I've built a machine for a dry-erase board. I have a typical gondola design with a servo actuator holding the gondola away from the surface when the pen is up.

As expected, the actuator erases thin lines when the gondola moves across areas already drawn. I know this is also an issue on other media (not dry-erase), if the actuator is dragging through wet ink, or otherwise defacing what's drawn.

I just wanted to start a thread on ideas for minimizing this problem.

I've filed the end of the actuator to a point, so the lines are thinner, which helps.

Has anyone made a utility to sort paths (either in the source SVG, or the generated Polargraph commands) from top to bottom, to minimize the overwrite? I know there are utilities to optimize the paths for less travel (which helps somewhat), but I haven't seen anything for sorting top-to-bottom.

I would expect on a vertical surface, the erasing problem would be less than what you're seeing on the tilted board. But then, adding the magnet would make it a problem again, necessitating the electromagnet scheme.

Does a dry-erase marker really need the pressure? I've seen plenty examples of dry-erase polargraphs that draw fine without extra pressure, so I'd expect not.

A simple solution to the erasing problem might be to glue a stainless steel ball bearing to the end of your servo horn. It might glide over the dried ink without harming it. Even better would be a ball bearing that actually rolls over the ink, which wouldn't be too hard to make. My whole gondola rolls on delrin ball bearings and it never smears ink anymore. I've never done dry-erase, so I've never had problems with the servo horn spoiling things.

The ball bearing works perfectly! I glued a 1/2" steel ball bearing to the servo horn, and it slides over the drawn lines without eroding them at all.

I have the board mounted vertically (without any tilt), and the dry-erase marker draws just fine. I have to have everything tuned pretty precisely, though, to keep the marker in contact with the surface over the span of the board while still keeping the marker away from the surface while lifted. That might be a little more difficult with the large 6' x 4' board at the office, so I may still have to use a small magnet there to stabilize it.

I also have a slight problem with bounce when the marker drops, and I think it's because the ball bearing has some mass to it, so it causes a little oscillation when it swings away from the board (and I have the pen up/down range as small as I can). This can make the marker tip bounce slightly at the start of a line. I'll look at the Polargraph server config and/or code and see if I can add a small delay after the pen drop (so the oscillation can settle before drawing).

FYI, you can change the speed of the pen lift with the penLiftSpeed variable (https://github.com/euphy/polargraph_server_a1/blob/master/polargraph_server_a1.ino#L134). You could add a delay to https://github.com/euphy/polargraph_server_a1/blob/master/penlift.ino#L95 to give a pause after each penlift.

delay(300);

would delay for 300ms which might help.

Gondola - and drawing - looks great, well done!

sn