Z Axis Voltage Adjustment to correct drift on G540

[bcurrey]

I built my CNC a couple years ago. Lately, I've noticed that if I run code that has heavy Z axis movement, it slowly lowers itself throughout the cut. In other words, if I set home to 3.5" above the table surface and run 75k lines of heavy Z movement, and send it back to home, the machine will be about 3" above the table surface.

I tested decreasing and increasing the Z axis speed/acceleration. It had no impact on this variance. I did the tests without the router on, so it was just the CNC moving to help reduce any influences on the machine.

It hit me today, that I upgraded my Z axis about a year ago.I'm beginning to think the Z axis isn't getting enough power to move the new and significantly heavier Z axis under increased use.

It's been a while since I've looked at the electrical on my machine. This CNC build is also the most advanced electrical work I've ever done, so I just want to make sure I'm thinking through my changes correctly before making a change.

• I was going to slightly increase the voltage on the backside of the G540 for the Z axis. Looking at diagrams, I believe there is a small screw that you can adjust to increase/decrease voltage
• Is there a way for me to measure the voltage while I adjust? If so, how?
• Is there a target voltage setting I should be aiming for?

Appreciate the help!

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[phomann]

The trim pots on the back of the G540 are to tune out any resonance in the motor. It is not to set the stepper motor voltage.
You need to ensure that you have set up the G540 to deliver the correct current to the motor. This is done with a current set resistor connected to pins 1 and 5 of the axis DB-9 connector.
Have a look at the manual for the drive to see how this is done.

Cheers
Peter


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[bcurrey]

The trim pots on the back of the G540 are to tune out any resonance in the motor. It is not to set the stepper motor voltage.
You need to ensure that you have set up the G540 to deliver the correct current to the motor. This is done with a current set resistor connected to pins 1 and 5 of the axis DB-9 connector.
Have a look at the manual for the drive to see how this is done.

Cheers
Peter

Thank you for the response! I did some research on this today, but unfortunately, I think it's confused me even more. For my setup, I'm using these cables . They come with 3.5 kOhm resistor resistors already installed between the 1 and 5 pins. Using that resistor, the the z axis is getting the max amount of power, correct?

Little more info about my setup: the Z axis motor I'm using is the one from Openbuilds high torque NEMA 23 step motors.

My Z axis setup is similar to this one: eBay Link


Do you have any other ideas of other things I can check? Thanks.

[peteeng]

Hi BC - Using the G540 with 3.5kohm resistors means the current is limited to 3.5amps on each axis. The dropping Z is unlikely to be electronic more likely to be mechanical, maybe. If you run a no load program does it do the same thing? Usually the z creeps the other way if you are doing lots of plunges as plunges require a lot of force and this can make the motor miss steps. Check the bearings and nut in the z axis to ensure they are freely moving. If they are tight then this could be why the unit is missing steps and slowly dropping. When the z axis is stationary the current to the motor is halved. If this current is not big enough and the weight of the z axis is high enough then gravity can pull it down. This is the only scenario I can think of that results in a downward drop. And this agrees with your thoughts about a heavier Z axis. You can remove the 3.5kohm limiting resistor from the Z axis motor and this will allow the full 4A current on that axis, maybe just what it needs. Otherwise you will need some sort of balancer on the axis, a spring a gas strut some pulleys and weights.....I haven't read the G540 manual in years but you maybe able to turn the current reduction off on that axis to see what happens, so when the axis is stationary it has full torque vs half torque? Peter

[CoAMarcus]

bcurrey,

You can run those motors without the 3.48K resistor on the DB9 connector. It sounds like your motor is losing torque when the drive goes into standby mode where it cuts power to 70% of set current. Is there a way to remove that 3.48K resistor? With that resistor no longer present the drive will not go into standby mode and apply full torque at all times to your Z axis motor. That will likely take care of the issue you are running into.

The Z axis is kind of a tricky one because you are fighting your normal torque battles with drag and mechanical resistance, but you then have to fight gravity as well. This means that the static torque load is going to be higher than most other axes and will increase with the dead weight of your cutting head.