R&D issue: Where should the knuckles be?

For discussion of hand designs.
Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

R&D issue: Where should the knuckles be?

Postby Jason M Bryant » Fri May 15, 2015 5:20 am

If you look at your hand, one of the first things you'll notice is that the fingers are different lengths. Look a little closer, and you'll notice that the pinky isn't actually that much shorter than the ringer finger, it's also that the knuckle at the palm is lower, lowering the whole finger.
I don't think this is inconsequential.

The Raptor hands have all the fingers mounted at the same place, which is quite different from our natural hands.

finger knuckle comparrison.jpg
finger knuckle comparrison.jpg (57.99 KiB) Viewed 1593 times

I believe this affects grip. I have a series of pictures to illustrate this point.

finger offsets.jpg
finger offsets.jpg (37.62 KiB) Viewed 1593 times

In picture one, I'm trying to hold the cup in a very unnatural way. Picture two shows why this is unnatural. It requires keeping the fingers straight up and down, perpendicular to the hand. The thumb is not moving directly against the fingers.

In picture three I'm holding the cup much more naturally. Picture four shows why this works. The fingers actually form a diagonal line against the rest of the hand. This makes the thumb and the fingers move directly against each other. When we discus an opposable grasp, we tend to focus on the thumb placement, but the finger placement can be improved also.

Just try picking up a cup and drinking from it in the position from picture number one. That's what we're asking our hand recipients to do.

In fact, one thing I often notice when I grab things with my test hand is that the ring and pinky fingers don't contribute much to the grasp. The object won't stay in the position of the cup in picture one, it twists away from the ring and pinky finger. I think this is part of the reason why.

The Raptor Hands put the fingers in a straight line. However, the Flexy Hand puts them at an angle. I've mocked up something to show what a Raptor style hand would look like with finger placement that is more similar to the Flexy hand.

hand shapes.jpg
hand shapes.jpg (59.28 KiB) Viewed 1593 times

I'm pretty sure I can fit the knuckle pins in, it would just take 3 instead of 2. I am working on a new hand design, and I'd really like to arrange the fingers in this way.

My concern is leaving enough space inside of the palm for the recipients. That's where I need some advice from people with more experience with recipients. I see three ways of changing the shape of the hand.

hand options.jpg
hand options.jpg (75.17 KiB) Viewed 1593 times

Option one would cut off a lot of space from the current design. Is that okay? Is that empty space for most recipients?

Option three just adds more space. Everyone who can use a Raptor now will be able to fit into this. However, this will leave empty space and elongate the hand, possibly enough to make it unwieldy.

Option two is in the middle. It cuts a little corner off the side of the hand on the pinky side and raises the index side a bit. This seems like my best bet.

By the way, I'll obviously need to make the inner cavity a straight angle in the finished hand, not a stairstep like in the pictures.

So, any opinions? People who have worked with the Flexy Hand, do you like this palm cavity shape better than the Raptor's? Does option two still cut off too much space?

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Skip Meetze
Posts: 33
Joined: Mon Feb 16, 2015 9:33 am
Location: Webster, NY

Re: R&D issue: Where should the knuckles be?

Postby Skip Meetze » Fri May 15, 2015 9:33 am

I definitely think you are onto something. At RIT, there is a student with a hand for an arm (as yet unpublished) with this more anthropomorphic knuckle pattern. She uses tiny cylindrical hinge pins and I'm looking forward to trying 1.75mm filament for hinge pins.

You might consider filament for your hinge pins and the 1.75mm diameter will make them easier to insert above the hand features for the adjacent digit.

Also, we are developing a Raptor Reloaded with a reoriented thumb as a part of a system that doesn't quite work the way we want it to, so we haven't published it either. We will hopefully be ready to publish soon!

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Joined: Tue Feb 17, 2015 12:02 am

Re: R&D issue: Where should the knuckles be?

Postby hertfordkc » Fri May 15, 2015 9:33 pm

Great line of thought. I'm assuming that this would operate in conjunction with a whippletree?
As to shape, I don't have an opinion. The current designs cater to the notion that the squared off design would simply answer most of the needs. However, the tapering of knuckles and the squareness of hands is all over the lot. My hand's knuckles are nearly straight across and my pinky is much shorter than my other fingers. I am now working with a recipient whose hand and knuckles are severely tapered.
Look forward to your progress.
Kent Carper

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Joined: Fri May 15, 2015 4:44 pm

Re: R&D issue: Where should the knuckles be?

Postby blanche » Mon May 18, 2015 1:51 pm

Hi Jason,
I believe I was able to upload my video to you tube. Here is the link!

Hope this clarifies my idea to you.

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: R&D issue: Where should the knuckles be?

Postby Jason M Bryant » Tue May 19, 2015 9:20 am

Hi Blanche. I watched your very interesting video.

One thing to keep in mind is that the hands we're making will have a human hand inside them. Making the metacarpals curve inward might be problematic with our designs since in the end the palm has to be a plastic cup that goes around a human palm.

I'll definitely keep this in mind. I'll let you know if I think of any way to account for that or if I have any questions.

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