Marker holder (for colouring feedstock)

We bought lots of white ABS with our shapercube.. Lots. But who wants
to print everything in white? After a while that got a
bit.. samey. So, an experiment, jam a coloured marker in the feedstock
inlet and see what happens. What happened was, coloured prints came
out. So the idea for the marker organ was born, a way to attached
several coloured markers to the feedstock, and pick which one, or
combination, is colouring the plastic.

This is the first attempt, a simple clamp over the feedstock which
holds a single marker against it.

Note: Please don't get discouraged by the wall of text. It's not that hard, I just wanted to document it fully!

Instructions:

Becuase there's no de-facto standard for the shape of markers, you'll
have to measure your marker and change the openscad model to fit.
Don't worry, it's pretty easy. If you want to be lazy, you can just
get the exact markers I used for this.

You can also change the size of the hardware you'll use to put it
together. Since it's currently set up for M6 bolts with square nuts
(because I had lots of them on hand), and because you need to modify
for marker size anyway, this is probably a good idea.

If you want to get the same markers I'm using, they appear to be
http://www.staples.co.uk/office-supplies/writing-correction/writing-instruments/markers/permanent-markers/staples-permanent-and-dry-proof-markers/permanent-markers-4-pack-assorted
. Oddly, this doesn't match the number on the packaging, which says
"item #11090".

Open the .scad file in your favorite text editor. First, the bolt
size: change the line that says "bolt_dia = 6;" to list the diameter
of your bolts, in mm. (Don't add an extra bit for clearance; you want
one of the bolts to cut it's own thread as you put it in.) Make sure
you haven't accidentally removed the semicolon on the end of the line.

Measure the diameter of the part of the pen that you want to grip
(just above the actual tip), in milimeters and put it in where it says
"pen_holder_bottom_id = 8.5;". As above, make sure you
don't delete the semicolon. You want this to be a tight fit, but not
so tight that you can't get the marker in at all. If you get it wrong
the first time, you can always print another one.

Next, measure the length of the part of the pen that you want to grip
-- the part that is no wider then the pen_holder_bottom_id. You
should include space for the tip of the marker in this measurement as
well. Err on the side of making it shorter then neccessary -- it
needs to be long enough to get a good grip, but not so long as to keep
the tip from actually touching the feedstock.

Next, run openscad. Do a file/open to get your file, then
design/compile and render. (It's normal for that to take several
seconds.) Then design/export as stl, and do whatever you'd normally
do to print an STL file.

Usage:

Clamp the two parts together with bolts, with your feedstock running
through the middle. Insert your marker and clamp it in using a bolt
through the side hole. Print something!