First prusa i3 and x axis bending?

Hi to all... Just constructed my first prusa i3 box variant and I am very impressed of how well it works, with one exception... trying to calibrate and level the bed I had always difference from left to right end even that the surface was flat.. so after many hours I found out that due to the WEIGHT of the mk8 extruder/head in the middle point the lower rod goes a little inside and the upper comes out producing drop of the nozzle of about 0.3mm... anyone faced the same problem or any suggestions how to fix that? I have used the printed parts from dx.com which goes to "heacent prusa i3" model, but on my box frame (12mm plywood) are also pretty solid stable... only the problem with x axis...

thanks in advance!

This is always a problem with end supported rods. You could try getting larger diameter rods so they flex less or convert to fully supported rails or linear guides.
Something like this [www.ebay.com] would work nicely, though it's probably bigger (wider) than needed. Look around on ebay for something the right length or a little longer- 15mm wide is plenty for an extruder carriage. Make sure it comes with a matching bearing block or two (two is better, because the used ones are often missing some of the balls from the bearing block and you can steal some from one to put into the other).

Many thanks for your answer!!!... Now its explained all!!... But in a closer look, i am also suspecting the printed parts... i found them in hurry to fit the box construction and they are PLA. I have read that ABS is more tough (and maybe hard too?).. Because i am suspecting the holes where the rods fit in.. What about rebuild the x parts with some longer side fittings for the rods? I don't believe that the 8mm rods can be bent from the weight of the extruder in such a "short" length! I am bending them with my hands of course pretty easily, but i think they accept more "z axis" force from the weight than the "y axis" (inside) due to the hanging of the upper rod... In any way things must be reconstructed even if i use thicker rods i need all x plastic parts and other bearings...

thank you!!

Just a small update on this... i noticed that the rods didn't have exact fit into the x axis ends... so i filled every hole with some 5-min epoxy glue and then pressed (really hard) the rods into the holes until the glue came out... after 10 mins , rods and the plastic parts were one body... after reassembling measured again the "error" across the x-axis on the bed and seems better.. normally around 0.2mm.. now this x-axis is a fixed one but still the head can be removed from the x-carriage.. for now i think its ok, maybe a next upgrade would be a 10 or 12mm rods and bearings for the x-axis hoping that i can reach the 0 error..

You might also try a Bowden type extruder first where the heavy filament feed parts are rigidly attached to the frame and only the much lighter hotend is moving and is supported by the rods. This would dramatically reduce the load on the rods and also allow higher printing speeds due to the reduced inertia in the X axis.

Quote
Ecky
You might also try a Bowden type extruder first where the heavy filament feed parts are rigidly attached to the frame and only the much lighter hotend is moving and is supported by the rods. This would dramatically reduce the load on the rods and also allow higher printing speeds due to the reduced inertia in the X axis.

Thanks for your suggestion, i have this on my mind too.. a little bit concerned only about the "hysterisis" of the feeding of the hotend due to the length of feeding pipe..

Sounds like you are on the right track lighten it up and figure out a way for the mount for the rods to be super tight. The rods will pull the mounts when they flex; you may want to print some tighter fitting ones once you get it working good enough, can always use a raft to get it working good enough to print some replacement pieces. Looks like the Z rods are pulled in slightly too, you may want check them out. With that design if the Z rods flex at all the X axis will be affected as well. Might just be an optical illusion in the photo though.

Thanks!... till now I see no problem from z rods.. and I mean the vertical "x" sides of the 20x20mm cube... they have pretty good and flat surface and all layers are the same... No I am facing another problem little different... If the part I am printing is starting at small size something goes wrong even with the temperatures or feeding of filament I don't know... the first 10-20 layers are not very well printed.... the filament seems soft (high temp?) and also maybe lack of filament as I see some like gaps... BUT after the 10-15 layers height , the printings are almost perfect! I am using 60-70 degrees for PLA & bed, and 190-205 for the hot end.. And another question.. MK8 extruder has two fans from which one actually cools the object... the other (on the heatsink) seems to cool the head and the filament before enter the hotend... So shouldn't they be separate connected so the one works at all time and the other (for the object cooling) work after some layers and at a lower speed?

Many thanks in advance!