Tube Versus Channel
 

As a relatively new member to the FRC community (this is my second year in the competition) I have been very closely looking at other teams' robots, as seeing how different teams design is rather interesting to me. While doing this, both online and at regional competitions, I have noticed several things about frames, which have gotten me thinking about robot design in the future, as my team is likely to design some sort of showbot over the summer for outreach, as well as participating in future FRC seasons.

The main thing that I have been wondering about is structural materials. Namely, I've noticed several distinct types:

a) Channel

b) Tube

c) 80/20 or other extrusions

Noticing these differences lead me to wonder:

Why do teams choose these different materials?

I have several theories on the benefits of each.

For channel, it may be for convenience, as, if I remember correctly, 1" channel is the typical kitbot construction. Additionally, this isn't an all together structurally unsound material. This also may be perceived as cutting weight. It also only requires one hole to be drilled through the material to bolt something to it.

For tube, some possible reasons may be that it is more structurally robust than channel. Also, there is more area to mount onto, which may be a benefit.

For 80/20, possible reasons may be the ease of prototyping with this material, as well as the excellent structural integrity it affords.

I am interested as to why different teams choose these building materials, as well as what benefits and disadvantages are seen in them. If any other team members could offer some advice to this humble sophomore, I would be much appreciative.

If anyone has already posted a thread to this effect (I didn't see one when I searched, but I may have missed it), I would also be perfectly grateful with a link to that thread.

Also, this is specifically for metal robots. I am aware that some teams make wooden or plastic robots, but as these are rather uncommon, I would appreciate it if any discussion focused on materials for metal robots.

Re: Tube Versus Channel
 

I think your hands down favorite/best material to construct a robot out of is waterjetted and bent aluminum sheet metal, since you can design it to the exact requirements you need, given you an optimum strength to weight ratio.

Lacking the fabrication support necessary for the above, 57 has settled on the kit frame with a chassis of 1"x1"x1/16" wall square aluminum tube, mostly held together with more 1/16" sheet plates riveted with 3/16" aluminum structural rivets. It's pretty lightweight, plenty strong if you're not dropping your robot off a cliff, and we can get 20' sticks of the stuff for $6$20 from a local supplier, even on a Saturday. Which is harder to say about AM channel or 80/20 or another T-slot extruded aluminum.

Thick wall AM style channel is heavier and tougher than we need for any framing purposes. Thin wall channel would be flimsier than tube, if lighter. Either of the u-channel options means you have an entire side that can be a pain to attach things to.

80/20 style extruded aluminum is good for prototyping and quickly adjusting parts, etc. Only problem is that it's heavier than the 1/16" wall tubing that we use, for no more benefit than the quick propotyping.

Re: Tube Versus Channel
 

Our frame is all aluminum tubing welded together. 1x1 1/16 wall for most of the upper structure, 1x2 1/16 wall for the base. As well as 1x2 1/8 wall for the base side beams that have the DTs mounted.

We do all of the welding in house and find that a welded tube frame is light and very stiff if done correctly.

It also gives us flexibility of the frame geometry and how we mount other components.

Re: Tube Versus Channel
 

Interesting... We use 1/8" wall square tube on a kitbot frame, and every year it seems like we bend or break at least one piece of it tubing -- making us leery of going any thinner. (Also, there's no such thing as a "local supplier" in Naples, NY... Unless you need a cow, STAT.)

Re: Tube Versus Channel
 

We used to go with all 1/8" wall as well. We eventually tired of being constantly overweight with frames that took up a distressingly large percentage of our weight allowance. Yes, they would probably fare well in a head-on collision with a small motorcycle, but that's obviously a bit overdesigned for most FRC games. Our biggest problem was that we usually ended up using the 1/8" in various locations for motor mounting bars, etc. Places that didn't need anything near as strong as 1/8".

So, we build of the kit frame, and figure on that taking any big impact loads. Anywhere we're not expecting to get hit gets the 1/16", and we look at using 1/8" if we're likely to be hit hard and often somewhere.


Although, this reminds me of the single biggest problem of using 1/16" wall tubing. If you're mounting something by bolting through the tubing, you need to be cautious about over-tightening your bolt, or you'll squish and deform your tube distressingly easily. We're constantly reining in the gorillas on the team that have a bad habit of severely over-tightening bolts and such.

Re: Tube Versus Channel
 

That's another fun problem with using tube, you need to plan your attaching points pretty early on. Welding (or riveting) a mounting tab to the tube is a good way to go.

We built a welded steel frame using mostly thinwall square tube and some angle and strap for this year's robot...no problems at all with the frame.

Re: Tube Versus Channel
 

You can either drill a large hole on one side of the tube so that you're only bolting through 1 face, you can use large washers, be careful with your tightening, some teams have had success with rivnuts, or you could try to find some other way to mount to the tube entirely (instead of bolting).

