I am doing a lot of research in anticipation of buying a bench milling machine and converting it to CNC.
One thing that I’ve come across is the weight of the head. In manual mode, the head is positioned above the work, and then the bit is lowered into the work by using a “quill,” which is basically a tube that holds the spindle, and can move up and down over a limited range. When converting the CNC, the quill is ignored, and locked into position, and the entire head is moved up and down. On the machine I am looking at, the head is around 160 pounds, and the ballscrew to move it needs to be really beefy, and requires a big motor. Also, the heavy head tends to fall if the Z-axis motor is turned off.
To fix those issues, people do two different things -
- Use gas springs to provide a lifting force to counteract most of the head weight.
- Use a weight and pull system to counterbalance the head.
The counterbalance system is probably simplest, but it makes me wonder - does the addition of another 160 pounds double the inertia? Meaning, if the head is commanded to go down and then quickly move up, will that motion be easier to perform with the gas spring system vs. the counterbalance. Or, does it not matter? If the motor and ballscrew is capable of moving a non-counterbalanced head, will any counterbalance always make it easier to move (even quick direction changes)?
I installed gas springs in my old mill. Granted, it was a much smaller machine than yours–the head probably weighed less than 50 lbs. But it was an easy installation, and the springs fit inside the column, with a little bracket to pull the head up.
I used a premade kit. My mill was one of those common Harbor Freight/Grizzly specials that all come from the same factory. So lots of third-party support. I didn’t do it for a CNC conversion, just convenience.
A counterbalance system will double the inertia. But, probably not an issue. Z travel speed is unlikely to be a serious issue; generally you spend far less time doing Z travel than XY. Often to where you’ll move Z by 0.05", then spend several minutes with XY movement.
I don’t think my new Tormach has any spring or counterbalance system. Sometimes it’s just a matter of having a beefy enough motor.
Thanks.
That’s what I figured, but I wanted to ask here.
Tormach, huh? Those are very nice, but out of my price range. Which one do you have?
PCNC 440 with all the trimmings. The power drawbar is very nice, as is the “Tormach Tooling System” toolholders. Just one button to change tools, with Z offsets handled automatically. It has the auto tool changer also, but it’s fiddly and I never got it working properly.
Good luck with your conversion, and keep us posted in the hobby thread. I thought about converting my old mill, but decided to splurge.