ Thread depth (without corner rounding) = Pitch * SQRT(1-(0.5^2))*2 = Pitch * 1.732051.

 

But both the crests and roots of the threads are flattened. The crest by 1/8th of the height and the root by 1/4 of the height. So, the actual thread height is 5/8th of the sharp tooth height.

 

So, finally the working thread depth is given by Pitch * 1.082532.

 

To calculate the tapping hole size for a given percentage engagement, use the formula

 

Tapping size = Major Diameter – (1.08253 * Pitch * Percent Engaged/100)

This is the formula given in Machinery’s Handbook.

 

So, for an Iso M8 coarse, pitch is 1.25 mm, major diameter = 8 mm and for say 60% engagement the tapping size is 7.188 mm. In reality, use a 7.2 mm tapping drill and this will give about 59% thread engagement.

 

If you don’t want to bother with calculations, Harold Hall’s website has a very good section on selecting tapping sizes showing a range of thread engagements.

 

Hope this helps.

 

Regards, Hugh

 

[David LittlewoodParticipant @davidlittlewood51847]

Jason said:

Have you measured commercial fixings lately, I’ve just measured half a dozen different M8 fixings and the average major dia is 7.8mm so does start to shake about a bit in a shallow 7.1mm hole And as for those far eastern hold down studs I could see them pluuling out.

Andrew you must have different books to me as mine gibe M8 core as 6.376 so clive may just squeeze a tap into a 6.4mm hole, getting it out is another matter

 

Jason – First point, on the shake, I think you are confusing two different things. The overall diameter of a male ISO thread is defined by truncation, whereas the intimacy of the mating flanks is determined by the depth to which the 60 degree sloping thread is cut. IOW, it is perfectly possible to have a closely mating set of thread flanks but with a male crest truncated to below the standard (nominal) diameter. Of course, the oriental bolts you refer to may have both problems, but it is the excessive thread depth rather than the lower major diameter which would cause slop.

 

Second point: the defined core diameter for ISO metric male threads is D – 1.227 x pitch, which for M8 x 1.25 is 6.47 mm. The nut minor diameter is D – 1.082 x pitch, i.e. 6.54 mm.

 

David

Edited By David Littlewood on 23/11/2011 12:49:55

Edited By David Littlewood on 23/11/2011 12:51:04

Edited By David Littlewood on 23/11/2011 13:03:55

[JasonBModerator @jasonb]

David I agree the first point regarding slop but it WILL affect pull out as those nicely mating flanks will be bearing on less metal so that affects engagement percentage. The only way to really measure these bolts accurately would be with thread wires as that thakes the crests out of the equasion.

 

Second point, looks like I need a new Zeuz book

 

J

[TerrydParticipant @terryd72465]

Hi All,

 

I have just come in on this thread and the interesting discussion. I’ve been working on the restoration of a Stuart Beam engine over the last few days and have been referring to the Andrew Smith guide to “Building the Stuart Beam Engine” and came across the following passage which you may find amusing. I have only used his reference to 2BA threads but he lists others.

 

(I have edited out the text that I pasted from Word as it lost any formatting and made little sense so I have replaced it with an image of the text)

 

Best regards

 

Terry

Edited By Terryd on 23/11/2011 13:32:49

[Chris TriceParticipant @christrice43267]

I think the other point to follow up with that is should the tap manufacturers make taps that suit machine screws that are inaccurately made by suppliers or make them accurate and get the bolt makers to raise their bar? I think there’s only one answer to that. The figures quoted obviously assume the screw maker is competent and not pandering to stupid people who struggle to put a bolt through a hole or don’t intend their bolts to perform important roles.

[David LittlewoodParticipant @davidlittlewood51847]

Jason, calculations show that the shear strength of the crests of a thread which is 1.0 diameters long in similar materials is about 2.2x the tensile strength of the bolt core. Thus even at this level of engagement the bolt core will always break before the threads will strip. Andrew Johntson’s tests, as he reported yesterday, bear this out. Even with a high tensile bolt in a mild steel female this is almost guaranteed to be the case. For a tapped hole of at least this (bolt) length in steel there is no advantage in going for more engagement.

 

For other circumstances this may need modification. For example, in thin sheet (less turns of thread) you need more crest strength (and may need to consider other measures). For cast iron, which is not strong in tension, I would go for 70-75% engagement, ditto for aluminium. But the figures quoted in TC’s book show all these percentages, and the discussion gives a good guide to these modifying factors.

 

David

Edited By David Littlewood on 23/11/2011 14:36:32

Edited By David Littlewood on 23/11/2011 14:37:39

[Chris TriceParticipant @christrice43267]

The easiest way to check would be to have a matched tap and die of the required ‘fit’ and spin both onto/into the bought items.

