On 01/12/2011 07:18, Spehro Pefhany wrote:
> I don't see a "32" on that drawing, I see #2 x 3/16"=20

Yes, the datasheet for the same box I have on my laptop actually has 32=20
x 3/16" instead (I did mention this in my original post, think I got my=20
datasheet from the product page on Farnell)
Must be an older one that I have, maybe they changed it because someone=20
like me couldn't figure it out :-)

> 32 threads per inch (North American screws use the reciprocal of the=20
> pitch) is certainly plausible- typically this type of screw has a much=20
> coarser pitch than the equivalent diameter machine screw. In any case,=20
> this would be approximately a M1.8 Thread Forming fastener, also=20
> called K18, but you'd have to verify the size to be sure. Pitch at=20
> 0.8mm is about right. Here's one maker, along with some design rules=20
> for various plastics:- http://www.semblex.com/files/PT-Semblex.pdf=20
> Also note the Hammond part number 1551ATS100 for 100 pieces of screw,=20
> about 6 cents each.. if you don't feel like screwing around with=20
> substitutes. I would not be particularly gobsmacked if they were=20
> actually metric-- K18 4.5 or 5mm.=20

Thanks, I will probably order some from Hammond too. I was manly=20
wondering how to figure these sizes out or find a good reference to save=20
time in future.
I seem to spend an inordinate amount of time on the mechanical aspects=20
of designs, so I figured it might be time to learn something about the=20
relevant bits and hopefully make the process less painful.

>> I have trawled through pages and pages of all the many different
>> "standards" - Whitworth, BAS, UTS, etc but I can't fit 32 into anything
>> that seems to fit properly.
> Thread forming screws designed for plastics have a completely different
> thread form from machine screws. Also different from those designed for
> metal. You might be able to use ones designed for metal, but they won't b=
e
> ideal.

Ah, I see. I'll see what I can find on threads for plastic.

>> The UTS (Hammond are US so I assumed this would be the most likely) is
> Hammond is Canadian, headquartered in Guelph, Ontario.,

Well, I was close.. :-)

>> specified as X-Y where X is the diameter and Y is the thread turns per
>> inch. 1/32 would give a diameter of 0.8mm and 3/16 means nothing in
>> terms of turns per inch. Even if you swap them round it doesn't make sen=
se.
>> Whilst linking to the above datasheet I noticed it's actually different
> >from the one I have, and gives #2 x 3/16" instead of the aforementioned
>> 32 x 3/16". This makes more sense as #2 equates to 2.18mm diameter
>> shank, but I'm still unsure as to the thread (not that it matters too
>> much as it's self tapping, but I'd like to know anyway)
>> I'm going to order some number 2 screws, but can anyone help me to
>> confirm exactly what the numbers mean for future reference?
> Hope this helps.

Yep it does - thanks a lot.

--=20
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