Well, I got bad news for those who bought the HK15298 servos for the forarms. It seems these digital servos have a restricted rotation from 0 to 90° which doesn’t let the fingers do their complet movement.
There is search going on at the moment to see if we could reprogram them to get a rotation of 0 to 145°, but it won’t be an easy thing, I think.

This has lead me to design another bracket system.

As it was in my previous version the servos were set sideways, and we could have a total of 5 servos.

The space between the bottom actuator and the robpart was really small

I wanted to create some printed actuator rings with a bigger diameter which would allow restricted servo rotation to get more pulling developpement. So I turned all the servos up,  this way I get more space and actually it all comes out more fonctionnal. I remember when I first designed the forarm brackets, how much time it took me to create it. It was insane.
This went actually pretty fast.

I have uploaded it on Thingiverse.
You can get here.
You don’t need to reprint the “robparts”, it can just fit in. You only need to remove two little parts with a plier and it all should go nicely. Fixing the fishing braid will be an easy task, done with a few nots.
The nice thing about this, is that we can have an extra servo in the forarm now. That’s a total of 6 servos. With a bit of redesign we could even have 7 servos in there.
We can actuate the thumb with this derivative or create something to move up and down the wrist.
I haven’ t tried to print it yet, since I don’t need it,  but I think it should be fine.
The assembly is pretty straight forward.
Have fun!

Comments 21

  1. Nice work Gael! The simplest solution always takes the longest. This design allows really nice access to all servos, and seems to make it easier to wire things up… Excellent!

  2. Printing the parts works fine. But as I managed to modify the HK15298 servos to 180 degrees I’ll stick with the current servo bed.

    Testing the new design for the first time while at the same time building the arms for the first time seems to be no good idea.

    Perhaps I’ll switch to the simpler bed and its extra servo if I modify the hands for the adjustable thumbs anyway. For that taking everything apart will be necessary anyway.

    • Hi, yes all together it might a bit confusing. I have read your solution on the forum about the Hk15298 servo, and it ssems a pretty good one. I need to find some of those resistors to make a test.

    • I found that the HK15298 are slightly higher than the MG995. The current design reqiures scraping and/or cutting off some small areas that prevent the HK15298 from fitting into place. But it can be done and fit good enough after manual modification.

      A redesign for HK15298 size would be nice but can wait until you have to rebuild new arms with HK15298.

      BTW: Did you get my mails?

  3. Gael:
    This is a great design. I have everything printed and cables run
    and almost assembled. Just one small problem.

    How are folks securing the servo bed into the forearm? I tried gluing it down but the torque pulling up just one finger broke it loose.

    • Hi, if you used ABS, with a few drops of acetone between the parts will glue/melt it securely. With PLA, a two component epoxy glue should do it for good.
      Did get the servo wires easily through the cavities? They seem to be a bit too small for some servos. I will check the STL.

  4. Gael:
    I had tried the ABS glue. There was just not enough contact surface it seemed to hold things. I ended up drilling two holes and putting M3 bolts through the outer cover.

    I had to take a dremel to the outlets for the servo wires.

    • Hi, I normally always print my design to make sure there isn’t too much problems… But I had never printed these parts before. It was initialy designed to solve a problem concerning the rotation degree of the HK15298 servo (only 90degrees)
      I have decided to finally print the servo bed yesterday. There is evidently problems with the part. I solved them, mostly for the outlets. I will look to see what can be done for the fixing. These minor changes take a lot of time to fix, and I would rather go on with the rest of the robot.
      It is now called RobServoBedV4 and is available on Thingiverse.
      Hope this helps.

Comments are closed.