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Robomower part 6 - Assembling the frame

This is getting fun! Putting the frame together is, for me anyway, a very important part of any project. Up until this point, the Robomower has been a stack of angle iron, a box of electronic bits, a crate full of wheelchair parts, and a rather daunting pile of random-looking fasteners. Today that will all change, and I will end up with something that looks like an insanely hot-rodded bed frame. For the longitudinal members, I used 48″ lengths of 2″ steel angle, 1/8″ thick. I am hoping this will give enough strength and stiffness. The cross-members are made from 1 1/2″ steel angle, still 1/8 thick. My first thought was to build the frame with the angle oriented such that the horizontal surface was on top, with the vertical surface dropping down, creating sort of a skirt. This would have the effect of visually lowering the top of the frame, as well as allowing me to hide the attach points for the mower hangers. Alas, this was not to be. With the casters mounted in the front corners of the frame, there was insufficient clearance for them to spin, and that is a deal breaker, so I flipped the angle over, which does make the frame look a little like it should be holding box springs, but I suspect it will all blend nicely when the thing is finished. Speaking of casters, I went through a bit of trial and error with those as well. Harbor Freight makes pneumatic casters in two sizes: 10: and 8″. The nearest Harbor Freight store only had the 10″, so that’s what I went for. In all honesty, I think I would have gone for them anyway. Once I got them home, however, something didn’t look right. I realized i should have done this sooner, but I then took a measurement of the Invacare motors, and determined that they would be supporting the frame deck at a height of 10″. The 10″ casters would support the deck at a height of 12″, so something had to change. I could have built a riser for the motors, but I wanted to keep the center of gravity of the bot as low as possible, so I swapped the 10″ casters for the 8″ at another HF. I was a little disappointed to see that the smaller casters did not have the cool, bulbous knobby tire that the 10″ has, but what are ya gonna do? In any case, the size is perfect, the bot sits dead level.

Robomower frame, Front view

Front view of the Robomower frame with casters and motors attached.


The construction is pretty simple, as these images show: The front crossbar forms the base of the frame. The caster plate acts as a gusset to provide some lateral rigidity. The slots in the plate are sized for 3/8″ bolts, so I got some 3/8 x 1″ carriage bolts, lock washers, and nylon nuts to hold everything together.
Front of Robomower frame

Construction detail of the front of the frame: The caster plates act as a gusset to keep everything rigid and aligned.


The longitudinal members were placed on top of the front cross member. I added a 1/8″ washer under the second longitudinal bolt hole to keep the spacing even, accounting for the fact that there is only one layer of steel at that point, vs. the two layers at the corner. I may add a flat 1/8 x 1″ additional cross member joining the rearward caster slots if I need the extra rigidity and have the space.

The rear followed essentially the same principle, except that the rear cross member is placed on top of the longitudinal member. I used the original mounting plate from the wheelchair to create a drilling pattern to mount the motors (I didn’t have any 1/8″ spacers on hand at the moment, which is why the mounting hole is empty in the picture).

Right motor mounting detail

Detail view of how I mounted the right motor and joined the frame at the same time.

One small snag: the mounting plate for the motors is wedge shaped, which means that the gear towers rise as they come away from the motor. The wheelchair mount had a curved arm which allowed the towers to tuck neatly up and in, out of harm’s way. Unfortunately, I was working with flat angle, and there was no way that I could find to orient the towers forward, which is where they would be best protected.

Rear view of Robomower frame

Rear view of the Robomower's frame. The gear towers projecting rearward has me a little worried.

The best solution I could come up with was to point the towers toward the rear of the frame, then slide them forward until they -almost- contacted the frame. This still left them pointing backward past the back edge of the frame, but they do angle upward. I am concerned about how unprotected they are, as if the mower runs off, say, a curb, or something of that approximate height, the towers will be the point of impact once the wheel has rolled over the obstacle. I am currently thinking that the hard plastic battery case will likely hang lower than the towers, and would provide a good mounting point for a skid plate of some sort. I like the progress the mower is showing.

Next step: mount the batteries and control electronics, and run the thing around!

Also coming soon: In every project, there is a moment where the reciprocating saw shows up…

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