[SCzEngrgGroup]

Last night, thanks to some prodding from a friend, I finally got off my lazy a$$ and designed up the stepper-driven power drawbar I've been wanting to build for the last several years. Today, between interruptions to help a neighbor build a dog run, I got most of the parts machined. All that's left is the coupling between the gearbox and the drawbar, a trivial lathe-turned part, and the spindle brake mechanism, which is also very simple. Then comes all the electronics (Gecko G201 and optical encoder I/F) and software to make it all work.

For anyone who hasn't kept up with the plan here, I've been wanting a power drawbar that would give me full drawbar tension required for TTS tooling under the worst case conditions (about 2500-3000# drawbar tension, which equates to about 25-30 ft-lbs drawbar torque), and would make it possible to "quick-change" both TTS tools (unscrew drawbar one turn), or full R8 tools (unscrew drawbar compeltely). Belleville washers were out, due to space constraints, and they would not allow R8 tools to benefit from the power drawbar functionality. So, my solution was a stepper motor, driving a planetary gearbox with a high reduction ratio (55:1 in my case). With the 400 oz-in NEMA34 stepper I have, this gives me up to 75 ft-lbs of torque to play with.

The air cylinders will provide ~200# of down-force, to pop the taper free, while springs will return the assembly to the up and disengaged position. The existing mechanical spindle brake will be mechanically activated by the PDB assembly coming down into engagement. The stepper will then unscrew the drawbar the required amount, as confirmed by feedback from the shaft encoder on the stepper. Stepper driver current limit will be controlled to provide maximum torque when loosening the drawbar, and limited torque, equating to 25 ft-lbs, then tightening the drawbar. The stepper will be driven by a spare axis of the CNC controller, under control of a macro.

With luck, I'll get the drawbar coupling done tomorrow, and be ready to give it a try as soon as I can get the electronics updated.

Regards,
Ray L.