I scrounged an old Rigid pipe threader, and it works a treat for bar up to 3/4" cold, and much bigger hot. The three-jaw chuck is not handy for holding square, but it is easy to make inserts from either 5C square collets (for small stock up to about 3/4") or fabricate larger ones from scrap. If you have a milling machine, you can make spiffy ones with square holes and hex perimeter. Alternately, you can get some really large hex nuts and scab in some welds inside the round hole to make it a square. The three-jaw chuck holds the hex nuts perfectly.
For the tailstock, I just clamp the threader to a piece of scrap wide-flange I-beam and scab together a holder fork that can be clamped wherever I want it. You can get fancy and make the tailstock slide to accomodate shortening when you twist really long sections, but I've never found the need.
One note about homemade twisters:
Pipe threaders, lathes, gear reduction units, etc, all will have some greater or lesser degree of "overrun" on the twisting when you shut off the power. It's pretty much inherent in any geared-down device; the momentum keeps it turning for another half or more revolution. Since this isn't always consistent, due to variables in hardness of the stock, heat levels, etc, it's a good idea to plan for some variation. If you can't tolerate a bit of uncertainty, then you need to build in a way to stop the machine on a dime.
Fancy twisters like the German machines will twist stock in any increment of a degree or so of twist, from what I hear. They also cost big bucks. For the small shop, you can put a brake on the twister head, create an automatic stop that drops in after X number of revolutions, or something similar. For myself, when the number of turnsis critical, I try to twist them cold and stop a bit short of the final number and finish turning the machine by hand with a big bar.
I scrounged an old Rigid
I scrounged an old Rigid pipe threader, and it works a treat for bar up to 3/4" cold, and much bigger hot. The three-jaw chuck is not handy for holding square, but it is easy to make inserts from either 5C square collets (for small stock up to about 3/4") or fabricate larger ones from scrap. If you have a milling machine, you can make spiffy ones with square holes and hex perimeter. Alternately, you can get some really large hex nuts and scab in some welds inside the round hole to make it a square. The three-jaw chuck holds the hex nuts perfectly.
For the tailstock, I just clamp the threader to a piece of scrap wide-flange I-beam and scab together a holder fork that can be clamped wherever I want it. You can get fancy and make the tailstock slide to accomodate shortening when you twist really long sections, but I've never found the need.
One note about homemade twisters:
Pipe threaders, lathes, gear reduction units, etc, all will have some greater or lesser degree of "overrun" on the twisting when you shut off the power. It's pretty much inherent in any geared-down device; the momentum keeps it turning for another half or more revolution. Since this isn't always consistent, due to variables in hardness of the stock, heat levels, etc, it's a good idea to plan for some variation. If you can't tolerate a bit of uncertainty, then you need to build in a way to stop the machine on a dime.
Fancy twisters like the German machines will twist stock in any increment of a degree or so of twist, from what I hear. They also cost big bucks. For the small shop, you can put a brake on the twister head, create an automatic stop that drops in after X number of revolutions, or something similar. For myself, when the number of turnsis critical, I try to twist them cold and stop a bit short of the final number and finish turning the machine by hand with a big bar.