I’ve just finished testing a pen plotter that I designed and built on my OF. I’m very happy with it. The key component is an MGN-9 linear bearing, with a 100 mm rail.
Now build a rotary automatic pen changer! 
Nice work! I love the innovation.
How did you select the “cutting depth”? Have you considered spring loading the pen?
It’s hard to see, but the pen IS spring loaded. The spring is mounted around the rod directly behind the pen. You can see the spring if you expand the picture.
Depth will depend a great deal on which spring you use. I selected an arbitrary machine Z-coordinate and worked from that, with a series of test drawings. I wound up using .050 inches.
To set it up, you run the Z carriage down to your selected machine coordinate, zero the work Z, and drop a pen in the holder so that the tip rests on the paper, and then tighten the setscrews. I’ve got a macro that remembers where the Z axis goes, because I never can.
Once it was working to my satisfaction, I saved the tool under “Special” in my Vectric tool list.
I’m hoping it will save me some wood for some project concepts I have in mind. Paper’s a lot cheaper.
I thought about an automatic pen changer, but then I’d have nothing to do!
Isn’t that the goal, I spend time doing things so that I have to do less things, then find even more things to do to keep me from having to do other things?
You’re making me dizzy…
Thanks for the clarification. I misunderstood the rod behind the pen. It appeared threaded, maybe for fine adjustment. Your spring method is good since it would track a surface better.
The rod is a chunk of 1/8" welding rod I had lying around. I did originally think a threaded rod would help with the tension adjustment, but it turned out to be unnecessary. I suspect the “threads” you are seeing is the spring.