10
CAPSTONE PROJECT: CREATING A ROBOT ARM

For your final exercise, you’ll show off the skills you learned in this book by designing a small robot arm. This arm, shown in Figure 10-1, uses a total of four small 9G hobby servos, which are motors you can rotate to specific positions with commands from a microcontroller development board. The completed robot arm will have a reach of about 7 inches. It won’t be able to lift anything particularly heavy or operate with a great deal of precision, but the relatively simple construction is a good introduction to robotics and multipart assemblies.

This chapter won’t guide you through every step of the modeling process. Instead, you’ll learn enough to determine how to model the parts yourself. We also won’t cover how to program this robot arm, as that’s a complex topic in its own right. But, if you use a popular microcontroller development board like an Arduino, you can find many tutorials online that thoroughly explain how to control servos.

Figure 10-1: The completed robot arm

For this project, you’ll need the following parts:

• Four 2.5 kg·cm stall torque 9G hobby servo motors (any brand will do)
• Arduino or similar microcontroller development board
• Sufficient 3D printer filament to print the parts, which should be less than 0.1 kg

This design doesn’t use any bearings, and you’ll only need the screws that come with your servo motors. A small amount of superglue will help hold the parts together, but you can also design a means of attaching them if you prefer.

Measuring and Modeling Your Servos

Although you’ll have to buy the servos you’ll use for this project, you’ll also want to model them. While most 9G hobby servos are very similar in size and shape, there are often small differences between models from different manufacturers. To ensure that everything fits together, you need to model your servos exactly as they are.

Many manufacturers provide drawings with the servo dimensions, so you should first check on their website to see if those are available. If not, you can use a set of digital calipers to measure the servos yourself. Digital calipers can measure the distance between features far more precisely than rulers or tape measures. You can find inexpensive calipers online for less than $30. As you measure, pay particular attention to the areas where the servo will mate with other parts, like the screw mounts and motor shaft.

Modeling the Base of the Robot Arm

With the servos modeled, you can begin designing the 3D-printed parts of the arm. The first one—the base shown in red in Figure 10-2—is a teardrop shape that keeps the arm from tipping over when it’s fully extended.

Figure 10-2: Model of the base, shown separated from the rest of the assembly

Start your design by creating a circle with a diameter of 60 mm; then elongate this to form the teardrop shape. The base should be wide enough to handle the load, but if your arm ever falls over, you can add additional elements to it for stability. The base should also be hollow. You’ll fasten the first servo, which controls the rotation of the shoulder of your robot arm, to the inside of the base with screws through holes on the tabs that extend from the body of the servo.

The top of the servo body, where the shaft comes out, should sit a millimeter or two above the top surface of the base for clearance. Finally, place a small cutout at the bottom for the servo cable to exit through; that way, the base can still sit flat on the surface you place it on.

Once you’ve designed the base, put it in an assembly with the first servo motor.

Shoulder Motor Mount

The next part you’ll model is the mount for the second servo motor, shown in Figure 10-3. This part will connect the first servo in the base to the second servo. The first servo rotates the entire arm relative to the base. The second servo tilts the arm up and down. The mount should cradle the second servo so it will lie on its side.

Figure 10-3: The second servo mount joins servos 1 and 2.

Create a hole in the center of the mount’s bottom for the first servo’s shaft to go through. The hole should also have a cavity on the top to keep the shaft screw’s head below the surface. Next, create two more small holes for the servo-mounting screws; align these with the holes on the servo. Finally, create a channel and hole to allow the servo cable to pass through the mount. Pay attention to the second servo’s orientation in Figure 10-3 and make sure your channel and hole are on the side where the servo cable will exit the mount.

Create another hole in the mount opposite the servo’s output shaft but directly along the shaft’s axis. You’ll use this hole for a pin that acts as a pivot point for the next segment. It must be on the same axis as the servo motor shaft to ensure smooth motion. Once you’ve modeled the mount, add it to your assembly.

Third Motor Mount and First Segment

This next part, shown in Figure 10-4, will act as both the first segment of the arm and as a mount for the third servo. It has parallel pivot points on each end—one for the second servo and one for the third servo. The pivot points are, on one side, the pin and hole, and, on the other side, the servo hub shaft and pin hole. You can reuse some of the geometry from the shoulder motor mount as a starting point, because you’ll mount the third servo just like the second servo.

