Thanks for all the ideas. I have not had a problem with pen adjustment yet as all my trials have been with the same size ping pong balls. I can imagine there could be an issue with the taper on the pens and it would not be hard to change the vertical pen arm to have a slot and thread two holes in the disc that goes on the stepper, I might give it a go on my next attempt. The plastic springy servo connector is interesting but I can see problems forgetting to set the correct S value and giving it a good stretch, I am playing with a few different bits of software to generate the gcode and in some I have to replace the Z moves with the correct S values. so far I have not had a problem with the pen going down and have tried a golf ball and it coped with the dimples ok, being aluminium and not plastic it has some weight to it. My major problem was with the play in the hinge, I am getting acceptable results now with my new version but I am sure it can be improved on further. I am lucky that I have access to CNC machines at work so I can try these ideas out in my dinner times and as its all just a hobby the budget is not such an issue, if it helps improve future versions then that's even better.