The Mystery Fist

With my group of students from the Ponts Paristech school we decided to build the MysteryFist as part of a course project that aimed to introduce us to robotics, laser cutting and 3D printing. So we decided to make this project fun by building a hand that flips over and gives a middle finger when you want to check it. Yes it's rude ... but this project allows us to tackle all the areas mentioned above. Indeed we used 3D printing and laser cutting for the structure and arduino to move the hand. The hand has 2 mobilities. The first one must simulate the rotation of the wrist and the second one must operate the middle finger that rises.

In order to make the MysteryFist you will need :

• 1x Arduino Uno board 
• 2x Servo motor (1 for the finger + 1 for the hand) with their tips
• 1x Obstacle avoidance sensor
• 1 x Breadboard
• 1x External power supply
• Some connecting wires 
• 1x 3D printer
• 1x laser cutter
• 1X scotch

The challenge in designing our mechanism was to correctly predict the rotation of the wrist and finger. For the wrist, a cross-shaped notch (possible tip for the servo motor) at the back is enough. As for the rotation of the finger, we have designed a cavity in the hand to make room for a servo motor that will operate the finger. A notch has been made in it to be able to turn it with a tip of our servo motor. It was also necessary to foresee the place for the wires and the obstacle avoidance sensor. All the elements are stored in the interior cavity of the hand and in a box at the back.

We have drawn sketches of a left fist and simplified them. On CATIA, we have drawn the sketch parallel to the palm of the hand and extruded this sketch. All the operations after were dependent on the previous one. It was a long task to design the hand so that the electric components could then be assembled (particularly the servomotor which lifts the finger). Indeed, we had to anticipate the spaces the components would occupy, but also and the way they would be placed. Then, we used Cura, a slicing software. It is not possible to print a horizontal surface without any pillar to support the plastic. We had to select "support" in the parameters. The supports were placed everywhere there was an horizontal surface. Due to the fact that our hand is hollow and has only an opening at the location of the middle finger, it would have been impossible to remove the supports after the print. We had to print the hand biased (the inclined plane at the base of the thumb). And we selected the parameter "support" and used the option which makes trees to support the surfaces. We selected a certain angle to delimit where to put the supports. Finally, the print was not perfect, we had to remove a piece above the index finger to make sure the servomotor could be placed.

The issue with the design of the finger is to leave a notch to accommodate the servo tip. The dimensions and the design of the finger had to be in line with the ones of the hand.

This step requires a good knowledge of the tolerance of the laser cut in order to properly size the edges of the parts that must fit together correctly. We also planed a base and a notch to let pass the servo that will operate the finger and the cables that go with it.

Like any good programmer on Arduino, before assembling, we tested all the sensors/motors. That's why we put the minimal tests for the servos and the obstacle avoidance sensor. For the final algorithm, it is just an "if" loop on the output of the sensor and we rotate the 2 servos with an offset so that it does not take out the finger or retract the finger while the hand turns.

The assembly is a sensitive stage. It is the step of disillusionment, everything does not go as planned. We had several problems, including the fact that the finger servo is impossible to place in the hand, the wrist axis is too weak to hold the hand, there are cables in all directions, the Arduino board cable does not fit in the box... We then had to solve these issues step by step.

First we cut a notch in the hand to put the servo, we also drilled a hole in the box to let a cable go through. Then we had to make correctly the connections inside the hand and in the box by attaching beforehand the sensor on the hand with tape. Additionally, we have to fix all the cables and the cards so that they do not move. Moreover, It is necessary to consolidate the axis of the wrist by screwing the tip and with adhesive tape. Once these steps are well respected, the hand is functional and ready to use!

IT WORKS !