FRC pocketing
 

What is pocketing (gussets) in FRC?

Re: FRC pocketing
 

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

Re: FRC pocketing
 

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.

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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

Re: FRC pocketing
 

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

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Or a CNC and a .125 end mill, which would leave you with at least .125 radii.

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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.

Re: FRC pocketing
 

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

Re: FRC pocketing
 

affectionately known to me as "swiss cheesing"
::safety::

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

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

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

I'll let you all sleep on it, and see if you can figure out what I'm getting at.

Re: FRC pocketing
 

Or you could explain your reasoning? Wouldn't that be more helpful to inexperienced readers?

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

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!

Re: FRC pocketing
 

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).

Re: FRC pocketing
 

These posts have pictures of some good examples of pocketing.

http://www.chiefdelphi.com/forums/sh...stcount1387268
http://www.chiefdelphi.com/forums/sh...stcount1396742
http://www.chiefdelphi.com/forums/sh....php?p=1308653

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.

Re: FRC pocketing
 

or forgetting a key feature like a second carriage. D'oh! Being underweight was really helpful for 3476's design iteration.

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Let us not forget the practice of pocketing the back of bumpers to fit over protrusions in the robot frame. This practice was essentially eliminated with the FRAME PERIMETER language and "minor protrusions' limited to 1/4" entries in past rules. 2016 rules may differ, please be sure to read everything in the robot section, twice!

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And yet, each of the materials you list (and probably every reasonable FRC construction material) can produce a better load-to-weight ratio when under compression through the use of pocketing than with solid convex parts.

Tubing, angle, c-channel, and many other extrusions and sheet metal folds are simply an example of pocketing in one dimension. Monocoque is another great example of sheet metal one-dimension pocketing.

Did you ever really look at a cantilever bridge? Here's a neat example. Can you even count how many levels of pocketing appear here? (This is a 1930s railroad bridge around which another bridge was added much more recently; I grew up five miles away from it and still cross it most Sundays.)

I encountered a new construction technique for pocketing a few weeks ago. I haven't done any 3-d printing, but one of the other departments at my office 3d-prints internal parts for oceanographic data collection systems. For many of them, they use solid shells for the boundaries, but in the gaps, they trace the printer thread at what looks like about five extrusion diameters apart, and rotate between 60 and 90 degrees between layers, producing an "open foam" pattern which is much lighter but about half as strong as a solid block would be. Bird bones.

Edit:Twice? Twice? I notice something I missed before every time I read the rules (latest yesterday). I must have read the 2015 rules at least 10 times before bag and tag and probably 30 times so far.

Re: FRC pocketing
 

You make valid points about pocketing. On 254 we feel that lots of teams just pocket away with no understanding of why they're doing it, other than to look cool. Individual teams have different resources. If a team doesn't have much in the way of CNC resources, they probably shouldn't be designing a diamond patterned belly pan...or pocketed gearbox plates.

But the two points above make zero sense to me.

1) If the part is going on a CNC, the time it takes to pocket is trivial in most scenarios. Our gearbox plates would be run maybe 3-4 minutes quicker if they weren't pocketed. The time to setup the machine is the biggest sink, not the run time. I don't see what laziness even has to do with it. It requires more effort to manually mill any kind of gearbox plate than it does to CNC mill it.

2) Your comment about "buying extra material just to throw it away" doesn't make any sense whatsoever. If my gearbox plate fits inside of a 6" long piece of .25" x 6" 6061 bar stock...how am I throwing material out? I'm certainly not pocketing material that is outside of the bounding box of the part. I'm pocketing material that is inside the perimeter of the part no matter what.

I'm glad that you think pocketing isn't necessary for your team, but 9/10 years 254 would not make weight without the heavy use of strategic pocketing.

Re: FRC pocketing
 

OK, now I'll take the opposite side:
Got no CNC. Got no manual mill. Got no plasma or laser cutter. Got no router. Got no sheet metal sponsor. Pocketing to us means a drill press or jig saw, or worse. Your 3-4 minutes would take us 3-4 days or more. You're on a different plane of existence from most of FRC.

This "makes no sense whatsoever" makes no sense whatsoever. If all goes well, there never was meterial outside of the bounding box of the part. However, every bit of material that was originally inside the bounding box of the part but cut away (i.e. in a pocket) is thrown away (or hopefully recycled, especially if aluminum). I certainly hope you don't discount the material originally in the pockets from your BoM; that would have violated the last several years of rules!

As I said earlier, you're on a different level of play; birds vs mammals. The key item between pocketing as practiced by top level teams like 254 and most rookie teams is that 254 (please correct me if I'm wrong on this) planned the pocketing from the get-go and knew how much strength would remain, whereas rookie teams pocket out of desperation, without a good understanding of what is acceptable vice what is excessive. For us mid-level teams, it makes much more sense to select the proper strength members than to engineer them by buying over and pocketing down.

