[normalform]

Hi, I recently built a CRP4848 and I've run into a problem with the machine--after a few minutes of cutting one of the X-motor's pinion starts climbing out of the rack on small incremental x-movements and after a missed tooth slaps back a tooth, thus ruining the cut.
It seems to be that the side that has the problem pulls the gantry out of square as the other motor moves less and then slaps back when it can't extend any further. I've run the program at different locations of the table to see if it's a damaged section of rail, I've switched motors to different sides, I've run different test programs, I've tried tightening/loosening the spring tensioners, lowering velocity and acceleration in motor tuning in mach3, but the problem remains.
I'm starting to think that there's a problem with my geckodrive--when I got it the A axis driver was dead so we sent it back for a replacement--maybe something is still off? I'll try switching axis cables around next time I can too, but in the mean time I'm posting this in the hope that it's something easily serviceable that I'm simply missing so I don't have to go through worst case scenario of sending the unit in. Could odd motor trimming also be a factor here?

Also, while I'm at it: sometimes when jogging in mach the machine will uncontrollably move for a few seconds even after I've stopped pressing buttons. Thankfully I haven't damaged anything but it's rather alarming and I haven't been able to figure this one out either, my assumption was that it was my computer but again if there's a solution out there I'm missing I'd love to know.


Thanks for any help or advice offered.

[jm82792]

What kind of control software are you running?
Perhaps you are getting software issues...

[CarveOne]

This is called "racking" and can be caused by binding due to a carriage being adjusted too tight on the rail, rail misalignment, parallel port signals being too low in voltage, or erroneous settings in Mach3.

If the mechanical issues are not corrected, changing software settings and swapping motors will not likely correct anything. Keep in mind that the bearing carriages need to roll effortlessly but not have excessive free-play. Having them a bit too loose is ok for troubleshooting your problem though. You can readjust them once you find and correct the problem.

To adjust the rail parallelism, loosen all bolts in one of the rails (the other rail becomes a reference rail) and move the gantry to one end of travel. Snug the end bolt enough to prevent the rail from moving, but will allow the rail to pivot a little. Move the gantry to the other end of travel and snug the nearest bolt a little. This should get it close to parallel, but not perfect. Move the gantry to mid travel and snug the nearest bolt. Now move the gantry along the rail and feel for tight spots. If you find any, loosen and re-snug the the nearest bolt until the gantry rolls easily along the rails end to end. Tighten all bolts and recheck for any tight spots again.

The rail may need shims under one or both ends to get the rail parallel to the other rail in the vertical (Z) axis if the frame is not perfectly true. Shims may be needed in other places if tightening a bolt creates a tight spot. This can be a repetitive effort. This procedure uses the gantry itself to calibrate the spacing of the rails. It's very difficult to align the rails with any other measurement method that I know of due to the close tolerances involved.

At higher feed rates your motors will have less torque available to overcome any binding that's present in the rail system. Binding (drag) can be bad enough to cause the pinion gear to climb out of the rack teeth. (Been there done that) Lubricating the racks can help the situation but won't cure it. Adding more tension does about the same. Neither of these are the real solution if binding is present.

When you are satisfied that the gantry moves freely and the bearing pre-loads are good, reattach the lead screws or pinion drives and run the machine to see if you still have the racking problem. If you do, then there will be other causes to investigated.

[Dragonfly]

I've seen similar machine with a stepper on each side but it has a synchronizing shaft between the driven pulleys on both sides. Also, if microstepping is used, are both drivers set to the same step?

[normalform]

This is called "racking" and can be caused by binding due to a carriage being adjusted too tight on the rail, rail misalignment, parallel port signals being too low in voltage, or erroneous settings in Mach3.

If the mechanical issues are not corrected, changing software settings and swapping motors will not likely correct anything. Keep in mind that the bearing carriages need to roll effortlessly but not have excessive free-play. Having them a bit too loose is ok for troubleshooting your problem though. You can readjust them once you find and correct the problem.

