I designed a pen that attaches to the Makita router/spindle (Model: RT0701C) for my Shapeoko CNC. The pen uses a gel refill that comes in various ink colors and line weights including .5mm and .7mm. It also houses a compression spring that is used to smooth out bumps as it draws on uneven surfaces. The spring loaded design plus the smooth movement of the gel inked tip; prevents tearing of the substrate.
To install the 3d printed gel pen; simply remove the nut and collet cone on the router. Then screw the pen onto the routers thread. The pen is secured and lines up perfectly with the spindle. This makes it easy to draw a design onto a surface and use it as a reference, before switching to a cutting bit to carve a design.
More Info: https://www.mmg1designs.com/3d-printed-gel-pen-for-makita-router-cnc-drawing-machine/