What is pocketing (gussets) in FRC?
Generally, for gussets, (and pocketing in general) you’re removing material from low stress points in order to save weight.
An example of pocketing are the front and back plates of this gearox:
Instead of being a solid plates, numerous holes are cut into them to reduce weight, while being careful not to over-reduce strength.
And hopefully ensuring you’ve accounted for manufacturability by adding radii in the corners of your pockets (like the above image does not have, not to call whoever made it out, its just a good example of whats missing).
Lasers. With a plasma or laser cutter this could be made exactly how it is shown.
Or a CNC and a .125 end mill, which would leave you with at least .125 radii.
It would still have a radius in the corner due to the laser/plasma diameter.
Also sharp corners represent infinite accell/decell to the machine (which has to ramp up/down). A curve allows the machine to be decelling in one axis while accelling in the other. Will actually reduce runtime appreciable in qty. This is a non-issue on an industrial sized laser, but for a smaller one it will be.
There are different ways to look at pocketing… another way is to see it as a mostly unnecessary process, required only if you designed something with the wrong material, or wrong shape.
But I’m lazy and cheap, so take this with a grain of salt
affectionately known to me as “swiss cheesing”
Gussets are completely different than pockets. Gussets (or more formally gusset plates) are pieces of plate metal which are used to join two or more beams or similar linear members and keep them at an appropriate angle to each other. Here’s the page of VexPro gusset plates, probably the most common COTS gussets in FRC. They are typically riveted onto both sides of VersFrame stock with 5/32" aluminum rivets.
The first gussets I encountered in FRC were welded into place, in the 2013 game specific drawings The term is first found on page 37, but gussets were used to keep each layer of the pyramid square.
Please provide some guidance to inexperienced teams on how to always design with the right material and right shape.
Otherwise, perhaps withhold advice like your previous post as it might send someone down a path they aren’t prepared for.
“Mr Forbes says we don’t need to do pocketing!” 140 lb robot
148 loves pocketing / trussing. We do it on everything. Maybe we’re just bad at designing with the right material & shape.
I don’t entirely disagree… but keep in mind that pocketing is basically changing the shape - and getting it closer to the right one You can of course take steps to be close to the right shape in the first place. Pocketing gets you closer.
It’s probably because 148 isn’t cheap or lazy :rolleyes:
One simple alternative: use thinner material without pocketing. I’ve seen a lot of pocketed designs in FRC that could have been made simpler and cheaper with practically no effect on strength (although, oftentimes can be made stronger) by using thinner material.
I’ll let you all sleep on it, and see if you can figure out what I’m getting at.
Or you could explain your reasoning? Wouldn’t that be more helpful to inexperienced readers?
The difference is easily explained by analogy to bones. While mammal limb bones are simple tubes with filled with marrow, they do not have any truss work within them. This is because mammals are not pushing the envelope on the tensile strength of calcium and phosphate salts as hard. Dinosaurs and birds however, push that envelope. Land dinosaurs came in sizes much larger than mastodons, and birds need to shave every available gram in order to improve performance in the air, so their bones feature (irregular organic) truss work. Here’s a web page comparing the structure of human arm and bird wing bones.
Or use a completely different material. Wood is nice for certain applications; PVC has its uses. You can’t forget about fiberglass/carbon fiber (not the same material, but I’m lumping them together) despite the extra precautions needed for them.
In short, here are some ways to avoid pocketing/swiss-cheesing:
–Change material. Aluminum fasteners and gears instead of steel ones, for example. PVC structure can be heavy, but try building it out of aluminum sometime.
–Use a different design.
–Use thinner material that doesn’t need to be pocketed.
There are also some other tricks not having to do with pocketing or making lots of holes.
I’ll provide a little history behind my comment, and why I think JVN’s hypothetical “Mr Forbes says we don’t need to do pocketing!” 140 lb robot is quite humorous.
I started working with team 1726 on the last weekend of build their rookie season. My son was on the team, and he asked me to come in and help them get the 140 lb robot down to 120 lbs, and they only had a few days to do this. So, I helped do a lot of pocketing and we got it to 119.9 lbs and all was well. Of course, I got hooked on the whole FRC thing… The next ten robots the team built, I was around at the beginning of the design process, and we didn’t ever build another robot where we had to chase weight by cutting material away like that. Instead, we kept track of weight from the beginning of the design. And we didn’t use a material because “that’s how robots are made”…we kept open minds, and used materials that met our requirements of cost, local availability, weight, strength, stiffness, ease of fabrication, suitability for that game, etc. It turns out you can use a wide variety of materials to build robots. We’ve used fiberglass, wood, steel, aluminum, polycarbonate, etc over the years.
Mainly, I see the extensive use of CNC material removal as kind of wasteful. This mostly has to do with the fact that I’m cheap and lazy.
If you have the resources to purchase extra material and then cut it away, then go for it!
I think the points you make here are valid for many teams, but there are two other points relevant teams should be aware of.
Each team has unique resource, so for many teams it might be more efficient at the team level to use 1/4" plate and pocket it every time than try to figure out lighter methods (ideally some compromise can be found here).
The other point is that there are some games where the top 10% of teams or so that try to do it all (or some large subset of doing it all) and to pull this off most of those teams (based on the above point) can’t hit weight without pocketing.
To sneak in a third point, there are reasons that coming way under weight can be valuable (namely faster acceleration and less battery use).
These posts have pictures of some good examples of pocketing.
It’s often used because a flat surface is needed (or is convenient for fabrication) or the thickness is needed for bearings or geometry, and then you cut away material to get a truss structure between the important (load bearing) points in the part. Reduces weight, can ease maintenance, and also looks kinda cool.