maxy

If you want to experiment, try my speedrefactor branch of CuraEngine on github. It does always smooth out extruder speed changes gradually (during the layer, not only on layer change). It works both for the first layer, and for sudden layer time changes. It helps for some objects. Here is an extreme one: (left: old code, which suddenly slows down, right: speedrefactor branch) It works well, and I still use this branch myself for printing. But I think I have broken dual-extruder support. Also, it does not help as much as I hoped, so I'm not pusing for upstream integration. (Note: the XY speed will still jump after the first layer. But it is now calculated such that the E speed does not jump.)

I don't know what the UM2 can do, but you're trying to squeeze 0.3*0.4*50 = 6.0mm^3/s through the nozzle, which may need too much force on the filament. Try to reduce the layer thickness to 0.2mm, or reduce the XY speed to 30mm/s, or if you really want to print that fast, increase the temperature.

You mean a hiccup in the updating of the temperature display? As long as the temperature was never wrong, I wouldn't worry. When the software asks for the temperature, I think it first has to wait for the current movement queue to finish before it gets an answer, which can take several seconds, depending on the move. If you see moderate temperature jumps it could be EMI, in which case you can try to move the termocouple wires as far away from the stepper motors as possible.

I put some drops of oil there, seems to have worked too.

Have you reduced the current of the motors already? That made the biggest noise difference for me. The defaults are a bit overpowered (probably to make them work even if you have quite a bit friction). Reduce the current until you lose steps, then increase it a little bit. You can find info down on that page: http://wiki.ultimaker.com/Electronics_build_guide

Well I set the max speed higher (to like 10000) using Daid's build me marlin website. So rest assured I am actually running 70 mm/s (or at least trying to, skipped steps aside). So could it be that everyone else is running 45mm/s?? That would make sense... Reading this thread made me curious... I tried Florian's settings above and got a nice string-free print. But I realize now that I was retracting at 45mm/s (which actually results in longer pauses while retracting). You can change the maximum without recompiling Marlin. Use the M503 command to list the settings (the defaults stored in flash I think). The command "M203 E1000" removes the limit. It may be worth to try decreasing the extruder acceleration limit too (not sure if this one actually has an effect, but it's extremely high compared to the other motors). I have my extruder stepper current turned down to get a more quiet machine (it was WAY too loud and rattling in the beginning). I can hold the wheel steady with a little bit of force to make it miss steps (with the default motor current I could not do that). So I had to quickly test the real 70 mm/s retraction - sometimes it did miss steps, sometimes I think it actually worked. I should mention that I have http://www.thingiverse.com/thing:25490 installed to reduce extruder friction. I could turn up the current to make it work, but it was too loud for my taste already. I'll stick with 45mm/s, which isn't exactly quiet neither, or maybe with something lower.