Losing Z height during cutting

[gr8ga]

Let me start off by saying the problem isn't occurring on a shopbot rather on another machine. My 2 shopbots are running just fine.
While cutting 3d files the z axis will randomly drop. How much it drops and where in a file it drops changes everytime. I'm cutting 2 pound foam so there is practically zero torque on the spindle. I swapped drivers with the W axis and the problem remained. So I'm assuming the driver is good. Although, I don't use the W axis so I wouldn't know if its driver was also bad--doesn't seem likely. I replaced the stepper motor and that didn't help. There is a tranformer between the 2 drivers, could it be bad? I also noticed that the cable to the motor is not shielded.
Any thoughts on a fix or troubleshooting would be greatly apprieciated.
Thanks,
Gregg

[Brady Watson]

Gregg,
It sounds to me like a mechanical issue. How is the Z axis mechanically driven?

If it uses a screw, check for play in the screw/nut assembly. These get packed with sawdust after a while and the nut can get worn giving you excessive backlash. Hold the Z and tell the machine to move up or down. Feel/listen for lack of movement, play and mechnical grinding.

If it uses rack & pinion, inspect the rack & pinion for wear marks. Wiggle the Z assembly up & down in verious places in it's travel. If you hear/feel dicernnable clicking/play, then chances are the pinion &/or rack needs to be replaced or the mesh of the 2 needs to be adjusted.

-B

[gr8ga]

It does use a screw. At your suggestion, I tried to find any mechanical issues. As far as I can tell everything is very solid. I can't feel any movement/backlash when the machine is at rest or in motion. It is a fairly heavy duty machine. I disconnected the screw from the stepper motor and ran the program (air cutting) and I couldn't hear any difference in sound. Thanks for the suggestion, the more I can narrow it down the better.

[Brady Watson]

How about electrical interference? If you are using a dust collection system, is it grounded completely from one end to the other? If not, static electricity can 'wick' into the motor control wires and cause interference with the electrical signal being sent to the motor.

What brand & model is the CNC?

-B

[gr8ga]

I'm told its from a now defunct company called Digital Imaging. I don't know what model it is (no markings). The stepper motor and driver are made by Anaheim Automation.
I bought it used about 6 months ago. I don't have a dust system installed on it. All the cables/powercords that run to the machine are bundled together. The Z cable is not shielded. Is it likely that it is receiving interference from other cables?
Gregg

[Brady Watson]

Gregg,
I doubt it is getting 'crossover' from other stepper wires, but I wouldn't totally rule it out as a possibility.

I know you said that you checked out the screw & nut under power...I would look at it again and really put your 'analytical' eye on it. With the power to the machine ON, try to rotate the screw by hand. It should be tight as the stepper is holding torque. It's tough to check the nut with the router etc attached to it, but try moving it up and down with force. Really try to twist that screw like you are TRYING to make it wiggle loose.

How does the screw mount to the stepper? Is it tight? Does it use a belt? Are the teeth in the pulleys clean?

Does the Z drop only when cutting THIS particular file, or does it happen on other files?

Is the screw protected with a boot or open to dust & contamination? Even gear racks will get clogged with foam and the pinion will drive over it and show a marked spot.

-B

[paco]

Why not take this opportunity to clean to whole thing; play, if any there is, will be even more obvious in a clean environment. A dial indicator can help in this kind of case...

[gr8ga]

After the 3rd conversation with the stepper motor tech support, I may have solved the issue. The motor may be sized to small to handle the weight of the head at higher speeds. I slowed the z speed down and ran a program with no loss of height. I'm not 100% sure this is the cause but it looks promising. The only problem is that I'm running the z at 20"/min. SLOOOOOW. I can run x & y at 500"/min (they use servos). If the size of the motor does end up being the culprit I will put on a bigger one.
Thanks for all the suggestions.
Gregg

[Brady Watson]

Thanks for the feedback...If you do some investigating I'm sure that you can swap out that small stepper for a larger one. It will depend on the Z axis motor driver and what amperage it can handle...Then you should be able to get faster speeds on the Z.

