Pen holder design factors

[Amin]

I thought I would summarize here my findings on factors behind a stable non-swinging pen holder (gondola). And hopefully I can get some feedback what I am doing wrong, because I feel like I am touching all these points without getting straight lines like the rest of you. I know the fact that I’m using spools and strings instead of pulleys and timing belts is not helping, but it seems even with this people are getting better results than I am.

Here’s a video of my pen holder in action: https://www.instagram.com/p/BhgQkx-Da0b/?taken-by=shazman.life

DESIGN FACTORS ON PEN HOLDER, AND MY RESPONSE:

1. the strings should meet on a single point – mine meet at 4cm distance, but is it really better to have them close to each other, or better to have them separated on the egdes of a larger surface gondola?

2. having at least three points of contact to the drawing surface – this is super hard to get right, any tips on a gondola configuration where this is the easiest?

3. proper amount of weights and weight distribution – I have tried everything from having weights on top of pen to below it. I am using about 50-100g of weight. How much weight are you using?

4. when using strings, like I do (and not a timing belt), if the strings are long and are bent through eye bolts, there might be some delay/friction that causes jerking? – In all fairness, I do have a quite big machine (3x3m). Is this the reason?

5. don’t go too fast – I can’t get stable drawings with speed and acceleration of >500. What speeds are you running your stuff on?

6. could it be that some string materials extend and contract more than others, and that this somehow adds to the swinging? – What string material do you use?

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3 good posts about pen holder design:

Drawbot: Pen Holder

Trying to improve the Makelangelo accuracy with v-groove bearings

Thoughts on Improving the Makelangelo Pen Holder

[Dan]

One thing you might try is a Y shape at the end of the strings. Notice how the official pen holder has the long arms that get wider as they approach the pen holder. The two triangles create stability by holding the two ends of the pen steady.

[Amin]

Hmm you mean the Makelangelo 5 pen holder? The arms do look like (extremely thin) triangles, but I can’t see how they support both ends of the pen – both arms seem to meet at roughly same place at middle of pen length.

What speed and acceleration settings are you running your stuff on?

[Dan]

yes, the M5 pen holder. even a small triangle is better than a straight line. holding from two points is better than one, and the further apart they get the more stable it becomes. (in general, with special exceptions, etc)

the triangles are positioned around the center of mass so that the pen holder hangs straight. they rotate around a line that is in the center of the pen holder, so that even when the arms move it doesn’t change the pen tip position. This is was a major change over the M3 where the belts met above the pen. I went through about 20 designs to fix the pendulum behaviour you are seeing. … and that’s why I applaud your can-do attitude. I’ve been working on the next pen holder model, maybe we can collaborate? I want to put in a chuck that can be changed out so the pen holder supports more sizes and styles of pens.

I run at the default settings.

[Amin]

Ok so you’ve found that the most impactful thing to do is having long arms that balance the pen over it’s entire length. Do you think the arm-to-pen bearing (or what it is you use) is necessary too? Thing is, I find super simple designs out there that seem to be very stable, I’m perplexed haha!

The other stuff, like having 3 points touching the drawing surface, or long string lengths don’t matter that much in your experience?

I can’t remember the default settings, but wasn’t it like 6500 something? Crazy! I can barely get stable over 500! 🙂

[Author]

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