Puncture through sheet metal for tapping

As an alternative to PEM nuts, steel sheets can be assembled by puncturing the lower sheet and tapping the short "tube" formed
by the ragged edges. What's the name for this technique ?
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Andrι Majorel <URL:http://www.teaser.fr/~amajorel/
"Buy in bulk, that's my advice." -- Lemmy
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It is called: puncturing the lower sheet and tapping the short "tube" formed by the ragged edges
Bob Swinney
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For tubing assemblies, we used to blind punch the top surface of the tube with no backing, got the shape you are referring to. Usually referred to as as "pierce and tap" or "extrude and tap"
You can get a much better form for the hole if you grind the end of the punch pin to a taper with the main section of the pin the size of the desired tap drill size. For the punch block, use a hole about tap drill size + 2x diameter + .010" or a bit more. Chamfer, round, and polish the hole edge to allow metal to ease over the corner and give thicker sides as well as minimize tearing.
Andre Majorel wrote:

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Yeah. If you Google "extruded nut" you'll find what you're looking for. This process is technically not extrusion or drawing. It's a bending operation.
I designed/built the tooling for such nuts during my apprenticeship. I recommend a roll-form tap. You should get roughly 30% more strength in the threads, which is important as there is sooo little meat in the wee tube.
We actually drilled a small hole in each nut before using the forming punch. We found that if there was no hole, you would end up with a ragged hole at the other end of the tube and it would typically split off-center. We only needed about 200pcs so we drilled them. In normal production, the hole would be pierced. Indeed, you can combine the pierce and forming punch. The pierce will not be supported, but it's only a little wee hole anyway (like 1mm pierce for an M6 thread).
Using 16ga mild steel sheet, we could easily get the forming punch using a 5 ton arbor press.
We used a urethane stripper cause it's really simple and the nut *will* get stuck on the punch without a stripper of some kind.
As I remember, we left the die hole sharp or perhaps with a slight countersink. This may or may not be important. The application was pretty relaxed so we didn't spend the time to test a sharp/countersunk/ radiused hole. YMMV

All good advice.
Regards,
Robin
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wrote:

Why not go with a riv-nut or a two piece threadsert rivet? Cost? Time?
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"Bob La Londe" wrote: Why not go with a riv-nut or a two piece threadsert rivet? Cost? Time? ^^^^^^^^^^^^^^^^^^ In an application where a drilled hole and self-tapping sheet metal screw are good enough, or almost good enough, this would be better. Threaded inserts would, of course, be better. They cost more and take longer. They also require access to the lower sheet prior to assembly.

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Did you mean tap drill size + 2x *thickness* + .010" ?

Thanks folks. The application is 19" cases.
The sides are usually pieces of relatively thick (perhaps 1.5 mm) steel sheet with four 1-cm wide "ears". The front panel and back panels can be attached to the corresponding ears with whatever screws suit you.
The top and and bottom covers are made of thin (<= 1 mm) sheet metal. They are often attached to the sides' top and bottom ears with button head screws. Obviously that's not ideal because when you insert or remove the unit from the rack, the screw heads scratch and get caught in the units above and below.
I'd like to use countersunk heads. Problem is, the head of an M3 screw is thicker than the covers by at least 0.5 mm. So I'm thinking of either making a 0.5 mm-deep countersink in a PEM nut or pierce and tap using an ad-hoc die with a 0.5-mm deep countersink.
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Andrι Majorel <URL:http://www.teaser.fr/~amajorel/
"Buy in bulk, that's my advice." -- Lemmy
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On Mon, 21 Jul 2008 10:45:35 +0000 (UTC), Andre Majorel

Are you familiar with undercut flatheads?
http://www.mcmaster.com/library/20050628/91099A155L.GIF
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Ned Simmons

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Nice ! McMaster-Carr are on the wrong side of the pond for me but I'll have to ask my supplier if they have anything like that. Thanks.
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Andrι Majorel <URL:http://www.teaser.fr/~amajorel/
"Buy in bulk, that's my advice." -- Lemmy
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Yeah, I caught the error later. my bad. indeed: 2x the material thickness.
You might have to do the countersink as a second op. Same press, same die block different pin, different hole in the block.
Andre Majorel wrote:

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Been a while for me too! You are correct, the shape of the punch is fairly critical to getting a consistent result. You want to leave a bit of a flat end on the punch pin to get a consistent starting hole and the taper is adjusted to maximize the usable hole length. Using a roll form tap is an excellent idea, this thing wants to be machine formed.
The results are fairly fussy about material thickness and ductility. The die/punch combination that worked great on the last run may be miserable on the next run.
snipped-for-privacy@gmail.com wrote:

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    --Check this out: http://www.formdrill.com/english/index.html     --Generic term maybe "flow drilling" or some such.. Here's a link to the video: http://www.formdrill.com/english/pop_formdrillvideo.htm
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"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
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interesting. I thought I'd buy a couple common size formdrills. Till I seen the price. Small sizes start at 45 euro up to 325 euro for large. The darn thing is just a center punch, I'll try to make my own. If anybody has one, I'd love to reverse engineer (that is copy) the angles.
karl
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wrote:

Drill/punch a small hole, pound a nail set into it to form a tapered hole, use a sheet metal screw.
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    --Well it's more than a center punch: for a #10 size bolt the recommended speed is something like 2700 rpm. And those suckers get *hot*! If ya try it with a centerpunch it may anneal the tip. Dunno what they make the formdrill out of, but the ones I have show no evidence of wear after a few hundred holes...
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"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
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Adamantium? :)
Probably a steel alloy or carbide-mix substance but I've not looked closely enough myself to know either...
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Regards,
Joe Agro, Jr.
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Couldn't sleep... (common problem for me) so I just read all I could find about flow and form drills. They are made of tungsten carbide. So, I'm going to grind one up. By chance, do you have an optical comparator so you could get a measurement of the angles involved? Also, they mention applying a paste after each cycle. Do you know what it is and where to get it?
If this trial don't work, I'll buy a set. I've got a project right now that needs this solution.
Karl
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wrote:

I went down and tried a few different shapes, mainly center punches made from broken taps. It looks like a sharp punch drills faster but raises a tube on the top surface as well as the bottom. A rounded tip with tapered sides, ground by hand from a broken drill bit, pushed the metal mostly out the bottom into a tube. The flange on the tool in the photo squashes down the raised metal on top, but for low volumes it might be easier to knock it down with a punch and hammer rather than grind away so much metal. You need a backup underneath or the sheet will wrinkle. A drilled pilot hole helped. Cutting oil was OK as a lube and the tool didn't get hot enough to make it smoke drilling one or two holes in 6061 Aluminum 0.062" thick.
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    --Actually there are two different style bits: one that produces the flange and one that cuts it off. Mostly I prefer to keep the flange on as it means no material removal and therefore less weakening of the part.
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Hacking the Trailing Edge! : through the moronosphere!"
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There is a competitor to them called (interesting enough) Flowdrill.
http://www.flowdrill.com /
Still expensive, but if you have a *lot* of these to do, then an automated process using these is a good option.
We've sold equipment to run them, and have learned it's a balancing act of sorts... If you go too slow or with too little feed (or the other way around) you really don't get what you want... But when set up correctly, this is a great option for thin material, pre-tapped holes.
It can be done on a drill press... But again, it's a balancing act and requires a good amount of HP / thrust.
Regards, Joe Agro, Jr. (800) 871-5022 01.908.542.0244 Automatic / Pneumatic Drills: http://www.AutoDrill.com Multiple Spindle Drills: http://www.Multi-Drill.com
V8013-R
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