Rubber Band Raptor Reloaded

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Jason M Bryant
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Rubber Band Raptor Reloaded

Postby Jason M Bryant » Sun Apr 26, 2015 2:44 pm

I have three hands in my home that I use for testing. I've rewired them over and over to try out new parts.

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The first is a white Raptor Classic. It uses rubber bands for the elastic. I just took regular rubber bands, cut them once each to make long strings, and wired the hand up.

The second hand is a red Raptor Reloaded. It uses the kind of elastic cord that is recommended by eNable.

The funny thing is, the rubber bands on the white hand work much better than the elastic cord. I have it strung up tightly enough so that the fingers pop fully upright, yet it is easy to close the fingers all the way. The hand with the elastic cord barely gets the finger fully upright, but takes a lot of force to get the fingers near fully closed. I can't really get them completely closed.

I did a little testing and found that rubber bands can stretch to about 500% of their original length, while the nice elastic cord only stretches to 150%. So obviously the elastic cord is hitting its limit. This is more of a problem on the smaller hands, which have less elastic to stretch. That might also help explain why I had an elastic cord break before any of the rubber bands broke. The rubber bands still have stretch to spare even when the fingers are fully closed, so they're not feeling the strain.

That led to me creating the third hand, a blue Raptor Reloaded with fingers based on the Falcon hand. It has studs on the fingers and palm that hold little rubber bands, the kind you get from your dentist to put on braces.

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I'd like to thank Adam Arabian and the Falcon team not only for the idea and the design work leading to this, but also for the support and information that he shared on the forum.

First off, I got the wrong dental elastic bands. Adam instructed me to get the medium strength, 6.5oz bands, but I got the medium 3.5 bands. Also, my studs are bigger than his, which stretches the bands more. Several bands have broken, but obviously that's because they're not the right bands. So I can't really talk about the durability of the bands just yet. I am trying to order some stronger, slightly longer bands. I'll post some more once I can test the hand with the proper bands.

At this point, I do have a few thoughts on the design. It seems to work pretty well. The dental elastic hand does not close as easily as the rubber band hand, but it does close easier than the elastic cord hand. I'll have to reevaluate that after I get stronger bands, but so far it looks good. Hopefully someone with a strength testing rig can make one and compare the force needed to close the different hands.

One interesting thing is that the force is distributed differently. At the very beginning of the motion, the dental elastic hands require a little more force. However, that force doesn't seem to increase much as I continue pushing the fingers closed. On the hand with elastic cord, it starts off very easy and linearly ramps up as you get closer to closing the fingers.

This might be improved by experimenting with different lengths of elastic bands. The ones I got were 3/16, which I believe means they have a diameter of 3/16 of an inch. There are many other sizes, and I'm trying to get 1/4 now. If they are longer, then they won't be stretched so much at their starting positions. That should make it easier to open the fingers for the full range of motion.

It's also worth noting that we'll probably need different sizes for different scales of hands. Here's a picture of a 175% percent finger that I made, which would fit an adult hand. As you can imagine, getting the rubber band on that was extremely difficult, and the band broke within a minute. If some people would volunteer to test this, hopefully we can try a few different sizes of dental bands on a few different scales of hands.

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Here's the big thing. These are *much* easier to put on. The goal of the Falcon Hand team was to make something that can be put on with one hand. It's not easy, but I managed to do that. That means that you can give your recipient a little bag of rubber bands, and they can fix any broken bands themselves.

I'm probably going to make this hand my new test hand. It is much easier to put on the dental bands, and I don't have to waste the elastic wire every time I restring a hand to try out a new part.

I'm pretty happy with the design of the fingers, although further testing will show if any tweaks are needed. The palm is a complete hack. I just stuck on the studs where they needed to be. If this were an actual attempt at making a hand with this system, the Raptor design would need a significant overhaul to accommodate the Falcon Hand elements. Maybe we'd call it the Raptor-F class.

However, it's good enough for testing the concept. If anyone would like to take a look, the files are here. I've got palms with and without support, for both left and right hands. Any parts that are not here (wrist pins, tensioner box, etc) are the same as the standard Raptor Reloaded, so you can just use those files.

https://www.dropbox.com/sh/eq6rdl6kffjt ... O3voa?dl=0

You'll need to get a hold of some dental elastics. Make sure that they are 6 to 6.5 onzes. The Falcon hand uses size 3/16, but I think this design needs 1/4.

All feedback is welcome. This design had some very nice useability and installation advantages over the current design, but we need to look for drawbacks. I don't really feel good about giving this to a recipient yet, but maybe a few people would like to print one out and see what they think.

