Rack Building Basics -- 90 degree miter
This is mitering for a basic 90 degree joint with 3/8" tubing.
The tube directly after cutting with a hacksaw. It's a little messy:

Make a notch in the centerline of the tubing using a file or hacksaw. This keeps the file centered in the next step:

File. A handy hint is that a 12" Nicholson *** file has a 3/8" diameter, so it makes a nice miter into 3/8" tubing. A 10" file works for 5/16", a 8" for 1/4" tubing.

Once filed you have a miter, but it's pretty ugly:
Clean up the inside of the miter with production cloth wrapped around a piece of tubing:

Then clean up the outside with production cloth:

Check the miter. It looks pretty good to me:
