Originally Posted by
pastera
I have a 7x12 and have turned some 6"+ parts from cold rolled steel (1.5" thick). This is not something I would want to do all the time. This was done with the parts on an arbor because my 4" chuck would not open enough to hold the parts. Even if it did the jaws would not clear the bed extended that much. Going with the 5" would not help for the same reason. If you were to make your own jaws for the 4" or 5" chuck you might be able to swing it but I would not trust it to anything other than plastics. Go with a 4 jaw if you do get another chuck.
With either the 5" or 4" chuck, you will not be able to part off a 6" part. The lathe simply too small to do this (the stock would hit the cross slide). If you just want to turn the diameter and add some simple grooves, the 7x12 will work.
Aaron