Figure 10-4: The third servo mount is also the first segment of the arm.

The distance from the second servo pivot axis to the third servo pivot axis should be about 70 mm. Give the other side a U shape so that it can fit around the second servo and mount. Create a hole on one side to fit over the servo shaft and a pin on the opposite side to fit into the hole on the mount. It’s a good idea to add a rib (the vertical bar in the middle of the part) to give the part some rigidity. Add it to the assembly model and make sure it can rotate unobstructed.

Fourth Motor Mount and Second Segment

As with the first segment model, you can start the second segment model by reusing some geometry. The area where the second segment mounts to the third servo is exactly the same as where the first segment mounts to the second servo. As you can see in Figure 10-5, the section you have to create from scratch is the other end, which attaches to the gripper mechanism.

Figure 10-5: Reuse the side of the second segment that’s the same as the first segment.

On the gripper mechanism side, create a hole, like the one you created on the top of the base, for mounting the fourth servo. At the end of that side, model two 45-degree protrusions with 4 mm diameter holes that will be the pivot points for the gripper arms. You should make each protrusion roughly 12 mm long. Once again, add the part to the assembly to make sure it can move freely.

The Gripper Mechanism

The robot arm’s gripper mechanism is the most complex piece of the entire project. A hub on the motor shaft connects to two short linkages, which in turn connect to the arms of the gripper. When the hub rotates, it pushes the linkages out, causing the arms to pivot and close.

The mechanism is complex because the lengths of servo hub arms, the linkages, and the “fingers” of the gripper will affect how well it operates. Get one of these items really wrong, and the entire gripper will jam up. For that reason, you’ll want to spend some time experimenting with different lengths in CAD before you actually 3D print any parts. Start with lengths that look similar to Figure 10-6 and add joints for them. Then, move the mechanism to see if it opens and closes fully. If it doesn’t, adjust the lengths and try again.

Figure 10-6: The length of the linkages determines how well the gripper will work.

When you’re modeling these, you’ll need to make one arm longer to avoid jamming. To compensate for this asymmetry, you can make one side of the center hub longer than the other. This allows the gripper to open by roughly equal amounts on each side.

Finally, take some care with the linkage pins and the holes they fit into. The pins need to have a large enough diameter to be strong, so they won’t break, and the fit needs to be loose enough to allow free movement. You can 3D print small discs to superglue onto the pins after assembly to hold the linkages in place, or you can design the pins with tabs so they snap into place.

Add the parts to your assembly when you’ve finished modeling them and make sure they all fit together.

Printing the Parts and Assembling the Robot Arm

You can print all of these parts in PLA or ABS on any hobby 3D printer, including a fused-filament fabrication (FFF) printer. Only the second segment will require support material during 3D printing. You’ll want to keep the parts’ density high enough to be strong but low enough to be lightweight. Somewhere between 25 percent and 50 percent infill density should work well.

Once you have all of your parts, assembling them is as easy as mounting the servos with the provided screws and snapping the pieces together. The second and third servo pivot mounts will fit tightly, but a little force should get them on. After you’ve placed each part onto the corresponding servo motor, use the included shaft screws to hold them on securely. Then just place a small dab of superglue on the gripper linkage pins to attach the discs and keep the pins from sliding out.

With the arm assembled, you can attach the servo cables to your microcontroller development board to control the arm. These 9G hobby servos use only a small amount of power, so you may not need a separate power supply; check the specs of your motors to see how much current they use and what your microcontroller development board can supply.

Summary

Before you started reading this book, you probably didn’t know how to begin modeling an entire robotic arm. Now, with only minimal guidance, you should have done just that by drawing on what you’ve learned. The project in this chapter shows just how much you can accomplish with the CAD skills you learned in this book.

You can continue to develop these skills as you go forward, whether for hobby projects or in your professional life. While this book didn’t cover some of Fusion 360’s most specialized tools, your working knowledge of the software and CAD modeling practices should allow you to understand them if you ever need them. By this point, you should understand how parametric CAD works. Getting good at 3D modeling requires more than simply knowing how to use a tool. It takes a specific kind of thinking and forethought. Now, take that thinking and apply it to your next project!