Here are a pair of rotary weld positioners that I built. They were 
designed so that they could be used as a pair of supports for welding
long pieces of tubing, clamp smaller objects in a single positioner and
weld with it's axis horizontal or flip them on their backs and
bolt a table on to weld smaller pieces. 

One unit is powered with a step motor that I scavenged at a junk sale. 
On the powered unit, the drive motor is electrically isolated from the
weld circuit to reduce the likelihood that the high frequency unit on 
my welder will cause problems with the step motor electronics. 

A large pillow block bearing was used as the basis of these pieces. 
Cutoffs of 1" wall steel DOM tubing were used for the spindles after 
turning them down to a light press fit into the bearing and 
drilling and tapping the shoulder area. The ID of the spindles is 2.5"
which should handle most of the stuff I commonly work. 1" square steel 
tubing was used to make the frames with a 1"x3" crossbar on the bottom
to bolt the pillow block to. A spring loaded aluminum wiper contacts
the center of the bearing to prevent weld current from damaging the
roller bearings.

Bob Harbour
wanderingmetalhead __at__ yahoo __dot__ com