There are a lot of terrific pen holder ideas in the forums but I did not see anything quite like this so i thought I’d post my simple build. I apologize if it has actually been done before.
If you need to draw a simple part outline, text outline, drawing your first crown, or drawing a rectangle to check the square of your machine, this pen cartridge holder might help. It took me less than an hour to assemble, has a 1/4 inch shank, and fits nicely into the 1/4 inch collet of my Dewalt 611 router. The other advantage is that it shows you exactly where the tool bit will go since it is mounted into the router.
It’s made simply by cuting and soldering pieces of telescoping brass tubing. The 2.75 inch ink cartridges are available from multiple sources on Amazon and cost less than 50 cents each when bought in packs of ten. I actually bought this set that has ten pens and ten refills. I put one pen with my passport, one with my notebook, one in the toolbox, etc.
https://www.amazon.com/gp/product/B08B4FCW58/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
It’s a small cartridge, so you won’t be using it to draw 4 foot by 8 foot photos, but it’s useful in that you don’t have to remove your router and install a separate pen holder to draw. It’s 3 inches overall, and 1.5 inches fits into the throat of the Dewalt 611. I don’t know about other routers. To assemble, I used a torch and silver solder, but I think an iron and electrical solder would probably work as well.
The spring is from a Pilot G2 07 pen (lucky find in my junk box) and fits perfectly into the 1/4 inch open end with just a little bit sticking out. I push the whole thing into the router which pre-compresses the little bit of spring and then I’m good to go. The cartridge sticks out about 3/8 inch, all of which can be compressed, so the more you lower your router, the harder the pen will press onto the drawing surface.
I hope this is useful, enjoy.
