Non-round screws?

Is there any reason why the threaded part of a machine screw should not be perfectly round? I've seen several small ones (6-32, 4-40) that were slightly triangle shaped, and I'd like to know if this is just a sign of poor quality or if there's a legitimate reason for making them that way.
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larrymoencurly wrote:

Depends on where you got them. Sometimes, those are "thread cutting" screws, if they also have a slight taper as well. Great for sheet metal and most plastics. Just drill the right size hole, position the parts and screw it together; no tapping of threads necessary.
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Usually those are 'self tapping' screws going into soft steel or plastic as far as I have seen.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
larrymoencurly wrote:

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See http://www.taptite.net/taptite2000.asp
Regards,
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larrymoencurly wrote:

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larrymoencurly wrote:

sounds like the screws that they use in plastics... it cuts its own thread(ever see a tap... its not perfectly round also)... the reason why is so the cut material can fall out and not interfere with the threads..
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On 23 Aug 2003 19:43:09 -0700, snipped-for-privacy@my-deja.com (larrymoencurly) wrote:

Never seen that. Some have suggested a self tapping screw but it could only be that if the forward edge of the missing metal had sharp edges that would have a cutting effect. It sounds like your bolts are triangular with rounded edges? If so it must just be a poor quality bolt.
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being triangular is not poor quality and that is a bad assumption , fact is that shape is normally material dependent and meant to be self cleaning on insertion .
snipped-for-privacy@mucks.net wrote:

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On Sun, 24 Aug 2003 13:48:45 GMT, "atec77>>"

Please explain this process of self cleaning. Post a link if you have one.
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On Sun, 24 Aug 2003 09:54:00 -0400, snipped-for-privacy@mucks.net wrote:

Following up my own post:
http://www.entegrafastener.com/products/threads/thrdfind.htm
Search the page for ... "in a rounded triangular"
" Provides a thread-forming action with no tearing. The trilobular shape also provides an anti-loosening action. "
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snipped-for-privacy@mucks.net wrote in message

You're right that these aren't self tapping. They're the chrome plated screws found on many computer cases and that go into holes that have already been tapped.
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right that these aren't self tapping. They're the chrome

If the screw taps its own threads how would you know the holes are already tapped?
Erich They are thread forming screws.
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larrymoencurly wrote:

Those are thread forming screws they use in some computer cases. Note the tapered end to get them started.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
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Approximately 8/23/03 19:43, larrymoencurly uttered for posterity:

It could be poor quality, you can pretty much judge that by just looking at the things and whether they all look uniform or more like mutations of each other. Some of the self tappers I've come across were vaguely triangular, easier to use.
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Lon Stowell wrote:

I used to work in a fastener plant and one of our products was a small machine screw that was intentionally slightly triangular-shaped. I'm not sure what the purpose of this was, but it was definitely intentional (as it's harder to make one like that than it is to make one perfectly round.)
nate
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Tri-lobular screws are not only thread forming, but are also thread-locking. In that some material will deform into the flat area and thus prevent the srew from unscrewing itself. No lockwasher or Liquid lock washer is needed Widely used on plastic and die cast aluminum

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The particular non-round screws I have go into common sheet steel computer cases with holes already threaded. The steel was pressed in slightly so that the threads extend about 1/16".
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larrymoencurly wrote:

That really depends on where you get your cases. I have seen lots of unassembled new cases with just the holes stamped into the sheet metal. Those triangle self tapping screws are needed for these type of cases.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's
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that's called a pierced hole. often the screws are used which are available or cheapest or both, not necessarily the 'proper' screws. sammm
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