So this is what i do, but it’s not super efficient. After making the 90 degree miter on the mill (see above pic), I rotate the tube 90 degrees and use the disc sander to start the counter-miter along the top centerline of the downtube (no picture of this, i’d need help to take that picture to be safe!). It’s difficult to get the downtube and sander perfectly square against each other, and I’ve yet to learn to NOT shave too much off before checking the miter in the fixture. Go slow is a good motto. Total bummer…I was using a True Temper OX Platinum 38mm downtube and will now need to use it on a smaller bike (oops!). It was mostly because I was trying to use the miter lengths from the BikeCad drawing instead of being more careful or conservative and inching up to the tube miter by filing/checking/filing/checking after the initial cut in the mill. I cut it too short by about 1cm somehow. That’s an expensive lesson. Had to use a 38mm Nova 9/6/9 tube, but no matter. It actually had much less of an arc when I inspected it on the granite table!
So, not learning from my recent mistake, I used the drawing to mark where the ‘top’ of the counter-miter will be along the centerline of the downtube and got a jumpstart on the miter using the disc sander. I use 180 grit paper but it’s still pretty easy to take off too much or not sand at the right angle! After getting a notch in the tube to work from, I loaded the tube (with the tube block) in the vise and took a 10″ half-round bastard file to the counter-miter until it looked good. Then it’s the normal run of – check in fixture, file some more, check… until it fits as perfectly as I can get it. It took me way less time on this frame than my first two but I’m sure there’s a better way. I have seen people use the mill to make this cut but that scares me right now, I’m just not that confident yet on the mill to try that out.