FRC pocketing

[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.

[GeeTwo]

Originally Posted by 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.

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:

Originally Posted by Al Skierkiewicz

2016 rules may differ, please be sure to read everything in the robot section, twice!

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.

[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.

[Ryan Dognaux]

Originally Posted by Cory

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.

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.

Originally Posted by Sanddrag

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.

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.

Originally Posted by Michael Hill

Does it work well? How many more files will we need to buy to get rid of the burrs?

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.

[Michael Hill]

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.

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

[GeeTwo]

Originally Posted by Alan Anderson

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

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.

Originally Posted by Ryan Dognaux

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

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:

Originally Posted by Everett33

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

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.