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Say you have a gearbox plate. Whether it was pocketed or not, it will end up on the robot as part of the gearbox. The material that is eventually pocketed was likely not going to be used elsewhere on the robot, so the fact that the material is there or not becomes irrelevant. It then becomes advantageous to just remove it in order to save weight - why not take every advantage you can get, right? I understand it's not much and for most teams there are better things to spend time on and ways to prevent this from becoming a large factor (my team did not pocket most plating on our robots), but if you can do it, might as well.

Re: FRC pocketing
 

Well you wouldn't be able to make a gearbox plate to pocket, without a mill, so this is kind of an apples and oranges comparison.

Unless you only make plates with right angles you're going to have material outside the bounding box of the part. It's a very loaded statement to say "if all goes well, you have no material outside the bounding box". Maybe that's true if your team's goal is to never make a non orthogonal part. That is not the case for many teams in FRC.

You also may consider it semantics, but "contouring" is absolutely different than pocketing and contouring is not under discussion here. Furthermore, teams can and do just bandsaw/belt sand the outer contour of their gearbox to reduce weight or provide clearance for other features of the robot.

Yes, material inside the bounding box of the part that you pocket out is thrown out...but MrForbes is making it sound like it's "extra" material that you didn't have to buy and you could have purchased a smaller piece and saved money. It's metal you already bought. It's metal that cannot be used for anything else. His statement makes absolutely no sense in that context.

Your last point is seemingly irrelevant...unless you're suggesting that you think 254 might be violating the rules. We are well aware of how the BOM works.

Re: FRC pocketing
 

I think MrForbes is talking more about using thinner, unpocketed (or minimally pocketed) material when he's talking about paying for material that is "thrown away," since you are clearly right in the context that you considered his comment. It obviously depends on the situation, but I'm sure we can agree that the time and money savings often make thinner material worth considering, especially if whatever part in question is being made on a drill press or manual mill (or even on the very low end CNC mill that my school has).

Re: FRC pocketing
 

More like cherries and pumpkins, but it's good that you understand that the situation is so different as to defy reasonable comparison.

You completely lost me on both of these points. Please elaborate.

As I read it, MrForbes' point is that you could have bought thinner material originally, and yes, saved money. The money for thicker stock would never have been spent, and this makes perfect sense in my context.

If you were referring to:
I certainly did not mean to imply that you were violating the rules, but rather to express confidence that you were not violating the rules.

Re: FRC pocketing
 

I mean, you don't NEED a mill to make a gearbox. It helps but it's certainly doable on a drill press, I've seen it done. Of course, you could pocket with a drill if you REALLY wanted to as well and just make your bot look like swiss cheese.

The big question is, if you don't have these resources and you don't know how to pocket wouldn't the resources being consumed be better placed by leveraging COTs components during the build season? [1] With the low cost of various planetary boxes and the wide range of reductions available in off the shelf spur gear boxes I question why any team that didn't make Einstein last year would need to design custom boxes [2].


I'll go back to writing code and stop nitpicking.



[1] Obviously not a question directed at you/254, more a general thought.

[2] From a functional standpoint, many of teams have the resources and knowhow so it make sense to use it to save weight/money but for teams that DON'T have resources to do it easily, use COTs.

Re: FRC pocketing
 

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.

Re: FRC pocketing
 

Selecting without knowing the engineering trade-offs and objective differences is simply guessing. Thinner material is also not the same as thicker material with pocketing/lightening/trussing/whatever you want to call it. Maintaining a larger cross-sectional area is extremely important, and pocketing can let you do some cool things to make parts stronger (in directions you care about) while also lighter than their thinner alternatives.

Re: FRC pocketing
 

Just to add to this:

The thickness of a plate has a cubic relationship to its rigidity. A "pocketed" plate of thicker material can have significantly more rigidity for the same weight as a thinner "non-pocketed" plate, depending on the thickness difference of course.

I don't really know why we are talking about making gearboxes and pocketing and then referencing lower level teams. A little insight low level teams don't make gearboxes. They use the toughbox they scavenged off a kit bot. Overweight? You don't "pocket" or "truss" you take a drill and swiss cheese until your under weight not to mention the fact that your doing this at your one and only regional's practice day.

This whole discussion doesn't make sense. Elite teams would have 200 pound robots without pocketing. Mid level teams don't pocket unless they have to because they don't yet have the resources to make it look nice and would rather focus on better material choice and low level teams swiss cheese.

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Tons of non-elite teams have access to a waterjet, plasma cutter, router, or similar (for example, my team). In that situation, it's well worth the time to remove excess material from the get-go.

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I'm not sure I would agree with "tons" of teams having access to CNC tools (at least in MN, I'm sure there are regional differences at play here), but I'd definitely agree that it's a significant number of teams, and that there's a huge difference between having access to CNC tools and being "elite".

Re: FRC pocketing
 

Hi all, just got my first pocketed gearbox plate off the mill. plz r8.

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Why do I get the feeling this wasn't made on a Haas? :P

How much does it weigh?

Re: FRC pocketing
 

Are you planning for a water game, or not?

