FRC pocketing

[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.

[Travis Covington]

Originally Posted by GeeTwo

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.

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.

[Deke]

Originally Posted by Bob Steele

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.

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.

[Sperkowsky]

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.

[Basel A]

Originally Posted by Sperkowsky

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.

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.

[cadandcookies]

Originally Posted by Basel A

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.

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".

[s_forbes]

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

[asid61]

Originally Posted by s_forbes

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

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

[mman1506]

Originally Posted by s_forbes

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

How much does it weigh?

[GeeTwo]

Originally Posted by s_forbes

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

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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.

[pmangels17]

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.

[Alan Anderson]

Originally Posted by GeeTwo

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?

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

[Ryan Dognaux]

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.

[sanddrag]

Originally Posted by Ryan Dognaux

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.

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.

[Cory]

Originally Posted by Ryan Dognaux

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.

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.