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metric taps

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colin calver22/11/2011 18:43:20
29 forum posts
4 photos
I'm a novice, so please be gentle. The drawing states " tap M8 ", easy I think, just buy a M8 tap but I now find there are two types fine and coarse. (1) Which do i need? (2) and why are there two thread forms ?
JasonB22/11/2011 18:53:03
19568 forum posts
2142 photos
1 articles
Unless it specifically states fine then you can 99% of the time assume it will be Metric Coarse.
The finer threads are used where you don't want the fixing working loose or need fine adjustment so for example you may find the fine specified on gland nuts, valve rods etc.
Also watch out when drilling your tapping hole as the finer tap needs a larger hole. As a rule of thumb with metric deduct the pitch from the diameter to get the tapping drill. So M8x1.25 needs a 6.75 hole but 6.8 is easier to get hold of. For the fine M8x1.0 you would drill 7.0mm.

Edited By JasonB on 22/11/2011 18:55:14

David Littlewood22/11/2011 19:45:19
533 forum posts
Jason is quite right that an unspecified metric thread can be assumed to be a coarse series one.
However, I strongly disagree with his suggestion to use a 6.8 mm drill. This will give you over 90% thread engagement and you will probably break the tap (or your wrist!) if you try to do it by hand. I just tapped 3 M8x1.25 threads in a 10 mm thick MS plate today, and I used a 7.1 mm tapping drill, which was plenty hard enough. This gives about 65% thread engagement, and the thread flanks will still be about 2.5 times stronger than the core of the screw.
Do yourself a big, big favour and buy a copy of "Drills, Taps and Dies" by Tubal Cain, No 12 in the Workshop Practice series. It's only about six quid and it will tell you almost anything you are likely to want to know on the subject. The tapping drill tables at the back are worth it alone.
For some strange reason, almost all other standard reference works give ludicrously small tapping size holes for metric coarse threads; these give 85-100% thread engagement and are probably aimed at industrial users, with sharp taps and torque-controlled machines. Machinery's Handbook even admits that aiming for these levels of engagement is unnecessary and risks breaking the tap, and that 60-70% is perfectly good, and then goes on to reproduce the same old table which does the exact opposite.
One of the reasons for this is that ISO metric taps have a diameter greater than the nominal figure (by about 11% of the thread pitch). Most other thread forms do not have this feature, and the commonly quoted tapping sizes are not quite so ridiculously small, though they are mostly still tighter than they need to be.

Edited By David Littlewood on 22/11/2011 19:47:52

JasonB22/11/2011 20:03:29
19568 forum posts
2142 photos
1 articles
Guess I've got strong wrists as I've been using dia-pitch for 30odd years ever since that was what I was tought and seldom break a tap.
mgnbuk22/11/2011 21:04:25
903 forum posts
65 photos
Guess I've got strong wrists as I've been using dia-pitch for 30odd years ever since that was what I was tought and seldom break a tap

Ditto. Well, except for the strong wrists bit (Carpal Tunnel decompressions both sides !)

I cannot recall seeing any instances of "oversize" tapping holes being recommended on manufacturing drawings - the "Thread diameter minus pitch, rounded to nearest standard size" rule seems pretty universal.

Clive Hartland22/11/2011 21:11:12
2630 forum posts
40 photos
The constant for Metric standard threads is:- Thread dia. x .8
An example is:- 8mm x .8 gives 6.4 for tapping size This is a lot different from 7.1mm quoted. I am sure the bolt would wobble through under its own wieght!
Robert Dodds22/11/2011 21:50:43
275 forum posts
39 photos
Guess I've got strong wrists as I've been using dia-pitch for 30odd years ever since that was what I was tought and seldom break a tap
The only good reason for further proliferation of thread standards was the fact that it was easier to remember / calculate tapping sizes in the new metric system than with the BSF, Whit , BA , UNC, ANF etc. but we did have letter drills and number drills in those days!
Nicholas Farr22/11/2011 22:02:33
2550 forum posts
1211 photos
Hi, well I've always used tapping drill sizes given in the Zues booklet, the metric revision one from 1972, and have tapped holes from 5mm up to 20mm in blind holes as well as through holes, using hand tap wrenches, in all sorts of machinery and thick steel plate, that you just can't put in the vice on the work bench, and like Jason, I seldom break a tap, and have never broken my wrists. Use a good tapping medium when tapping, and you shouldn't have too much problem.
Regards Nick.
David Littlewood22/11/2011 22:12:28
533 forum posts
Well folks, I suggest you actually read the references I gave and then try it yourselves. I have to say "Seldom break a tap" doesn't really cut it for me. Since using the tables I referred to (since about 20 years ago) I have broken precisely one tap - an M3 tap, doing it by hand, and almost certainly because I bent it. Broke plenty before when using the figures in the Zeus booklet. As for 6.4 mm, that is actually smaller than the core of an M8 male thread - insanity.
And no Clive, the bolt definitely does not wobble at even 50% thread engagement. If you look at the form of the nut/bolt threads you will see that is no more likely than if you use the quite unnecessary 90% thread engagement.

