Help Needed: Where and How to Add Threaded Elastic Wire in InMoov Hand (v2)? + SG90 Fit & Tendon Issues

[AwesomeCoolDude]

Hi everyone,

I'm currently building the InMoov hand (version i2) and I’ve already printed all the parts. I noticed on the website it says:

“ With small servos of the JX-PDI1109MG or SG90 type, placed in an adapter and mounted in the palm instead of the standard servos. (In this case we do not put the springs, but elastic thread)  ”  

I’m planning to use SG90 servo motors mounted in the palm, and I’m using fishing line for tendon control, but I’m a bit confused about how and where exactly to add the threaded elastic wire. 

• Do I run the elastic thread alongside the fishing line, or does it go on a separate path?

• Should it be tied to the same fingertip holes or somewhere else?

• Any tips on what type or size of elastic thread works best?

• Also, there’s only one tendon hole per finger, and it’s just the size of the fishing line—how are people managing to route both lines through?

Also, I’m facing two additional problems and would really appreciate some advice on:

1. Finger movement issue:
   Even when I pull the fishing line via coding the servo, the finger barely moves. I suspect I’ve tightened the fishing line incorrectly or routed it the wrong way.
   What’s the correct method to tighten the fishing line so it provides proper motion? 
2. Fitting the SG90 in the printed adapter:
   The servo doesn't slide in easily. I tried using a soldering iron to widen the opening slightly, but I’m worried that’s not the right approach.
   Any tips for cleanly and safely fitting SG90s into the adapter?  

Lastly—are there any videos, step-by-step guides, or tutorials for assembling the i2 version of the InMoov hand with palm-mounted servos? I couldn’t find much specific to this configuration.

Thanks in advance.

[Mr Trurth]

did you make some progress since April?
the sg90 have enough force?
Did you find some video with paml-mounted servo?

[Abraham Sleem]

After much trial and error I succeeded with this. 

Used:

PDI-1109 servos - quieter and stronger. The SG90 weak, loud and burn out easily. But ~10x cost. 

Braided Fishing Line - 200lb 0.75mm; i remember they advised against the other type as it may stretch

Elastic thread "Elastic Bracelet String Total 330 Feet,1.0mm..." from amazon. Just not sure if it was 1.0mm or more; either way, I doubled up on the elasticity by looping it though the distal fingertip and tying it off at the base of the hand. Of note here is that I had to drill my own hole in the distal finger tip to thread it through as the "spring hole" was too thin and often ruptured. 

I tried springs and they worked OK but were stiffer and tended to lock more often. Also harder to source. 

Regarding the model of the servo adaptor - I was not getting enough rotation across the circumference of the adaptor and the fingers were not closing completely. 

Made my own model. It gets a little more circumference / distance because it pulls it at an oblique angle rather than circular. 

See the model attached. Ignore the hole on the side halfway up and thread the fishing line through the lowest hole in the "leg" then up the center hole. Then add a screw through the middle to secure the adaptor to the servo horn and servo. Watch as the finger closes completely when you go from 0 to 180. Adjust for tension.