[cnc1000]

Hi,
Here's my setup: Bridgeport Series 1 cnc with Mach3, Geckos, original steppers, 70V supply, XP laptop, smooth stepper.

X and y work perfectly. I can do a job and note that the mechanical counters return to exactly their original numbers after I'm done.

Z, however is goofy. It loses steps during a simple program, but never ever if I just to a jog 0.1" either way. I can do that all day and never lose a step. What is different about Z motion within a program? The simple program is just 4 or 5 linear cuts in a loop that starts off 0.1" above the part, does a .050 cut and then another one. I command it to return to the original position and it's usually off by quite a bit, like .045"

I have tried slowing the feed speed down to a crawl, 2"/min. No improvement. Slowed down and sped up acceleration in Motor Tuning to no avail. Speeds on each axis set to 20 "/min.

How can this be? The pendant jogs are perfect, but a z motion with F2.0 quivers and loses steps. I see the z motor shaft quivering back and forth in program moves but rock solid in pendant jogs.

Flummoxed.

???

cnc1000

[awerby]

If this happens during a dry run, then the Z axis is trying to move too fast. Try lowering the acceleration values in Motor Tuning (they should be about 10% of the total velocity, but if that doesn't fix the problem, cut them in half, to 5%). If the dry run works but the motor still loses position during an actual cutting routine, then it might be that you're plunging into the material too abruptly, and stalling the motor.

[cnc1000]

Hi,
Yes, it is happening during a dry run. Speeds are set to 20 "/min, accel is only 0.5. That should be plenty slow, shouldn't it? This problem does not happen if I jog. What is different about jogging? Since it jogs perfectly, I don't suspect the motor, or the drive current, or voltage. Something about the pulses must be different, but what?
Thanks for the quick reply.

If this happens during a dry run, then the Z axis is trying to move too fast. Try lowering the acceleration values in Motor Tuning (they should be about 10% of the total velocity, but if that doesn't fix the problem, cut them in half, to 5%). If the dry run works but the motor still loses position during an actual cutting routine, then it might be that you're plunging into the material too abruptly, and stalling the motor.

[awerby]

Yes, that does seem slow enough; maybe too slow. What happens if you sent the acceleration to 2? Have you tried swapping the leads from the Z axis to another one? (Turn off the machine and let the caps drain before you do that). Then command some moves in both axes using the MDI function, to see if the problem stays with the Z axis or moves to the other one. Check for lost steps by zeroing to a certain position, and then returning to it and seeing if it agrees with the DRO.

[cnc1000]

Hi,
I will certainly try increasing the acceleration, but I think I already did that, with no improvement. The motors are hard wired into place and it's going to be hard to swap them. But since jogging the Z axis works perfectly, I think I can safely assume that the motor is ok. The one common denominator here is that z works UNLESS I try to move it within a program. Maybe it has something to do with the accel ramps and not getting to full speed before it has to start slowing down... By only jogging 0.1", maybe I never get up to high speed, where it starts losing steps. I think I can configure the jog distance in the CONFIG file, temporarily. Might set it to an inch to see if it still works ok. I'll report back. Thanks.

I just spotted that I can do a jog of one inch from the front panel MPG. I'll do that shortly.

[cnc1000]

I think the config file for Smooth Stepper may not be optimized right. I'll try to post a screen shot. Turns out, I can't post. Screen shot is too big. Here are the salient parameters:

Controller freq 500 Hz

Max step freq is 32 KHz on x,y,and z axis

Number of dat points Mach should pre-calculate 1023

I did mess with Z motor tuning and tried to speed it up and it won't go any faster. I increased accel and it does accel faster but speed is not increased. Seems to be maxed out.

New info: I have 200 step steppers (original motors) and Geckos are in 10 microstep mode. Ball screws are 5 turns per inch. There is some pulley ratio between the motor and ball nut. I will find it out.

I'll reread the Mach manual on setting up config screen and Smooth Stepper config.

[cnc1000]

Figured it out. Gecko drive was running at 7A, it's max current spec. Using original 8A per phase steppers on Bridgeport. No problem, right? Wrong. The phases are each wired in series, which apparently alters the amperage specification for the 8A motor, taking it down to 70%, or 5.6A. I was hitting the motors too hard, apparently. Dropped to 5A and all is well, now.

After talking to people at Mach, Smooth Stepper, PMDX and others, the most common thing they see is people retrofitting an old cnc and once they see motion, they call it good and move to software and other tasks in the rebuild. I'm guilty as charged. This hardware problem looked for all the world like a software problem to everyone. I guess the takeaway lesson is to keep on the hardware until it's fully understood and implemented. Only then, move to the rest of the project.

[awerby]

Thanks for reporting back. So your fix involved installing a current-limiting resistor? It's good that you got your parts from suppliers who could actually provide some after-sale tech support. Many people here go with the cheapest possible parts they can find on Ebay or whatever, and then are stuck with a machine that won't work right, and nobody to tell them why.

[cnc1000]

The fix was not a classical current limiting resistor, but a control resistor that goes on the Gecko drive itself to just 'tell' the Gecko how much current to use. I looked at the drives closer and while they are Geckos, the name imprinted on them says RMS Technologies. Same hardware, exactly. RMS Tech was bought recently by Lin Engineering and it was a Lin engineer by the name of Dave that suggested the fix. I was very impressed with him.