Just to say how easy it is....
I used ¼" steel threaded rod on my Curved rod tracker & found that the 45 Kg gas cylinders gave exactly the correct radius required.
Just bent the 3 ft rod by hand & used the "best bit."
Pics in the "Show us your equipment" thread, somewhere.
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http://www.iceinspace.com.au/forum/a...owse.php?a=553
With a little help from a friend with a lathe, (for the bearing set-up), the whole thing was completed in 4 days.