Retractable Octagonal Pen

My first attempt at close-fitting threads in Fusion 360! This will fit pretty well any smooth pen refill. 

The pen refill I modelled this after is 145mm long, 3mm diameter, and is smooth, no tabs. The hole is 3.5mm so you may be able to use larger ones, or maybe ones with some textures.

Print as oriented in STLs and the photo. The insert prints on the flat side, and the pen cap and end need to be printed vertically with brims. Make sure Detect Thin Walls is on.

Push the refill all the way into the insert, and twist to keep in with a bit of friction. Screw the cap on, and slide on the pen body. Depending on tolerances you may need to work them in an out a few times. It may be a bit tight, but some twisting and pressure clicks the two ends together. 

I'm spoiled with a well-tuned Prusa Mini so tolerances are pretty close. But, if anyone has issues I can try making more room between the parts. 

I also added a dimpled pen body, it will print with holes to show the insert colour. Due to the holes, do be careful when working in the insert.

Shorter pens should be fine, just need to twist it a little more to get out. Any longer, and either you could trim the refill, or elongate the pen body and insert in your slicer.

Printed in PLA, but any filament works. 0.2mm layers should work fine but I like the smoothness of 0.1mm.