The Canadian Finger Trap

I designed and fabricated an elastically-averaged coupling (EAC) that could hold a part concentrically within it, but I did not have an application in mind when I was developing this idea. My earlier concepts were inspired by drill-chucks and collet systems, but wanted to avoid the complexity of their construction. Finally, I thought of a Chinese Finger Trap, and how the individual helical strands come together to tighten about the finger. Working from this idea, my final concept incorporated beam pairs that were helically arranged to grip a part.

Stiffness, accuracy, and repeatability of the 3D-printer part were predicted by conducting Monte-Carlo simulations of 100 permutations of coupling and part pairings, assuming a normal distribution of three errors: run-out of the gripped part (σ= 0.01 mm), error of in-plane x and y printing (0.05 mm), a normal z printing (0.25 mm). The coupling outperformed my expectations, with a measured angular accuracy (0.07° measured) and radial accuracy of (85 ± 57 um)


  • Concept Development
  • Sketch Modelling
  • Design and Manufacturing
  • Testing

Summary Design Spreadsheets Matlab Model Drawings