The transmission references the components that go from the motor to power the top joint, which acts as a knee. It was determined that to provide the necessary torque to the knee to walk (a fast movement) and climb stairs (a slow movement) the transmission needs to have a gear ratio of 500:1. The drill provides a transmission of 65:1, so the gear ratio from the motor housing to the top joint will need to be 7:1. This is accomplished through two stages. You can learn more about how gears work here.
To complete this subsystem, you will need to disassemble the Makita drill. Please make sure that you have completed all other mechanical assembly steps before removing the motor and transmission from the Makita drill.
Step 0: Review design files and print motor and power electronics housing
Please review the design and print files here. Please print the motor housing (2 parts, exterior and interior sides), power electronics housing (2 parts, side 1 and side 2), magnet holder (2 parts, holder and alignment guide), and two copies of the tap guide. The motor housing should be printed such that the internal features are facing up. The power electronics housing can be printed in most orientations.
Step 1: Prepare 17T Gear
The 17T gear attaches to the output of the drill planetary transmission. This is a 1/2-20 thread. In contrast, the 17T gear has an 8mm bore. Thus, this step outlines the process of drilling a 29/64th inch hole into the 17T gear and then cutting threads into the gear with a 1/2-20 inch tap. The drill size is smaller to account for the minor diameter of the 1/2-20 thread pitch. More information on tapping is here. To complete this step, you will need a power drill, a 29/64th inch drill bit, the 17T gear, a clamp or vise, and two copies of the tap drill guide.
Step 2: Cut and Glue Transmission Shafts
The secondary gear stage transmits power from the output of the planetary transmission to the knee joint. It also provides a 7:1 reduction ratio to create approximately a 500:1 total reduction (accounting for the 65:1 planetary transmission). The transmission system uses two shafts to help transmit power. The first shaft is made of steel and in the shape of a 0.375" hexagon. It will connect to the 52T and 18T gear. The second shaft is made of aluminum and is in the shape of a 0.5" hexagon. It connects to the 45T gear and also the flange hubs. This shaft interfaces between the shank and the thigh frame. Thus, it forms the knee joint. A small diametrically polarized magnet is glued to the shaft. This magnet is used in conjunction with the magnetometer sensor (installed during system integration) to measure the angle of the knee joint.
In this step, you will cut the shafts to length with a hack saw and also glue the 52T gear to the 0.375" steel shaft. To complete this step, you need a hack saw, a ruler, a file, a marker, a clamp or vise (to hold the material while it is being cut), super glue (or another metal adhesive), the 52T gear, the diametrically polarized magnet, the magnet holder (2 parts), the 0.375" shaft and the 0.5" shaft.
Step 3: Remove support material from 3D prints
The following video will demonstrate how to remove support material from the 3D prints. To complete this step, you will need the 3D printed frame for the motor housing (2 pieces), the 3D printed battery mount (2 pieces), 2.5 mm and 4 mm drill bit and various tools (pick, pliers, safety gloves, etc.) to remove the support material.
Step 4: Remove motor and transmission from power drill
The following video will demonstrate how to remove the motor and transmission from the power drill. Do not proceed if you have not completed all prior mechanical assembly processes. To complete this step, you will need an 8 mm Allen wrench, a Phillips screw driver, needle nose pliers, and the power drill.
Step 5: Motor, Transmission, and Power Electronics Integration
The following video demonstrates the process of assembling the motor and planetary transmission unit and the integration of this unit with the battery and motor controller. This is the main power unit of the motor and converts chemical power (stored in the battery), into first electrical power (voltage and current), and then into mechanical power (torque and angular velocity). To complete this step, you will need the drill motor, drill planetary transmission, the 3D printed motor housing (2 pieces), the 3D printed battery housing (2 pieces), 4 M2.5 by 14 mm socket cap screws, 6 M2.5 by 30 mm socket cap screws, and a 2 mm Allen wrench.
Step 6: 17T Gear Install
Content to be added.