Drawing pictures in a CAD program is fun, but when the rubber meets the road and you start fabricating something, you quickly notice some areas that weren’t too well thought out. Lately I’ve been backfilling those issues as we discover them at the shop. Here are some things I’ve tweaked over the past few weeks. Hopefully we’ll have a finished robot lawn mower soon!
Mower Deck to Chassis Interface
When the deck is stationary in my CAD program, chains look like a great way to support it. I can flip the model upside down and they don’t even move! But when the robot lawn mower starts rolling in reality, what keeps the deck from swaying all over the place? Well, if it hangs from chains, the answer is nothing.
Unfortunately, the chassis weldment and the mower deck weldments are pretty much complete. So whatever fix we come up with has to interface with those features like the chain did. The solution? Turnbuckles! Some really small turnbuckles, to be exact.
The eye on these little guys is 0.26in ID, perfect for the 1/4-20 screw I had planned on using. The length is adjustable from 3.375in to 4.625in long. They’re rated for 36lb, a strangely specific number, but with three of them they should work fine. The mower deck weighs just over 30lb.
Steel Mower Blades
Another issue the shop made me aware of was the mower blades. I don’t remember if I mentioned it or not, but the reason I designed the robot lawn mower out of aluminum was to avoid any compass interference issues. You may recall I ditched the compass a few months ago, but I never went back and changed the design to steel.
The shop thought that aluminum mower blades were a goofy idea. They’re not wrong, but at least I had a reason for making them that way. Kenny Trussell discovered that when the blades spool up to speed, they interact with the earth’s magnetic field in a way that skews your apparent compass heading.
Making them out of aluminum would avoid that issue, as they’d be non-ferrous. But since we’re not using the compass anymore, it seemed like a reasonable change to make. Besides, all the mower blades you see out in the wild are made from steel.
And if there’s one thing I’ve learned working with fabrication guys, if they make a suggestion that doesn’t impact your design significantly and doesn’t cost much, change it. It’s an easy way to show them you value their input, and they’ll do whatever it takes to get your design working now that their finger prints are on not just the physical parts, but the design, too.
A Legit GNSS Antenna Enclosure
On the wheelchair robot, I had the two GNSS modules velcroed to the top of the robot. That doesn’t seem befitting of a robot I’ve spent a year and a few thousand dollars making. So I designed a small 3D printed enclosure for the RTK GNSS antenna and the UBlox M8N module. It sits on top of a small ground plane disk, mounting to the lid of the electronics enclosure.
Everybody I talk to says you need a really good ground plane for your antennas. That’s what the circular disk is below the enclosure. The screws for the lid of the enclosure are plastic. Hopefully this doesn’t create any reception issues. I also hope that the antennas don’t have to be perfectly concentric with the ground plane. If anybody has experience with ground planes, I’ll take any advice or feedback you can give me!
Mower Deck Discharge Chute
For some reason I had it on the left side of the mower deck. The shop mentioned that most mowers have the discharge chute on the right side of the deck. I don’t want people latching on to minor quirks of my design, so changing it to the right side seemed like a good idea.
Mower Deck Progress
Here’s the progress on the mower deck last time I visited the shop. We’re a few roll formed parts short of a robot lawn mower!