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  • Author Author: balearicdynamics
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Art-a-Tronic Episode 4

balearicdynamics

Introduction

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In this fourth episode, we see the completion of the torso movement of the Art-a-Tronic mannequin. The solution adopted seems working fine with good mechanic stability. The only problem was finding a way to get fast a considerable number of steel spheres to make the Lazy Susy bearing support; unfortunately the Amazon order of the 3.5 mm diameter 1000 sphere set has been delayed and I can't expect to be delivered before next April, 15. I thought what is the mechanism that contains this kind of spheres, cheap and easy to disassemble then finally I solved it in a sport mega discount. At the ridiculous price of 7 Euro, I bought two freewheels replacements for bicycles. Not difficult to open it with the Dremel metal cutter and like magic I have found the treasure: some hundred of 3.5 mm diameter steel spheres. Let me show you how I have completed this part.

All the material shown in these posts are available on the GitHub repository https://github.com/alicemirror/mannequin  released as open source, license LGPL 3.0

Moving the Torso: Parts Design

As already mentioned in the previous post, the 3D printed part has been designed with Fusion 360.

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The two above images show a more detailed rendering of the stepper motor support discussed in the previous post.

The base

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The base of the rotating support has four holes to keep it in place with the central hole of the same diameter of the mannequin shaft. the engraved guide on the internal diameter of the base is 3 mm depth; it is half of the Lazy Susy bearing guide. The rotating support has the same to keep the spheres in place.

The Rotating Support

Below, the top and bottom rendering of the rotating plate, coaxial to the fixed base.

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The fixation method is the same as the bottom base, through four holes screwed to the bottom plane of the dorso. The rotating support is coaxial to the base. The first rendering shows the bottom with the second half of the guide for the bearings sphere. The hole in the middle of the side perimeter will be used to screw the clamp of the teeth belt transmission. The second rendering – the side of the rotating plate adhering to the bottom of the torso – shows the 2 mm rectangular hole for the nut of the clamps blocking screw. The nut will be kept imprisoned when the support is screwed to the body. The sketch below shows the detail of this part.

 

The clamp

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As the rotation of the torso is not complete, the teeth belt does not need a 360 DEG full revolution, so the belt is fixed to the rotating plate by a clamp to keep the teeth belt with the right tension.

Assembling the Rotating Support

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The Base and the Body

To assemble the base has not been too complex; as it is impossible to use nuts in the internal side of the legs I have used parker screws. The height of the base has been calculated to have the rotating part at the same height of the teeth pulley on the motor shaft.

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I met some difficult to center the 3D printed base to the shaft; as shown in the above images, the base of the shat is a threaded bar about half of the diameter of the shaft.

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The further step has been to cut the bottom of the torso to be sure I was following the right assembly procedure. I never had seen how the interior of the mannequin body is built. Frankly, I never saw the interior of a mannequin. This surgical operation revealed that the body is 5 mm thick; this measure influences the length of the locking screws of the rotating support.

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At this point, it is time to test the fixed base with the steel spheres.

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The design was right and the Lazy Susy bearing works as expected.

The Rotating Support

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The images above show how the teeth belt is fixed to the rotating platform with the clamp. One of the two sides of the belt is locked with a cable tie while the other side is locked closing the clamp when the belt has the right tension connected to the motor pulley.

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All the parts fit perfectly and motion is smooth. Next episode, the motor in action!

 

Update: Moving mechanics and more in the next episode coming soon!  Don't miss it

 

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  • Enrico,

     

    I like the idea of making your own bearing mechanism, that's something I might try now. It will be interesting to see if any of the ball bearings ever escape.

     

    Dubbie

  • in reply to dubbie

    As a matter of fact no. Important is to calculate correctly the dimensions.

     

    Top and bottom engravings in the design are from a torus 3 mm diameter. Balls are 3.5 So as a matter of fact, when the two parts are one pressed over the other there is about 0.4-0.5 mm open. That is just what is needed to give smooth motion and to keep the balls in place. If needed / possible you should lock the upper part with a top ring on the shaft. Here I can't so in case of an earthquake, maybe the torso goes away and the balls are lost. I am thinking on how to fix the torso with a locked on the top of the shaft. But in this special case mechanics is strongly constrained by the "ancient' features of the vintage subject.

     

    Enrico

  • in reply to balearicdynamics

    Enrico,

     

    I'm definitely going to have to have a go at making something similar. I have been thinking of 3D printing a dome head for a walking robot and I have been thinking how to support the weight of the head without it all resting on the axle of the motor. This looks to be a good solution. I have been thinking of using a flat 3D printed washer which does work but ball bearings would have even lower friction. The 5 mm ball bearing seem pretty cheap as well as they are used in bicycles so there are plenty around.

     

    Dubbie

  • in reply to dubbie

    Here it was not easy to find the ball bearings but as I wrote (maybe) in the post I have just got them from a couple of freewheels for a bicycle. Not difficult to cut the aluminum internal ring with the dremel. Not easy to take together all the balls when your freewheel spontaneous disassemble image

     

    This solution as also discussed some post ago with is the solution adopted in small and big rotating platforms. I have also found this in an Ikea rotating wooden fruit platform. And it is not difficult to make with a 3D printer.

     

    Enrico

Comment
  • in reply to dubbie

    Here it was not easy to find the ball bearings but as I wrote (maybe) in the post I have just got them from a couple of freewheels for a bicycle. Not difficult to cut the aluminum internal ring with the dremel. Not easy to take together all the balls when your freewheel spontaneous disassemble image

     

    This solution as also discussed some post ago with is the solution adopted in small and big rotating platforms. I have also found this in an Ikea rotating wooden fruit platform. And it is not difficult to make with a 3D printer.

     

    Enrico

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