I made a plot clock too... Thanks joo! http://www.thingiverse.com/thing:248009 I had a hard time with a few things, so I documented my process in the instructions. Hopefully these will help someone else. My modification is really minor, and it looks like there are others that do the same. I created a scad file of the part that connects to the arm servos that fits the single horn that came with my servos and used the following projects in addition to my horn part. I also modified the ino file to use some different constants that made it easier for me during the calibration process. See the instructions for more on this. Video of it in action: https://goo.gl/photos/qACTKfDJhuQvSEaRA You will adjust some numbers in the ino file to get these arms to move correctly, then you will turn off calibration and adjust some more. The lines you will be adjusting are: #define SERVOFAKTORLEFT xxx #define SERVOFAKTORRIGHT xxx #define SERVOLEFTNULL xxx #define SERVORIGHTNULL xxx #define LIFT0 xxx #define LIFT1 xxx #define LIFT2 xxx Here is what mine looks like during calibration: https://goo.gl/photos/JceryoG6WuWsPQMy6 I just played around with these numbers, then uploaded to see the results. Once you are happy with these, comment out the CALIBRATION line and upload again. At this point, you should see the arm go up to the upper right, wipe back and forth to "erase", then draw the canned time "19:38" from the hardcoded numbers in the ino file. It will repeat this once per minute, adding a minute to the time until you stop it. This is where you can adjust the LIFTx numbers and get the pen to come down right and clar the top of the holder just right.