Spiral tube weld.
https://s3-us-west-1.amazonaws.com/h...piral_weld.jpg
Previously:
https://www.homemadetools.net/forum/...2791#post97149
https://www.homemadetools.net/forum/...251#post141954
https://www.homemadetools.net/forum/...371#post146495
https://www.homemadetools.net/forum/...537#post132301
https://www.homemadetools.net/forum/...279#post103846