Z axis repeatability

[JERRIMY]

Hello!
I have a CNC Crafter, a 14" x 20" x 3" cnc router that is run with Ability Systems HPGL Controller V 2.07 for 2.5D cutting and also for diamond drag point etching. I am very happy with the machine except that the Z axis must be homed frequently to maintain cutting depth. The hardware for each axis are Applied Motion Products model 7080 open frame driver and 5034-349 stepper motor with independant power supplies. The one mechanical issue I have with the machine is that the Z axis bearings are worn where they are used frequently. Would this cause repeatability problems? Are there any other areas I should investigate. Any suggestions would be appreciated. Thank you.

[trubleshtr]

Your worn bearings maybe causing backlash. I am not familiar with cnc crafter, but loss of position repeatability is usually slop in the axis.

[JERRIMY]

Thank you trubleshtr. As per your advice I replaced the Z-axis bearings, the most logical place to start, but still have repeatability issues. What could the next step be?

[trubleshtr]

Are you losing steps on Z, when cutting/engraving? If you are pushing the machine too hard, or your tools are becoming dull then the cutting forces are greater, perhaps the axiz is losing position as it does not have enough strength to maintain the needed position?
If you simply touch off a work piece with the tip of the cutter and move z up and then ask it to return to the exact touch position, does it repeat ok? IF so, you are probably losing position while cutting under load. If it does not repeat (z) just by simply going up and down to the same point over and over, then, maybe you have some kind of binding or drag in the axiz set-up? ie. slides, packed up dirt ect slowing things down. It could also mean the z axis stepper is weak, or perhaps missing a stepping signal, ie broken wire, interferance on the wires from other elec. wires.
Maybe you can post a pic or two of your set-up?
I am just rambling out things that are coming to my mind, which is not a very methodical way to troubleshoot a problem

[teebee]

Is there a brake on the Z axis?
What kind of driver are You using? some reduce current to the motor coils after some time without new steps and so the holding torque decreases... So if You cut in a plane (constant Z) the cuting forces cane cause the axis to backdrive and so You lose positioning precision.
Try to check out the driver and maybe think of adding a brake.

[Davidh]

Hello J.

How about mounting a dial gauge on the z axis, then you might see the point at which the problem occurs.

Maybe too a spring loaded scribe would help. Then you know it is not bottoming out and stalling a few steps off.

[ger21]

Slow the speed and accel of the Z-axis in half, and see if it still does it.

[JERRIMY]

Thank's to all of your suggestions. The bearings on all three axis are Bishop-Wiscarver DualVee. Easy to use and easy to adjust. The drag point is spring loaded. There is no brake. I did try mounting a dial indicator and had similar results with and without the drag point load, still losing position. I think we have narrowed this down to a weak motor. I may try to swap motors from X to Z to double check.

[Jarwalcot]

Here is a backlash test for a turning center that you can use as a troubleshooting template/guide... This method should give you more feedback... I will also include a procedure for replacing thrustbearings. Thanks to Steve Solaris at MTS for putting this together.

Do you have a local company that does ballbar testing? Might save you some time and money having it done... Just a thought

Good Luck,

[Jarwalcot]

Ballscrew Thurstbearing Replacement Procedure was to large - 640 KB.. Email me an address I can send it to.

Regards,

[JERRIMY]

Thanks evveryone for their suggestions. I installed the new motor today and am up an running again. The only issue with the installation was in wiring. The replacement motor is an 8 wire and the original was a 4 wire. Fortunately the manufacturer's web site was very helpful. BTW the machine BEARINGMAN is selling is visually identical to the machine I have. His machine has Microkinetics motors and drivers instead of the Applied Motion Products parts that I have. http://www.cnczone.com/classifieds/s...p/product/1052

[brockadeau]

Can anyone out there help me with the settings for the dip switches on the x drive of a cnc-crafter contoller model 1914. I bought it used without the x axis working properly. I thought it was just a loose allen set screw, but the x axis is still not working. All the dip switches on the x axis are set to the right, and since there is a more elaborate pattern on the y and z axis dip switches, I am now assuming someone tried to reset these without luck and just put them all in a row. any info would be much appreciated-- thanks-- Jack