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!
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!