I am not in favor of moving the knee as the Z axis of a CNC mill.
The mass that you're trying to move is not in consequential. Even if you counter weight it, you're now DOUBLING the inertia of the system because BOTH the knee and the counterweight add up to inertia that has to be controlled. Eventually, friction comes into play and that too increases as you add more inertia.
It has been proven time and time again that reducing weight is better than increasing power when it comes to trying to make automobiles perform better. Since the mill and the auto follow the same laws of physics and inertia, the same logic should prevail.
It is hard enough to drive a spindle quill precisely with a Z axis drive and these have low mass and friction. I don't see how adding mass and inertia and friction (there is a bunch in the knee), will make it easier than driving the quill.
I do hope that folks who have done Z axis motion with the knee will chime in. Maybe they have found the holy grail on how to do it. I still contend that controlling anything with less mass and/or inertia will be easier that something with more of the same.  |