Calibrating the build plate/build plate shaking during printing?

[juj 0]

Hi there,

I got my new Ultimaker 2 this week, and I've been able to do some nice prints already (https://dl.dropboxusercontent.com/u/40949268/dump/CIMG0031.JPG and https://dl.dropboxusercontent.com/u/40949268/dump/CIMG0056.JPG). Today I noticed while observing it printing that the nozzle of the printer sometimes hits the layer of material underneath it, enough to make an audible tap/click and shake the material a bit. Also, the build plate occasionally shakes a little as a result. I made a video to attempt to demonstrate: https://dl.dropboxusercontent.com/u/40949268/dump/VID_20150513_235625.mp4 (2:00 minutes, 170MB), though not sure if it shows there very well. Does this sound unusual?

I attempted calibrating the build plate two times, but the same effect does seem to persist. How thick paper do you use when calibrating? In my calibration I've used one layer of A4 paper, which wikipedia says to be about 0.05mm thick. Is that too thin in general, I wonder if I should instead use a thicker sheet of paper for calibration?

Thanks!