Tuesday, March 3, 2015

Making of a new distal pen arm

Guest blog from tml

As you may already know, eggs are mainly drawn upon by using Sharpie pens.  The pen holder piece of the EggBot is designed to hold ultra fine point Sharpie pens. This is quite a good decision as Sharpie pens are both cheap and available everywhere.... except for Hungary. We don't have them.

On our recent trip to London we checked out a stationery and were lucky enough to find a 24-pack of fine point Sharpie pens at a very discounted price. Their lines are not as thin as those of the ultra fines but Kitti thought that they were good for painting infills so we bought them.

Back in Budapest  she tried using them, without much success; as it turned out, fine point Sharpie pens are somewhat thicker than the ultra find point pens, so they did not fit into the  pen holder arm that came with the EggBot Deluxe kit, as seen on the picture below.

Fail. After taking a look at the pen arm I thought that drilling could help but I am not a big fan of destructive methods and drilling a larger hole would have made the pen arm for the ultra fine point Sharpie pens less ideal.

3D printing to the rescue!

My next idea was that I just 3D print that particular part of the pen arm using my emaker Huxley 3D printer I have since 2011. 

The pen holding piece of the EggBot is an L shaped piece consisting basically of 2 wooden pieces, both screwed to an L-shaped, bent metal piece.

The original 

At first I thought that I would only print the particular piece holding the pen, but after looking at the part  for a while I decided that it would be better to print the entire L-shaped pen holder arm in  one piece. That way there are no bolts and metals, and plastic sheets, no unnecessary dimensions of freedom. 

I did not feel like redesigning it too much so I decided to copy the idea of the  arm's design completely. I used a pink ruler to take approximate measurements of the original piece. As it is not a very complicated part, I decided to use OpenSCAD for creating a 3D model of it. I use OpenSCAD whenever I can as you can create 3D objects in a programmatic way, by writing a simple basic-like code for generating primitives and applying transformations and boolean operations. The main advantage of this is that you can use constants and variables which makes it easy to generate multiple versions of the same object with slightly different parameters, like one arm for the fine Sharpie, one for the ultra fine Sharpie, which are basically the same, only the pen diameter changes.

I had to make a few changes to the original design, like I had to rethink the way the thumbscrew is fitted (as the original EggBot Kit part had a metal piece there which I didn't). I solved it by using a nut and bolt.
Alternative thumbscrew. The plastic is housing an M3 nut. 
There was  only one conceptual  modification I made to the original design: I wanted to create a pen holder which can fit both kinds of Sharpies nicely and that switching between the 2 kinds of pens does not require a screwdriver. I also wanted the pens to fit tightly. You can see on the picture what I came up with: I modularised the part of the arm that has the hole for the pen and made it replaceable.

The final, printed piece, glued together, with the servo motor mounted.
One pen extension for the ultra fine point Sharpie (diameter = 11mm)
and one for the fine point Sharpie (diameter = 11.5 mm)
After the design was done, I started the usual iterative process; print, try, adjust variables, print, try again, adjust variables, until everything is perfect. I needed about 4 iterations to find the perfect numbers. The biggest challenge was to get the size of the shaft for the nut that holds the thumbscrew correct I also ended up with cutting my one-piece pen arm into 2 pieces and gluing them together. The reason for this is that vertical holes print much nicer than horizontal holes.

Here is a picture of what it looks like when fitted on the EggBot:

Eggbot with custom pen arm

The thing files are available on Thingiverse.

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