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Zen Webb asks good questions.  Take this one, for instance.

Can you elaborate a little bit on the gondola design you used? I see multiple versions in the photos, but I’m personally pretty interested in the simple ring design. It looks like it might be 3D-printed, or did you source it from someplace else?

Yes, it’s 3D printed.  So far every version has been.  While I use the ring a lot, it’s far from perfect.  A good pen holder should

…have the two strings meet at a single point, or as close as possible.  The moment they separate the math gets really ugly.

…deals with friction.  Friction causes the pen to drag and lean.  If I tell the robot to draw a square corner and it comes out rounded then I know my pen is dragging because it never reached the corner.  The pen has to stay at a right angle to the drawing surface.  So far I’ve found that having at least three points of contact is enough to eliminate the problem.  That’s why I tape my business card to an eye bolt on the bottom of the ring – the bottom edge of the card forms a large contact area with very little friction.

…deals with gravity.  If the pen is balanced wrong it may point up or down.  If it points up then it might go dry.  If it points down then it might have extra friction when moving downwards, causing the pen to skip and create a dotted line.

…make it easy to switch pens.  Not only should it be easy to replace a pen but every pen should “lock” into the pen holder at the same distance and angle from the drawing surface.  In order to simplify this problem I only use one kind of pen that comes in many colors.

…works on both vertical and slanted surfaces up to a maximum of 10 degrees.

If you have some ideas about how to make a better pen holder, please contact me.  I’d love to print your idea and try it out.

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