PM25MV CNC conversion Z-Axis help

[justlie]

Hi there, I have a PM25MV that I converted to CNC. Below is a brief overview of the parts I am using:

PM25MV (late 2015 model)
Leadshine MX3660 controller
Keling 570oz Nema23 stepper for X & Y
Keling 906oz Nema34 stepper for Z (8 wire motor wired in series)
48v 12.5A Switching Power supply
C7 Ballscrews 16mm for all 3 axis.
Latest Mach3 (licensed)

Everything works well with the exception when I run longer programs, the Z-Axis randomly loses steps. Its hard to catch the machine doing it in person but I think the Z-axis motor is stalling for a second when lifting the head. I already reduced the acceleration and velocity in Mach3 with no change in behavior. It typically looses about 0.1" in height.

I am not sure if its a temp issue with prolonged operation as it typically only happens 20-30min into the operation.

Measured temps:
Z motor: 110-120F
Head: 130-140F

The controller is warm, not hot. Didn't measure it yet but much cooler than the Z motor.
The power supply is cold to the touch, not getting warm at all really.

Mach3 Motor Settings:

X: Steps: 1016 Velocity: 120 Acceleration: 4 G: 0.0103607 Step pulse: 0 Dir Pulse: 0
Y: Steps: 1016 Velocity: 120 Acceleration: 4 G: 0.0103607 Step pulse: 0 Dir Pulse: 0
Z: Steps: 1016 Velocity: 60 Acceleration: 2 G: 0.0051803 Step pulse: 0 Dir Pulse: 0

All running at 25khz.

I reduced the Z axis Velocity and Acceleration even more than the current setting but its still happening. The axis moves freely without any issues or hangups by hand or when i used mach. I am at a loss here and hoping for some input from other folks.

Thanks

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[109jb]

One thing is having the motor wired in series will reduce the potential torque at speed of the motor. Wired in series the inductance is very high and torque as speed increases will suffer. I would run it in parallel with that driver set for the maximum 6.0A output. I think you would be better off in that regard than series at lower current.

[109jb]

Looking at the torque curves on the Automation Tech site, the torque in parallel at 300 rpm (about 60 IPM) is about 480 oz-in, compared to 290 oz-in in series. So you are only getting about 60% of the torque at your rapid speed that you could be. The torque in paralle would not drop to 290 until about 500 rpm, which could give you 100 IPM rapids. Even at that you are slow. My G0704 rapids 180IPM all day long with a more basic driver using the 570 oz-in motor with no counterweight or helper springs.. How tight are your gibs?

Also, the motor temp that you cited (110-120F) is nothing. Stepper motors are commonly rated for case temps up to 100 deg C (212 F). I see a lot of worry about stepper temp that I believe is unfounded.

[justlie]

Looking at the torque curves on the Automation Tech site, the torque in parallel at 300 rpm (about 60 IPM) is about 480 oz-in, compared to 290 oz-in in series. So you are only getting about 60% of the torque at your rapid speed that you could be. The torque in paralle would not drop to 290 until about 500 rpm, which could give you 100 IPM rapids. Even at that you are slow. My G0704 rapids 180IPM all day long with a more basic driver using the 570 oz-in motor with no counterweight or helper springs.. How tight are your gibs?

Also, the motor temp that you cited (110-120F) is nothing. Stepper motors are commonly rated for case temps up to 100 deg C (212 F). I see a lot of worry about stepper temp that I believe is unfounded.

I am not sure how I would quantify on how tight the gibs are. I listed the temps just for the fact that I have seen a few threads where people brought up the issue of it. It didn't seem to me that the motor was getting to hot. How much impact does the head casting temperature have on the gibs?

One thing is having the motor wired in series will reduce the potential torque at speed of the motor. Wired in series the inductance is very high and torque as speed increases will suffer. I would run it in parallel with that driver set for the maximum 6.0A output. I think you would be better off in that regard than series at lower current.

I'll re-wire it and see if that helps. I figured that series would give me better low end torque for directional changes especially for the Z-axis.

I appreciate the input.

[109jb]

I'll re-wire it and see if that helps. I figured that series would give me better low end torque for directional changes especially for the Z-axis.

I appreciate the input.

The motor is rated for the same holding torque (0 RPM whether wired in parallel or series. The only difference here is how much current the motor can take in each case. This means the torque to change directions is the same in either case because the motor has to decelerate to 0 rpm first and then accelerate from 0 rpm during a direction change. Where parallel gets you an advantage is that wired this way the inductance is a lot lower. Given the same input voltage a higher inductance motor will drop torque more rapidly that a lower inductance motor. To overcome this we raise the voltage. Higher voltage (within reason) and lower inductance at rated current means slower drop in torque as rpm increases. For you z axis motor the series inductance is 13.2 mH, so the optimum voltage is about 116 V. Wired in parallel the inductance is 3.3 mH and the optimum voltage is about 58 V.

[justlie]

The motor is rated for the same holding torque (0 RPM whether wired in parallel or series. The only difference here is how much current the motor can take in each case. This means the torque to change directions is the same in either case because the motor has to decelerate to 0 rpm first and then accelerate from 0 rpm during a direction change. Where parallel gets you an advantage is that wired this way the inductance is a lot lower. Given the same input voltage a higher inductance motor will drop torque more rapidly that a lower inductance motor. To overcome this we raise the voltage. Higher voltage (within reason) and lower inductance at rated current means slower drop in torque as rpm increases. For you z axis motor the series inductance is 13.2 mH, so the optimum voltage is about 116 V. Wired in parallel the inductance is 3.3 mH and the optimum voltage is about 58 V.

I re-wired the motor and let it do a 40 min program that had issues in the past in free air and it didn't lose any steps. I'm going to try to run the program with material tonight to test.

Thanks for the help. Much appreciated.

[109jb]

Glad it worked.