I got the mower deck installed this afternoon. I finally have a robot mower! I chose not to install the mower blades for the time being. I figure now is a really bad time to end up in a hospital, and I can still test the motor functionality without them.
I was worried that there wouldn’t be much clearance between the deck and the front swivel casters and that the front motor would interfere with the battery bay. The clearances in these areas were pretty tight in the CAD model and I was expecting some variance in the parts. Everything fit together pretty well though.
One thing didn’t though:
I incorrectly sized the cable glands for the mower deck motor cables. I could barley cram both cables through the gland without the bushing. But after some finagling I was able to get it squeezed through there.
I picked the smaller size on purpose thinking it would be tough to get the bushing to seal around two cables. The two wires are 0.18in diameter with the insulation, which means I should have used a 0.375in ID bushing.
But plugging that size cable gland into the CAD model looked pretty ridiculous with lots of empty space in the bushing on both sides of the two cables. So I undersized it, and here we are. Lesson learned: your CAD model can be deceptive when dealing with flexible materials.
The turnbuckles worked pretty slick for leveling the mower deck. You could pretty easily get your fingers in there to rotate them and adjust things. Getting the deck mounted though was a two person job. I had to ask Mrs. Mower for help. Once you have one of the rear turnbuckles attached the rest is pretty easy.
Unfortunately, the deck does still sway a little when driving the robot around. Nothing terrible, but not perfect either. I think I will try to make a custom turnbuckle for the front deck attach point. There’s a lot of motion at the joint between the turnbuckle and the linkage.
I finally have all the parts I need to start putting the robot mower together. And for better or worse, my company has furloughed us for the next four weeks. Here’s hoping I get to cut some grass over the next few weeks!
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!
Back when I was ordering hardware for the robot lawn mower, I came across a smoking deal on some 5/8-11 X 5.5in long socket head cap screws. I was browsing Grainger’s website and they had a deal for a box of five for $1.92. Hot dog! Those things are $4+ a pop at most other places. I placed an order without thinking twice.
I opened the box up today when I was putting the front caster assembly together and found this newspaper page stuffed inside the box. Talk about a blast from the past. I assume this means these screws sat in that box for almost 12 years before they sold. No wonder they were on sale!
Looking at the picture of those girl scouts got me thinking about how short life is. There’s a good chance those girls are probably out of college by now. I wonder if any of them even remember the Girl Scout Sunday events on March 11, 2007. It was probably a big deal at the time, but 12 years later, I’m sure it’s but a distant memory to most of them.
Seeing this newspaper page was a good reminder to cherish what’s really important in life: family and friends. The Mower Project is a lot of fun, but without good friends and family to share my successes, failures, dreams, and goals, it’s all a very empty exercise.
And beyond that, it’s sobering to look forward and think about what will really matter 12 years from now. Who knows where the mower project will take me? The work I put in here could be a defunct blog 12 years from now. It could be something else, I’m not sure what. But if it comes at the expense of time spent with family and friends, it will surely not have been worth the effort.
This Christmas, I hope you all have a wonderful time with those closest to you, and I hope you make some beautiful memories with your loved ones, on which you’ll look back on 12 years from now and smile. That’s a project worth every minute.
I’ve received a few of the weldments back from the shop. While I wait for them to finish fabricating the mower deck weldment I’ve started to put some components together.
Back when I installed the wheel encoders on the wheel chair motors, I stupidly drilled a hole through the dust cover on the back of the motor so I could run data wires to the encoder. In hindsight, I should have run them through the little sleeve that the power and brake wires were routed through.
I had to take the brake off to put the encoder on the motor anyway, so there was a perfect amount of space for the encoder wires once the brake wires were removed from the sleeve.
Because you can’t undrill a hole I purchased a pair of cheap gear motors off eBay for $80. I mostly wanted them for the motor dust cover, but it will be nice to have spare parts on hand in case I need them down the road.
I took the aluminum back piece off the motors and removed the two white wires you see in the picture. The hole you see them sticking through was where I routed the data cables for the encoder.
Pro tip for dealing with these motors: There are two Philips head M5x150 screws holding the aluminum back piece to the mounting plate. These screws have lock washers under them. The screws are ridiculously soft and easy to strip the heads on. If you want to remove them so they’re still reusable, it’s best to use an impact driver. It’s extremely easy to strip them using a screwdriver.
I managed to strip the screws on both motors before I drilled them out and discovered this, so heads up to anyone modifying the motors like I am here. I ordered replacements that were socket head cap screws instead, hoping to avoid this issue in the future.
Once you have the aluminum back piece off, you’ll see wires inside like this:
The inside is going to be quite dusty with a lot of little brush particles inside. I blew it out with compressed air after taking this picture.
You can pull the white wires through pretty easily, but I had to bend the black wire terminal so I could get access to the hole to feed the encoder data cables to. I also ended up removing the brushes so I’d have more room to work.
Once you’ve got the white brake wires removed, you can pretty easily push the encoder wires through. The end result looked like this:
One thing I realized doing this is that it would have been pretty easy to drill holes into the aluminum back piece for screwing the encoder base down. I selected an adhesive backed encoder because I didn’t want to mess with it. But going to the trouble to take it apart like this changes that calculus. If I find myself doing this again, I’ll order an encoder that has clearance holes for mounting screws.
