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.

1. 3d printable parts: http://www.thingiverse.com/thing:250204
2. Expo marker arm: http://www.thingiverse.com/thing:261999
3. Battery holder: http://www.thingiverse.com/thing:456900

Video of it in action:
https://goo.gl/photos/qACTKfDJhuQvSEaRA

Printing

1. Print the servohorn.stl (x2)
2. Print all but the servo arms and arm that holds the marker from http://www.thingiverse.com/thing:250204
3. Print the Expo parts from http://www.thingiverse.com/thing:261999
4. Optionally print the battery holder (in ABS!) from http://www.thingiverse.com/thing:456900 and wire it for series operation. If you don't do this, you will need a power supply for your arduino that has sufficient ampherage to drive all 3 servos. My USB port was not enough to power the servos, and it took a while to figure out why it would start to move, then restart.

Electronics Assembly

1. I created a servo header complex by soldering (sloppily, don't judge!) 4 x 3-pin headers together with some wire (see picture), but you could also use a breadboard. The servo's red and brown or black go to +5V and GND. Each servo's yellow wire goes to it's pin
   That is really all you need to do for the sample clock. A real time clock would be a good thing, or you can look at some of the other plotclock mods out there for other ideas (serial time, drawing, etc)

Mechanical Assembly

1. Put the printed parts of the base together and sand / file as appropriate to get them to fit together. They should be snug. Use Joo's skp file as a guide for where parts go.
2. use a 3mm drill bit or file to make sure that the arm part holes all have enough give to allow them to move freely with M3 bolts attached.
3. attach the servo horns to my part with a wire or small screw
4. attach the servo horn to the lifter servo and make sure it rotates from basically flat toward the other servos up to 90 degrees.
5. attach the driving arm for the lifter servo to the horn with a small screw.
6. attach all the arms to one another (refer to all the pictures there are on these plot clocks. It is especially important with the expo marker to make sure the expo arm is on the bottom side so the marker can reach the drawing surface.
7. I used self-locking nuts to fasten the bolts so that they won't work off while the arms are moving.
8. rotate the arm servos by hand counter-clockwise to the end stop.
9. attach the arm parts with the left arm facing left 100 degrees (counter clockwise of flat) and the right arm facing straight up. don't screw them on until you do the calibration.
10. I did not put in the clip for the pen/eraser head, since it does not quite fit. I will upload a new clip when I get one that works for me.
11. Use a rubber band or elastic string to attach a paper towel or other soft eraser like material to the expo pen holder. Trim anything that comes up over the sides so the pen won't catch on anything.

Calibration

1. manually position the arms so that the left arm is roughly horizontal and the right arm is vertical and the bottom servo so the arms are well above the surface
2. upload the ino file to the arduino (with calibration uncommented).
3. What you should see is that the two arms move back and forth.

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

• If you increase the LIFTx numbers, the arms should get closer to the platform.
• if you increase the SERVOFAKTORxxx numbers, a bigger angle will be used in the arms
• if you increase the SERVOXXXNULL numbers, the point at which the servos stop changes

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.