Almost jigless bicycle frame building

Step 7 Another way to hold a tube

Here's another way to hold a tube at a given height that I've used.

Clamp a piece of angle iron to your base, and then clamp the tube to the angle iron upright using a metal spring clamp. You could also use a small C-clamp, if you file the flat end of the C-shaped part of the clamp to have a round profile to fit inside the tube without denting it.