Working near aluminum angle?
Answers: You might want to rob it to a sheet metal shop & have them use their "brake" to tighten up the angle a bit. This would be about a 1 minute assignment for them & they'll probably charge you about $5.00 (to be in motion into their coffee fund) if anything.
You are pretty much stuck with that angle. If you grind on that aluminum, it will distort and wont be stable. Kind of close to releasing internal stresses in it due to the "skin" on the aluminum.
I would bring that back and see if you can attain a perfect 90 from the place you get it. Take a Try Square along and check the whole supply.
You steal that to a shop and its $50 to reform and it will not stay in attendance perfectly at 90 degree.
Have you though of a drill and tap a few 1//4-28 screw on the near boundary of the angle leg spaced along the 24 dimension and to put a 3 or 4 jack screws within there so you could adjust to a foolproof 90 against your mounting surface?
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