Edited By David Littlewood on 22/11/2011 22:13:06

Andrew Johnston22/11/2011 22:32:59
5828 forum posts
662 photos
I did an experiment on tapping sizes a couple of years ago for a critical M4 thread. The internal thread was blind, in aluminium, grade 6082, and the bolts were socket head, grade 8.8 steel.
I drilled holes of 3.4, 3.5 and 3.6mm diameter, ie, thread depths of about 80%, 65% and 53% respectively. The holes were hand tapped using a Dormer tap. I then screwed in a bolt until something broke. I tried 10 times with the hole drilled 3.6mm and the bolt broke every time. I didn't bother testing the smaller holes.
The tapping hole size depends not only on the depth of engagement required, but also on the material and the style of tap used. It is normally left to the discretion of the machinist.
According to my book on threads the minor diameter for an M8x1.25 screw is 6.47mm, so a 6.4mm tapping drill is going to be pretty snug?
Robert Dodds22/11/2011 22:49:49
275 forum posts
39 photos
The standards and specs for threads were laid down for serious use in industrial and commercial applications sometimes with life threatening implications.
The depth of thread engagement set by the tapping drill size impacts on the cross section of thread available for shear in the axial plane through the bolt, in other words, when the bolt is in tension. It can sometimes be compensated by extra thread length, but non of this affects the fit of the two mating threads. Hence DL's comments on wobble.
In terms of holding strength thread engagement depth has little influence, this being more to do with the root diameter of the bolt, either in shear for holding or tension for an axial load (where the thread depth is also implicated)
As modellers we very seldom test any of these threads to even a fraction of their true potential strength and can get away with exaggerated amounts of "disengagement" that would be cause for rejection in some industries. It is certainly lighter on the pocket to "drill big and cut easy"
David Littlewood22/11/2011 23:31:50
533 forum posts
To add to the above points, it is worth looking at the effect of varying drill diameter/thread engagement on the torque required for tapping. The book I quoted in my first post above has an interesting graph (p 64) for this on Unified threads. Taking 50% engagement as 1.0 torque, the figure for 60% is 2.0, for 70% is 3.5, for 80% is 6.0 and for 90% it is off the scale. Now bearing in mind that for a thread of at least 1 x diameter long the fixing is at least twice as likely to fail in tension as in thread shear even at 50%, this means that by following the Zeus figure you are up to 6 times more likely to break a tap than you need. For Whitworth and BA forms the effect is slightly more, and for ISO metric threads it is even more so (in large part because of the oversized tap diameter I mentioned before). Because of the form of the ISO thread pattern, you cannot even get more than 82% thread engagement (because there are voids at top and bottom of the threads), anything above this is simply digging out metal to no purpose.
I suspect the reason why some people get away with it as much as they do is that drills, unless in prime condition, usually drill oversize. If re-sharpened by hand and used without a pilot hole this is likely to be at least one size larger, probably more. I always use a pilot hole (sometimes two) and don't resharpen small drills - not worth the time and effort.
Chris Trice23/11/2011 01:21:28
1362 forum posts
9 photos
I'm afraid I'm 100% behind David on this. Since reading the Tubal Cain book, my record of tap breakage has virtually ceased. The Zeuss tables quote drill sizes for tapping on industrial rigid machinery, very rare in amateur model engineering circles and not hand tapping. As he explains, when you tap a thread, part of thread forming is done by extrusion, particuarly in ductile materials. This is often the cause of tap breakage as it causes the user to over torque the tap as the drilled hole closes up on the tap. It's easily tested by drilling a hole, tapping it and then trying to poke the same drill through the tapped hole. The core has usually closed up slightly. The graphs displayed in the book show the correlation between reducing the thread engagement (drilling larger) against tap breakage against lost strength in the thread. 65% engagement still gives an internal thread stronger than the core strength of the screw but reduces dramatically the torque being applied to the tap and the saving in tap breakage. That not withstanding, it's a fascinating read about the principles of what you're trying to do and I'd argue the best of the Workshop Series books.
alan frost23/11/2011 02:08:59
137 forum posts
3 photos
I think it was H.Hall or Tubal Cain who pointed out that for all types of common thread 60% engagement gives a fixing of about 95% of the strength of 100% engagement. I can't remember the exact figures but the above are probably pretty close.
I do have strong wrists (sign of a mis -spent youth -?-) but am lazy and much prefer tapping at 60% or thereabouts engagement-gives you a bit more choice on tapping drill size too as I'll accept for 95% of my applications any engagement between say 50% and 80%.
And remember every time you use a commercial nut and bolt you're probably around 75% engagement. Not much point killing yourself tapping the rest of the machine at over 85%.

