This topic is very interesting to me. I have limited experience with these conversions and spent lots of time considering air cylinders, counterweights, gas struts and bigger servos with brakes. I chose to use the gas struts configured so I would have some downward force and a closed loop stepper. My conversion was just completed, so unfortunately, I have not had a chance to use the mill yet.
The head weight is about 325 lbs. The two gas struts exert a force ranging from 100 to 140 lbs each over their range of motion. This means that at the top of the Z-axis the stepper sees a head weight of about 125 lbs. At the bottom the Z-axis the stepper sees a head weight of 45 lbs (assuming linearity for simplicity).
Here is how I visualize the effect of the gas strut:
My mill has 17.5 inches of Z height. If I mill a part that is 5 inches tall, and the part is in a vice, the cutting tools will be working primarily at the bottom portion of the z-axis and vary in height 5 inches. The stepper would see a change in head weight of about 23 lbs. As I mentioned, this is all new to me, but I would believe that during normal milling operation, on a milling machine that is perfectly counter weighted, the vertical tool load would vary far greater than 23 lbs as the cutting tool cuts through metal during a host of milling operations. Therefore, I can't see how a change of 23 lbs caused by gas struts would have much effect.
Please let me know your thoughts.... I find this very interesting.
Z Axis Counter weight. How much weight??
Moderator: cnckeith