Fall 2016 Prosthetic Hand: 3D Printing and Modifications

The design of the “Tact hand” was chosen from the instructable.com website [1] since it uses fully 3D printed parts. It weighs close to 500 grams and appears to look like a human hand. The design also suited our budget as it was a low cost solution to build the hand. All the stl files for 3D printing was available to begin printing the parts. [2] It also gives instruction for assembling the entire hand. There are close to 35 pieces of parts to print out for the hand. The hand only came with stl files that can be 3D printed, what that means was I could not modify the parts. That was be a serious problem as I proceed with the testing of the hand to grip a 1.5 lbs drink and the wrist rotation. I spent many hours researching to find a way to use the stl files in Solidworks for editing. Unfortunately, there was no way around it. The .stl files would not upload in Solidworks due to the fact that each piece had many geometric faces. The software was unable to trace the faces. So, instead, I had to create my own designs in Solidworks, based upon the tact hand 3D prints from the .stl files.

After printing out all the parts and assembling the hand for the electronic testing, we came across several problems. First, was that the fingers don’t have enough grip to lift any object at all for testing. Second, the thumb position was off and wasn’t able to do a pinch motion with the index finger. Third, the spool that connects to the shaft of each DC motors wasn’t strong enough when being pulled by the motor; it tore up the 3D printed part and lost rotation of the motor to pull on the fingers for movement. Fourth, the thumb location was too small to be inserted into a cup to grip the object. These are just some of the initial problems I had to face and needed to find solutions for. With that in mind I was required to use Solidworks to modify some parts to fix the issues. I begin to model the palm to relocate the screws for the servo installation so the thumb could be aligned exactly with the index finger. Another modification was extending the thumb to sit at a wider position to allow more room for insertion and grip of a medium sized drink from McDonald. For these problems, I cut rubber gripped groves and insert them to the tip of all fingers and thumb for better friction and I used fiberglass and proxy to enhance the spool for a sturdier pull. With these modification, we could redo the testing to see if there were any more issues. At this point, the hand could pick up a cup, do the pinch to grab a fry and use 3 fingers to hold the burger. It was a success, but another problem occurred, the DC motors gears started to break and would not able to pull on the string for the fingers movement. I had to replace close to 10 DC motors throughout the whole progress. I concluded that perhaps those DC motors were not meant to operate such high load.