Bed level variates along Y axis

The bed was never level along the Y axis from day one. At first I thought it was an issue with the 4 corner clips and was waiting to implement Tesseract's(Jeff) mod to fix it.

As the issue was bad enough to prevent me from printing even small object often, I asked for Jeff's help.

We had a skype video call and tried different things. Boro glass turned upside-down, rotated 90 degrees. No change. Boro+heating element removed. No change.

We came to the conclusion that it has possibly to do with the Y rails.

On the first video, using the dial(in millimeters-one full turn=1mm) I start from Y200(extruder all the way to the back). The bed first gradyally drops about 0.25mm and about 50mm before the Y-home positions it rapidly jumps up by about 0.5mm! Again this big jump happens during the last 5cm(50mm).



To show that it hasn't to do with the boro, ther is a second video where I have laid a laser pointer on the bed(along Y-axis) ponting to the wall, and you can see the dot moving on the wall. So it is not a curve on the bed, rather than the whole bed rotating as it slides along the rails.

 

Removed boro/heat element and woodboard, looking to tighten the pulley(regarding different problem, see http://forums.robo3dprinter.com/index.php?threads/layers-slipping.1113/).

While I was at it, I thought id test each rail individually to check the levelness end to end.

Here are the videos:

Right one


Left one


As you can see, the right one is completely off when close to the very back of the printer. It is even visible without the dial, if you look at the rail closely on the video.

The left one moved up too, but less and on this one its wehn reaching the front of the printer.

One more thing I noticed on the left rail, is that when pushed all the way back, if pushed up, the top bit moves a bit in reference to the bottom part. Something that is not happening on the right rail. Here's a vid.