Please HELP!  I am pulling my hair out…
I have been having this problem now for a while, but have been able to avoid it until now.  It seems that my Z-Axis is drifting or loosing steps.  What makes it more difficult to figure out is that it seems to be a math error of some kind as the results I can duplicate accurately. 

I am thinking this is a Mach 3 issue since I can duplicate the results.

Here is an example….  I am cutting a 3d part, I can let it cut the file out and when it is finished, it thinks the z axis is at 0.0, when it is actually at 0.25.  I can cut the part again after re-zeroing it and I end up at pretty much the same offset.  I am doing parallel finishing; I can see the bit is gradually and smoothly cutting lower and lower.  There is no big jump; it just seems to smoothly drift.
I can also zero the machine and toggle the head up and down about 15-20 times and will check zero and it will be off again.  I can do this at any speed setting and I let it completely stop before changing direction and I still produce the same results. 
This is what I have tried so far with no change….
Tried changing frequencies to 3 or 4 different settings.
Tried the shirline ½ step mode.
Tried setting the pulses under motor tuning to 5, then to 15, then to 20.
Tried changing the number of pulses per inch from 3200 (Setting I was running at) to 2000, to 1600, to 1000 and I still see the same results.

I am running the Joes 4x4 hybrid machine.  You can see it here
http://www.saberfire.com/gallery/Projects/CNCMachine/

I am Running Mach 3 with the Hobby CNC Pro kit with the 305/oz motors.  I have the jumpers set to quarter step.
I am not sure what else to do here?

Thanks,
Bobby

Bobby,

Sounds to me like a classic case of lost steps.
A common cause, when this happens with the Z axis only, is binding or tight spots in the screw or slides. Try halving the velocity and acceleration, in motor tuning, and see if it still happens (the effect of this will be to increase stepper motor torque). If this cures it then experiment to achieve the best reliable settings.

Tweakie.

I wonder at this bit

Tried changing the number of pulses per inch from 3200 (Setting I was running at) to 2000, to 1600, to 1000 and I still see the same results.

I cant see how you can get the same results (0.25 higher) with different steps per unit unless the cutter is being pushed into the collet by that amount.
Try running the file without material and see if its still high at the end.
Hood

Bobby,

Sounds to me like a classic case of lost steps.
A common cause, when this happens with the Z axis only, is binding or tight spots in the screw or slides. Try halving the velocity and acceleration, in motor tuning, and see if it still happens (the effect of this will be to increase stepper motor torque). If this cures it then experiment to achieve the best reliable settings.

Tweakie.

I have tied slowing the speed down to to a painfully slow 20 inches per minute and let the machine stop before changing directions, and I still see the issue.  What bothers me is that this seems to be a math error.  It happens all the time and happens consistently.  I.E.  I can cut the same part over and over again and each part will be identical with the axis slowly drifting on every single one by the same amount.

I wonder at this bit

Quote from: bbristow on July 08, 2010, 04:16:05 AM

Tried changing the number of pulses per inch from 3200 (Setting I was running at) to 2000, to 1600, to 1000 and I still see the same results.

I cant see how you can get the same results (0.25 higher) with different steps per unit unless the cutter is being pushed into the collet by that amount.
Try running the file without material and see if its still high at the end.
Hood

Sorry, let my clarify.....  It is not off by the same amount when I change steps, it is just off no matter what the setting is, but always slips further down.  I.E...So when it thinks it is at 0.75 the bit is really at .72.  I can move the head up and down without cutting anything about 10 times and see that it is off.

What bothers me is that this seems to be a math error.

This is unlikely - there are so many users for so long, if this was the case it would have been discovered and corrected a long long while ago.

I can move the head up and down without cutting anything about 10 times and see that it is off.

Are you saying that just jogging the Z axis up and down produces this error ?.

Tweakie.

Yes, just moving the head up and down as little as 10-20 times I can see that I am off.

I think I am back to what I thought earlier - lost steps.

Tightness, binding, excess speed and electrical noise are all possible causes but electrical noise usually affects more than just one axis.
If you are absolutely sure it is not binding or excess friction on the axis then I would suspect the motor or possibly a loose coupling on the screw.

Tweakie.

I think I am back to what I thought earlier - lost steps.

Tightness, binding, excess speed and electrical noise are all possible causes but electrical noise usually affects more than just one axis.
If you are absolutely sure it is not binding or excess friction on the axis then I would suspect the motor or possibly a loose coupling on the screw.

Tweakie.

I have the power for the motors and controller going through a power conditioner as a friend of mine had a problem with noise, but that showed up as random z spikes in the cut files. 

If I am loosing steps, what would cause the Z to loose steps so precisely?

As Tweakie says it is almost certainlylost steps, could be binding, noise or even a bad drive or motor. Cant see it being a setup issue but if you want to attach your xml I will have a look through it.

As for your Z test, if you move up and down 20 times at rapid, then same at half speed and same at a crawl is it always the same amount out?
If you then move a longer distance doing the same 20 moves is it the same amount out?
Hood