This is a good start. Your design process is fairly sound and it seems like you're focusing on making improvements to an existing design where it makes sense to and keeping the rest. Not bad.
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Originally Posted by dmaciel10123
The point of the design is to make it easier to change out wheels, even possibly have the side modules on the side ready for a quick change when needed.
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I think you probably don't need the entire module to come out as one piece to do this. You could save some weight by removing the inner plate and just using the tubing for that purpose. Just put the robot on blocks and remove the outer plate when you want to change wheels. Making the outer plate itself removable is a bit of a challenge but shouldn't be too hard. Hint: Drop some riv-nuts in your tube for the outer plate standoffs to thread into rather than making them coaxial with the inner frame standoffs.
Alternately, for the outer wheels you could do a "tube axle" - a 1/2" OD, 1/4" ID piece of round with a 1/4-20 bolt running through it. This lets you assemble the wheel, pulley, and spacers onto the axle on the bench, then you "just" slide the entire assembly in place from underneath or above. Pop the 1/4-20 bolt through once it's aligned and you're done. There isn't really an elegant way to do this with the direct driven wheel though, unless you want to slide the gearboxes out to change this wheel!
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The outside plates are intended to be made of steel rather than aluminum to provide extra strength, but all of the other parts are aluminum.
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As a general rule, you want to get your strength from your cross section, not your material choice. Rather than upgrade the flat plate from one material to another, make the plate not flat anymore. Can you use aluminum C-channel here? C-channel is much stiffer than flat plate because of the flanges, and thus is less prone to bending. It is also only marginally heavier than a flat plate of aluminum, while a flat plate of steel is much heavier. It's more strength and weight efficient to change your geometry than it is to change your material.