To adjust the rail parallelism, loosen all bolts in one of the rails (the other rail becomes a reference rail) and move the gantry to one end of travel. Snug the end bolt enough to prevent the rail from moving, but will allow the rail to pivot a little. Move the gantry to the other end of travel and snug the nearest bolt a little. This should get it close to parallel, but not perfect. Move the gantry to mid travel and snug the nearest bolt. Now move the gantry along the rail and feel for tight spots. If you find any, loosen and re-snug the the nearest bolt until the gantry rolls easily along the rails end to end. Tighten all bolts and recheck for any tight spots again.

The rail may need shims under one or both ends to get the rail parallel to the other rail in the vertical (Z) axis if the frame is not perfectly true. Shims may be needed in other places if tightening a bolt creates a tight spot. This can be a repetitive effort. This procedure uses the gantry itself to calibrate the spacing of the rails. It's very difficult to align the rails with any other measurement method that I know of due to the close tolerances involved.

At higher feed rates your motors will have less torque available to overcome any binding that's present in the rail system. Binding (drag) can be bad enough to cause the pinion gear to climb out of the rack teeth. (Been there done that) Lubricating the racks can help the situation but won't cure it. Adding more tension does about the same. Neither of these are the real solution if binding is present.

When you are satisfied that the gantry moves freely and the bearing pre-loads are good, reattach the lead screws or pinion drives and run the machine to see if you still have the racking problem. If you do, then there will be other causes to investigated.

Phew, thanks for the long post--these tips will come in handy and I'll go through all of these to just make sure, but right now the gantry rolls perfectly with no binding. I'll look into the parallel port voltage too.

jm82792, I'm running mach3 and dragonfly, the stats in motor tuning for the two motors are the same.

I think I'm going to look into some synchronizing apparatus.

[CarveOne]

I have two machines, a 3' x 4' with 5 start ACME screws and a 5' x12' with R&P drives from CNCRP. Neither of them use any syncronization schemes other than in Mach3. They will run at least 6 hours with no signs of anything being wrong or racking. Before running long jobs I move the gantry up to a pair of mechanical stops to check that the gantry is still square. Mach3 can do auto squaring with the 2010 screen set by ger21.

When your machine is working right there is little or no need for forcing the alignment with mechanical means. As long as both motors get the same number of pulses and there are no mechanical problems they will track each other reliably. I had major racking problems when I first installed the R&P drives. It's documented in my CarveOne's Worktable CNC Machine build log. It's all part of the " fun" of learning this stuff. After almost giving up, it turned out to be a Mach3 active-high/active-low setting that was wrong for one of the motors.

[normalform]

CarveOne you may have just solved it for me, I checked the active-high/active-low setting for my motor outputs and yes indeed there was the discrepancy between motors I'd been looking for, duh! I'm glad it did turn out to be something so simple that I overlooked it. When I have time to check I'll confirm the fix. I'll definitely check out your build log, too--you've had some great advice to give and I'm sure that thread is chock full of good stuff too. I've looked into ger21's screenset before and i'm definitely going to pick that up as well.

[CarveOne]

CarveOne you may have just solved it for me, I checked the active-high/active-low setting for my motor outputs and yes indeed there was the discrepancy between motors I'd been looking for, duh! I'm glad it did turn out to be something so simple that I overlooked it. When I have time to check I'll confirm the fix. I'll definitely check out your build log, too--you've had some great advice to give and I'm sure that thread is chock full of good stuff too. I've looked into ger21's screenset before and i'm definitely going to pick that up as well.

In my case, I fretted over it for nearly four months and finally Mariss Freimanis at Geckodrives magically appeared and gave me a list of things to check. The signal voltages at one of the G203V drives was too low. That led back to Mach3 where I discovered that one of the motors (A axis) was set for active low. Toggling that one setting is why I'm still here making stuff.

That build log is a long one. It morphs from a modified 4' x 8' work table into a steel frame machine by undergoing multiple major upgrades as I found new problems and found money to correct them.