Improved version of second Tulio Marble Machine 3D print model

The Marble Machine from Tulio is very impressive. A huge job, that runs nearly perfectly. I changed two little things:

• The button you should turn to lift the marble up.
• The helix that actually lifts up the marble.

The axis joining these two parts had a very small triangular section of the original design. It breaks itself very easily, especially when a marble blocks the helix at its bottom. I also added a curved arrow on the button, to help users knowing how to lift up marbles.