I made this because I am an engineer who works away alot and the machine I work on have several loose nuts that need removing and rather than get my whole tool kit out it is easy enough to do it with just a 6mm socket. This pen now always me to fill out my paper work and remove or add the nuts. Fits standard 6mm square sockets. Can also add screw driver attachment to it :) I have also added the SCAD file so that you can easilt customise the cylinder size for the type of refill you have. Print the PenSocketcylinder with no supports but use supports on the PenTop and also print it at least 50% fill (the more the better really)