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Skip Meetze
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Re: Rubber Band Raptor Reloaded

Postby Skip Meetze » Sun Apr 26, 2015 3:40 pm

Way to go Jason! I'm looking forward to trying this. I liked a lot of things about the fingers from the Falcon Hand when I played with it many months ago (during my pivoting thumb experiments). I abandoned my testing due to difficulty I had in printing the little grooves for the elastics with my primitive (3-year-old built-from-a-kit) printer. The hands I hacked together worked well for all the reasons you site. I believe the elastics will have to be replaced often since they seem to deteriorate in the air (sunlight?), but this may not be a problem since they are not expensive and are easy to replace.

I'm looking forward to trying your re-mix as soon as I can.

Peregrine Hawthorn
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Re: Rubber Band Raptor Reloaded

Postby Peregrine Hawthorn » Sun Apr 26, 2015 4:43 pm

I'll admit, I was skeptical when you said you were trying rubber bands, thinking that the exposed rubber would likely be fragile and vulnerable, so it's a pleasant surprise to hear that they're MORE durable than the elastic. I hadn't even considered liking for an elastic that stretches more to deal with the ramping resistance.

I think I'll try hooking up one of my Raptors with some rubber bands and see how it performs.
Heavy duty hand user, and co-designer of the Talon.

I break hands and then tell you how I did it.

droomurray
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Re: Rubber Band Raptor Reloaded

Postby droomurray » Sun Apr 26, 2015 9:09 pm

Just looking at these, and it seems the rating is based on the stretch size, so I will see if I can put a table together to correlate, when you were advised 6.5oz based on what size was that do you know ?

To explain better, I am looking at these - http://www.ebay.co.uk/itm/Orthodontic-E ... 4ad6134683

But note that the pull strength varies with stretch distance, any idea's or will I just have to buy LOADS and try them out :-/

Drew.

AdamArabian
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Re: Rubber Band Raptor Reloaded

Postby AdamArabian » Sun Apr 26, 2015 9:33 pm

The ones we designed the falcon hand to were 3/8" but since you guys are redesigning around the raptor fingers it can be any length really so long as you get the separation right. In general you just want the separation to be juuuuust a bit greater than the neutral length of the rubber band. 3/8" is the ID if I recall correctly (I'm not at the university at the moment and can't confirm) but the solidworks files have the point-to-point distance on the posts at I think 11mm (again, can't confirm thats based on recollection, though that could be measured in software from the falcon files).

If the hand gets scaled up it does increase that separation but not enormously - we were looking at probably needing two finger sizes, one for 70-100%, the second from 100 on.

Oh, and the reason we used heavy duty ones (6.5oz) was to deal with wear and abrasion - they are fairly thick diameter so they hold up really well. The lighter ones just abraded down too quickly.

I think this is what we used most recently:
http://www.amazon.com/Clear-Oz-Orthodon ... ubber+band

But if you can get them the latex ones are actually better (natural amber colored). You can get them internationally but not much in the US due to concerns about latex allergies but they hold up better (just the properties of the material).

I'm SO glad the design is providing a framework for experimentation! Sorry the posts were too small for your printers - at the time we only had access to a high end printer which can do all sorts of stuff; we now have a bunch of others (printrbot and ultimaker) and are experimenting with the DFMA elements of 3d printing this stuff.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Sun Apr 26, 2015 11:23 pm

Thanks Adam, that's great information.

Drew, thanks a ton for offering to put together a chart. Hopefully Adam's advice narrows down how many kinds you need to buy.

Since the 6.5 oz bands are thicker, that might be an issue. Obviously the channels around the posts are only so big. Hopefully I can get some soon and try it out. I'll work more on the design as issues come up. I'm already thinking of lengthening the channel just a smidgen without lengthening the posts. An extra milimeter will make it easier to put the bands on 100% hands.

As you can see by the picture of the 175% finger, the posts and the channels get ridiculously big as the hand reaches adult sizes. This is actually a regular rubber band, double looped, and it fits in there with room to spare. Ideally, we'd use a parametric design to make the posts and channels grow slower than the fingers. For now, let's just see what we can do with this.

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AdamArabian
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Re: Rubber Band Raptor Reloaded

Postby AdamArabian » Wed Apr 29, 2015 5:08 am

As a side note, we are also trying out the bowden cable designs advocated by (I think) Peter Binkley. I like the dental elastics a lot but for the folks in the middle east we are primarily designing for aesthetics are key.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Wed Apr 29, 2015 11:48 am

Someone on an R&D hangout mentioned that same issue. They said that someone in the middle east put a latex glove over a flexihand and was very happy with ho natural it looked.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Fri May 01, 2015 11:26 am

I got new rubber bands. Unfortunately they're the same strength as before, 3.5onz. I have to try and find another company in China that I can order rubber bands from.

