Wednesday, January 15, 2014

prosthetic hand: milwaukee prt.8

As I mentioned before, I'm waiting on supplies for the first hand that we are going to build for Shea. Tonight I thought I might take a little time and get the drawings that Adream and I made into Rhino along with the 3D scans and then I could scale components easily to things in this digital file. I figured I might as well use the beast hand to figure out scaling of models and designs that we might explore in the future.  I plan on making Shea a Beast Hand to go along with a basic Robohand and then she can try both hands out to see what she prefers and how we can adapt things to suit her needs. Jorge has a procedure that he listed in the Beast Hand documentation on how to scale the hand and this is all done in the slicing software, but I thought I would just scale the hand digitally as compared to the hand drawings and 3D scans in Rhino before I slice and print. Again, this would give me the opportunity to use for future modeling around Shea's digital hand/arm.

I'll walk you through my process.

Rhino 3D has a feature called "background bitmap" that allows you to bring a jpg image into the drawing space that you can use for reference. Adream and I had drawn Shea's outline on a piece of paper and then I in turn scanned this. This is the image I placed inside Rhino as a background bitmap. (I often use this method when working digitally as it allows me to draw without the burden of the vast virtual space staring back at me; it can be intimidating to only have the mouse and screen between myself and the image in my head). I drew a few vector reference lines over my image and then scaled the Background Bitmap image to correspond with the actual dimensions in the drawing that I scanned. 

I had previously dropped the Beast Hand .stl files into Rhino. Each component was set up in their own layer so I could hide and show things as needed. I measured the Beast Hand Gauntlet width as my first reference point as I know the hinge will occur near the pivot of Shea's wrist, which I had measured and marked in the initial drawing. As you can see the imported model was much smaller at this location than what Shea's wrist is. I knew that I would need to scale the Beast Hand, but I would also want to allow for the thickness of the padding I will be using.

Once the padding thickness was calculated and added to the width of Shea's wrist, then I needed to scale all of the Beast Hand parts to match.

All of the parts were selected (this is important as you want to make sure all parts are scaled equally), and then I scaled the parts at the gauntlet pivot based on the measurement that I had calculated to account for wrist width and foam allowance.

The parts were now the  desired scale that would match Shea's hand/arm.

Then I adjusted the angle of the model to match that of my Background Bitmap image. In this case I wanted to see if Shea's partial thumb would fit into the palm section of the Beast Hand or if I would need to adjust the model. I also wanted to make sure that Shea would have sufficient space at the end of her hand and allow me to pad the inside of the Palm section thoroughly to prevent rubbing. 

As you can see, Shea's thumb sticks out, but she had it flexed out a bit when we drew the template and I drew a little wide as well. In this particular image the palm section is slid a bit too far up on the hand, so I moved it down enough to allow for interior padding. In addition, her thumb fit inside the bulge on the left hand of the model once I did this. You can see where the tracing shows through the holes that are at the end of the palm section near the finger knuckle mounts. This shows that the model is too far up on the hand. I adjusted this, but didn't get a picture of it. Sorry.

In these two pictures, you can see that I brought the 3D scan of Shea's arm into Rhino and started to compare things there. The 3D scan was scaled to match my drawing template at the key locations shown earlier. You can see here that I had moved the palm section down the hand a bit and then the gauntlet pivot is no longer aligned. I wanted to try and keep the Beast Hand pivot directly inline with Shea's wrist pivot. I did end up moving the gauntlet down a bit but I think I'm still really close to her wrist pivot. If this is a problem, I can always extend the pivot mounts in the model.

Next step is to flip the finger and thumb section over and align their joints.

Before copying the fingers, I took some measurements from the tip of the finger to the base of the last joint. I wanted to see if the length of the finger matched Shea's finger length on her unaffected hand. I brought Shea's unaffected hand 3D scan into the program, scaled it based on my template measurements and proceeded. Surprisingly, the Beast Hand middle finger was just one millimeter longer than what I had measured on her unaffected hand. Once the finger was aligned, I simply copied and pasted the other three fingers and placed them in relation to the palm section joints.

I am curious if anyone has made different length fingers? I know that Bob Roth did on the hand he made, but I was curious if there was an advantage to having all of the fingers the same length or if this is out of convenience for scaling and printing? I know that modeling in parametric software like Solidworks would make this easier. There is no worry about changing pivot hole diameters etc. when you are able to regenerate the model after making subtle changes. This is something that is not possible with the .stl files that we are all passing around. Anyway, I'm just curious if there is a benefit to same length fingers?

At this point everything is scaled to the proper size and I can now re-export the scaled .stl files and get them ready to print. I just received a few packages so, the supplies are starting to role in for the thermoplastic hand. I'll be ready to meet up with Shea again soon!

This last series of shots are just to show the addition of each section in wireframe and rendered views. I thought Shea might like to see this since she had to put up with us taking quite a few scans. 

I hope you're liking this Shea, as I think this is pretty cool to see virtually. Can't imagine how cool it will be to see for real!!

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