[JasonBModerator @jasonb]

Still no answer to what the total engagement percentage is based on.

 

For example some are suggesting drilling the tapping hole to give 65% engagement, now if we take our cheap and nasty bolt that is 7.8mm OD that is only 85% of the male part of the thread.

 

Multipy these two together .65x.85 and you have only got a combind engagement of 55% so we are getting close to the Tensile strength of the bolt 2.2 x 55% = 1.2, not a very big factor of safety.

 

Still I’m happy that my traction engine cylinder is fixed to the boiler with the ISO suggested engagement

 

Chris the easiest way to good fitting threads is to tap your hole and make all the male fixings yourself, adjusting the die to give a nice fit, must confess I don’t do it on all my models.

 

J

[Hugh GilhespieParticipant @hughgilhespie56163]

Hi Jason

 

Still no answer to what the total engagement percentage is based on.

 

Oops – I thought I did

 

Regards, Hugh

[JasonBModerator @jasonb]

Was it this bit Hugh?

 

“for an Iso M8 coarse, pitch is 1.25 mm, major diameter = 8 mm and for say 60% engagement the tapping size is 7.188 mm. In reality, use a 7.2 mm tapping drill and this will give about 59% thread engagement.”

 

As I say that does not take into account the male thread, and looking at TC book he says “A die MUST cut a full thread” my capitals.

 

Now as you know I like to spend Wednesday evenings in the shed so what better way to put all this theory into practice and I did as TC says in his book to drill a hole, tap it, put in my chosen fixing and see what happens.

 

So I found a bit of 10 mm steel bar, drilled 1/4″ and then opened out to 7.1 whithe what looked like a new drill and it was also still sticky with the original oil.

 

I then tapped it M8 x 1.25 and also counterbored it 2mm deep so the hole was 1xd deep.

 

Next I grabbed an average bolt, as you can see it measures 7.8mm

 

Slipped a washer on the bolt, screwed it in by hand then got a standard ratchet and tightened it up.

 

Result one stripped thread IN THE BAR and slight damage to the bolt. I rest my case

 

As I say any thread consists of two parts and its no good just making one easy to tap without considering the other

 

J

 

 

 

Edited By JasonB on 23/11/2011 17:51:35

[Raymond AndersonParticipant @raymondanderson34407]

Hi All,

I have just been informed that the “normal” tapping drill charts are for use in ordinary mild steel, tougher materials require a different tap, for instance a Dormer yellow ring tap E348 in size M10 is slightly larger in Ø than the equivalent Black ring E201.

That is another facet of material specific taps.

Regards,

Raymond.

[colin calverParticipant @colincalver]

Hi,

and I cogitated for a few days, spent a couple of hours on google looking for difference between fine and coarse, before asking question as I did not want to look stupid. Fantastic response. Thanks

[Dennis RaynerParticipant @dennisrayner52782]

As a relative novice to these matters I would like to ask if this “percentage engagement” has any effect on sealing the thread. At the moment I am busily screwing fiitings into a backhead using Loctite 242 primarily as a seal. Does the percentage engagement affect the gap to be filled by the Loctite?

[Chris TriceParticipant @christrice43267]

Jason, the figures are based on ISO metric threads to B.S. 4827 standards.

[Chris TriceParticipant @christrice43267]

The question of undersized bolts does impact on the total % thread engagement and what the tapping drill size needs to be to achieve a given % but unless the tap matches the bolt for being undersize, the bolt manufacturers failing is actually gifted onto the tap maker by making the tap perform a function under higher torque than it is obligated to deal with. It’s a bit like blaming a car manufacturer because the independent makers car accessory won’t fit without altering the car.

[Francois MeunierParticipant @francoismeunier96697]

Hi
Listen my friends, this fellow has just a hole to drill and tap M8.
drill 6.75 to 7.1 according to hardness or drill bits available or tap quality( Carbon steel, HSS, new, worn, Chinese, British…) or type (hand tap, machine, gun, roll, three or four fluted..straight or helicoidal) the discussion is not over !
I agree with JasonB that commercial M8 screws & bolts are smaller than the ISO standard. nuts and bolts found in supermarkets & DIY stores give together a sloppy fit, IMO.

Zephyrin

[Terry LaneParticipant @terrylane]

And there, gentle reader, is the simple and practical answer!

[TerrydParticipant @terryd72465]

Posted by Robin Woodward on 23/11/2011 20:36:11:

Hi All,

 

…………………….. On a lot of the imported machines I have come across over the past twenty years I have found 1/4-20 UNC bolts with 10mm A/F heads on, I am sure knowing the USA that their SAE standard would definitely not call for a Metric head. To which standard these bolts were made is unknown to me, but none the less they are in the market place, further more when I replaced them with a standard UNC bolt the improvement in fit was very noticeable……………………………..

