Fixturing jig

I'm still wrestling with getting the rear subframe welded to the drop tabs. I spent a log time with magnets and clamps, trying to get a stable assembly of the subframe and the tabs that I could tack up in the frame, then remove to complete welding. I had no luck, and even cursing at it didn't help, so I decide to regroup and find another way to fixture the tabs.

I need them to be the right distance apart, both side to side and fore/aft, and I need the top edges to be in the same plane. I decided to try 3D printing these little jigs, which I will position and screw to a piece of plywood.

The idea is that I will clamp the plywood to the frame and use the frame and the transmission subframe to position the drop tabs, then screw the 3D printed jigs to the plywood. This should give me a repeatable position for the tabs, which I can remove from the car and move to my welding table.

This is effectively upside-down in the frame of the car, but orientation doesn't matter right now - just the dimensions I mentioned.

The plywood board is clamped to the frame of the car. The blue plastic jigs hold the tabs. I've used the transmission subframe to get everything in position, with some clamps and spacers to help align everything.

And here's the finished fixture, with the tabs in their slots. This can be repeatably placed in the frame, so the width is right if nothing else.

I will next finish prepping the subframe and the tabs and get it all aligned, then tack it together and test-fit it in the car. I think it should work; then I can finish weld it, bolt it back to the bottom of the transmission, and tack it into the frame.