Twist Pen
This pen is operated by twisting the front and back sections. It features a triple-start square thread as the drive mechanism.
Figure 1: Section View of Pen.
Design
I designed this pen to actuate without a gap opening up anywhere, like you would expect with a screw mechanism. There is a square shaft that transmits the torque from the outer housing to the screw, which is what pushes the ink forward. I added a spring around the square shaft to keep the outer housing pressed against the front housing.
Figure 2: Triple Start Square Threads.
Machining
The most challenging part of machining this pen was making the triple start square threads. I machined both the inside and outside threads with a custom single point threading tool. I machined all parts of this pen on my South Bend manual lathe.