I've been wanting a power drawbar for a long time. I looked for one but none are available so last summer... decided to make one but due to time available, I have not touched it since end of last summer. Below link is to my design... it uses a lever to mulitple the force at the draw bar. I'm only using a 3 1/2" air cylinder that is rated for around 850lb but I can get double that with the lever. The biggest problem is the limited space available. 3 1/2" is the largest that will fit in the motor cage and still allow the motor belt tension to be adjusted. I have mounted it on the Tormach and tried it... but my mount points are at the 45 degree angree on each side of the motor mount plate and I need to add one or 2 more at the end. The way it sits right now, when the power drawbar is activated, it will tilt the entire air cylinder unit forward.
http://www.cnczone.com/forums/showthread.php?p=504052
Another problem as many of you have mentioned is the force needed at the drawbar. I am not an engineer so I did it by trial and error. I had a 1200lb spring washer in there and if I tighten it to 1/2 compression... it will cut with 1/2" Endmill with depth of 1/4"... I did not try any more than that. I think, and don't hold me to this number, around 1500 to 1800lb washers should do the trick. Seeing the recent thread... I got my intereste back and hope to have some time this weekend to start playing with it again. I bought a bunch of spring washers and will first try to see if I can use a 1800lb washer to cut 1/2" depth using 1/2" Endmill. This is probabily the max I will ever cut and if it passes this test... at least for me, I'll be happy.
If someone comes up with one that will work... I'll still buy it... I have very limited time to work on projects as it is and even less time to get my equipment working so if I can spend a little extra... I rather buy it.