Oh god. This group *is* active

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Oh god. This group *is* active Suzy 01-08-2008
Posted by David Littlewood on January 17, 2008, 7:42 pm
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>On Thu, 17 Jan 2008 01:51:39 +0000, David Littlewood

[Some stuff snipped]
>
>>
>>Personally, I go for about 65%, unless the above factors suggest
>>otherwise, and use stub drills kept mostly for this purpose.
>
>Then stop recommending 50.8%.
>
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.
>
>>
>>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).
>
>
>
>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!
>
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
--
David Littlewood

Posted by Tom on January 18, 2008, 12:48 am
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 David Littlewood wrote:

>
>> On Thu, 17 Jan 2008 01:51:39 +0000, David Littlewood
>
>
> [Some stuff snipped]
>
>>
>>>
>>> Personally, I go for about 65%, unless the above factors suggest
>>> otherwise, and use stub drills kept mostly for this purpose.
>>
>>
>> Then stop recommending 50.8%.
>>
> 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.
>
>>
>>>
>>> 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).
>>
>>
>>
>>
>> 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!
>>
> 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

Posted by Austin Shackles on January 18, 2008, 5:21 am
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enlightened us thusly:

>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?

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.

--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
------------------------------------------------
>> http://www.schlockmercenary.com/ << ...and Kill them.
a webcartoon by Howard Tayler; I like it, maybe you will too!

Posted by Wayne Weedon on January 18, 2008, 6:37 am
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Austin Shackles wrote:

> 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.


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......

Posted by Tony Jeffree on January 18, 2008, 6:54 am
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wrote:

>Look up any respected tapping table, and they will quote 5mm for M6x1.

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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