• Bad resolution, but it is also too complicated :(

    Adam • 06/01/2021 at 20:43 • 0 comments

    Hammer_mold

    Foam hammer

    I was inspired by pin impression toys (The toy where you put something like your hand on it and the pins make the shape of your hand). If foam could be injected into the shape that the pins make, then parts could be made section-by-section, layer-by-layer. Multiple molds could be suspended from a “skycam” type system of pulleys, allowing for very large parts to be made. I 3D printed rectangular shafts and glued them to the ends of pins on a pin impression toy. The shafts were 1 cm x 1 cm x 25 cm and I took out 4/5 of the pins. Rubber bands were weaved through the pins such that, when pulled tight, the bands kept the pins from moving. You can see in the picture, a small hammer is used to make the cavity for a test print. 

    How to actuate such small pins?

    The distance between the pins sets the resolution of the print, so better resolution requires many pins bunched close together. Conventional stepper motors would be too large and too expensive to actuate so many pins, so I came up with a different method to actuate the pins. I took cellphone vibrating motors (~$0.1 each and about the size of a dime), they are pressed against the pin which causes friction between the pin and motor. When powered, the vibration of the motor breaks the static friction and the pin moves down by gravity. The pin is coated with a thin polymer, and is in a metal sheath, creating a cylindrical capacitor. As the pin moves into the sheath, the overlapping area between the pin and the sheath changes and therefore the capacitance changes. Measuring the capacitance gives the location of the pin. Instead of using gravity, one could theoretically build a single manifold chamber behind the pins. Pressurizing or pulling a vacuum on the back of the pins would push/pull the pins that are vibrating. You could switch between push/pull so quickly that the pins look as though they are moving in opposite directions simultaneously.

    Here is a video of it in action. I apply five volts to the motor and the pin slides down. Once the battery is disconnected, the pin stops.

    The result: Injected foam made a pretty similar shape. It was difficult to get the foam to stay in the cavity (you can see the over fill), the foam stuck to the shafts, and the resolution isn’t very good at 1 cm x 1cm. Too complicated (would also require a “skycam” type positioning system), bad resolution.