Has anyone used a counterweight system for the Z axis? Mine is a bit heavy after I switched to a spindle motor and went a bit overboard beefing up the holder and added a dust collection system. If there’s no power going to the stepper motor it falls.
Unlike the MPCNC, falling Z is pretty common on the LowRider, so it is mentioned fairly frequently. The problem with a counterweight is that you are adding still more mass at the core. All that mass has to be accelerated and decelerated with every direction change. You are probably already experiencing feedrate limitations due to the current mass of your router and vacuum.
There are some potential things (with downsides) that you can do to mitigate/solve the issue:
- Attach the dust shoe to the core instead of the Z axis. This might not reduce the weight enough to solve your problem, but it is suggested even for lighter weight MPCNC builds.
- Go to a 1-start instead of a 4-steart lead screw for the Z axis. This means you will need to turn the lead screw 4X more to move the same distance. For most builds this means a substantial reduction in the max Z feedrate, though that can be overcome somewhat by running your motors at a higher voltage and/or running a faster control board.
- Use a solenoid breaking system. When the power is lost, a break is applied to the Z axis. I’ve skimmed topics that have mentioned this kind of system for the LowRider, but I’ve not seen any specifics of someone who has implemented one. Apparently this kind of breaking solution is not uncommon for large CNC machines.
Edit: You can use g-code to control what and when steppers are engaged. You could add end g-code automatically to your scripts that disengaged the X and Y steppers but leave Z engaged. This would allow you to manually pull your spindle away from the piece and potentially block the Z axis. Blocks are mentioned fairly frequently as a “solution” for the Z axis falling on the LowRider. Of course this does not solve the problem of the spindle falling in an emergency stop situation.
Yup, many of the machines in my plant are some sort of custom made cnc with all vertical axis having brakes. Most of the bigger servos can be ordered with an electric brake built in. Some of the older machines have been retrofitted with pneumatic brakes to stop z drop. The downside to the pneumatic brake is you’d need a compressor running to supply constant air pressure, or your brake will kick in mid route. I’ve seen electric ones, but haven’t used one, yet…
Was thinking about this post the other night, then found these… nema17 with electric brake. $60 each. Still tempted to make a vertical mpcnc or lowrider. Well, at least 30 degrees from vertical… would save a ton of floor space.
What about some sort of constant force spring to apply the force rather than a mass+pulley setup? I’m not really sure how much force/mass we’re talking here, but it looks like you can get springs that are significantly stronger than their weight.
Those don’t exist, right? The force from a spring is proportional to its distance from the rest position. If it is 2" down, it will expert twice the force as 1" down. If you had a very long spring (or a long coil), you could have something that was 200" away from rest and 201" would be about the same.
An ideal spring is like that, but real springs can be different, in this case by design. It’s not actually a constant force, but over the main operating range (after a transition region from 0 force to the constant region) these springs apparently exert an approximately constant torque. I just saw they existed today, so this is basically all I’ve learned of them.
It looks like they are using my idea (haha, jk) of a long coil to get near constant performance.
A torsion spring and a spiral shaped pulley could accomplish something similar, where the radius of the pulley is inversely proportional to the torque.
I had thought about trying to build one to offset the weight of my Z axis, but I got distracted as I always do.
The only problem would be the swinging weight causing other issues. An electric brake has no swinging mass.
We have an older machine that is my model for a vertical mpcnc or lr. The frame is 30 degrees from vertical. We added cables pulleys and weights to off set the enormous weight of the x axis. The weights only move with the axis in one direction, up or down. The y axis carries the z axis independently of the x axis. Assume someone originally had seriously miscalculated the mass for the servo brake capacity. With the counter weights, the brake has not slipped.
Just a thought…
Or pipe dream…
Or straight up delusion… lol
BTW I believe there are constant force springs in each of our beloved tape measures to pull the tape back. I have used them before when I first got out of college and worked at International Harvester on very large farm tractors like forever ago.