Thanks Eddy for going to the trouble to take photos. Now makes sense. I have already got some steel castor wheels (won't burn with plasma). Will look at Felder? design and also exploring a less complicated solution (than my original) I found on another welding forum where castor wheels were simply slightly raised above ground and then lowered via adjusting the legs, albeit this approach may be more tedious to get it mobile. Will dwell on both solutions!