Making a 2-Speed Custom Gearbox
DESCRIPTION
This tutorial will show how I created a custom 2-speed shifting gearbox. As a student in the FIRST Robotics Competition (FRC), I became heavily involved with designing and building my team’s drivetrain and chassis. Though the chassis was custom designed and fabricated, we always used commercially produced gearboxes due to time and resource constraints. As I looked at some other teams’ robots, I was amazed by their beautiful custom gearboxes that perfectly fulfilled their design goals. Seeing these other designs inspired me to try making one of my own, even though producing it would be impractical for my team during the actual build season.
This project was really just me having fun before I graduated from my high school, where I had access to the machining equipment necessary to complete the project. Unfortunately, I did not have enough time to finish all of the parts. However, I do have enough to give a very clear idea of what it will look like. Though the gearbox was designed to be used on a FRC robot, it will probably never see use.
This tutorial was made through the Autodesk FIRST High School Intern program.
Materials
Aluminum Plate
Aluminum Tube
Aluminum Hex Bar
Gears
Bearings
Motors
Bolts
The PDF attached to this step is a complete Bill of Materials for this project. It includes information such as the quantity, cost, material type, and vendor for many of the required parts.
Tools
CNC Mill
Lathe
Band Saw
3D Printer
Socket Wrench
Hex Wrench
Pliers
The design of this gearbox was heavily inspired by other FRC teams who posted pictures or full 3D models of their designs. Admittedly, I am not the most creative designer (steal from the best, invent the rest, right?), so the inspiration is fairly obvious. In particular, the design was inspired by Team 254 and Team 973, who have been gracious enough to share their designs.
I designed my gearbox entirely in Autodesk Inventor before purchasing a single part. I started by choosing a gear ratio (which you can learn more about here), and then ensured that there would be no clearance issues between the rotating parts using the sketch shown in the second picture. This sketch also helped me define the exact shape of the gearbox plates. From there, I designed the rest of the gearbox, part by part. Throughout the design process, I had to ensure that I could make each part that I designed on the tools available to me.
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There were a total of four parts that needed to be milled, each of which was distinct. Using a CNC mill to machine the parts requires the creation of G-code to control the tool. I did this by taking the 3D models I created in Inventor and importing them into the Computer Aided Manufacturing (CAM) program MasterCAM. After defining how I wanted the mill to machine my parts, I set them up on the CNC mill in the first picture. The next four pictures are of the parts I made using the mill. Finally, the sixth picture is of a workholding jig I printed out on a 3D printer to hold one of the parts while it was being machined.
Next I had to make several parts on a lathe. These parts were the spacers that separate the gearbox and the axles that the gears spin on. I used the small bench top lathe pictured to machine all of these parts.
Photo Credit: http://www.grizzly.com/products/G8688
The final set of parts were made using a 3D printer. This manufacturing method was chosen because the parts would not experience high loads and because they may have been tricky to machine using traditional methods.
Photo Credit: http://www.rapidreadytech.com/2012/07/university-of-nevada-reno-opens-3d-printing-to-student-body/
These two pictures are of all of the parts I purchased, instead of machined, to make this gearbox. The three suppliers I purchased parts from are McMaster Carr, AndyMark, and WestCoast Products.
Now, we can begin assembling the gearbox.
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First, we must assemble the motor. The second picture shows how to place the plastic spacer on the shaft and insert the key into the motor shaft's keyway. Now, the 12 tooth pinion gear can be slid on, followed by pressing on the retainer ring using a 3/8" socket wrench, as shown in pictures three through five. Repeat this process for each of the CIM motors.
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Next, we install the motors on the aluminum plate. Simply insert the motors into their holes, line up the screw holes, and tighten a self locking 10-32x.5" screw into each of the holes, as shown in the first three pictures.
We also have to press several 3/8" bearings into place. Pictures four through six show the three locations where these bearings are installed.
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Now we will assemble the output shaft and place it in the gearbox. First, we must install the e-clip ring in the center of the shaft. I did this by pressing down on the ring with the shaft against a hard surface, as shown in the first picture. Pictures two and three show how I slid the dog shifting gear, 45 tooth dog gear, and small 3D printed spacer onto one end of the shaft. Next, I slid the output shaft into the lower bearing on the previously assembly plate, as shown in picture four. Finally, pictures five through seven show how to install the 60 tooth dog gear and 16 tooth sprockets on the 1/2" hex shaft.
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In this step, we will assemble the rest of the gearbox case. First, I pressed the final 3/8" bearing into the center top hole shown in the first picture. In the second and third pictures, I placed the aluminum spacers in their holes on the first plate and then aligned the second plate with the spacers. The gearbox plates have shoulders for the spacers to ensure that the plates are aligned. Next, I slid the four 3.5" 10-32 screws through the four holes shown in the fourth picture. In the fifth picture, I used a 5/32 hex wrench to tighten the bolts that screwed into the motors. Then, I used a socket wrench to install nylock nuts on the remaining two bolts. This setup and its result are shown in the sixth and seventh pictures.
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Now we will install the bearing block, which will support the output shaft and would be used in connecting the gearbox on a robot. I first slid the large 3D printed spacer on the hex shaft, as shown in picture one. Next, I slid the bearing block into the milled shoulder on the front gearbox plate. I then inserted and tightened down the two 1.5" 10-32 countersunk screws using a 1/8" hex wrench, as shown in the third picture. Finally, I installed the 1/2" hex bearing by sliding it onto the shaft and into the the bearing raceway on the bearing block. This process is shown in the fourth and fifth pictures.
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In this final step, we will install the wheel and several snap rings. First, I pressed on the 3/8" snap ring at the end of the output shaft shown in the first picture by pressing down on it with a flat bladed screwdriver, as shown in the second picture. I then slid the 4" aluminum wheel onto the output shaft as seen in the third picture. Finally, I installed the 1/2" snap ring at the front of the output shaft. I did this by first placing it on the grove as in picture four and then pressing down on it with a flat object, such as a screwdriver.
The installation of the 1/2" snap ring is as close to completion of the gearbox as this tutorial can go. Unfortunately, I ran out of time to manufacture several of the necessary parts, leaving the gearbox in an incomplete state.
Here are some lessons I learned from making this gearbox:
- Ensure that you have sized holes properly for bolts and motors - I ended up having to modify the holes the CIMs are mounted in by manually sanding them down.
- Control tool chatter when milling parts - resulted in a few parts with bad surface finishes and even caused some parts to be over-sized
- When used properly, CAD is an incredibly powerful tool. Because of it, I didn't have to redo a single part for reasons other than manufacturing errors.