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Y Axis Slipping

Posted by usslindstrom 
Y Axis Slipping
September 09, 2015 11:29AM
3d Printer Experts,

I've been having some nasty Y Axis slipping for the last couple of weeks. During a print, the printer seems to hiccup on the bed movement, and ends up ruining everything.

I've been going over the forums here for quite a while, and have come across many others with similar issues such as this. The printer in question is a Prusa i3, w/Arduino/RAMPS 1.4.

Based on the other forum posts, I'll answer inline with what I've done to check the particular item:

Belts and pulleys are too loose.
I have double-triple-quadrouple checked the belts. They have very little, to no slack at all. Also, to make sure there isn't any gear slippage, I ended up using some gorilla glue to keep it in place on the motor. Everything checks fine.

Belts and pulleys are too tight.
Again, I double-triple-quadrouple checked the belts. The belts are snug, but not too tight. You can pluck them, but they're not so tight they even make any guitar-like sound. They are very consistent in their play. I have tried both some belt tensioners and without.

Bed has too much friction.
The Y Axis is aligned parallel very nicely (measured with digital calipers). But also, the fluid-moving test where you move the bed back and forth to ensure the proper distance between Y-Min and Y-Max stays the same, and the bed isn't moving against itself. Everything is lined up perfectly. In order to ensure the smooth operation, I've also been using automotive grease on the smooth rods, which is an amazing pair and allows free and perfect moving of the bed.

Heated bed wires get caught on something (other wires / printer frame / etc).
I have routed the wires very nicely, and they are 100% free and clear of any obstructions. They move in each direction required without interference from anything else. No snags.

Stepper motor getting to hot and skipping steps.
No matter how long I print, the motors do not get too hot to touch. They get a tiny bit luke warm, as expected - but nothing out of the realm of being fully acceptable. Just to be on the safe side, I went ahead and purchased some aluminum heat transfer blocks that I slapped on the ends of all the motors. Motors aren't getting hot at all.

Firmware acceleration settings are too high.
In all of the write-ups I've seen regarding this issue, the acceleration settings within firmware were set to 1000 or more for each of the Axis. I have followed the answers in those threads to lower this setting to 500.

Stepper motor driver (polulo) aren't configured correctly *THIS is probably the reason as I see it now.
Finding awesome information on the configuration of these things is far and few between. Most are written with some awesome equations, and expect that you have an electrical engineering background to understand how to apply the proper voltage in the pots to achieve 1A worth of current. Since, this math is just short of adding Greek symbols - I went the route of what some YouTubers did, and drop the pots all the way down until the motors on each of the Axis' fail to move... Then, slowly creep up until everything moves without issue (adding a nudge for the heavy loads). But of course, all of the settings on this I've done to this point haven't given any joy and the Y Axis still continues to skip during prints.

Stepper drivers are bad.
I purchased a handful of spare drivers, and have experimented with a couple. All have the same results.

So, I've done quite a bit of research on this topic, but am completely still missing the boat on something. If you guys wouldn't mind, could you share your insight on what you possibly have done to fix this or something similar? Any help would be GREATLY appreciated. Thanks.
Re: Y Axis Slipping
September 09, 2015 12:09PM
*Forgot to mention another potential issue and what I've done to check.

Stepper drivers are overheating.
They do not get too hot to touch, but to be safe, they also have their own little heat-sinks installed w/thermal tape. I also have a fan blowing directly on them all.
Re: Y Axis Slipping
September 09, 2015 12:26PM
It's quite likely that increasing the Y motor current would solve the problem. Either get yourself a multimeter and learn now to set Vref to get the current you want, or get a modern electronics board with software-adjustable motor current.

Heatsinks on top of the stepper motor chips are not very effective, because the chip is designed to be cooled through the PCB and not through the plastic top of the chip. They may even be making matters worse by preventing the air fro the fan getting to the PCB. Most modern electronics boards have integrated driver chips, and generally cool them much better than plug-in drivers do.

