Rivnuts
If you've ever seen the most recommended post on this part of the forum, you'd probably see a thing or two about PEM self-clinching nuts. They're nice, but they're not exactly an easy install job, and require a bit of tooling to get right. Some quick research tells me that you need about 3500 lbs of force on each nut to install them properly on steel, and I did not have access to the kind of punch to install those to spec, so I opted to make use of "normal" rivet nut or rivnuts, which are far more abundant and easy to find. They rely on the buckling of a midsection on the threaded rivet to provide the backstop on the rivet and anchor it in place, which is pretty easy to get right and can easily be done at home.
I initially bought a pack of aluminum M3 rivnuts. This was a terrible decision--I will strongly urge anyone who thinks to use rivnuts to go for the steel variety. The aluminum threads strip far too easily, and are nigh impossible to install correctly. Even with steel nuts, the installation of fasteners were not exactly easy... I initially tried to use the tooling that typically is used for this application, where you have a bolt (M3 in this case) against a thrust bearing, which allows you to hold the bearing in place and use the threads to pull the rivet into place. I ended up with issues accurately and reliably getting the rivets to install, and the threads weren't too happy. The twisting action of the thread-pull method doesn't seem all that great.
Then I attempted to use a drill press with a dowel pin to use as an improvised hydraulic press, but that resulted in what you see below...
All too often I came across this defect, in which the pressure applied was off the axis of the nut, causing it to buckle to the sides. From what I could see, it was hard to get uniform axial loading on the rivet, and often the rivet would buckle off axis like seen here. I eventually figured out how to install these nicely, and reliably:
A washer, a dowel pin partly turned down to M3 internal size ~2.8mm, and a 123 block all in a milling vice does the trick. You have to align the dowel pin through the nut, with the hole of the washer aligned over a hole in the 123 block. This solves a couple of different problems:
- The dowel pin provides an internal "guide" for the rivet to ride along as it is crushed.
- Using a vice prevents the pressure from backing out as you install.
- You can go nice and slow, monitoring the rivet as you go--much easier to get it right.
Once aligned, this setup gives very reliable, clean results by simply crushing the nuts into place. I just used a dowel pin that was lying around (and it was hardened steel--the lathe wasn't all that happy) but you could probably just grind down a regular M3 socket head screw to get the exact same results.
This is about as far as you want to go. You can give it a little more, but avoid over-crushing the nuts. The result is a very nicely aligned, secure installation of threaded fasteners fixed to your sheet metal. You can install some as test pieces to see how far you want to go, I opted to go as far as the seam lines.
With all fasteners installed, it's time for a test assembly...
We're still missing the plywood front panel, but this is pretty dang good. I'm not getting extreme blowout anywhere like I used in my older design and I've got nice clean angles everywhere! Very happy with the results. Looking like a nice case.
