Hello there!
I always wanted to build a MPCNC but never really got into it. Recently I picked one up that was already built.
I have some cnc expirience from my first home built cnc from 20 years ago, but haven`t touched it since about 15 years, so I am more of an slightly advanced cnc beginner but a mpcnc noob.
My problem is: The cnc runs fine, I never have an issue when I move it manually. I also milled some small projects with no problems. But it happened to me twice now, when I am milling larger pockets, at the end of the pocket my Z axis does not move up high enough and that ruins my workpiece. I can noticably hear the step loss, right when it starts to move up. It sounds like the Z axis is stuck and the stepper can’t move it at first.
But my router motor is quite heavy (3.2kg). With the board turned off, the Z axis moves down by itself due to the weight. So it seems to me it runs mechanically fine. Maybe my stepper is just to weak? I am using this stepper motor for Z: https://www.omc-stepperonline.com/nema-17-bipolar-59ncm-84oz-in-2a-42x48mm-4-wires-w-1m-cable-connector-17hs19-2004s1
The driver board is a CNC shield, I do not know the model of the drivers, but they seem to be the typical chinese drivers with the violet PCB, have a heat sink on them and the Z axis is adjusted to 0,6V.
My settings are
800steps/turn
8mm/turn
480mm/min
inertia 85%
I am using Estlcam software.
I got these settings from the previous owner who built the cnc, but he just used it twice, maybe they are totally off?
Do you have any suggestions on what stepper to use or what setting? If you need further information, please let me know. As mentioned, I am pretty much a noob when it comes to mpcnc and I am pretty overwhelmed by all the information I read in the last couple days and all the trouble shooting guides.
Lost Z steps can occur for a variety of reasons. The two most common are 1) the grub screws on the connector between the stepper motor and the lead screw are loose, and 2) the feedrate you are using for Z is too high. The maximum feedrate for Z is significantly lower than for X and Y. With your heavy router 480mm/min may be too fast.
Your heavy router may be the issue. I assume you are using DRV8825 drivers. What current setting are you using on the stepper driver to Z? The stepper motor can handle more current than the V1 recommended settings, but at some point, the heat from the stepper motor will impact the PLA. If you go the route of increasing the current, a cork gasket between the motor and the PLA might mitigate heat issues.
Another solution is to go to a 2-start lead screw. This will impact the maximum feedrate for Z, but it will increase the Z torque. It should also keep your router from dropping.
I would add to the above list of reasons that acceleration is too high. Keeping the Z feed slow and the acceleration smooth, it should be quite strong.
The screws are tight on the connector. When this error occurs I can hear the motor skipping. It tries to turn but i only hear it knocking.
I will decrease the feedrate eaven more. The interesting thing is, this does not happen when I move it manually. Everytime it happpened, it was after milling out a pocket for a while. Maybe because the stepper was warmed up by then?
Yes they look like the DRV8825 drivers. They were set to 0,7V (Motor connected) which equals out to 1.4A if I understood that right. I saw some videos and read some articles about adjusting the reference voltage and they mentioned that 0.7V is the upper limit with additional heatsinks.
I crancked it up to 0.9V for now and I will keep an eye on the temp. The motors should be good for 2A. I also have a fan in my control unit housing that blows directly on the heatsinks.
I also set the control board from 1/4steps to 1/2 steps. Maybe that increases the torque also.
I can now feel a lot more force pushing up the axis. Before i could easily prevent the axis from moving up by stopping it with my finger. I am sure if I would have added the weight of lets say a soda can to my Z axis it would have stopped moving up.
I also ordered a Nema17 with 71Ncm instead of the 59Ncm if that all does not get me further.
I also set the control board from 1/4steps to 1/2 steps.
I don’t know the impact on torque, but the V1 maintained Marlin firmware runs the drivers at 1/32 microstepping. I’m concerned about your accuracy if you are running at 1/2 steps.
Losing a bit of torque when the stepper motors heat makes sense. It looks like you were running near the Z limit with your heavier router, so even a minor loss of torque might lose steps.
If the actions you are taking do not solve your problem, then consider a 2-start lead screw. It will double the torque of your Z axis. Note your router is twice the weight of the recommended Mikita router.
There are a few other reasons steps are lost that I’ve not listed. They are not a comfortable fit with the description of your problem, but if changes you are making do not solve the issue, we can explore them.
he should also be running active cooling for the drivers & steppers, a 4040 fan blowing air on one side of the stepper and upping the amps a little more should be sufficient. back in the days y replaced my z steppers with the beefier nema17, that solved all my issues due to the weight. active cooling is a must or he will melt/deform things