A few years ago, I needed to make a cut with a 1/64" slitting saw very close to my vise, so I made a custom arbor. It had a cap about 3mm thick held on with an M4 (I think) countersunk screw. Running the saw had a tendency to tighten the screw, so it was sometimes hard to remove. Then a few weeks ago, I forgot to tighten the gibs before making a cut and the saw jammed while running. That really tightened the screw, to the point that when I tried to remove it, I made a helical hex wrench.
I tried using an impact driver to remove it, and instead snapped the bit off.
So I put the arbor in the lathe and faced down the cap until I could grab what remained of the screw and remove it with pliers, and sat down in FreeCAD to design a new cap and modify the arbor to match. If I hadn’t just been Making a rotary broach. Twice I might have just moved to a much larger countersink screw, maybe M8 (which takes a 5mm socket), but since I can now broach a 6mm hex hole about 9mm deep, that would give me a lot more torque for unscrewing, and I can make the whole thing in one part.
Section view showing the deep broach:
Because I designed the new cap with a thin 2mm rim, I chose tough 4140 steel instead of mild steel, even though I had a chunk of 1144 handy that would work just fine. In retrospect, I would not do that again, it’s just not necessary, and if I ever crash this into my vise jaws, it would be better for it to be made of mild steel. And it turned out that 4140, being tough, is tough to broach as well. I may have spent 20 minutes slowly broaching and checking progress and occasionally drilling out chips hoping to reduce load and looking for any damage to the broach and diagnosing that the high pressure had stalled my electronic lead screw… 
The only real design mistake I noticed was that I forgot that my M12x1.5 tap isn’t a bottoming tap, so the thread wasn’t fully formed to depth. That meant that I had to truncate the threads a bit to be able to screw it in.
I have modified the design for the arbor to drill and tap much deeper to accommodate non-bottoming taps, so that anyone else using this design won’t have that problem.
Showing off a 1/64" blade:
It is such a perfect fit for 1" blades that I can rest it on the blade to take this picture without it popping off, yet it doesn’t distort the blade and is not difficult to remove:
As I improve this design, I plan to keep pushing changes here:
I’ve already simplified the design relative to the images I show here.
The current FreeCAD files are also available here:
Arbor.FCStd (292.2 KB)
Cap.FCStd (610.9 KB)
Here are my drawings, such as they are:
Cap.pdf (125.4 KB)
Arbor.pdf (94.1 KB)
