As we explore LEGO art machines, we've been trying to figure out the best way to connect markers to the technic pieces in a strong yet flexible way. While we recently learned about a set of LEGO markers that connect to the bricks, for the activity on the floor, we wanted to use standard markers. Over the past few weeks, we've been experimenting with bringing in outside materials and creating custom 3D printed parts for that purpose.
For the first drawing machine experiments, we attached markers with masking tape, pipe cleaners, and binder clips in a similar fashion to regular scribbling machines. While this worked ok, using tape goes against one of the qualities that we like about LEGO art machines, the ability for systematic iteration using the technic pins. 
We've had a 3D printer in our prototyping space for about a year and are just starting to get a sense of it's potential. We've made a few small exhibit parts and tested the machine out with visitors for a beetleblocks workshop, but overall we haven't found too many uses for digital fabrication technologies. However, for the LEGO art machines activity, it seemed like an interesting challenge to try to create a pen holder. It turned to be a great chance to take advantage of way a 3D printer supports the rapid prototyping process and allows for the ability to quickly share designs.
Nicole created the first version of the marker holder with a hole the same diameter as the small thin crayola markers with the idea that a marker would press fit into the part. It was a happy discovery that the tapered shape of the tall markers made it easy to slide them into the ring. The 3D printed piece had two holes on the back to fit the technic pins that attach the markers to beams and bricks. 
As we started using the new 3D printed part, we posted some of our art machines on twitter and started up a online conversation with several educators who had done work building with LEGO. It was pretty amazing to be able to send others the file and within a couple of hours get to see them using it to make their own art machines. This collaborative aspect of 3D printing was something that I hadn't thought about before and it really opened my eyes to the possibilities for shared prototyping. As we began to use the part, we quickly saw improvements that we could make and began to quickly adjust the file to make new iterations.
The first thing that we thought to change on the pen holder was to add a plus shaped back with four holes so that we could orient the markers in two directions. Quickly that led to thinking about the brush holder on the watercolor bot and adding a set screw that could be adjusted to hold different sized makers and change the height of the drawing implement. 
One challenge with the piece is that we had to use a tap to thread the inside of the hole as we were unable to create a file with printed threads (something that we hope work on for the next version). In the first workshop we tried with the rest of the Tinkering Studio team, we noticed that the metal screws added too much weight to the system, so we swapped it out for nylon screws for our next floor test.
This version seems to work great so far and really opened up the possibilities to explore different ways of drawing, including using paint and other mark making tools. 
It seems like there's a lot of interest for others to make LEGO art machines so we've uploaded both the basic pen holder and the set screw version to thingiverse as STL files. This is our first attempt at using the website so we hope to get feedback on how the print works on different machines. As well, it would be really great if other designers and educators took the design and modified it to suit their purposes.
As we've continued to think about the elements of the LEGO art machine workshop it's been fun to get the opportunity to experiment with 3D printing and learn about different qualities of the tool. And for visitors to the Tinkering Studio workshop, hopefully with the right pen holder, they will be able to spend less time on getting the attachment point right and get to tinker more with the motions, mechanisms, and artistic qualities of their creations.