Angle grinder flat cut-off jig
I used this big carbide tipped dry cut saw to make most of the cuts when I built my lawn tractor trailer. It worked well, but I had the workpiece insufficiently supported and it got loose on several cuts, which resulted in about a third of the teeth on the blade getting chipped or broken.
Small thin angle grinder cut-off disks work really well for cutting off metal, and it occurred to me that a small angle grinder, in a jig that worked more like a sliding miter saw, could also be used to make flat cut-offs on larger workpieces.
Then I had the idea of just sliding the grinder's guard along a table to achieve parallel motion, with the stock to be cut mounted vertically.
I see a lot of people using small angle grinders without a guard. I think this is partly because you can easily buy 5-inch (127 mm) cut-off disks, but the small angle grinders are typically just 4.5", so the disks won't fit in the guard. Fortunately, the ones from Canadian Tire have about 1/4" margin around the guard, so a 5" disk will just barely fit within the guard, with less than half a millimeter to spare. It's tricky to get the disks on the arbour though, they need to be flexed a little to get them into the guard.
Looking at it closely, there is part of the cut that is about 1 mm lower. I'm not sure if this was because of the heat, or because I didn't hold the grinder flat enough, or maybe because of pre-existing stresses in the metal.
At any rate, for a piece this complicated, the cut-off was satisfyingly flat.
The angle grinder disk had about 1.5 mm of clearance inside the guard after these cuts, so possibly, the disk got about 5% used up from this cut. These disks don't last particularly long. But they are cheap and not as easily ruined as carbide tipped metal saw blades.