FIRST Robotics Sprocket

FIRST provides opportunities for middle school and high school students to work together and build robots – what could be cooler! We’ve helped a variety of FIRST LEGO League (FLL), FIRST Tech Challenge (FTC), and FIRST Robotics Challenge (FRC) teams over the years. We were asked if we could help them make some custom sized chain idlers because the correct sized ones didn’t exist. They drew up a file making sure to make proper gear teeth and we’re ready to go. If you didn’t know, you can also borrow a model from McMaster-Carr if the correct size exists.

BILL OF MATERIALS

• 1/8” Sheet of Aluminum at least 3” x 3”
• 25 size Chain (so you can check sprocket fit)
• 1/8” End Mill
• 1/4” End Mill (not required but makes center cut quicker)

TOOLS

• CNC cutting machine - like a PCNC 440
• Windows computer running CamBam (or your own CAD to CAM solution)
• Vise or other workholding solution (vacuum table, double sided tape, etc work better for sheets)

STEP BY STEP

1. The first step is to fixture your plate to hold it properly during cutting. We ended up just gently squeezing it in the vise, but that causes deflection and makes it more difficult to control the depth per pass. I’d recommend an alternative fixturing strategy for better results. You’ll notice in our CAM step we ended up cutting 0.020” deeper than the stock thickness due to the deformation. Once you have the part secured in the vise, set up your 1/4” and 1/8” tools and input the correct tool numbers and offsets into CamBam. Our first operation will cut the hub out of the center of the part with the 1/4" end mill. We’re cutting the center first to maintain stiffness on the press-fit clearance – the chain ring geometry isn’t nearly as critical. Our first pass removes all of the material except for 3 tabs that are cut off in our finishing pass. By leaving the ring center, we keep some stability until the very last passes.

2. Once the center hub is completed, swap tools to the 1/8” cutter and cut out the perimeter.

3. We set the finish passes to utilize the 1/8” cutter, but also to save you another tool change.
4. After the sprockets are cut out you’ll need to deburr the edges. If you have a polisher/tumbler, you’ll have a quick time for clean up, otherwise get out the round file and break those edges.The chain will wear them down quick enough if you’re lazy, but that's not recommended.Lastly press (or drop) the sprocket on the bearing, if you have too large a clearance, use some Loctite retaining compound to take up the slop.
5. Check the chain fit by looping chain all the way around the sprocket – don’t just check a few links as it won’t be obvious if it was cut deep enough until you’ve made a complete circle around the sprocket.
6. Install the sprocket on your robot, and get into the FIRST arena!