The mechanic design of the machine is actually pretty easy, but it can be a little tricky to explain/understand. Here is a step by step explanation:

  • The DC Motor activate teh rotation of the M8 shaft
  • The shaft is connected to the outer ring, so the outer ring spins
  • The inner ring itself is connected to the outer ring via the steel balls, and they spin together
  • On the "control turret" there is a fixed Pinion
  • On the inner ring there is a large bevel gear
  • The pinion and the bevel gear are connected
  • When the rings assembly rotate via the DC Motor, the inner ring is forced to turn inside the outer ring because the bevel gear have to turn around the pinion, which is fixed
  • Rendering of pinion and bevel gear from Rhino 3D model

    Bevel Gear and Pinion

    The gears were designed in Rhinoceros starting by a very useful Gear Generator on which I calculated two normal gears. Then I imported the pdf in Rhino and used it as a reference to draw the actual pinion and bevel gear. The pinion has 11 teeth, while the Bevel gear has 77, thus making it a 1/7 ratio.

    Gear Generator template

    Pinion and bevel gear detail

    Pinion and bevel gear detail

    3D printing the bevel gear

    The bevel gear was way too large to fit in any 3D printer I had available at the Fab Lab, so I had to split it in fourth. I designed a joint that would make the gear consistent once assembled, and I put pairs of little bulges (from one to four) on each side of the parts as a reference to find the right couples.

    Parts being created in the printer

    Bevel gear parts printed, reference bulges are visible

    Joint between two parts, the fit is tight enough to make it robust but easy to disassemble

    Assembling the bevel gear

    The bevel gear rise for 1,5mm over the inner ring to match the size of the pinion, that's why the inner ring is assembled using longer M6 bolts. Two 5mm lasercut acrylic spacer are placed on every bolt, then the bevel gear is layed on the bolts and secured with nuts.

    Inserting the lasercut acrylic spacers

    Despite this picture, the right number of spacers is two

    Motor housing and Decoupling Ring

    The motor is kept in place inside the turret with four 100mm long M6 bolts and it is connected to the M8 shaft with a custom connector. In the beginning the connector was a single piece with a M8 nut housing on one side and a custom housing on the other. The problem was that when the motor was off, it couldn't rotate because of the warm gear inside and so couldn't the machine. So I came up with a decoupling ring whit two different housing element and one ring that can slide connecting or deconnecting the two.

    Inside the turret, the motor is connected with four long M6 bolts

    Printed decoupling ring parts

    Decoupling ring motor housing and sliding connector ring

    How the three parts fit together

    Final assembly, coupled