Hi all, just noticed this thread, and I have a couple of questions to raise about the automatic drawbar design (both your design and the Z-bot, since they appear to work the same way):
Firstly, if the piston, through the cam, is applying 1000lbs force to the top of the drawbar, doesn't that load get transferred to the spindle bearings at the same time? The Z-bot certainly seems this way, since the cylinder is bolted to the head. It would be kinder to the bearings to arrange the mechanism to push the drawbar out whilst holding the spindle.
And secondly, is it safe to assume that if a rigid drawbar is holding a tool in with a force of 600lbs, then springs to generate the same force will be adequate? Because with a rigid drawbar, if you manage to pull the tool even a little way out then the force required to pull any further will increase very rapidly (thousands of pounds within a few thou, most likely.) With springs, loaded to their recommended deflection at 600lbs, the force required the further the tool is pulled out will only increase a couple of hundred pounds until the washers are completely compressed. So, with a rigid drawbar it may take a few thousand pounds pull on the tool to loosen the collet enough to release the tool; but with the springs it may only need 900lbs, even for an instant, to pull the collet easily far enough out of the taper to release the toolholder.
I have no idea if this is actually the case, but it's what my intuition tells me.