Tapping thin sheet metal for machine screws

Hi,
I notice that the cases for CDROM drives etc. used in PCs are constructed of relatively thin sheet metal (1/32" perhaps) yet are able to support M3x.5 machine screws. The hole for the screw has what looks like a 1/16" - 1/8" long threaded tube on the inside of the case that basically supports most of the screw threads. Is this done with a special tool or is this "tube" brazed over the hole then tapped? Normally, if one uses a sharp drill bit then taps anything 1/32" thin there is very little thread to support a machine screw regardless of the TPI.

oparr wrote:
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If on close inspection it looks like that "threaded tube" is contiguous with the sheet metal then it was probably made with a piercing punch which dragged the sheet metal along with it to create the tube.
If it doesn't look contiguous, they may be using one of the several brands of commercial fasteners which press into an oversized hole in the sheet metal and leave a flat surface on one side. Here's an example:
http://www.accuratescrew.com/CatalogPage.aspx?ProdCat=NUTFLSHA
If you've only got a few pieces to make you can try putting the sheet metal on the end grain of a block of soft wood and whacking a skinny sharp punch (like an ice pick) through it to make an extruded hole of about the right size, then tap it.
HTH
Jeff

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This can work really well. Having experimented with it on a big project years ago (re-bodying my dad's trailer), I found that a very abrupt point on the punch, not too sharp, did the best job of drawing the sheet metal into a fairly long "tube." Then I put a drop of Loctite on each screw before screwing it in, to keep it from vibrating out. Twenty-plus years later, every screw was still in place.
Ed Huntress

Ed Huntress wrote:
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Gotta like that. Oh, it has been used since then? ;~)
michael
chuckle...

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point on
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into a
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They used it a few times a year for around 20 years or so after I worked on it, and then they sold it. 'Don't know where it is now. Maybe in the Bermuda Triangle. <g>
BTW, my dad bought the skin from the original maker of the trailer, although I can't remember who it was. If I were doing it today I'd use those aircraft-quality pop rivets. As cheap as I am today, though, I was even cheaper then, when I was in high school. I used stainless self-tapping screws. There were cap strips meant to cover the fasteners but I never put them on. I had caulked the seams and it looked fine without them.
Ed Huntress

oparr wrote:
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Looking at a spare CD drive, I see it's not seperate pieces for the threaded holes. The thin (18 gage, I think) metal is made thicker at the hole by swaging it--punching a die into it to form a lip, which gives enough for a couple threads.
Ken Grunke SW Wisconsin http://www.token.crwoodturner.com/shop /

And the threads are probably formed instead of being cut. I havn't tried form taps in this method but they will most likely work. Find the tap drill size for it, and make a tapered punch to suit, and a die with rounded corners to give you the lead in. (dia= punch+ 2xthickness)
Ken Grunke wrote:
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Thanks to all for your help. On further inspection of the CDROM case it does appear that the "tube" was extruded using a punch or punch/die combo of sorts. Will follow up on Machineman's suggestions. Probably try a thread-cutting tap after punching since I have it already. Can always get a thread-forming tap if the extruded metal is too thin. Hopefully, the extra torque required for forming won't cause fatigue if the metal is too thin.
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    --Sounds like they're using some form of "nutserts". You can get a thing that looks sorta like a pop riveter to do these at home. *However* if the thing you're fastening with these is going to require frequent assembly/disassembly or if it's subject to vibrations of any kind I'd advise against using these, as they can come loose in their holes and that will give you endless headaches. A better fastening method can be done with "formdrill" type bits, but this op must be done in a milling machine, as it requires high rpm and quite a bit of force; more than can be done with a hand drill.

oparr wrote:
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If your application requires more strength (thread engagement) than that, use something like a riv-nut. See <http://www.metalworking.com/dropbox/_2000_retired_files/thrdsrt.txt
Ted