This OpenSCAD script will generate an upgrade that allows the makerbot to hold a pen (or cutter or laser or drill or other tool) so it can be used as a plotter, this will hold the pen in place with elastic bands so that you don't need the unicorn upgrade or careful vertical alignment and levelling your build platform. Basically pen plotting will no longer be such a chore!

Current Version: 0.3
Date: 2012-04-30 ISO Standard YYYY_MM_DD format

Todo:
add option to add mouse-ears to prevent curling during printing
add option to have triangular supports at side of an addon piece for 3rd rear M5 bolt hole
make rubber band holes bigger!
Increase base thickness at ends or make glue-on covers for holding M5 nuts in place

Warning:
None

Changes from previous versions:
0.3

• added triangular support ribs to side of tower
• properly implemented maximum_pen_diameter
• made walls thicker and pen size larger after the printed holder of version 0.2 failed structurally around trapped nuts during assembly
• changed base shape to make more efficient use of material
  0.2
• moved bottom of all parts to build platform z=0 level
• rounded ends or main support bar
• fixed one slightly large node
• adjusted m3 nuts to be 2.1*nominal thread diameter
• changed variable names to reduce confusion/ambiguity
• added three square nodes on intelocking cylinders to further constrain motion
• Replaced hard coded numbers with named variables
• rotated hexagonal cutouts on inside of holder to ensure 2-manifoldness and inmprove printability
  0.1
• First attempt

1. Change openscad script variables to set the maximum pen size you want (in this case it is 20mm diameter)
2. Modify any other variables you want (such as nut and bolt size if you aren't using m5 and m3 or control variables to set what parts you want to print)
3. Print (you may need to rotate and move it about on your build platform)
4. Cleanup (paying particular attention to the nodes on the pen holder so that it slides smoothly in the tower, acetone helps immensely!)
5. Insert trapped nuts in pen holder and base (may need superglue/acetone/ABS glue to hold them securely)
6. Slide holder into tower of base and insert bolts through slots on base tower into holes on pen holder and tighten until they are threaded into their respective nuts
7. Fold rubber band into tre-foil shape and pass each of the 3 loops from underneath the base through the 3 small holes in the base and loop over the heads of the bolts, checking that it pulls the penholder towards the base without being too stiff or loose, you may need to use a different size rubber band or loop over some of the top and some of the bottom bolts to get good a tension (see photo).
8. Remove the current print head from your printer and install the penholder using M5 bolts (having your K-type thermocouple split with THE CORRECT K-type connectors helps here!)
9. Slide pen into holder and tighten bolts until the pen is held securely and vertically (this may damage your pen slightly!
10. Use UechiMike's "Thing-O-Matic G-Code Extension for Inkscape" http://www.thingiverse.com/thing:16424 (or something similar) to turn your 3d printer into a pro doodler!