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: Y Axis Slipping
September 09, 2015 03:39PM
What speeds are you printing at? (Including infill and movement speed)

I had a similar issue that seemed to pop up out of nowhere. It just started skipping on the Y axis one day.

It drove me insane. I tried everything from realigning the rails, to replacing the drivers with drv8825's, and from lowering my exceleration, to replacing the motor itself with a bigger one that has higher torque (all of which made it a little better, but the problem was still there). I ended up solving it by turning down my non-extrusion movement speed. Haven't had it skip since then.
Re: Y Axis Slipping
September 09, 2015 04:01PM
Interesting note for the heat sink thought process you just mentioned. I didn't know that about these chips, that the heat dissipation is actually more important under vice above. I figured they were just like any standard passive-cooling required setup. - Thanks for that info.

It shouldn't have much affect though, and I imagine the heat sinks are helping though.

But I'm more than willing to try anything for sure. One of these tests, I'll peel off the heat sink, leave the fan on, and just let'r rip.

I've got a voltmeter, so when I get home tonight - I'll verify the stepper voltage. I've been watching the videos like crazy, and the one specifically from Pololu themselves is very educational: [www.youtube.com]

But that's where I'll need a nudge (apologies in being so needy, but THANKS for the suggestions). The motors I have are these:

4x NEMA 17 2.6 kg/cm

And the one that's currently skipping

1x NEMA 17 4.8 kg/cm

For the smaller ones (4x), the FolgerTech manual said to configure them to around 350 for their output. Haven't had any issues with these. For the larger one (the one that's skipping), I started at 400 and onwards up to 500, all of which would skip at certain rare times. And that's where the fun started, with all of the troubleshooting steps listed above. Many Youtube videos show the way of calibrating, is what I mentioned earlier about turning them all the way down until the carriages don't move, then slowly incrementing it back up until you get motion without vibration (if that makes sense).

In any case, please bear with me - as I'm most certainly not an electrical engineer... But to get that bad boy (the larger NEMA motor) to stop being angry at me, what value do you think I should put in the vref?
Re: Y Axis Slipping
September 09, 2015 04:02PM
Just saw your post "TheJones" - that's another option as well. Thanks for the feedback.

I believe my non-printing movement speeds are 100mm/s <-- Don't quote me on this, since I'm not currently in front of the unit, but I believe this is what it was set at. Definitely worth a shot to slow it down to 60 or even lower.
Re: Y Axis Slipping
September 10, 2015 09:42AM
Took both your guys' suggestions.

Re-adjusted the pots according to VRef, and slowed down the traverse speed of everything. I was right in my assumption that moves were set at 100mm/s for non prints... I brought that down to 60.

For the VRef, using the method where you turn them all the way down, then creep them up until the motor is able to move; This put the vref hovering about 400 or so on the Y stepper driver. I incremented it upwards a few iterations, stopping at 500, and then at 750.

Both ended up still skipping a bit into the print and offsetting the Y. Although, the 750 was considerably better as it appeared to only have skipped once.

So.... it looks like I need to go BIGGER! grinning smiley - But if you guys don't mind me asking, how much is too much? I'm afraid of frying the little stepper driver, or more probable - burning out the motor if I crack-addict the power going to the thing. What value do you guys think I should shoot for with this motor (NEMA 17 4.8 kg/cm)?

Also, side note, you mentioned a board that can do everything in software and has all of this crap integrated. What are the options for a Prusa in using one of those?

Again, sorry for being needy - you guys are what makes this community outstanding! smiling smiley

Thanks for the help.
Re: Y Axis Slipping
September 10, 2015 09:53AM
I see some very good suggestions for you and about the only thing I haven't noticed mentioned was XY Jerk in the firmware. I had to lower mine to 10 in order to stop Y drift. Hope you get it figured out.
Re: Y Axis Slipping
September 10, 2015 10:13AM
Will do when I get back home to make the adjustment. Thanks for the input.
Re: Y Axis Slipping
September 14, 2015 09:06AM
That was it guys. Thanks!

Looks like a combination of all the above items - finally made it so the Y stopped slipping.
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