After I had everything wired up, I tested the encoder to make sure it was working well. Nothing like having to tear down a motor after it’s already on the robot to fix a loose wire.
I also wanted to make sure that running the data cables next to the power supply cables wouldn’t cause any issues. I didn’t find any during the bench test. Fingers crossed none pop up in the field either.
I used 5/8-11 screws for all the connections in the front caster assembly. I wanted to standardize on one size so I could buy several of one type of lock nut. Unfortunately the width of a 5/8-11 lock nut is 0.9375in and I don’t have a wrench that size. I also don’t have a hex wrench for the socket head cap screws either. The picture above shows everything hand tightened. I’ll have to go pick up the right tools to get this all put together.
I had a chance to drop by the shop yesterday, and things are progressing nicely! The chassis weldment is complete except for a little grinding and cleanup, and the front caster weldment is almost finished. It is very exciting to finally see the autonomous lawn mower jumping off the screen and into reality.
I spent a lot of time making detail drawings of each part, weldment, and assembly. You can never be too clear or explicit when making something complex. Unfortunately I think the pile of drawings scared off a lot of more than qualified welders and mom and pop fabrication shops.
I’m very thankful I found someone willing to take on the challenge. But even with very detailed drawings, things can still go wrong. For example, below is one of my drawings for the tube shown in the picture above.
That 55° is geometrically correct. But when you’re using a miter saw to make an angle cut on the tube, what angle should you set the saw to? The correct answer is 35°. In this case, my drawing was actually a little misleading, while technically correct. Lesson learned: if you’re dimensioning a miter cut, it’s best to show the angle the should be set to avoid any ambiguity.
Another lesson learned is to always plan for 50% or more material than the design calls for. The tightwad in me ordered exactly what I thought the shop would need with an extra 0.5in on the ends. In hindsight, that’s a recipe for extra trips to the metal yard to get material for the inevitable mistakes caused by my own sloppy drawings.
One other good engineering practice: always collect your old drawings. We went through a few producibility changes over the past few weeks, and when I dropped by the shop yesterday, I noticed a few old drawings floating around. Round those suckers up! At a minimum, mark them void. The last thing I want to do is pay for parts that I can’t even use because I changed the design.
If you spend a fair amount of time in a CAD program working on the same thing for more than a few weeks, you start losing a sense of the scale of things. On the computer screen, this weldment looked pretty substantial, but in real life, it’s actually pretty small. That battery bay is going to be very tight. I really hope I dimensioned it correctly.
Maybe this spring I’ll have something to actually cut grass with!
To kick off this Labor Day weekend I did some shopping. Wichita has a cool little place aptly named The Yard, which sells everything from screws, casters, foam, you name it. And because most of it is surplus, the prices are great, too.
When I design things, I start out planning to source everything new, and I record the price of every component I call out on the design. This information was immensely helpful as I dug through bins of screws. I had a price to beat on every component I was shopping for.
And as usual, The Yard came through. For example, a 5in long socket head cap screw, 5/8-11 thread was $5.80 through McMaster. The same screw at The Yard was $1.99. I found some 5/8-11 lock nuts for $0.68 a piece. McMaster quoted me $0.92 a piece. This was typical of all the hardware I was able to find there. Shopping at The Yard also saves me shipping, a cost I’d incur buying through McMaster or some other online source.
The Yard even cut some 3/16 chain for me. They charged me for two feet at a total cost of $2.50. I even got to keep the extra links. Not too shabby!
Those pennies add up. There are some components I’m going to have to shell out a lot of money for, like the hubs for my drive wheels. The bad boys below cost just shy of $50 a piece, and after shipping it was $120 to get them.
Harbor Freight had a sale on their cheap 10in diameter wheels, and I picked up four of them for $3.99. However, I think Harbor Freight is always having a sale on those tires. Their regular price is still dirt cheap.
Each tire comes with two hub pieces. One hub piece has two flanged bearings pressed into it, the other has no bearings. You can see the two hub pieces from one tire in the picture below.
I took two hub pieces with no bearings in them and made a “drive wheel” tire. I took the remaining two hub pieces with the bearings in them and popped one of the two bearings out. You don’t need a total of four bearings for one tire, and I have plans for these extra bearings.
I had to drill a hole in the hub piece with the bearings so the nipple on the innertube had a place to protrude from. You can see the hole in the hub without the bearings in the picture above.
After I drilled the holes and put the wheels back together I had a pair of “caster wheels”, a pair of “drive wheels”, and four extra flange bearings. I had to bend the innertube nipple on the caster wheels to get a bicycle hand pump on the nipple so I could inflate them.
The four extra flange bearings are going to be used to mount the stem of the casters into.
The Yard also has a vast selection of steel and aluminum raw stock. They didn’t have the 2in X 2in X 0.25in square aluminum tube I was after, but they did have some 2in X 2in X 0.1875in square tube. I think this will be easier to weld to 0.125in thick aluminum sheet metal anyway, so I had them cut several sticks for me.
My only complaint about The Yard is that they won’t do angle cuts for me. I’m either going to have to get my miter box and hack saw out, or find someone with a nice band saw to get them cut.
Overall, I think this is a good start to getting the prototype robot lawn mower fabricated.