The new bands I got *are* longer. These are 1/4, which still seems to be more than tight enough to straighten the fingers. I'd like to try 5/16 and 3/8. I wouldn't be surprised at all if 3/8 works fine. That's the largest size I've seen, is there anything larger?

It's actually a good thing that I've had to use the weaker bands. I've seen many bands fray and break, and I've seen that they fray in the same place every time. I've got a pretty good idea where the bands are rubbing against the plastic, so I think I can improve those spots a little. In the end it will be a better hand for it.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Sun May 03, 2015 1:18 pm

I have added some improvements to to the new hand.

First, I haven't had any rubber bands break since I switched to the larger size. This isn't even the strength I want, or even as large as I think they can be, so that's very encouraging.

After that, I changed some angles of some surfaces and rounded off some corners. The places where the rubber bands were wearing out should cause less friction now.

The biggest thing is that I've added in the tweaks to the wiring system that make the fingers close at the palm first, then at the fingertips. I've uploaded some video on the google group.

https://plus.google.com/115790229936940 ... kzXA5Ni4Ue

Perhaps I'm getting overly excited because I'm so impressed with the thing I made, but this feels like a significant improvement. I think this gives a small improvement both to picking up large objects and picking up small objects.

Obviously that will require testing, starting with maker testing. This is still a test, not a new hand. If things go as well as I'm hoping, we'll need to build a new hand from the ground up, not just hack off parts of the Raptor Reloaded and stick on new parts.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Fri May 22, 2015 9:11 am

GreggDennison, you mentioned in another thread that you'd like the posts to be a little taller to make it easier to get the bands on. I can make some of the posts taller, but not all of them. Which ones gave you trouble?

I'm guessing that the thumb posts were the easiest, but the palm posts for the middle and ring fingers were the hardest.

GreggDennison
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Re: Rubber Band Raptor Reloaded

Postby GreggDennison » Fri May 22, 2015 9:29 am

You are correct about the middle and ring finger but it was the finger post that was hardest. Could just the finger posts be higher, maybe a 2-4 mm? It would allow the band to be started easier. The flatness of how they are now makes it difficult to even start one side. Especially for Luke. The index, pinky and thumb weren't that bad for him, but you fix one finger, you fix them all. Thanks.

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Fri May 22, 2015 12:04 pm

I can raise the posts on the proximal phlanges, but not on the fingertips. That's the side it prints on, so it all has to be level.

My method is to start the rubber band on the fingertip, then pull the finger down a bit and slip the band over the phalanx post.

GreggDennison
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Re: Rubber Band Raptor Reloaded

Postby GreggDennison » Sat May 23, 2015 1:51 am

It's usually the phalanges side we have more problems with. Probably due more to him flipping the fingers back and forth when bored.

StephenDavies
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Re: Rubber Band Raptor Reloaded

Postby StephenDavies » Fri May 29, 2015 4:42 pm

I've been trying the dental band hand. I'm gonna wear it myself (no left hand) and test it out. Unfortunately I didn't have any dental bands, and wasn't sure of which ones I needed for a a150% hand. My daughter is into loom bands, and I thought i'd try those. They've turned out pretty good.
Image

Don't know how long they will last, but they are easily available and quick to replace.

GreggDennison
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Re: Rubber Band Raptor Reloaded

Postby GreggDennison » Fri May 29, 2015 5:00 pm

I use the loom bands, we just double them up for each band. They've worked great so far.

Ianadan
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Re: Rubber Band Raptor Reloaded

Postby Ianadan » Sat Jun 27, 2015 11:03 am

Guys

I love the 'streamlineliness' of this. Is there a Thumbless Palm version of this?

I have a young girl that I'm making for and I'd like to try this version

Take care
Ian

Jason M Bryant
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Re: Rubber Band Raptor Reloaded

Postby Jason M Bryant » Sun Jun 28, 2015 3:24 pm

Hi Ianadan.

This hand was just an experiment. The lessons that I learned from this were used to help develop the Phoenix hand. You can find another thread about that in the beta section of the forum. It isn't ready for recipient testing yet, but we'll be sure to make an announcement when it is.

There's no thumbless version of the Phoenix Hand yet, but that will be a priority. I might be able to start work on that this summer.


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