Step 15: Assemble the gondola

Picture of Assemble the gondola

The gondola is the pen holder. There's a few alternative designs out there for them, including this 3D printable one that seems to do the business nicely. My design is much heavier, and has a hollow centre so that the pen can always be in the exact point where the cords converge. In practice, I'm unclear about how much difference this makes, but it makes me feel good.

I made the first one from corrugated cardboard, and a blank CD, stuck to some ball bearings (see the last picture on this step). Later I graduated onto some fancy-dan laser cut parts (available through ponoko, or you can grab the source from the github repo), but the principle is the same. I've attached the design in an EPS on a ponoko P1 sized board.

The parts just slide together, and then onto a length of brass pipe (see parts list). The laser cut parts have nodes in them that will need a little filing to get them on. Just be careful because the acrylic is pretty brittle. It should all push-fit together, but if it gets too loose, a few dabs of glue will keep it together. I usually put a bead of glue around the hole in the stabiliser, and then another couple of dots around the hole in the final spacer ring.

The sequence is, from bottom to top:

  1. Big stabiliser
  2. Empty bearing
  3. Spacer ring
  4. Bearing with straight cord hanger arm
  5. Spacer ring
  6. Bearing with offset cord hanger arm
  7. Spacer ring (this is the one I glue)
  8. And a plywood ring as a decorative touch
eyeh28091 year ago
quick question, Is the distance from the drawing surface to the sprocket the same as the distance from the surface to place between the 2nd and 3rd bearing(where the strings connect) on the gondola?
The reason i ask is because my current setup makes the gondola slightly wobbbly, it certainly produces great results, although I would like to refine it. (and the grid gets a bit off toward the bottom because of this) could you send me a picture of a side view of the gondola with a pen drawing to help me solve this problem, or is there anything that you might suggest?

On a side not:
Thank you so much for this detailed instructable on how this amazing creation of yours can be built, I'm only a fifteen year old in high school, but being a maker is my hobby, and this is my first major personal project, and you have helped out so much! Thank you thank you thank you again!
Euphy (author)  eyeh28091 year ago
Thanks! Your gondola is looking like my first cardboard one - love it. Yours is a good question. I don't have a pic from the side, but in principle you are right - the intention was that the centre of the weight of the gondola would be directly below the sprockets. On mine with three bearings, it was designed to be in-line with the middle one. On yours with two bearings, I would have designed it just as you have, and made it in-line with the gap inbetween the two bearings.

In practice it's not as simple as that. With a sloping surface, that straight in-line arrangement only works at one particular point on the drawing surface. Move any higher and the gondola ends up hanging in mid-air, any lower and the cords end up pulling the gondola into the surface, which makes it more wobbly about its tip. That's my theory for why it gets so wobbly at the bottom.

The stabiliser (the blank CD) was my solution to that - it just stopped the pen tilting so much at the bottom of the drawing. Makerblock has done this great round-up of pen holder designs: http://makerblock.com/2013/03/a-study-of-drawing-robot-pen-holders-and-design-considerations/. Dan Royer's gondola stands out amongst them as being a good balance of pragmatism and features. Kongorilla (on the polargraph forum - http://www.polargraph.co.uk/forum/polargraphs-group2/building-hardware-forum3/no-stabilizing-plate-gondola-experiments-thread246/) has done some recent experiments with a stabiliser-free design too, but I think came to a similar conclusion: It gets wobbly at the bottom.
i seemed to have forgotten to click the upload button for the pictures, here they are
2013-12-09 22.40.23.jpg2013-12-09 22.41.13.jpg2013-12-09 22.41.58.jpg
kvods1 year ago
If I have 608zz bearings instead of 6003z bearings would that make a big difference? They are the same dimensions..
Euphy (author)  kvods1 year ago
If they're the same dimensions, then I guess it would make no difference at all!
sleepybrett3 years ago
It might be nice if you had a photograph or a diagram that shows which parts go where on the gondola. From photograph #1 it seems like it the order is:
* plate
* empty bearing
* bearing with 'short arm facing up'
* acrylic spacer x 2
* bearing with 'long arm facing down'
* acrylic spacer
* wooden cap

Is this correct? If so I'm having a problem with non flat hanging on a vertical surface. Suggestions?
Euphy (author)  sleepybrett3 years ago
Hi Brett, sorry it wasn't clear, you've got it right except there's a spacer between the empty bearing and the bearing with the short arm, and only one spacer where you've got a double.

The wooden cap is purely decorative. I usually glue the last spacer on the top so I've got something reliable to push against when I'm handling it.

I guess it is hanging tilted so that the pen is pointed downwards, and only the top-edge of the stabiliser is against the surface? I would fix this by installing the pen, and sticking some more blu-tack on the end of the pen until it balances. Alternatively, you could move the cord hangers onto the next bearing down - so the empty bearing was the top-most one. This will have the effect of moving the gondola away from the surface (which is why I haven't done it), but you might be ok.