Severe skipping at low speed on mini kossel

I've been working through most of this build on my own for a while and I'm trying to be more engaged online; if this is the wrong sub-forum for this please let me know...

I built a Mini Kossel (kit from makergeeks..) running Marlin and initially suffered a lot of the problems others did with this kit, albeit unknowingly at the time. I've always struggled with print speeds. The direct drive extruder pushing 3mm could only support prints around 10-15mm/s (just like advertised! /s). After slowly printing a geared & hobbed extruder, replacing ball joints & arms, and a few odds and ends, I thought I covered all outstanding issues... But now things go pear shaped when print speeds get around 30mm/s, usually around layer 5 to 10 there is some catastrophic skipping, grinding the head into the model or just shifting the print off to the side (usually along Y axis, but certainly not always). I had really hoping the geared extruder, which can now push without skipping, would allow me to get to the 40-50mm/s range; I don't think that's unreasonable given what i've seen about this delta design. I understand the trade offs with speed/accuracy, but I cannot print faster that 25mm/s if I want any kind of confidence the part will complete. This goes for different sizes models, different layer heights, etc.

Reducing acceleration to around 1100 has helped make things more graceful and allows me to print around 40mm/s, but the failure happens after a few layers. As far as the stepper controllers, only the extruder controller ever gets warm, the axis controllers never even get warm. Setting the controllers to 0.4V like "everyone" recommends leaves the motors getting VERY hot. The motors have a 0.4A rating, which after going through the RepRap wiki, would put my control voltage around 0.15V. The stepper controllers can barely go down this low. It helps the motor temp, but the skipping come easily and the motors don't have much holding power. I've been trying to find a balance around 0.25V but i'm really starting to get the impression this kit wasn't balanced out (i've tried control voltages ranging from 0.15V - 0.6V).

I've ordered a new set up motors from TriD that are 1.5A and I hope that will allow me to run the setup cooler.

So other than tearing it all apart when I get new motors, is there anything else I can do to troubleshoot this? Why is everything skipping when controllers are not hot? Am I completely off base thinking 40-50mm/s @ 0.2mm layer height on a mini kossel is a reasonable goal? Adjusting the acceleration was revelatory and maybe there is a few other settings that I'm overlooking...

Sorry for the length, i've been bottling up all these comments and questions.

I'll add that i've printed from my PC desktop, macbook and SD card with similar results. I was reading there might be a command buffer issue; still trying to validate that idea...

I had the exact same problem with a direct drive set up, and like you managed to get a geared extruder printed well enough to get me going. Anyway, is it skipping because the nozzle is catching on high spots, or is it skipping for no visible reason? Have you ensured that nothing is binding? I've found that skipping on my Rostock is usually due to it hitting high spots on the print, or having things bind slightly because belts weren't tight enough or bushings weren't tensioned right etc... Now that my extruder is calibrated to retract the correct amount and my first layer goes down cleanly, I don't have any issues with blobbing or high spots.

As your motors have such a low current rating, it's likely that they have high inductance. This would mean that they need higher voltage to move at higher speeds, and if you are using a 12V supply as most people do, that may not be enough.

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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].

I have the belts as tight as i dare; defiantly can 'pluck' them. I'm not really sure if its catching on high spots, I initially thought not since I would see gradual layer shifting (few mm at a time) at 'medium' speeds, pushed a little further and I struggle to tell if it hit something and then went off, or went off and then hit the model. The nozzle has both ground into the model AND stuttered up vertically; this makes me thing it wasn't a height issue.

However i'm not doing a full 1mm lift between retractions and will see how that goes.

Yes I have a 12v PS, 20A. The 'kit' came with a 12v 5A that wasn't able to heat the hotend beyond 110C. Heats fine now...



I have these other steppers which i'll try after holiday. Moving to 24v would require a different thermistor and some changes to the RAMPS board, no?

Bummer to hear about another user with such issues from this kit. I went a different route: convert to 1.75mm hotend etc. I haven't even had a chance to push the print speed yet, I'm still getting this kit to just plain work.

I noticed that one of my rails has a spot where the bearings feel like they get stuck. That may be due to losing a ball or two when the carriage slid to the end during unpacking. Do the rails and carriages feel smooth across the range you are working in?

Does your 12V 20A PSU have a voltage adjustment pot? The ones that are widely available on eBay do. If so then you should be able to turn it up to 14V or so, and not need to change anything else. However, if you have an LCD screen powered from the 5V RAMPS output, best to check that the voltage regulator on the Mega isn't getting too hot to touch.

Edited 1 time(s). Last edit at 12/31/2014 04:08AM by dc42.

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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].