Z axis jamming inconsistently
Hello,
I recently assembled a Core One kit as an upgrade for a Mini I've had for 5 yrs (so relatively experienced). After a bit of troubleshooting the Z axis constantly jamming (lubed the crap out of the Z axis rods), it is now able to print shorter models but past ~150mm it continues to sporadically jam. Occasionally it goes perfectly fine (see the left white sword tip), sometimes there's banding (see right sword tip and sword body), and sometimes the Z axis completely jams leading to goop/spaghetti and the print "finishing" higher than it should be (see silver rod and broad sword segments).
Does anyone have any insight into what could be causing this and why it's so inconsistent? When it jams, the front right motor continues to move for a bit. I can typically unjam it just by going into settings and manually moving the bed up and down. The threaded & linear rods are coated in grease. Any help or insight into the jamming would be much appreciated. Thanks.
RE: Z axis jamming inconsistently
You are probably seeing the effect of mechanical tensions in the Z drive, due to a less-than ideal assembly procedure in the kit instructions.
When assembling the printer according to Prusa's instructions, the trapezoidal nuts which travel on the lead screws are attached first, in some "random" positions. (There is quite a bit of play in their holes for the M3 screws.) Then, in a later step, the smooth vertical rods are jammed in, and force the heatbed into a lateral position. That may result in mechanical tension in the Z drive, especially in the lower bed positions where the lead screws cannot compensate by tilting sideways. Letting the trapezoidal nuts find consistent, tension-free positions is quick and should prevent step loss in the lower bed positions:
To fix this, move the heatbed all the way down. Loosen the 3 * 2 M3 screws that hold the trapezoidal nuts in place. That will allow the nuts to settle in lateral positions which are in agreement with the bed position enforced by the smooth vertical rods. Then re-tighten the M3 screws. Do not overtighten them, you don't want to deform the black plastic nuts.
You may also want to lightly grease the leadscrews with the grease that came with the kit -- especially if you get "groaning" noises when the heatbed moves up or down over longer distances.
RE:
In addition to aligning the nuts, you might also check whether "Z-axis calibration" resulted in a print sheet that is truly horizontal (the main board cannot control the three steppers individually, they are "synchronized" via mechanical end stops. Those aren't particularly precise and possibly not even at the same height, see printed mods).
You can perform this alignment manually by (after homing) moving the table to Z=1, then putting a piece of cardboard e.g. business card between nozzle and table. Now turn screws manually (with motors engaged, skipping steps) until you get a consistent nozzle distance when moving the print head around (motors disengaged and card below the nozzle to protect the printbed) near all three spindles.
If the table is not perpendicular in both axes, it adds up to the tension on the trapezoidal nuts.
RE: Z axis jamming inconsistently
Thanks but I already did both of those, that's how I got it unjammed in the first place. I've even done it a couple times but it hasn't seemed to help past that 150mm mark or lead to any more consistency
RE: Z axis jamming inconsistently
Thanks but I already did both of those, that's how I got it unjammed in the first place. I've even done it a couple times but it hasn't seemed to help past that 150mm mark or lead to any more consistency
Not sure what "both of those" means, since between @mnentwig and myself we suggested three things to do. I would consider the "loosen the M3 mounting screws for the trapezoidal nuts" to be the most important step. It is important to do that with the bed moved fully down -- that is the most critical range, where the flex mounts for the motors can't help to relax some of the tension.
If that really does not help: A couple of users have reported recently that they received bad trapezoidal nuts, which would not thread onto the lead screw at all or move very stiffly. To test, you can entirely remove the M3 screws which hold the nuts, and rotate the nuts by hand. They should all move very easily.
RE: Z axis jamming inconsistently
Initially my back Z axis motor was not perfectly vertical (skewed very slightly left-to-right). This was rubbing on the holder as the bed position changed, causing some noise and unnecessary friction.
I saw this by running the z-axis top-to-bottom and saw the position in the top holder hole shift. I'm using motor mounts with heat staked screws, and I suspect one of them wasn't fully flush with the surface of the holder. In any case, just a bit of adjustment of the screws (loosen the left, tighten the right) and the position remained constant within that top holder.
Note that some people have published top holders with bearings. @jurgen-7 has pointed out several times that overconstraint on that shaft is a poor idea.