 

Rob

 

Hi Rob,

 

It is interesting to note that MG and Morris cars used metric threads with Whitworth heads until around 1955. Perhaps this was because most mechanics would not have metric spanners in those days? So much for standardisation – perhaps it didn’t matter when there was less international trade.

 

Regards

 

T

[David Paterson 4Participant @davidpaterson4]

Terry,

 

I was shy about throwing this in – my first car as a young officer cadet at Duntroon was a ’54 TF.the distributor and some of the engine bolts were this mix of metric and BSW. The doco at the time attributed it to a French influence of all things and the system was referred to as ‘Nuffields mad metric’

 

Incidentally – the car also lead to many other interesting engineering questions – Why do the English drink warm beer? They keep it in Lucas fridges.

 

Dave

 

 

 

[JasonBModerator @jasonb]

Just an update, I was not quite correct in what I said. Infact there was no visible damage to the bolt, on returning to the workshop I unwound the remains of the female thread from the bolt which looked as good as new (well as good as a poor quality one)

 

I should also add that contary to what some said the undersize bolt was a sloppy fit in the thread. Also although there were some markings on the bolt head there were no grade marks so I didn’t cheat and knowingly use a 10.8 grade bolt.

 

 

 

J

[Anonymous]

Just when you thought it was safe to come out from behind the settee……..I thought I’d try and reproduce Jason’s results. Here’s a picture:

 

I drilled a couple of holes in a piece of steel plate from the scrap bin. No idea what the material is, but probably hot rolled low carbon steel. Thickness measured as 7.98mm. I drilled the holes with a 7.1mm drill straight off, no pilot hole. I forgot to measure the first hole, but the second measured 7.18mm before tapping. The holes were tapped with a Dormer spiral flute tap; because it was the first M8 tap I found in the drawer.

 

First I tried a SHCS. I did it up as tight as I could with a hex wrench, followed by a Mole wrench. I ended up gripping the head in the vice and belting the plate with a hammer. Hence the damage at the bottom right of plate in the picture. Eventually the screw started to extrude material from the plate and the threads in the plate started to strip. It also damaged the crest of the threads on the bolt. The bolt grade was 12.9.

 

Then I tried a stainless steel bolt, says A4-70 on the head, from RS in the second hole. Using a large adjustable spanner the bolt failed. The thread in the plate looks fine.

 

So, a score draw. I expect everybody will have their own views, so I’m not going to draw any public conclusions.

 

Regards,

 

Andrew

[JasonBModerator @jasonb]

Andrew, could you let us know what the two fixings measure over the thread crests? would be interested to know.

 

J

[TerrydParticipant @terryd72465]

Posted by Andrew Johnston on 24/11/2011 19:39:46:

Just when you thought it was safe to come out from behind the settee……..

 

…………….So, a score draw. I expect everybody will have their own views, so I’m not going to draw any public conclusions.

 

Regards,

 

Andrew

 

 

Thanks Andrew,

 

that will do for my models. Perhaps if I ever work for NASA or the NPL I may need different standards?

 

T

[VersabossParticipant @versaboss]

Well, I also did an experiment. But before you should know the following: I do, more or less regularly, work for customers. I always use the ‘standard’ tapping diameters for metric threads, that is dia – pitch. Found in all the tables I have, and also written on a large plate on my Fehlmann drill (on which the tapping is done, under power). I have (mandatory) to check the threads with thread gauges. Yes, I have some taps which cut oversize threads! .

So I took a scrap of Aluminium, 20 mm thick, and bored 2 holes, 6.8 and 7.1 mm. I did not measure them, but due to the drills I used they are spot on. Tapping both M8, and checking with the gauge. Lo and behold, both are perfect! But, and this is a big but for me, looking down into the hole the lager shows distinctive truncated thread. From former (bad) experience I know that my customer would not be pleased with them!!! .

Conclusion: for my own purposes, and in the more difficult materials, I would also drill a gnat’s whisker larger next time.

Greetings, Hansrudolf

[Anonymous]

Posted by JasonB on 24/11/2011 19:56:50:

Andrew, could you let us know what the two fixings measure over the thread crests? would be interested to know.

 

J

 

For the SHCS I can’t find a new one in my box; M8 is not a size I tend to use. The ‘used’ one measures 7.85mm across the (damaged) crests. A new M8 stainless screw measures 7.9mm at the start of the thread, 7.87mm in the middle and 7.75mm just under the hex head. Both threads are rolled.

 

The broken portion of the stainless steel screw measures about the same as the new part; 7.9mm at the start, 7.85mm in the middle and 7.75mm under the head.

 

As an aside the SHCS is very consistent both axially and radially. The stainless steel screw not only varies axially the readings vary by 0.05mm or more radially.

 

Regards,

 

Andrew

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