I'm quite impressed by my Robot Drawing arm, but the pen mounting could be better. The problem is that the pen is held in place by a single bolt which doesn't stop it from wobbling when the arm changes direction. One of the great things about having a 3D printer is that I can solve these problems by designing and making a better one. You can see the design for mark 1 above. It has a thicker ring which will grip the pen over more of its length. I've got a plan to fit "rings" on the pens so that I can easily swap them in and out without having to worry about alignment. I'm also going to experiment with tiny magnets to see if I can use them to hold things in place. Such fun.