Oh god. This group *is* active

I know 3.3 mm is the preferred drilling for a 4 mm tap, but in practice is 3.5 mm OK? My supplier only stocks "standard" taps (no starters or
bottomers)
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I meant "Oh *good* of course!

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You were right the first time ;-)
Regards, Tony
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Suzy wrote:

How long is the piece of string ;)
What are you wanting to tap into? I tend to use a bigger drill when working with softer aluminium as I find the tap tends to bind less, and pushes up the tops of the thread anyway. And for a hard material like steel, while you will get better strength with a smaller hole, it still depends a bit on how clean the tap cuts.
It also depends on how accurately your drill cuts as well, a 3.3mm drill may well cut a 3.5mm hole if slightly miss ground.
So it's a little difficult to give an accurate answer on the information available.
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Lester Caine - G8HFL
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Thanks Lester. I'm drilling aluminium and plastic (I know -- which plastic? Answer: I don't know but it's stock moulding from a hardware store, 15mm X 20mm section). 3.5 mm has worked perfectly in practice but as I'm writing an article on the project I did not want to mislead my (non model engineering) readers
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Suzy wrote:

Plastic mouldings are a bit variable on 'density' but certainly on the softer materials the slightly larger drill is safe enough. I suspect you could probably simply screw an M4 bolt into it. In practice on the plastic boxes I tend to NOT tap all the way through, so that the bolt or spacer IS cutting the last couple of threads itself. But on the softer materials, being a little over enthusiastic fitting an M3 or M4 screw can easily strip the threads anyway so it is worth ensuring that the drill IS cutting 3.5mm and that the tap is cutting clean.
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(snip)

Hi. The tone here is so different to that other ng that shall be nameless...
73 OM!

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Suzy wrote:

Use 3.3 mm for Al and plastic, if at all possible. 3.5 mm will mostly work, but it will be about 1/2 as strong.
[note for list techies - the strength of the fastener is the proper metric, not the thread engagement - this can in most cases be linearly related, to a good approximation, to the strength of the material]
are 3.3 mm drills available in the UK ... ? yes, but not as readily as 3.5 mm ones.
-- Peter Fairbrother
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Not sure I understand the final para, but I disagree with your recommendation of 3.3mm, for *any* material.
A threaded fastening, if overstressed, will fail in one of two ways. Either the core of the screw will fail in tension, or the threads will fail in shear ("stripping"). For a given thread type, you have no control over the tensile strength of the male thread core, so the aim is to make the shear strength of the thread flanks at least as high, with a margin for error.
For those interested, I would refer them (again) to Chapter 8 of Tubal Cain's "Drills, Taps and Dies", No 12 in the Workshop Practice series. In essence:
(1) There is absolutely no point in going above 82% thread engagement, no further increase in shear strength is obtained.
(2) At 82% thread engagement, the shear strength of the thread flanks of a screw engaged in a nut of thickness equal to screw diameter is 300% of the tensile strength of the core.
(3) At *50%* thread engagement, the thread strength is still about 200% of the core strength. At 60% it is about 250%.
(4) At 80% thread engagement the torque required is *six times* that required for 50% thread engagement, and the risk of tap breakage is thus considerably greater (I won't say 6 times greater, as the risk of bending the tap does not vary in the same way if you are doing it by hand).
(5) Clearly, for thin nuts, or tapping directly into thin sheet material, less threads may be engaged, and the above may need to be modified accordingly.
(6) The above are for BA threads; he states that the position is even more extreme for ISO metric threads, as the tap diameter actually *exceeds* the nominal thread size, so the tapping torque will be higher.
(7) For tapping mild steel or other ductile material, extrusion will typically give 5% more thread engagement than the tapping hole diameter suggests. Conversely, you drill (if it is not properly sharpened, and no pilot hole is used) could easily be 5% over size.
Two further points. Although TC does not say so directly, it appears these calculations assumed mild steel; he certainly assumes the materials are identical. For tapping into aluminium, modify accordingly; Al alloys vary quite a lot, but typically have about 2/3 the yield strength of free cutting (EN1A, 220M07) mild steel - so even with these, 50% thread engagement will comfortably exceed the core strength of a mild steel screw. Conversely, for a high tensile bolt, the core strength will be higher and there may be more benefit in using a slightly higher thread engagement.
Personally, I go for about 65%, unless the above factors suggest otherwise, and use stub drills kept mostly for this purpose.
Going back to your suggested 3.3mm, this will give over 90% thread engagement; tapping torque will be through the roof, and tap breakage (even in Al) a real risk - unless the drill lets you off the hook by drilling oversize. 3.5mm will give you 65% thread engagement, which will be way more than enough unless you are only engaging a couple of threads (in which case you should probably be using a bush or some other method).
David
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David Littlewood

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On Thu, 17 Jan 2008 01:51:39 +0000, David Littlewood

3.3mm is the recommended tapping drill for M4 coarse and still only gives 71% thread engagement. If you want your products to break, that's up to you. But please allow that the rest of us will use the correct tapping drills for the threads that we cut.

