A top tube and down tube are born. This is how I miter these tubes using a fixture I designed and had manufactured locally. From my drawing of the frame I’ll get the angles and center-to-center dimensions of both tubes. The fixture is attached to the compound of the South Bend 13” lathe. I’ll use the power feed to control the cut. This method has work very well for me, and I’m able to miter thin walled heat treated tubes without a problem. This whole procedure takes less than a half-hour.