The goal was to build a mechanism big enough to teach and be handled by fourth-graders.
Collaborators: Ryan Anderson, Scott Taysom, Steve White, Brettany Rupert, Clifton Dudley, Wyatt Felt
Here's a look at the materials we used:
Giant Spring (taken from a broken fan base)
clear tube: 30"
plexiglass: .25" or .375" thick, 5" long .5" (2 of them) wide
plexiglass: whatever thick and abt 2.75" dia
1" PVC pipe abt 3'
upper pusher - 2.25" diameter plastic 8" long
lower pusher - 2.25" diameter plastic: 7.5" long
tip - 2.75"+dia 4.5"long
spacer - 2 1/4" by 1.5" long
set screws - various sizes
Big ol' Marker
Small spring and guide rod for cams
~$60, weighs in at 4 lbs
Step 1: How it works
When the pen is pressed to the change point, the lower cam clears the guide pin (Fig. 1b). The force of the compressed spring is larger than the friction on the inclined surfaces of the cams, and the lower cam rotates as it retracts into the extended position (Fig. 1c).
By compressing the spring past the change point again, the lower cam rotates and the upper cam is guided back into the retracted position (Fig. 1d). The stroke length from the retracted to extended position is 1”.