3-DOF Robotic Arm Assembly Guide

This is a 3-degree-of-freedom (3-DOF) robotic arm designed for ST3215 bus servo motors and intended as a hands-on platform for learning robotics. This guide covers only the mechanical assembly, so you can skip the process of designing the structure from scratch and focus instead on the electronics, control, and programming.

In the picture below, the robotic arm is mounted on an RC car. However, this build includes only the robotic arm itself. You will need to design your own base for your specific project and use M3 screws to mount the arm to it.

All 4.5 mm holes are designed to accept M3 threaded inserts.

3D Printed Parts

• 3x ST3215_Mount.stl
• 1x hub_adapter_motor_1.stl
• 1x link_1.stl
• 2x hub_adapter_motor_2_and_3.stl
• 1x link_2.stl
• 1x link_3.stl

Other Required Parts

• 3x Metal 25T servo disc horns
• Assorted M3 screws (a kit with multiple lengths is recommended)
• M3 heat-set threaded inserts
• 3x ST3215 bus servo motors
• 1x ST3215 bus servo adapter

ST3215 Motor Mount

The ST3215_Mount.stl part is a mounting bracket for the ST3215 bus servo motor. It allows the motor to be secured using standard M3 screws.

Since this project uses three motors, you will need to print three copies of this part.

The ST3215 motors include four small mounting screws. Use these screws to secure the motor to the mount.

Hub Adapters

The ST3215 motor comes with a metal lever horn, which is not ideal for this project. Instead, use standard metal 25T disc horns, as shown below.

Before attaching each disc horn, move the motor to its desired "zero" position. Ask yourself:

If this motor is at angle zero, how should the disc horn be oriented?

Repeat this calibration for all three motors before assembling the robotic arm.

It is helpful to mark the zero position on each disc horn with a permanent marker.

The files hub_adapter_motor_1.stl and hub_adapter_motor_2_and_3.stl are adapters that connect the robotic arm links to the servo horns.

Each adapter is mounted to the disc horn using M3 screws.

Attach:

• hub_adapter_motor_1.stl to Motor 1
• hub_adapter_motor_2_and_3.stl to Motors 2 and 3

Link 1

Attach link_1.stl to the hub_adapter_motor_1.stl.

Link 2

Set the Motor 1 + Link 1 assembly aside for now.

Take the second motor assembly and attach hub_adapter_motor_2_and_3.stl to the servo horn.

Attach link_2.stl to the adapter on Motor 2.

Link 3

Take the third motor and mount assembly and attach it to the end of link_2.stl.

Attach another hub_adapter_motor_2_and_3.stl to the third motor.

Finally, attach link_3.stl to the hub adapter on Motor 3.

Mounting to Your Base

Mount the Motor 1 + Link 1 assembly to the base of your project.

The motor mount uses four M3 holes arranged in a 45 mm × 20 mm rectangular pattern.

Final Assembly

Attach the Link 2 + Link 3 assembly to the end of Link 1.

Your 3-DOF robotic arm is now fully assembled and ready to be integrated into your project.