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Feb 3 16 2:35 PM

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PROBLEM
It can be hard to tie off the tendons in the right position. Also, over time the tendons can become loose.

CHALLENGE
To find a way to adjust the tightness of the tendons after tying them.

REQUIREMENTS 
1. Must be easily manufacturable using a desktop 3D printer
  • So other manufacturing tools aren't required beyond the tools used to make the hand itself
2. The tension of the tendon has to be adjustable easily.
  • Any change needs to be an improvement on the current method. 
​3. The tendon must be secure when not being adjusted.
  • If the tendon slips or releases at a certain load, grip will be limited to that load. 
4. Must ultise the same tendon line.
  • Because it has proven to be very strong and durable.
5. The solution must be licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
  • To allow the solution to be used across all future Open Bionics development and any other collaborative 3D modelling projects. 

SUGGESTED BRIEF
To design PLA fingernails that slot into the fingertips of the hand, and provide an anchorpoint for the string that also allows it to be tightened.

1. The fingernail should have a concealed loop underneath it onto which the tendon can attach.
2. The fingernail should be FDM printable in PLA plastic.
3. The fingernail should have a feature that enables the tendon to be tightened easily without tools (perhaps by popping it out of the fingertip and rotating it).
4. The fingernail should make the hand assembly process easier.
5. The fingernail should be designed in Blender.
6. The fingernail should be designed keeping in mind that the fingertips may also have silicone pads on them and should be adaptable such that both features can be added simultaneously.
7. The fingernail should work nicely without the tendon slipping off it if you try


FURTHER SUGGESTIONS 
1. Perhaps take this on at the same time as the silicone fingertips if you're feeling brave.
2. If designed correctly the nail could have an additional benefit of helping the hand pick up things like cards off a table.

Last Edited By: JonathanRaines Feb 3 16 4:46 PM. Edited 2 times

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#1 [url]

Feb 3 16 4:54 PM

Example

We've got a version of this working in the lab but it still uses a knot at each end of the tendon. Adjustment requires releasing a knot and re-tying. 

The advantages are it is better cosmetically and the fingertip knot is tied when the finger is bent (there is better access). 

However, the knots can slip slightly and slack the tendon. Tying with a bent finger is also quite difficult. 

Here's the blender file. The Nail Tendon Terminator should be printed in PLA, flat side to the bed. The finger should be printed in Ninjaflex. 

https://drive.google.com/open?id=0B8EyMlhsU65QQ0tKeW82WTl3dmM

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#2 [url]

Feb 3 16 5:00 PM

You would need a screwdriver, but incorporating something like this may work http://www.ebay.co.uk/itm/CLUCH-BRAKE-HANDLEBAR-LEVERS-SOLDERLESS-NIPPLE-THROTTLE-CABLE-END-FERRULE-BULLET-/270863507139?var=&hash=item3f10b91ec3:m:mkBxIndIGMFeZXFrw-cK-mA , or a barrel type tensioner, though I havent found one small enough.

http://www.hobbyking.com/hobbyking/store/__28966__Throttle_Linkage_Collet_A3015.html might work too

Last Edited By: jaundice Feb 4 16 9:35 AM. Edited 1 time.

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#3 [url]

Feb 8 16 11:08 AM

Now that I have the hand, and shark line, I can see that the throttle cable collets probably wouldn't work, they are too big and the line is too thin, though I did wonder if the ends of the line could be dipped in abs/acetone to make them more clampable. I haven't done the tendons in my hand yet, but I might steal some of my daughters beads and see if I can get a workable belay type system using them. Another embryonic idea comes from the clicker tensioner in my daughters roller boots which looks like http://www.rollersnakes.co.uk/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/b/u/burton-custom-2015-bindings-magentlemen-s360707mag-02.1431.jpg basically the same locking mechanism as a zip tie.

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#4 [url]

Feb 9 16 10:22 AM

I had a bit of success with the beads, I found some rounded cube beads (like a dice) about 8mm edge to edge. I drilled a hole perpendicular to the hole that was already in it. Then I threaded the line through one face and out of the side (so it turns 90 degrees inside the cube) wound the line around itself 3 times(at the point that it first enters the cube) and then pushed the line back up through the cube (still at the point it first enters the cube) and straight on through. I have these resting outside the fingers at the tie point with long excess stings. if you wrap the strings around long nosed pliers to grip it you can push the cubes back down the line to tighten. I haven't tested it with any real load, but I haven't encounter any slippage yet. I was just testing with a very knackered 9v battery (~7.3v) so it was struggling to run the motors anyway. After all that though I am wondering if just having a split post (or a post with a hole) from the fingernail down into a retaining ring in the finger may just be a lot easier :) The line is fairly fine there would be space for quite a lot of wrapping..

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#5 [url]

Feb 11 16 5:37 PM

jaundice wrote:
Now that I have the hand, and shark line, I can see that the throttle cable collets probably wouldn't work, they are too big and the line is too thin, though I did wonder if the ends of the line could be dipped in abs/acetone to make them more clampable. I haven't done the tendons in my hand yet, but I might steal some of my daughters beads and see if I can get a workable belay type system using them. Another embryonic idea comes from the clicker tensioner in my daughters roller boots which looks like http://www.rollersnakes.co.uk/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/b/u/burton-custom-2015-bindings-magentlemen-s360707mag-02.1431.jpg basically the same locking mechanism as a zip tie.

