A client recently requested a repeatable positioning device for a bio-medical system. Normally, this is a ‘turn the crank’ design, where the biggest challenge is sorting through previously-used kinematic configurations and applying the most appropriate one. This project was more interesting though. First of all, the environment precluded the use of springs, pins, and metal fasteners, which limited some of my favorite flexure-based options. What’s more, the device under test couldn’t move in the Z direction, it had to slide in from the side to find its nesting position. What a great problem!
After first running through my mental list of kinematic solutions and finding nothing appropriate, I turned to “The Elements of Mechanical Design” by James G. Skakoon.
This handy little reference is one of those books that should be on every mechanical design engineer’s bookshelf. It’s succinct and to the point, but without ending up as a dry compendium of everything any engineer has ever known, but wanted to forget.
There in the ‘exact constraint’ section was the picture I had been looking for. This configuration can be customized with a pin to provide the nesting force and a set of over-hanging wedges to hold the piece flat against the table.
Working from this inspiration, the rest of the design fell in to place rather easily. The lesson here is that the right collection of reference material, coupled with the experience to know how to use it, can make all the difference between a successful design (and a happy customer!), and a mediocre one.