Team 1768's 2016 Technical Notes

[snekiam]

What led you to using such thick metal for your bot?

[Zac]

The 4 main plates that make up the frame were 0.25" aluminum (edit: the outside plates were actually 3/16" thick for weight). 0.25" plates support bearings very nicely. Additionally given the rough nature of the game we felt comfortable with 0.25" as it was a thickness we had used before. We did lighten the plates fairly aggressively. Given that just being able to drive in stronghold meant being able to score many many points it seemed like a reasonable place to spend some of the weight budget.

Additionally these plates are very much an integral piece of the robot, it seemed as though every shaft of every mechanism found its way through one of them. These plates were responsible for many center to center distances for the 20-something (26? 27?) timing belts on the robot. We didn't want them bending, because replacing them was all but impossible in a FRC environment.

Elsewhere on the robot, you will find thin walled tubing. The climber mechanism is 0.0625" wall thickness, for each of the 3 tubes on each side (2x2, 1.5x1.5, and 1x1)

Additionally there is a 2x2 that connects each side of the drivetrain, also 0.0625" wall thickness.

~Zac

[snekiam]

Ah, makes sense. I'm a programmer most of the time, and am trying to learn more about mechanical stuff. Thanks for the detailed response !

[Greg Woelki]

Originally Posted by snekiam

What led you to using such thick metal for your bot?

Zac gave a pretty thorough explanation above, but I'll add on a little. The reasons why we didn't make each particular 1/4" part thinner vary, but here are a few notable ones:

• 1/4" thickness is convenient with most bearings we use
• 8-32 bolts can be tapped into the side of 1/4" plate without leaving too thin a wall
• Section stiffness is proportional to thickness cubed, so when stiffness against bending is important, it's better to lighten thick material vs. using thin material that's light to start with
• Tapping into the face of 1/4" plate with 8-32's and (to a lesser degree) 10-32's approaches the tensile strength of the bolt itself
• For the hood specifically, we wanted a large surface area in contact with the boulders to minimize any damage to them

I'm sure there are a few places where we would have been just fine with thinner plate, but, in the end, we were able to fit in all the functionality we wanted under 120lbs, so there wasn't much of a cost to us. Additionally, waste not want not on those 4'x4's .

On the whole thick-vs-thin topic, I'm a really big fan of thin-wall tubes, but thick-and-lightened is often a good way to go for plates.

Originally Posted by Zac

The 4 main plates that make up the frame were 0.25" aluminum.

The outer two are 3/16", actually

[Zac]

Originally Posted by Greg Woelki

The outer two are 3/16", actually

My bad, edited.

It is worth mentioning that while it did all come in under 120 pounds, it was tight for much of the season, going 3/16 on the side plates was a weight saver, and a large on in comparison to some of the other strategies employed on this robot to save the last couple grams.

~Zac

[Chris is me]

This might have been the most elegantly designed robot in New England this year. I think this is the robot a lot of top teams (mine included) should have tried to build this year. Great job, and beautiful design and machining!

As for the technical discussion on plate thicknesses - 1/4" plate driven design is certainly a viable method for building robots, but it presents some advantages and disadvantages versus both tube and bent sheet metal based designs. They are essentially the three main design paradigms in FRC robot design, and whether or not one is better than the other is dependent on your resources.

The big advantages of 1/4" plate based designs are in design flexibility (you can basically put any feature anywhere you want and just design the plates to support it, it's kind of cool) and some perks in construction and assembly (tapped holes, better bearing fits, etc). The big downsides are in having to remove more material to match the weight of other design styles and requiring a higher part count (standoffs, etc) to get comparable rigidity.

If you have a team with good lathework (for standoffs and the like) and more "2-D" manufacturing resources like a waterjet or laser where cutting times for intricate patterns are still fairly quick (and cutting tube isn't ideal), 1/4" plate designs play well into those resources. If you can add precision bending to that playbook, you can use thinner plate + flanges to save some lightening and effort, but not every team has these resources. A lot of it is also designer preference and expertise.