Can't level bed on Prusa i2

Hi everyone,

After a bit of a hiatus, I'm trying to get my reprap running again. Right now I'm having issues with leveling my print bed.

The print bed is a heated bed + piece of glass covered in blue tape. Printer is an i2 so no upper build plate, just heated bed sitting on the pen springs. When following the leveling guide on the wiki, I run into the issue where I can't seem to get even the first corner properly level. I tightened the screws, set the endstop so the nozzle was just above the bed and brought the front-left corner up so that I could fit a piece of paper between the bed and nozzle but there was still some drag. After that, I move the Z axis up a few mm then re-home and the level is completely out again. No matter how many times I try, when I re-home in Z the level is consistently off.

I've tried just doing the best I can, but my prints are very inconsistent, sometimes sticking too well, or not sticking at all.

Is there something obvious I'm missing here? I've check the following:

- Glass seems to be flat (checked with straight edge)
- Endstop (mechanical) is screwed down tightly, so it shouldn't be moving as the X motor mount hits it)
- Y rods appear parallel (checked with level and measured with calipers the distance from each rod to the desk)
- X axis is level (checked with level)

Currently I'm using Nophead's Z couplers and these Z screw isolators to eliminate some Z wobble I was seeing.

Any insight or suggestions would be greatly appreciated.

Thanks!

Some things to check:

1) Defective end stop switch.
2) Bad wiring on the switch
3) Connector to the Ramps (or what ever) defective
4) Blown input on the Mega (or what ever)

Put an ohm meter on the control board end of the switch with everything powered down. Switch closed should read one ohm or less. Unplug the switch. Connect the ohm meter to the connector on the switch wires. Switch open should read open circuit (> 1 meg ohm). That checks most of what's above except for the mechanical operation of the switch.

The normal way to check the switch mechanically involves either a micrometer or something like it. You run the piston in until the switch closes, note the reading. Run it out, switch opens, note reading (if it matters). Step and repeat the process for repeatability. With the right little fixture (and a micrometer) it's not real hard to do. Without the fixture - not quite so easy. With perfectly adequate switches selling for <$2, it's cheaper / easier to swap out the switch.

um...

First, if you have heated bed you dont need painter tape. for pla use mix of water and cheapest wood glue (white).

why am i telling you that? its easier to level on glass then on painter tape.

about leveling:

-lower your Z axis zero-ing speed (if speed to high it can happen that z axis crashes into endstop and then moves limit switch. try very slow settings like 20-30 mm/min then increase)

-lower your z axis acceleration (it can happend that one motor mises steps if acceleration is to high, limit switch is on one side so that can be reason. lower it to some ridiciolus values, then you will se whats happening, increase step by step if everything is fine.)

-check if your limit switch mount is fixed tight enough.

-put some grease on threaded rod, graphite is the best.

-i see you have solved couplers, check them out once again.(when bed is leveled, take permanent marker and mark motor shaft-coupler-threaded rod with one line. mark both sides on same place. this will also be indicator if one motor misses steps. marks must be all time on same place for both sides)

-set all limit switches to normally closed. why? if normally opened, and you have some connection/switch problems the axis will collapse when zeroing. if normally closed, and inverted in firmware, and you have some problems with wiring, firmware detects limit switch active and you know something is wrong without any consequence

Edited 1 time(s). Last edit at 11/25/2013 08:10AM by woo.

OK this is a bit of an outside chance, but it just this morning happened to me, kept on getting it level over-and-over and it kept going out of true every time I moved anything, was pulling my hair out trying to fix it...... Turns out the threaded rod holding the extruder to the X-carriage had worked loose so it could shift slightly as soon as the motors started moving it with any speed! I'm reassembling now with a dab of non-permenant threadlocker on everything

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Prussa i2 Comissioned 4-Nov-2013
Bitcoin : 1Jcrg5czBtNfKyS4sCjFAamJTcSYdTGk9h

The same sort of thing can happen if stuff is loose and the printer rocks back and forth as it tries to print. Mechanical stability is the first thing on the list in any printer check out. Belt tension and drive nut seating (the 5mm nuts on the thread rods..) is close after that. Things seem to come loose at the worst times on these printers ....

Thanks for all of the suggestions!

So I tried pretty much everything suggested here. I lowered the Z home speed and Z acceleration, tightened the end stop mount, checked the Z couplers, all to no avail. So far as I can tell everything is tightened up, but I'm seriously considering taking it all apart and re-assembling with threadlocker like Azrael_UK said.

The one thing I am noticing is some binding in the Z axis on the X idler side, which seems to be caused by my 210mm threaded rod not being perfectly straight. When I move the Z axis, I can see the end of the rod wobbling around. I wonder if this could be the issue? I'm using top and bottom nuts on my Z axis, but no spring tensioning (I should really get some springs for it). The Z motors are also not screwed down.

So it looks like it was probably a combination of all of your suggestions. I ended up just taking the whole thing apart and re-assembling with a bit of lock-tite on all of the nuts, and proper lube on all of the axes (white lithium for smooth rods, graphite aka. a 8B pencil for the Z threaded rods). After that the issue seems to have pretty much resolved itself, I can get much closer to proper level now. Now to continue the endless tweaking and calibration....

Thanks guys!

Your local hardware store will sell you a lifetime supply of graphite powder for a couple of bucks. It's usualy in the same area as the front door locks.