Two spindles hold two borosilicate glass tubes on centre and rotate them at the same speed in the same direction.
I use various sized torches to weld the tubes together, shape them, &c. By plugging the end of one tube and fitting a blowhose and swivel to the end of the other tube, when they are welded to together this creates a 'sealed' tube, allowing the heated glass tube to be expanded (by blowing into the mouthpiece of the blowtube).
600mm between centres
110 volt, draws 0.7 amps at 10 rpm, 1.4 amps at 30 rpm
Fun and satisfying to build, this machine helps me a lot to weld tubes together on centre that I could never do by hand.