FRC pocketing

[first_newbie]

What is pocketing (gussets) in FRC?

[Sohaib]

Generally, for gussets, (and pocketing in general) you're removing material from low stress points in order to save weight.

[asid61]

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.

[Brandon Holley]

Originally Posted by asid61

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).

-Brando

[Owen Busler]

Originally Posted by Brandon Holley

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).

-Brando

Lasers. With a plasma or laser cutter this could be made exactly how it is shown.

[Rosiebotboss]

Originally Posted by Owen Busler

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.

[AdamHeard]

Originally Posted by Owen Busler

Lasers. With a plasma or laser cutter this could be made exactly how it is shown.

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.

[MrForbes]

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

[Joe Derrick]

affectionately known to me as "swiss cheesing"

[JVN]

Originally Posted by MrForbes

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

Jim,
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.

-John

[TheOtherGuy]

Originally Posted by JVN

Jim,
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.

-John

It's probably because 148 isn't cheap or lazy

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.

[GeeTwo]

Originally Posted by MrForbes

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

Originally Posted by JVN

Jim,
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.

-John

Originally Posted by TheOtherGuy

It's probably because 148 isn't cheap or lazy

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.

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.

[EricH]

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.

[MrForbes]

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!

[Bob Steele]

Originally Posted by TheOtherGuy

It's probably because 148 isn't cheap or lazy

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 don't really see using thinner material as the same as "pocketing" A part that is thinner does not have the same structural integrity as a piece of thicker material that has been "pocketed" down to the same weight.

Our team carefully looks at the lines of force in different pieces and designs trusswork that supports the loads put on the part. We (like JVN) call it trussing. You take out material that doesn't add to the strength you are looking for. We also use this same information when we place fasteners. The trusses work WITH the fasteners and need to be designed together.

One can just look for places to make holes but it is really necessary to look at the stress on a given part to know that you have to be able to deal with the various forces being applied (both torsion and compression)

I am confused by the vocabulary anyway, I was always under the impression that "pocketing" was material removal from non-stress areas without making a hole. Just a thinning of the part in non-stress areas. This is often down in castings to save material. They weren't holes. I am probably wrong but that is what I grew up thinking. But then again what is a hole? I can't really dig a hole in the ground if it goes all the way though... I guess I am making pockets in the ground.

We are blessed with the opportunity to do sheet metal designs because of the equipment we have at the school (most notably a waterjet and a brake)
Students are taught how to design trusses that make a part lighter but still strong enough for the application.

Material use is important and there are good lessons to be learned about this.
I guess that the major reason we can do this is because of the predominance of aviation related mentors that we have working with the students. In aircraft, weight is an incredibly important factor.