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Posted by Lester Caine on January 8, 2008, 2:05 pm
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Suzy wrote:
>> Suzy wrote:
>>> 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)
>> 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.
> 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
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.
--
Lester Caine - G8HFL
-----------------------------
Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact
L.S.Caine Electronic Services - http://home.lsces.co.uk
MEDW - http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/
Firebird - http://www.firebirdsql.org/index.php
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Posted by Suzy on January 8, 2008, 3:02 pm
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(snip)
> Lester Caine - G8HFL
> -----------------------------
Hi. The tone here is so different to that other ng that shall be nameless...
73 OM!
> Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact
> L.S.Caine Electronic Services -
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Posted by Peter Fairbrother on January 16, 2008, 6:04 pm
Please log in for more thread options Suzy wrote:
>> Suzy wrote:
>>> 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)
>> 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.
>>
>> --
>> Lester Caine - G8HFL
>> -----------------------------
>> Contact - http://home.lsces.co.uk/lsces/wiki/?page=contact
>> L.S.Caine Electronic Services - http://home.lsces.co.uk
>> MEDW - http://home.lsces.co.uk/ModelEngineersDigitalWorkshop/
>> Firebird - http://www.firebirdsql.org/index.php
>
> 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
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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Posted by David Littlewood on January 16, 2008, 8:51 pm
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>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]
>
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
--
David Littlewood
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Posted by Mark Rand on January 17, 2008, 6:46 pm
Please log in for more thread options On Thu, 17 Jan 2008 01:51:39 +0000, David Littlewood
>
>>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.
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.
>>
>>
>>[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]
>>
>Not sure I understand the final para, but I disagree with your
>recommendation of 3.3mm, for *any* material.
See above.
>
>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%.
If you get an undersized or worn screw. you will get stripped threads.
>
>(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).
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.
>
>(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.
For sheet, use thread forming taps. WITH THE CORRECT SISE DRILLS.
>
>(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.
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?
>
>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%.
>
>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!
Mark Rand
RTFM
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