See above.

If you get an undersized or worn screw. you will get stripped threads.

Taps are designed to cut with the standard sized holes. If you suffer tap breakage, look at your technique first. Don't resort to over sized tapping drills.

For sheet, use thread forming taps. WITH THE CORRECT SISE DRILLS.

Do you measure all of your screws before use? How can you verify that you are not using under-sized screws in you over sized holes, apart form the failure of the part?

Then stop recommending 50.8%.

Show your working!
OD=4mm Core dia = 3.141mm
4-3.141=.859
4-3.3=0.7
.7/.859*100.49%
3.3mm is the bloody recommended tapping drill size for an M4 Coarse thread!
Mark Rand RTFM
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[Some stuff snipped]

I didn't *recommend* it, I simply showed some calculations -not mine, but those of a very highly respected engineer. Neither he (pretty authoritative) nor I (relatively insignificant) recommended 50%, just showed it was perfectly viable.

Not by people whose views I respect.
You assume that 100% thread engagement involves full contact at root and crest. This is a very bad idea, and not in accordance with the standards. If you actually look at the thread geometry (which varies according to whether you are looking at ISO metric, BSW, BA etc, but the same general principle applies to all) you will see that the correct calculation is more complex than the simplistic one above.
I don't really see how I can explain it further, without diagrams - and frankly, I don't see why I should, I've given you the reference, check it for yourself. I put forward these thoughts to add to the debate, if you don't like them you can have a full refund of what you paid for them.
David
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David Littlewood

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David Littlewood wrote:

> LOL So you know people who scorn Machinery's Handbook? Who believe maker's like Dormer don't know what they're talking even though they make the taps? You're never heard of arriving at the tapping size for metric threads by subtracting the pitch from the diameter? Your daytime occupation is?
Tom
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enlightened us thusly:

pitch-diameter is a useful rule of thumb but taking one from the air, M6x1 is normally reckoned to want 4.8mm drill, and the rule-of-thumb gets you 5mm, which is a fair bit less thread on one that size.
It really all depends on material and what the job's doing. if it's low load stuff in steel, 50% thread is going to be fine. If you're talking soft material with hard screws, then you want as much thread as you can reasonably get, and decent grade of correct-size and fully-threaded bolts to boot.
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Austin Shackles. www.ddol-las.net my opinions are just that
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Austin Shackles wrote:

Look up any respected tapping table, and they will quote 5mm for M6x1. Not sure I have ever seen 4.8 recommended.
I do a LOT of machine tapping and always use recommended UNLESS it's some awkward material like 316 Stainless. Or today 310 Stainless.
Last week was 1000's of holes (M4 and 1/8"BSP) in cast acrylic. There was no way I was going to use anything other than recommended.
Over the last few years I have done more form tapping than I ever did, and thats yet another story. Even 300 series stainless form taps quite well.
Wayne......
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wrote:

I guess that may be true for some limited definition of "any respected tapping table". However, the smallest diameter recommended in Tubal Cain's book is 5.1mm (for 85% engagement).
Regards, Tony
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Tony Jeffree wrote:

> Depends on whether you place an individual's opinion over either, a well established reference source i.e MH, or a manufacturer like Dormer. Somehow I know where my preference lies.
Tom
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wrote:

Actually, he quotes this as the British Standard preferred size, not as his personal opinion. So the "well established reference source" in this case is a British Standard.
Regards, Tony
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Tony Jeffree wrote:

> Whereas MH as regards: British Standard Tapping Drill Sizes for ISO Metric Coarse Pitch Series Threads [Supplement No. 1 (I967) to BS 1157: 1965)      Quotes 6 x 1 @ 5mm with 81.5% as the recommended and 5.1 as the alternative @ 73.4%. % as the theoretical.
Interesting. :-)
Tom
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Austin Shackles wrote:

Really? Can you cite a source that recommends 4.8mm? Without going to far from my desk MH & Dormer both quote 5mm which gives 81.5% Why would you need greater engagement?
Tom
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enlightened us thusly:

I think it was a table on the back of a steel rule. Hardly a guaranteed source, I admit. Presumably it represents 100% engagement, assuming Dormer et al have it right.
and yes, probably unnecessary. There is, though, the point someone made about inferior or worn screws - if I'm making a threaded boss for example with an M6 hole in it, then I don't know what quality or condition the screw used will be. If I drill with a 4.8 the chances are the hole is 4.9 anyway... and the tap seems to cope (in mild steel).
Tapping M5 into the head of a stainless A4 bolt is a bit more questionable. Luckily, I don't have to do that often.
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