Limn - Pen Plotter with Automatic Tool Changer (Partial Release)

Here's my attempt at building a Pen Plotter with automatic tool changes and alignment. It's built with many parts taken from old 3D printers.

I've shared extensive build details in this Hackaday Post as well as some demos on Youtube.

Key Features:

• Runs on standard software stack: Klipper, PrusaSlicer, and some Python code.

• Ability to use various pen/tools (eg. a fineliner, ball-point, markers, paint markers, etc.) in the same plot session.

• With a specialized Alignment bed and RFID tags on the tools the plots are fully aligned.

This is a work in progress and as such this documentation is currently incomplete. I'd like to focus on the toolhead, tools, and tool aligment in this post.

The current design has many, many printed parts, therefore I am only providing STEP files for now. I've identified key areas to simplify the design in future.

The Toolhead:

• It uses a small stepper motor to actuate a "key" that locks in the pen tools for plotting.

• It is also the Z axis providing automatic bed-leveling and pen up/down motion. It's a lead-screw based motion driven by a NEMA8 Stepper and 30mm M3 screw.

• A three-point kinematic coupling provides robust, wobble-free docking/undocking process.

• The coupling also provides power and a signal path so the tools can communicate with the plotter should it be necessary.

• The coupling was directly inpired from RatRig/E3D system, just minituarized further,

• The whole axis can be actuated manually, that is, even when the plotter is switched off.

• An accelerometer is also added for input shaping.

Alignment bed:

• It is quite a challenge to manually center all the tools for a plot, therefore I've built an Alignment bed that uses resistive touch screen and some simple math to obtain XYZ offsets for each tool.

• Once the offsets are obtained they are written to the tool's RFID tag, and the Alignment bed may be removed.

• The electronic part of this consists of 3 RP2040 microcontrollers, Resistive Touch panel reclaimed from old car GPS units, and it communicates with Klipper through magnetic pogo pins.

• My analysis showed an error of <0.2mm with the Resistive touch screen, and the  sensor marked "B" (a FSR matrix) is not yet utilized.

Plotting bed:

• A magnetic bed holds a sheet of A5 size paper -- largest I could do with my current 3D Printer.

Tools and Holders:

• The tools are specific to the pens, in the sense that whether the pen with specifc diameter can fit.

• I've created tool holders for Staedtler Fineliners, Stabio Fineliners, and Posca Paint markers.

• The maximum diameter of a Pen has to be under 20mm.

• I've also experimented successfully with tethered tools where instead of a pen a USB webcam was docked.

• Each tool requires a RFID/NFC tag to remember its parameters.

Software and Design files

• I've posted a walkthrough of the design files here.

• I've also published the klipper configurations, klipper extensions, and micropython firmware on GitHub:

  • https://github.com/prashnts/limn?tab=readme-ov-file

Conclusion

• I'm currently unable to provide step-by-step instructions to recreate my plotter design because many design decisions were taken based on what I had in stock and there's a lot to be improved/optimized.

• I believe that my main contribution for the community is the toolhead -- it can be adapted and used on other CoreXY systems.

  • The toolhead is strong enough that it has survived crashing onto things and so on.

  • It is quite small.

  • The Key, while tiny, has also suvived a lot of mishaps.

• My intention with providing all the files is so that one can build upon or based on my work. My work wouldn't be possible without learning from others.

Acknowledgments:

• RatRig introduced me to the concept of kinematic coupling and how to achieve repeatability. 

  • https://www.printables.com/model/137147-ratrig-vcore-3-tool-changer

• This Post on Reddit which finally gave me the push to make my own plotter.

• Klipper and its documentation + hackability.

• Several CoreXY based designs such as (https://www.printables.com/model/734327-extruh-pen-plotter)