Edited By alan frost on 23/11/2011 02:14:29

Tel23/11/2011 02:27:23
157 forum posts
28 photos
I too have been using the been using dia-pitch calc for more years than I care to think about with no problems, the exception being where the pitch is, say, 1.25 I will use a 1.3, ie to the nearest higher 0.1 mm

Edited By Tel on 23/11/2011 02:28:10

Chris Trice23/11/2011 02:49:59
1362 forum posts
9 photos
Internal nut threads are stronger than external screw threads because the forces acting on them are in compression. That's why you can have a high tensile bolt with a mild steel nut. 80% thread engagement takes three times as much torque and risk of tap breakage than 60%. 90% doubles it to six times. 65% is more than adequate for virtually all metal rising to 70% for larger holes. The Tubal Cain recommended figures are 2.65mm for M3, 3.5mm for M4, 4.4mm for M5, 5.25mm for M6, 7mm for M8 and 8.8mm for M10. For the interested or doubters, I'd urge grabbing a bit of scrap and trying it. The other thing to consider is that Zeus tables assume a new sharp tap. If there's even a suggestion of bluntness, the torque figures rise significantly.
David Paterson 423/11/2011 03:24:56
83 forum posts
8 photos
Thanks all - best discussion of detailed issue I've read for a while
BERTO23/11/2011 03:50:06
46 forum posts
Hi Colin.
I agree that jasons rule of thumb will get you by or out of trouble 99% of the time and do stick with the coarse thread unless specified different .
I also agree with David in regards to Tubal cains book .
You did not mention what material you are cutting a thread in ?
I have found the workshop practice series both informative and very usefull and there are over 40 different publications under the WPS banner .
Two i recommend are Metalworkers Data Book - WPS #42 by Harold Hall & Model Engineers Handbook by Tubal Cain .
Well actually i could recommend any book with the name of these authors on it and there are a few in the WPS series !
There was a series of wall charts produced by MEW that i use most of the time and i keep these handy in the workshop as it saves getting my good books as mentioned above dirty !
If you don't own a tap and die set you shoud be able to get a small tap set that has the correct drill for each size tap , usually has 4,5,6,8,10 mm etc and may be of use to you .
Regards ..


JasonB23/11/2011 08:08:45
19568 forum posts
2142 photos
1 articles
Can I ask those who use 65% or whatever how that is calculated? is it base on the male thread actually being the nominal size?
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
I'll stick to using what I said as its close to the BSI recommended of 6.75 especially if I'm putting up structural steelwork, I'm sure the Structural engineer and BCO would not be happy if I deviated. Funny enough teh BSI alternative is a tighter hole at 6.6mm not looser.
mick23/11/2011 08:14:01
398 forum posts
44 photos
British Standard tapping sizes give about 75% engagement, but for materials like stainless steel and other tensile metals you are allowed up to 50% in order to give the tap a fightinging chance. These relaxed tapping sizes are in constant use for smaller diameters in the high vacuum industary, where all machining standards are extreamly high.

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