-B

[ed_conley]

Gregg & Brady:

I'm having a very similar problem with my Alpha. I'm losing my Z height during cuttings, I had been referring to this as bit slippage but now sure it is loss of Z height. With the help of SB tech, I've done a number of things in attempt to cure the problem even some of those you mentioned,ie, new bits, new collets, new collet nuts, clean machine, checked bearings, checked groundings, etc. But after going 6 hours of cutting last night only to loose the entire job, I'm getting pretty frustrated. I have the 3HP Columbo Spindle on my machine. Any other suggestions?

Ed.

[paco]

Ed,

vacuum hold down?

[Brady Watson]

Ed,
What happens when the control box is on & you try to move the z-axis T-rail up & down? Do you hear clicking? Is there play in the T-rail/gear rack?

Are you using dust collection? Is it grounded with a bare copper wire from the foot to the DC? High voltage static will sometimes cause positional errors & other little gremlins.

As Paco pointed out...Are you using vacuum? If so, are you cutting through it? (more kerfs = more leakage as you cut) Is the releasing of vacuum causing the material to creep up in Z height?

-B

[ed_conley]

Dust collection is properly grounded. Both my PRT and Alpha are on the same DC and not having any issues with the PRT. Vacuum hold down is not the issue as I'm loosing as much as 3/4 of an inch whether the vacuum is on or off.

Just did a test on the Z axis by moving it up and down 60 times at 3"/sec in a pecking drill motion and no loss of Z height. No clicking or grinding noise is evident; movement is very smooth. Appears to be no play in the gear.

I'm open to any suggestions. Thanks
Ed.

[fleinbach]

I also had a problem with the Z axis slipping two days ago and and eventually narrowed my problem down to bit slippage. I was using a 1 1/4 inch core box bit and at times it was going to deep into the material. At first I grabbed the bit and tried moving it, it seemed solid. After running the file again I had the same problem. I monitored everything closely and could not determine that the Z axis was slipping so I checked the bit again. By putting a block of wood against the bit and pushing as hard as I could I was able to move the bit farther up into the chuck. I put the wrenches back on it and really tightened the chuck. I had no problem after that.

[ed_conley]

Frank:

I originally thought mine was a bit slippage problem. In fact, I went about a month thinking it was that. But,found it to be a problem with the Z.

Ed.

[phil_o]

I'm having a problem losing Z height. The Y value is also not staying true. I recently installed a new 4g board and I have a HSD 2.2 hp spindle. I ran a couple of files that took approx 10 minutes each and had no noticable problems. The I ran a large file over 200,000 lines of code estimated to run for almost three hours. At approximately line 98,000 I realized that it wasn't making the shallow cuts. I stopped the file and rezeroed the Z. It was off by .124". I started the file at line 80,000 and soon realized that the Y was out too. I rezeroed X,Y and Z. X was fine, Y was off by ay least .5", Z was out by .025". I checked to be sure the collet was tight, it was. I started again at line 80,000. At about line 104,000 I realized the Z was out again. Y was also out by about .4" and Z was out by .040"

[oscarg1971]

Oh man, I'm having the same issue. It just ruined a carving that I was doing for the Austin Camp. It had been going on for about 7 hours. It had started good and a couple of hours in, I supposed, I wasn't here, the Z had stepped up about .01". This happened during the final pass. I thought I would go ahead and start it over and run it overnight. But when I went to JH, it ended up some where else on the wood from where I zeroed it out before. So I couldn't continue and had to trash it. Before, I've been able to zero the Z and run it again and it will run fine. Any clues as to why it zeroed the XY somewhere else?? -Oscar

[scottcox]

I once saw that Z step-up issue on a 3d piece that I did. I reran it and it did it in the exact same place. I isolated the single line of code (sharp drop after upward ramping while at full x move speed) and repeated it and it even did it while air-cutting. So I decreased all 3 move speeds slightly and it worked fine. I've since slowed my z speeds for most cuts and haven't seen that again.

[ken_r]

I am having the same problem with my Z axis, I cut a 3D file of 27,000 lines and as usual I make my last line as JH,
I wanted run a final pass at the same height up the middle of my stock but when I reloaded the file with out even turning off the vacuum or spindle the first cut was 0.125 deeper than it should be.
Very frustrating!!! any ideas?
It happend with very next model aswell.
Thanks in advance for any help.

[dgs]

Hi ,I am having problems ,when I jog home my z is not going the full distance ,if i move home or jog z to 0 it works fine . but jog home comand does not . my jog speed is at 4 inches/sec