Section 2:

Arm Assembly

Goal #1: Create the Finger Mechanism

The first goal of the arm assembly was to create an iterative design for the finger mechanism. A crucial part to expression is to move all five fingers independently, so it was critical to have each finger be independently controlled. Similar to the head assembly, I wanted to challenge myself and use minimal fasteners, such a screws. Because of the nature of FDM 3-D printing, this goal was very feasible. The biggest challenge was determining how to move the fingers in any orientation using only one servo. The solution is inspired by stringed puppet techniques, using a thin string connected to the servo to open and close the finger. Addition, small rubber bands are used to act as "tendons", and create a resistance to this movement. This allows desired position of the fingers to be maintained, and allow the finger to return to its fully opened state, regardless of hand orientation.

A test finger was created and printed to prove that this design works, and the process of creation and testing is shown below. The test finger does not contain any aesthetic elements, such as fillets or the correct lengths of the "bone" pieces.

These images show the first test finger, without any rubber bands, strings, or servo motors. The finger consists of 3 "bones", and 3 joints, and therefore has 3 degrees of freedom.

This image shows the attachment of one set of rubber bands to the finger to create the resistance to motion. Each contains a small hook for the rubber bands to attach to.

First Finger Test

This video shows the first test of the finger mechanism.

This image shows the CAD model of the test finger, rendered from SolidWorks. This render is not off the final finger design, and is missing some aesthetic elements.

Goal #2: Create a Hand Model

The second goal was to create a print in place hand assembly, creating the entire hand in a single print. Since Project Progress is a full size animatronic, measurements needed to be obtained to create a human sized-hand and finger dimensions. These dimensions were obtained from the National Institute of Standards and Technology. This website contained anthropometric data for all hand dimensions. When creating the fingers for the hand model, the original test finger was duplicated, and the bones of the fingers were adjusted in length based on the data. The hand model is an assembly of each finger and the hand base. The colors of the render show the boundaries of the different models. This was done since each finger is technically its own system. At the base of the hand are 3 holes, as the intent is for the wrist mechanism to attach here.

Hand Assembly Printing & Testing

As previously mentioned, minimal assembly is needed for the hand mechanism, as it is a single print in place part. The images and video below show the assembly right off the 3D-Printer, as no supports were used in the joint areas. Supports were only needed at the finger tip areas and in the holes where the wrist will attach. These areas were later sanded down to create a smooth surface. Each joint is free moving in this stage, as no rubber bands have been attached to the hooks of each "bone". The hand took roughly 10 hours to print.

First Hand Print

This video shows the joints of each finger working as intended, having the full rotational freedom as designed.

Four Finger Testing

This video shows the first test of all the fingers, except for the thumb. Each finger was connected using the rubber bands to act as tendons, and strings attached to the SG90 micro servos. These were selected due to size, cost, and low force needed to move the fingers.

First Thumb Test

The last finger to test was the thumb. unlike the other fingers, the thumb for Project Progress only has two joint, and is orientated slightly different. The original design had the servo arm rotating into the hand. Due to errors in printing, the servo arm rotates away from the hand.

Goal #3: Wrist Mechanism and Forearm

The third goal was to design the wrist mechanism to attach to the hand, and the forearm section of the arm. For the wrist, only 1 degree of freedom was utilized initially, rather than 2 in a normal human hand. This was done to reduce complexity in the design, and the lack of necessity in the second degree of freedom. However, it was determined that this added degree of freedom was necessary in later development. The wrist utilizes a bevel gear to drive the rotation, as the servo is 90 degrees off-axis from the joint. To create these gears, the SolidWorks toolbox was used. This tool allows for the creation of standard gears using predetermined parameters. Using McMaster-Carr, standard gears were selected and imported into SolidWorks. Similar to the hand design, anthropomorphic data was used for the dimensions of the wrist and forearms.

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