The making of a large gear

I made a cool thing recently, and figured some friends here might appreciate a little tale of “how it’s made”.

For no reason at all, we desired a big ring gear this season. There are other (and less complicated) ways of doing this, certainly. 3D printing belt/gear profiles in sections, routering something with an appropriate profile to mesh with steel pins in a 3D-printed hub (a la 971), etc. But hey, we’ve got the time and the capability, so why not.

Here at WPI we have the Washburn Shops, where we’ve been making neat things since 1868. A far cry from 160 years ago, now we have 9 Haas VMCs, 4 Haas turning centers, a Accutex wire EDM, and much more. 4th axis, 5th axis, live tooling, robots. Got some cool stuff.

So, the gear. 285 tooth, 20dp angle. 13" ID, roughly 14.35" OD. There’s a few stages to getting this made, so I’ll walk through each.

First, choosing processes and making some CAM. We knew this was a great candidate for a wire EDM part - high accuracy, great tolerancing on small features, can cut with essentially no radius. Our wire EDM is an Accutex GE-43SA, for those playing along at home. We also need to do some milling - this part fit great on our Haas VM-2.

With something this big that is being cut out completely, we need to not fully cut out the part so it doesn’t fall through and pinch the wire. We figured we could cut the gear in “quadrants” (we had to do it in halves anyways because of travel limitation on our wire EDM) and leave one tooth still connected. Then, we can come back in with an 1/8" end mill and cut the remaining tooth and also contour out the center to size.

Some screenshots from ESPRIT, our CAM program we use here.

This is a simulation of it running.

The first operation was in our VM-2. First, we faced one edge to use for angular location later on. Then, we drilled the clearance holes for the #8 fasteners that attach the gear to the turntable, bored a hole in the center for locating on the wire EDM, and finally drilled some entry holes. What are entry holes, you might ask? Our wire can’t just go through material at will. We either need to enter from an edge (and “burn in”), or drill some holes that we tell our machine to feed the wire through. Burning in from an edge takes time and uses up wire, so it made sense to drill some entry holes and tell the machine to feed the wire there at the start of each contour.

Time to wire EDM! First, we made sure that the machined edge was straight using a dial indicator. We probed the part, adjusted some settings, and we’re off!

Cut one side, flip, re-indicate, probe, and press cycle start again! Wire EDM is an extraordinarily time-consuming process. All in all, it ended up being about 6 hours of cut time for the two programs.

Fresh off the wire EDM, here’s how we were looking. You can see that the gear is still captured by four of the teeth we didn’t cut out.

Now, it’s time to finish up in the VM-2.

Here’s our strategy for cutting out each tooth.

We put our plate in and locate appropriately. Any angular error is going to give us trouble - those gear teeth need to be exactly where we expect them to be. We pick up the center from the bore we created earlier.

Once we cut each tooth out, we can change some clamps around and cut out the center!

Not bad!

This is probably the coolest thing I’ve made in some time, and it was a whole lot of learning (and fun). Hope you enjoy the writeup, feel free to drop any questions you have below!


Super cool!


Like, is this actually something planned to be put on 190’s bot? Or does “no reason” mean that it was a proof of concept or the like for a manufacturing process for something else? Or can you not answer those questions yet?

Looking forward to seeing it in action at Central Mass, assuming it or something similar will be on your robot there!


Banana for scale?

Looks great!


We’re planning on using this (or something extremely similar) in our competition robot this season. Like @ndp mentioned, there are a ton of ways to get the same engineering function but the Wire EDM was too enticing!


Thanks David, means a lot. I remember just being blown away by the stuff that 696 was making back when I was in high school, and was always a bit jealous of the tools y’all had :slight_smile:

Like @zach_boyer said, this will most likely be on the robot. We thought about cutting a segment of profile to make sure everything meshed well. In retrospect, I probably would have done that first, as it’s a pretty big gamble to sink as much time as I did into something without a quick sanity check. But in our experience the wire parts are pretty dead-on if you do it right, so luckily it worked out.

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Neat! EDM’s are great


Thought that was our 2020 turret gear for a second there

Very cool process, and informative write-up. Thank you for sharing!

So, your team is planning to drive a bicycle wheel at 26 ft/sec? :wink:


26ft/sec isn’t particularly fast for a bicycle wheel (depending on the load attached I suppose). If we’re talking about arelatively small 18" wheel, that’s only ~300rpm (VERY rough math).

6000 RPM x (1min/60sec) x (14/285) x 24inch x (1ft/12inch) x pi() x 85% = ~26 ft/sec.

Cruising at 85% of free speed seems about right. 24 inch wheel is small for a bicycle, but more likely to fit an FRC robot design. Could go faster with a 12T pinion.

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Ah good call. I forgot to include the motor->giant gear ratio… that certainly affects the calculation a bit. Thanks for the correction


Thanks dude! We got it 2ish years ago and now it probably gets used near every day. Definitely was a huge value-add for our shop.

Thanks for posting the process, Nick! Super cool! I’m excited to see 190 feature the largest swerve drive ever. :wink:


Swerve unicycle.


Aren’t unicycles always “swerve”?


Brilliant!!! I like very much the process and you definitely have access to neat tools. What is wonderful with this method is you can make gear rings with inside teeth, essential for planetary gearboxes and torque converters. I also cut gears on a homemade machine, the old fashion way, with involute cutters #1 to #8, alas no gear ring cutting possibility for me…



3419 has our own very big gear this year - a slice of a 278 tooth 10DP gear. Our process was a bit simpler: send it to the good folks at SendCutSend to make for us. I believe they laser cut it.

There are two of these on the robot. What could they be for??? :slight_smile:


I can’t say I’m surprised this service exists, but it’s honestly the first I’m hearing about it! If you don’t mind me asking, how much did it cost for that gear slice? I’m trying to judge their pricing against a couple local options, but I don’t have anything to send them to quote.

It is made of 3/8” thick steel and cost $48 each.