[Solved] Extruder pausing changing directions and leaving blobs  

RE: Extruder pausing changing directions and leaving blobs

Start simple; check your bearings.

Turn the power off and move the extruder both directions. They should feel the same, with no resistance.

If you do feel binding, try loosening the four screws holding the bearing cover on. It is easy to get too tight.

RE: Extruder pausing changing directions and leaving blobs

Seems to me, that you ancounter a physical obstacle (tight belt or stuck bearings issue ) they slow down the x movement, but the printer does not recognize it and it does not slow down the extrusion creating the blobs.

What are your belt tension numbers from support menu? Obvious numbers for x and y should be between 260 - 300. If they are beelow 220 or even less your belts are too tight. You need to losen them. 

Next thing check please the crash counters whether they are 0. When you willtry to print I bet your printer will detect x crash.

Let us please know.

RE: Extruder pausing changing directions and leaving blobs

Btw. What Firmware release do you have? Try to flash the newest one or reset the printer to factory settings.

But I do not suspect this is a firmware issue.

RE: Extruder pausing changing directions and leaving blobs

Along with everything said above I would also check to make sure the drive pulley on the X axis motor is tight. 

RE: Extruder pausing changing directions and leaving blobs

@zoltan

Is crash detection enabled? If not many questionable things can get by.

RE: Extruder pausing changing directions and leaving blobs

Thank you for all the replies.  I think my problem is bad linear bearings.  I loosened the back cover a quarter turn for each screw, then manually pushed the carriage back and forth. When changing directions, it first moves easily, then catches and moves harder.  It happens in both directions and it doesn't matter where the carriage is located on the rail when the direction is changed.  I assume it has something to do with the ball bearings shifting back and forth inside.  I did not notice this during assembly, but maybe I lost some ball bearings while putting it together.

I will order a set of replacement bearings today, but it will probably be a week before they get shipped to me.

RE: Extruder pausing changing directions and leaving blobs

As a temporary solution to get you going - put some light oil on the rail. Work the extruder back and forth.

And maybe loosen those screws another 1/4 turn.

Note: there is nothing magic about Prusa bearings. Any good quality from Amazon would do.

RE: Extruder pausing changing directions and leaving blobs

@robert-rmm200

Thanks.  I ordered some cheap bearings from Amazon.  They cost less than $10 with free shipping for 12 bearings, so I am not expecting high quality.  I also placed an order for Super Lube from Amazon while I was there.  I'll try lubing it up and see how it goes.

McMaster sells 8mm ID bearings that are much more expensive.  I assume that they are higher quality given the cost differential.  Most of the 8mm ID bearings have a 16mm OD shell, but the ones used in the Prusa kit have a 15mm OD shell.  I did find one that has a 15mm OD shell:

https://www.mcmaster.com/63255k45

That part has the same length and width as the original bearings, so I assume it is a drop in replacement.  3 of these bearings cost around $60 with shipping.  I went ahead and ordered 3 of those as well.  If I have to take my printer apart, I'll compare the cheap and expensive ones on the x-axis motion.  Given that I spent $800 on this printer kit and many hours assembling it, I'd rather use the best bearings I can before I put the printer back together.

RE: Extruder pausing changing directions and leaving blobs

Grab the Thingiverse grease injector and cap also. Fits on the Superlube tube. Works - Super...

Follow some YouTube videos on packing the new ones. Start with an alcohol soak to get the shipping oil or whatever out first.

RE: Extruder pausing changing directions and leaving blobs

Ooof, you overpaid quite a bit. IIRC Misumi (the go-to bearing replacement which is already a significant step up over stock) are only around 5-10 bucks a pop.  I browsed mcmaster before and they don't have much in the way of linear bearings within the "3d printer" range, and what they do have is stupidly overpriced. 

RE: Extruder pausing changing directions and leaving blobs

The bearings were the problem, and the $20 bearings from McMaster are better than the $1 bearings from Amazon.  I just got finished assembling everything, and it moves smoothly by hand.  I won't be able to calibrate and do testing until tomorrow, but it seems like the problem is solved.  I also packed the bearings with super lube before sliding them on the rails.

For anyone who is interested, the bearings sold by McMaster have the manufacturer information on the packing.  They are Nippon Bearing model SM-8.  They are also sold elsewhere (including Amazon), but typically cost about $20 each.  Outfitting the entire printer with them would be costly.

RE: Extruder pausing changing directions and leaving blobs

This is now solved.  Thanks for all the help.

After going through the set up and calibration, I successfully printed out the Prusa logo part that came loaded on the SD card.  The part looks good.  The stock X-axis linear bearings were the problem, but I may have lost ball bearings during assembly.  I doubt I needed the expensive bearings, but they work nicely.

One thing I learned the hard way: smudges of Super Lube are terrible to have on your printing surface. 😀  It didn't print properly until I cleaned the steel surface 3 times.

RE: Extruder pausing changing directions and leaving blobs

Great to hear you your troubles are resoled.

Have a good time and happy printing.

RE: Extruder pausing changing directions and leaving blobs

Note to self: Keep SuperLube off the build plate.  I can see how that would be a PITA to clean up.