We actually tried just using cable ties as tendons (the added benefit of that is they have some push strength as well as pull) but we found to get them to be strong enough they have to be quite big. We used the clicking mechanism cut off from the end of another cable tie as the fastening mechanism, and you could just tighten them until they were tight enough then use a blog of super glue if you wanted to secure them in place. It could have been awesome, but they turned out to be too weak (even military grade).

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#6 [url]

Feb 12 16 9:46 AM

@Joel where was the weak point, the clicker mechanism or the actual track? For zip ties I was thinking to cut the clicker off the track, make a hole near the cut end of the track with a hot needle or even a small drill bit and thread the shark line through that. Inside the finger I thought there could be an insert: a U shaped tube with a bulkhead near the fingertip end, slotted to allow the track to pass through where it would go through the clicker and be trimmed off. You would then be able to tighten the tendon, loosening it would require a new track, tbh I don't know if there would be enough track to make it sufficiently tensionable, probably only15-25mm. Wrapping the line around a post, while lo-tec seems simpler and with more adjustability, though in less precise steps.

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#7 [url]

Feb 12 16 1:33 PM

We could design a mechanism the depth of the finger that would twist the shark line around a circular track. I will upload a design concept later. The basic idea would be to have teeth on one end that would lock into slots within the finger. The shark line would tie into a hole on a circular track on the opposite end of the teeth. To tighten to twist the line around the track then place the whole thing into back into the teeth.

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#8 [url]

Feb 12 16 3:12 PM

@TThomas, that sounds interesting and would give more strength to the winding track/post. It would also give more precision since it would be governed by the number of teeth rather than the diameter of the post.

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#10 [url]

Feb 25 16 2:56 AM

 I also like the idea of a racheting mechanism, but I'm curious just what kind of forces a mechanism would have to be designed to. Has anyone measured the amount of tension present in the line(s)?

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#12 [url]

Feb 27 16 6:47 PM

I put together a mockup of what a rope cleat might look like for the fingernail termination. The basic idea it to just add lashings until you've got the tension about right. The design should be able to be modified for more adjustability by adding more "cleats"

image

I put a model up on sketchfab:

https://skfb.ly/LxHV
 

Last Edited By: jimshealy Feb 27 16 6:50 PM. Edited 1 time.

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#14 [url]

Feb 29 16 3:27 PM

Hi jimshealy, 

Great idea! (Also nicely presented, we've since signed up for sketchfab). I printed your mockup today and did a bit of testing. It's certainly a lot easier to adjust than a knot! It also appears to be able to take a decent amount of load, although I haven't measured that with a meter yet. 

The holes for tying off the tendon were a little small (I just drilled them out on this one), They probably need to be 2 mm wide to get the line through the printed part. 
The only other comment I have was that it requires about 20 mm of tendon to make each turn on the cleat. That's the amount the motors contract, in other words enough to close the whole finger. Can you think of a way to give finer control? I suppose making the whole cleat smaller is an option, but then we run into printer resolution issues (can't print small enough). 
imageimage 

 
 Again, thanks a lot for getting involved and doing some modelling. There's a good chance we'll implement a future version of this on the hands. All the best, 
Jonathan

Last Edited By: JonathanRaines Feb 29 16 3:41 PM. Edited 2 times.

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#15 [url]

Mar 1 16 4:03 PM

Hey, Thanks for giving it a try! The notch in the back of the part was actually to allow a "half turn" of the tendon line.

I've got a few ideas to allow much finer control, but it's mostly changing the shape of where the line is wrapped. This was mostly a quick mockup to portray the idea. (not dimensional)

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#18 [url]

Mar 2 16 4:57 PM

Hi jimshelaly,

I've printed your update, and it's a substantial improvement. There is much finer control of length now, it's down to about 5 mm increments. The point where the tendon pulls on the nail is now much more central as well and so there is less twisting. The loop you designed for tying off the tendon was a bit small to come out. I know these are just rough prototypes and not to scale but due to the nature of this object, it’s quite close to how small we can print on FDM machines. However, getting them done in SLS is an option. I’ve included a photo with a £2 for scale, the nail on the right (for comparison) is our current nail that has to be tied off with a knot. The good news is that it works without needing to tie off the tendon, it’s held just by friction. The only room for improvement I see is perhaps making the four trenches around the sides a little larger and maybe angling the edges a bit to try trapping the tendon when it sits in them. If the tendon was trapped in the notch it would make assembly easier.

imageimage

Thanks for the good work!

All the best,

Jonathan

Last Edited By: OMac Mar 3 16 2:56 PM. Edited 1 time.

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#19 [url]

Mar 2 16 9:24 PM

I keep forgetting just how small it is in real life! For my own reference, what size nozzle do you have on your machine (.4mm standard size?) I can try to size the features to the nozzle for better printing.

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#20 [url]

Mar 3 16 9:10 AM

Everything does always look huge on a computer screen! Our smallest nozzle is 0.4 mm. Also on internal features (holes and notches etc) I find making them 0.1 mm to 0.2 mm bigger than desired is required. 

Last Edited By: OMac Mar 3 16 2:57 PM. Edited 1 time.

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