Help... I'm losing my Z-axis zero slowly

[Sciome]

I'm losing my Z-axis zero slowly. It drops about 0.01inches per minute. This is my first CNC build, I"m close to getting it to work well, but I need some help. I have an XY table and a stationary Z axis (well the spindle moves in Z... ) .

I previously had issues with my Z axis losing zero all at once when my Nema-17 stepper had trouble lifting my 3HP spindle. Since then I've replaced the Nema-17 with a 280oz/in Nema-23. It is working much better, but it loses zero very slowly. I was cutting the V-bit demo on the Artcam software. and the perimeter circle cuts are gettting deeper and deeper. By the time I get to the center section, the Z axis is about 0.1inches below the zero (I stopped and cut, moved everyhing back to zero and it was off by 0.1inches on the Z). The X axis and Y axis are perfect.

Is my problem the 280oz/in Nema-23 stepper isn't capable of lifting my 3HP spindle all time? Am I missing steps from the controller? How do you know if you are missing steps?

I currently have all my axis set to 30ipm.

[jsheerin]

How heavy is the moving part of your z axis, what kind of screw and nut are you using, what motor controller, is the z axis counter balanced, and what is your acceleration set at? Also if you have a torque curve for your motor, that could help.

A lot of times you can hear your motor miss steps, but if it only misses a few while your spindle is running, you might not hear it.

Lacking all the information I asked for, I'd guess you're missing steps. Try turning your acceleration down and see if things improve.

[Khalid]

In addition to Jsheerin, what power supply you are using , what stepper driver you are using...are you using unipolar or bipolar driver...

[Sciome]

My spindle, motor and mount are probably 13-15lbs. I'm using a 10tpi single flight screw, I'm not sure of the nut, but is designed for the screw. The motor controller is a KL-4030 from Keling inc. The power supply is 36V, 8.8Amps DC (keling KL-320-36). I'm using Mach 3, with a KL-DB25 break out board.

The Z axis was set at 20ipm and 3in/sec/sec acceleration (~0.1sec to get to 20ipm according to the graph in Mach3. (i had it set to 30 and ~0.5secs on the graph, on a seperate test, same problem at 30ipm and 20ipm). This acceleration seems conservative to me. What do you think?

I don't have a torque curve for the motor (checked, didn't see one).

I did find one problem, the Z axis motor controller was set for 1.6ampls (for my old nema-17 stepper), I increased it to the 2.7 amps for my nema 23 stepper. Hopefully this will be the problem.

And thank you for your help so far. Its nice to be able to communicate with someone about CNC. Everyone I know doesn't even know what a CNC is... Although they seem impressed when I show them.

__________________

[Sciome]

I tried it with a very slow acceleration, 0.2 seconds to reach 20ipm. It took alot longer to run, and it still lost the Z zero. (This was done with the currect amp setting on the Z axis.).

How to you build a counter balance? Put a weight over a pulley attached to the spindle?

Thanks,
Kevin

[ger21]

Try increasing the pulse widths in motor tuning to 10. (Both step and dir)

[jsheerin]

At 30ipm, you should have tons of force available even if it's a screw with tons of friction.

To make a counterbalance, yes, you can do a weight and pulley. Another way is an air cylinder with a reservoir, or a gas spring.

[Claude Boudreau]

Do you loose steps with no load on the spindle?
As stepper torque increase at lower speed, buying a ballscrew 5tpi could be a good investment.
Im unfamiliar with single flight screw but if they are less efficient than ballscrew,it would increase your chances of sucess.

Can you turn your axes freely by hand?

[Sciome]

okay another night, and I'm still losing my Z axis zero.

I can turn the Z axis by hand with the power off, but when it is plugged in, i can't budge it.

I took off the 270oz/in nema 23 and put on a 500oz/in Nema-23 (triple checked the settings, 3.0amp, 1.8°/pulse, 200pulses per rev, 10 revs (threads) per inch, 1/4 microstepping, 8000 pulses per inch, checked with a ruler). Ran the G-code with my spindle in the air (not cutting wood), I still lost my Z Zero (about the same amount).

Ran without microstepping... (1:1), I lost my Z axis and it got worse (about 4 times as bad, I was using 1/4 microstepping... )

I took off the spindle an ran, with 1/4microstepping (which was working better), I lost Z zero (typical amount).

I set the pulse width to 10 for step and dir pulses, re-ran in the air with no spindle. (not horrible, but lost 0.05 in in 20 minutes...)

So far the only thing that seemed to effect the lose on the Z axis was changing the microstepping to 1/4 to 1/1 (made it worse). So.... Can I microstep at 1/64 and put the steps per inch at 128000/in? seems a little crazy. if I do, i'm guessing i need to put the step and dir pulses back to 5 microseconds...

I did double check all my connections... I can't find a thing wrong.

Can anyone recommend a good ball screw? I'm getting frustrated, maybe I just need to start replacing stuff to see if it starts to work.

[Khalid]

How easy you can turn the lead screw by hand with all stuff mounted on Z-axes?... I can easily turn the lead screw with only two fingers of my right hand (I am right handed)...

If all the stuff is heavy then try to install counter weight..
If all the stuff is very tightened then little loosen it..
If all the stuff is Dry then lubricate it..

Using 500 Oz-in Motors will not give you good results if its not matched with the power supply and your controller.. Just check what voltage and amperes of these 500 Oz-in motors?

[FandZ]

I've had a problem with my Z axis dropping twice. The first time was a coupler. I was using just the lock collar but not the set screw. It was causing it to slowly drop. The other time two of my stepper wires were touching every now and then and was causing erratic Z movements.

[zool]

Are the router bit shanks clean?

Is the collet clean?