Spectra

After experimenting with rc hobby servos, I've decided that, for my project, I'll probably use a combination of hobby servos for the neck and head and cable tendons with a motor bank for the legs. 

Initially, I had planned to use very thin stainless steel wire cable for the tendon, but, after I received my sample order of wire cable, I quickly realized that, in spite of its strength, the steel cable wasn't designed to handle sharp bends around small pulleys.  In my search for an alternative, I found a master's thesis on the internet by Catherine Anderson entitled "The Design of a Compact Actuator System for a Robotic Wrist/hand" that was very helpful.  Among the several materials mentioned in this document I was most impressed with an artificial fiber called Spectra.  Spectra is an Ultra-High Molecular Weight Polyethelene that's available at a relatively low cost.  According to Catherine's thesis, the only potential issues with Spectra is its tendency to have high "creep" under extreme stress or heat.  Given that I am making a robotic tortoise, I don't think there will be many issues with stress or heat.

When I searched for Spectra on the Internet, I mostly found websites that only offered it in the form of thicker ropes.  To get an idea of what Spectra fiber was like I decided to order a short length of the thicker Spectra rope from McMaster-Carr.  The following set of photographs document my dissection of the rope and the process I followed to reassemble the dissected pieces into thinner cords suitable for use in my robotic tortoise.

The Spectra rope, as received from McMaster-Carr, consisted of what I assume to be a central core of braided Spectra fiber bundles surrounded by an outer shell of unknown material.  The outer shell can be easily slid off the core material as shown in this photo. 

This is a pile of the core fibers after I removed the outer shell and unbraided the fiber bundles of a 4 foot length of Spectra rope.  The fiber is not unlike thick dental floss, but with a heavier waxier feel. 

To make cords of the proper thickness, I selected one of the fiber bundles and split it with my fingers into four quarters.

I next took two of those four-foot quarters and tied them together at one end with an over-hand knot, so that I essentially had one very thin eight-foot piece of Spectra fiber.  I attached one end of the eight-foot fiber to a doorknob in my living room and the other end to my Dremel tool and used the Dremel tool to twist the fiber (see Photo). 

When the fibers were twisted enough that they began to kink in multiple places, I then carefully folded the eight-foot length of fiber in half and allowed the two halved to twist against each other in the reverse direction. 

The resulting product was a four-foot length of very thin Spectra cord.