I made this parametric model for 3d printed tube blocks with mating tube miters modeled in. The idea was to 3d print them, fasten them over 4130 tubes and use the printed blocks as guides for hand filing. The flat side on the miter intersects with the center axes of both tubes, so it works as a reference surface. To really make these work well, you’d need a really big set of calipers. Large vernier calipers are definitely accurate enough for welding fabrication work and would be a good fit for this application.
Paul Carson Designs 