I had a crazy idea for a completely different kind of power drawbar a couple of years back, and yesterday I finally had the time to build one. I specifically wanted to be able to use both TTS tooling, as well as R8. This meant Belleville spring designs were out, as it is very difficult to provide enough drawbar tension to properly retain TTS tooling under worst-case conditions (about 2500+ pounds drawbar tension, or 25+ ft-lbs drawbar torque is required). To change out a TTS tool, the drawbar only needs to be backed off about one turn, while for R8 tooling it needs to be backed out 8-10 turns.
So, here's my crazy idea - use a stepper or servo motor, driving through a high-ratio planetary gearbox. This provides very high torque capability, and makes it easy to control how much the drawbar is turned, by driving the stepper as another axis from the CNC controller. A little E-Baying got me a 55:1 NEMA34 gearbox capable of 75 ft-lbs. I picked up a 400 oz-in NEMA34 stepper, which lets me use the whole 75 ft-lbs if necessary. Mechanically, the thing is dead simple - The stepper and gearbox are mounted to a floating plate which is moved up and down by air cylinders. A 3/4" socket is attached to the gearbox output shaft by by a simple home-made coupling, and either a mechanical or pneumatic mechanism is used to actuate the spindle brake. I got a dual-shaft stepper so I can put a rotary encoder on the shaft, to monitor actual movement, if needed. Everything can be controlled by a simple macro (or, in my case, a simple KFlop C program).
Here's how it will operate:
1) Spindle stops, spindle brake is applied
2) The PDB is dropped down, with low air pressure, and the stepper is rotated slowly, until the socket and drawbar hexes align and engage. A micro-switch will signal that the socket is fully engaged.
3) The stepper is turned at high current/torque to loosen the drawbar by the required amount - one turn for TTS tools, 8-10 turns for R8 tools.
4) Once the new tool is mounted, the stepper is turned at reduced current/torque (corresponding to 25 ft-lbs drawbar torque), until motion stops.
5) The PDB is raise up, and the spindle brake is released.
The photos below show the complete mechanical assembly, missing only the engagement microswitch and spindle brake actuator. Here's a short video showing the PDB moving up and down while mounted to the mill:
http://www.youtube.com/watch?v=saGv_deZmL0&feature=youtu.beI just spun up the stepper for the first time. On Monday, I hope to get the software going, and be able to "use it in anger" for the first time.
Regards,
Ray L.