Three-Groove Kinematic Coupling Fabrication

My original plan was to machine my coupling from plywood and glue in steel contact elements. Unfortunately, the CNC router in the Makerworkshop is down for repairs. Inspired by Prof. Slocum’s demonstration using bagels and fruits, I decided to make a version of my design using soft materials. I thought this would be sufficient to help me build intuition. I had already acquired all the materials and generated toolpaths for the original plywood and steel design, so I plan to also produce that once the machine is back up. A performance comparison between my soft materials mockup and the final version would be interesting.

Melon Coupling Fabrication

I had a perfectly ripe honeydew melon I was looking forward to eat, and decided to borrow a small part of it to build my mockup. I cut the melon into 2 circular discs and punched holes in one of them, placing each hole on a vertex of an equilateral triangle. I then “transferred punch” those holes onto the other disc using a chopstick, thereby laying out the positions of the matching vee grooves. Cutting the vee grooves was a good chance for me to practice my knife skills.

I had hoped to glue the bearing balls to the melon disc over the holes. I had even whipped up a batch of starch-based glue to try this (gelatinizing cornstarch is a popular cooking technique in Chinese cuisine!). Unfortunately, the moist surface of the melon didn’t take well to adhesives, and I had to fall back on making the top half of the coupling from cardboard, to which I glued the steel balls.

Repeatability Testing

Melon Kinematic Coupling Repeatability Test from Shien Yang Lee on Vimeo (CC-BY-SA 4.0)

I tested the cardboard-melon coupling for angular repeatability using a laser pointer projecting onto a wall 59-inches away. The maximum spread over 5 engage-disengage cycles was 5/8″. Converting this sine error to an angular error using simple trigonometry, we find that the coupling is repeatable to within 0.607°. Extremely impressive for a coupling made out of ripe (so soft!) melon and cardboard with minimal measuring. I think this demonstrates how robust the concept of exact constraint design is against fabrication and material inconsistencies.

Melon coupling repeatability test calculation

Machined Plywood + Steel Coupling

Update: See the fabrication and testing of my revised (non-melon) kinematic coupling here.

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