Edit: Did you pre-treat the plate before milling it? Untreated sponges are quite pliant, and will not serve as a geabox plate, nor do they machine to tolerance. We usually temper our sponges by saturating them with warm, dirty water, and allowing them to air dry to improve stiffness. :rolleyes:

Re: FRC pocketing
 

There are times where pocketing is the best practice, and some times when you don't need it. There is a reason steel bridges are built with beams that have space between them, and not thinner-but-of-equal-mass plates. If you are optimizing for weight, your best bet is to add structure that provides support only where you need structure, and not support every load in every direction, because that would be what I would call egregious. Sure, sometimes a solid plate can be the right decision (under the right circumstances), but to say that either you should pocket everything or nothing is extremely misleading for 99% of teams.

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I see a bunch of trusses at various scales, but no pocketing jumps out at me. Am I missing something?

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http://www.harborfreight.com/34-in-2...-pc-68113.html

95% of FRC teams should have a set of these and use them often. We had a lot of 1/8th inch 2" x 1" aluminum last year that got cheesed significantly by some of our students that needed something to do.

We always try to come in under weight and adding some lightening holes only helps in a lot of areas of the robot. Cheese early, cheese often.

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That Harbor Freight set is pretty inexpensive. Does it work well? For anyone looking for a high-quality hole saw set, I've been very happy with this set.

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You should investigate using 1/16 wall tube if you frequently have this problem. It's going to be stronger than a heavily pocketed out 1/8" wall tube.

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I just bought that exact set the other day for our team. It was the most reasonable one with a 1 1/8" saw. We were hoping to use it mostly on wood, but since the teeth are HSS, I thought it had potential aluminum use. Does it work well? How many more files will we need to buy to get rid of the burrs? :-P

Re: FRC pocketing
 

They're functionally the same, though how you get there is different. There are definitely pockets on the bridge - large oval ones about a foot and a half wide and three feet long. Some (especially those on the old middle segment with the train) are really intended as access holes for riveting, but the ones on the outer tubes are just there to reduce weight. The easiest examples to see flank the vertical beam that drops from that large multi-directional joint to near the center of the white boxcar.

While we haven't done any lightening holes the past three years, we've found that cutting large holes in 1/8" or thinner aluminum (e.g. for pneumatic cylinder and bearing mounts) is easier, faster, and cleaner with a spade bit than with a hole saw. If access allows, we score the outer circle on one side, then complete the cut on the other.


Edit based on a later post: Very cool; halfway between the spade bit and the hole saw - with carbide teeth. I noticed on the Menard site that it's intended for making knockouts in steel boxes, and useful for stainless. As such, it should have an excellent lifespan on aluminum.

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I am not sure on the lifespan yet, but I have had very good success using this tool for drilling through 1/8" and 1/16" aluminum with a cordless drill. It produces a very nice hole in a very short time without the need for any cutting fluid.

http://www.menards.com/main/electric...80219544263520

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For those considering .125 wall versus .063, by the time you pocket .125 to be the same weight as .063 unpocketed it will actually be weaker.

Thin wall tubing is one of the best things for saving weight.

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I wish VEX Pro sold VersaFrame without the holes in it.

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Coast Aluminum will sell you that... for a much better price.

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Absolutely! Without the holes, there are a great number of suppliers available, many at a much lower cost. The value added of VersaFrame is in the pre-drilled holes, which should improve consistency between our practice and competition robots.

Thanks for this we are ordering one.

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I assume you are talking about the outside dimensions, and not the 0.100" thickness.

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With 0.1 or 0.040 wall thickness?

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.060" & .125"

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Totally agree - we had already purchased a lot of 1/8" thickness material and didn't have 1/16th yet. We later swapped our elevator railing to 1/16" thickness with no holes and loved it.

We pretty much used all 1/8th 2" x 1" on our drive train though and used the hole saws to take some material out. It's just too convenient for us to go any thinner on the drive since we can press bearings in as needed and not really need to worry about adding extra material.

The answer, as with everything from Harbor Freight, is it depends on the application. We cheesed some thin plate material this year with them and we also cut holes in 1/8" thick aluminum with them. With the thicker material I'd highly recommend vigorous use of cutting fluid. After about 100 holes over two robots this year we killed one of them, which isn't really surprising. For the price they do a decent job.

That's when you break out your cheap angle grinder that you also purchased at Harbor Freight :) http://www.harborfreight.com/4-12-in...der-69645.html - obviously doesn't work too well for the interior burrs but we weren't too concerned with those. Can hit those with your Dremel tool as needed.

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I thoroughly recommend purchasing one or two deburring knives. One of my favorite tools in the lab.

Not the one we have, but virtually the same
http://www.homedepot.com/p/HDX-Debur...X090/204218603

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This is the best post. I hope to see you at Flagstaff in March.:p

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I'm sure I'll find you first, assuming you guys make more awesome gearboxes! I haven't had a chance to see one in person. They look much better than my collection of sponges.