What is wrong with this pursa?

Hello,

I have just builded my prusa mendel and I'm still struggling to print a decent object....

I can't understand what more I need to calibrate. The printer starts printing aparently OK, but at a certain point it creates an offset on the X/Y axis and ruins everything... This image is a good example of what I mean:

[www.dropbox.com]

Any idea of what am I doing wrong?

Thanks,
Joaoabs

You are dropping x and or y steps. You might need to boost the signal from your stepper drivers. If you have a ramps board you have pololu drivers and these have a trimpot to adjust output current. Try Turning these up a bit. The setting of the trimpot is a balancing act between dropping steps when too low and motors overheating when too high. It might take some experimentation to find the right setting.

I see that the issue is only seen on a single axis'. On top of that, the lost steps are only in a single direction. If the end stop on the X axis is found on the right side of this image, then i think you are getting false EndStop trips on X.

Solution: Disable endstop sensing for everything other than Homing. Some firmwares have this setting in the configuration.h.

I had this issue too, and thought it was the steppers overheating or something, but as soon as endstops were disabled in firmware, i was a happy printing machine.

I had something similar going on with my printer, it ended up being that my belts were too loose, so quick movements would cause the gear to "skip" over the belt - particularly on prints with a larger footprint. The belts weren't saggy loose mind you, just a little bit loose, but that was enough to experience some skips.

You can monkey with slowing down the acceleration speeds, etc. (which I did), but the big fix was the belts getting tightened. Since I've done that, I haven't had any prints with those offset layers.

Also make sure your grub screws are tightened. That got me early on.

putting a dob of nail polish on every grub screw really helps. i had my extruder grub screws coming loose on prints longer than 7 hours. a dob of nail polish on the top part of the thread and on side side when tight tops them vibrating loose.

you can use lock tight but nail polish is a lot cheaper and could already be in your house

It could also be your stepper drivers getting too hot. They shut down automatically if they get too hot and they come back on when they cool sufficiently. This can be a fraction of a second later.
I need fan cooling and heat sinks on mine for current settings over 1A and they are genuine pololus, the black edition with 4oz copper.
Try running your steppers at 70% of their rated current. This should give 70% of rated torque but keep the temperature rise under 40° which gives a 65°C operating temperature.

And make sure belts are running straight, my x-belt wasn't and my prints looked like that.