I think the backplane was just regular steel, my main problem was trying to use the Klein triple-tap. But perhaps I need better drill bits too. I don't think I could have even drilled the holes without tapping within 15 minutes, the way I was doing it. I used the smallest bit 1/16" to drill all the way through the steel, so that the hole placement would be as accurate as possible. Then I used the 9/64 (I think) to drill a little over halfway through, then turned the sheet over and drilled the rest of the way from the other side. I drilled from both sides to avoid burrs of metal sticking out. I probably should have used a bigger drill bit before the tap, because if I recall correctly, I actually tapped them out to 8-32 first, then 10-32. That's an extra step I shouldn't have done.
