Oh god. This group *is* active

Have to agree with Mark. With a tapping hole size of 3.3mm, theoretical thread depth is 81.5% So with a tapping hole size of 3.5mm giving a thread depth of .25mm as against .35 of the 3.3mm hole. %= .25/.35 x 81.5 = 58.21%

Tom

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

It's probably better called a bolt, unless you screw it into something.

That's the real difference.

If you can screw it into something it's a screw - if something (eg a nut) is screwed onto it, it's a bolt. Yes, that means that the same object can be both bolt and screw, depending on usage.

Otherwise, it's a screw if it's tiny, and a bolt if it's large. Shanks are not really relevant.

as I'll

Got 2? :)

actually has an overall diam of

Yes. It's everyone's experience.

The 60 degree triangular sides of the threads of a metric screw or bolt would meet at a point - but in practice they are rounded off, for several reasons - they don't need to be sharp being one, but also the nut/inside_thread is strengthened by being smaller inside, and rounded.

The 4 mm nominal size of an M4 bolt/screw is the size that the thread point edges would be if the threads met at a point - which they don't, they are rounded off at some radius I'd have to look up - 8

Also, it's not unusual for inside threads to be a bit undersize, and inside threads to be a bit over, so they'll always fit - 8

8 - see the appropriate ISO standard (if interested) ...

Second question. The Australian word is centrepunch and

We have centrepunches, and automatic centerpunches, both. But not "sprint"?

-- Peter Fairbrother

In article , Peter Fairbrother writes

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

It is actually *larger* inside (i.e. to tap an M4 nut, the tap is more than 4.0mm in diameter. The nut thread is generously rounded (to reduce stress cracking) outside the major diameter of the male thread, to avoid interference and to ensure that even after tooling wear the clearance is adequate.

No, this is not true either; the nominal major diameter is 1/8 of the pitch below this (notional) meeting point of the thread flanks. The standard calls for a flat (also of 1/8 pitch wide) at the crest; any rounding (which is usually used) has to be within this profile, but usually the "peak" of this rounded crest is close to nominal size. One of the major advantages of the ISO form is that interference of crests and roots is a virtual impossibility, so closer fits are possible in routine manufacture.

David

In article , David Littlewood writes

Let me correct my own post (before anyone else does). The major diameter is 1/8 of the theoretical thread *height* below the meeting point of the thread flanks, not 1/8 of the thread pitch. With a 60 degree thread angle, the theoretical thread height is P*cos 60, or 0.866*P, so the thread crest is 0.866*0.125 or 0.106*P below the meeting point.

David

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=81.49%

3.3mm is the bloody recommended tapping drill size for an M4 Coarse thread!

Mark Rand RTFM

In article , Mark Rand writes

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

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

On or around Fri, 18 Jan 2008 18:48:59 +1300, Tom 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.

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......

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

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

In article , Tom writes

Well, I suggest you read what Machinery's Handbook actually *says* before getting too dogmatic. I quote from my 25th edition, page 1815:

"Tapping troubles are sometimes caused by tap drills that are too small in diameter. The tap drill should not be smaller than is necessary to give the required strength to the thread as even a very small decrease in the diameter of the drill will increase the torque required and the possibility of broken taps. Tests have shown that any increase in the percentage of full thread over 60% does not significantly increase the strength of the thread. Often, a 55 to 60% thread is satisfactory, although 75% threads are commonly used to provide a measure of safety...... In general, when the engagement length is more than 1.5 times the nominal diameter a 50 or 55% thread is satisfactory."

Interestingly, they later quote (page 1827) the BS recommendations (which typically give 81.5% engagement for the "recommended" and 70% for the "alternative" sizes ) without referring back to this advice, which is of course not consistent with the BS numbers.

And of course, far be it from me to wonder if a manufacturer of taps may not be entirely bothered if you break rather more taps than you need to...

Irrelevant.

David

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

But you are. You are telling people to use 3.5mm which is incorrect for an M4x0.7 thread.

Have you actually looked at a tapping drill table form M4x0.7? Can you provide a cite for _any_ one that recommends 3.5? 3.3mm is the correct tapping drill for the thread and is what the taps are designed for.

Ask them again...

Mark Rand RTFM

So? LOL For all your quoting I notice your not keen to quote MH's tapping size for the thread in question.

Why should the BS standard adhere to some advice in a US publication? >

As for not using a tap maker's chart because they're in the business of making taps, ergo they quote incorrect tapping sizes as they prefer that you break them, obviously you're not in an occupation that requires quality to sell.

Tom

In article , Mark Rand writes

[snip]

Only by your definition of "incorrect", which seems top be "does the job with a fraction of the risk of breaking the tap".

I already did. As I said in my earlier reply to another dogmatic follower of BS/manufacturers' tables: has it occurred to you that tap manufacturers probably care rather less than most people if you break more taps?

I suspect that the BS tables are designed to suite industrial manufacturing, where torque control is standard and significantly reduces the risks of breakage. In the home workshop, where torque control is virtually unknown, and you probably don't have a gross of replacement taps on the shelf, but do have time to drill holes in stages to get them the right size, things are different.

Right, got a ouija board?

Look, this debate is getting stupid. You don't have to agree with me, but you (and the others who also keep on parroting the BS recommendations) might at least have the courtesy to read the references I have quoted and say why you disagree with them. My views are much less important than theirs, which are (in the case of TC) extremely well supported by diagrams etc. All *I* can say is that I have followed these recommendations for years and *never* had a stripped thread.

Also, remember that you learn most when people disagree with you, provided you actually debate, not simply repeat your own views without listening. Either you learn something you didn't know, or you go back and validate your own assumptions, and understand them better. I personally don't care whether you agree with me or not, you are free to use whatever drill size you want, and if it works for you, good. I simply pass on these thoughts to try to add to the understanding of the OP and others who may not be so experienced.

David

As I said below, they quote the BS table, which as you well know gives

3.3 (recommended) and 3.4 (alternative). What I am pointing out is that their actual advice is in conflict with the BS table they print. What part of that do you find difficult to understand? And *I* notice you don't actually respond to the point they make.

Ask the people who write them. As I said elsewhere, I suspect that the BS figures are aimed at industrial production, with torque control.

I always know when someone descends to ad hominem attacks that they have run out of arguments. Since none of your above remarks is even remotely helpful or responsive to the points I have raised, there is clearly no point in us continuing.

David