Re: Tube Versus Channel
 

(Last thread derailing, I swear)
Rivnuts are definitely cool and we've used them before. Did I mention we have gorillas on our team? Last year they cross threaded a bumper attachment screw in a rivnut, then spun the rivnut. We missed a match getting that bumper off and replacing that rivnut. So we're running reversible bumpers now and are a little more cautious of rivnuts.

Re: Tube Versus Channel
 

It appears like most teams use welded aluminum extrusion, if not using the kitbot frame. The most common sizes appear to be 2x1x1/8 on the base, and 1x1x1/16 on the superstructure.

My team has took the rather unusual step of using rivets last year to assemble the frame. We used almost exclusively 1/16" aluminium tubing, with 1/8" gusset plates that riveted the whole thing together. A variety of sizes of rivets were used, from 1/8" aluminium to the huge 1/4" steel. The rivets were a weight saving measure after trying to use steel bolts.

I would recommend rivets to a team with an accurate mill and a desire to save weight. With an accurate mill, the gussets and the frame members can be made so the frame is perfectly straight (we have had some problems with crooked welded frames in the past). Also, rivets allowed us to use almost exclusively 1/16" tubing, which saves weight, but is near impossible to weld. I estimated that this saved us 5-8lbs.

If you do decide to rivet (or to bolt and then are forced to go to rivets), keep in mind that you have to size the rivet for how much material it is gripping. Not just any size will do. Also, 3/16" rivets work in #10 holes.

Re: Tube Versus Channel
 

Although they aren't very well known, rivnuts can make for a very neat solution in my experience. When I went around asking for rivnuts at SVR, people looked at me like I was crazy. I'm glad to hear someone has heard of them.

Re: Tube Versus Channel
 

We also like aluminum tube. Really, the best thing to use is 1 x 2 aluminum tube, welded. 1/8 in thick for drive base, and basically 1/16 for everything else. If used correctly it is perfect. Minimize the amount of frame material you need. Look at any 254 robot, and you will see how simple the frame actually is. It makes for lighter robots, and you really don't need anything more. Also, 1/4 in, waterjetted, aluminum plate goes very nicely with this. It makes building 20 times easier, and reduces the amount of tubing you need. Plus, you can make custom gearboxes. Really, it pays off to get a 2D sponsor, as they can turnaround parts amazingly quickly.

On the subject of sheetmetal, while a lot of good teams use it, there are plenty of good robots built out of tube, as long as you streamline the design process(254 again comes to mind). The nice thing is that the robot comes together lighter, more quickly, and more accurately than an aluminum tube robot. On the other hand, you need to have a good team of CADders, and have the sponsor to be able to do it.

Re: Tube Versus Channel
 

2 questions.

1) Is your 1/4" plate .25", or is it like ours and ranges from .22" to .24"? That made our gearboxes this year a little difficult to get fitting correctly (if you've seen the gearboxes, you know what I mean)
2) Besides custom gearboxes, how else do you use it in your frame? We have a lot more frame right now than I'd like, but I can't see how 1/4" plate would make it significantly better.

Re: Tube Versus Channel
 

If your 1/4 plate is .22 to .24, you ordered a gauge thickness rather than 1/4 plate.

1/8x1 tube is virtually the same weight as 1" 80/20.

This year we used a lot of 1/8x1 angle. Half the weight of the equivalent channel & allows bolts to be properly torqued without crushing the section.

We use mostly 1/4-20 bolts. A little heavier than other options, but you don't have to worry about the inexperienced over torquing them. Carroll Smith has a great book about fasteners "Nuts, Bolts, Fasteners and Plumbing Handbook"

It is all about the details & trade offs.

Re: Tube Versus Channel
 

Another semi-endorsement for rivnuts. They are really cool things, in theory. However, beware of using in 1/16th wall stock -- very easy to spin them, and then you have a mess. Installation is also an art! Too much force, thinking to make a better seat, tends to mess up the threads, especially in aforementioned 1/16th wall. Still, they're pretty neat and tidy when done well!

Re: Tube Versus Channel
 

1 question that I have that I have not seen mentioned is price. What is the cost of the tube versus the kit bot c-channel, which is included with the KOP. If it is expensive, I don't see much bonos to it over C-Channel, which works fine for most uses (at least for 3397). We used it on our chassis and tower and electronics mount. It is light and pretty durable. So, unless it has a VERY good benefit, I do not see the point of buying the tube.

Re: Tube Versus Channel
 

Do you have any advice or books/white papers/etc to recommend concerning designing for 1/16" tube? We're pretty much all 1/8" tube or angle for everything on our bots. I've heard great things about 1/16", but we've warped welded 1/8" chassis occasionally so there's some hesitancy.

Price:
From what I've found it's really very, very location-based. We used to buy everything from McMaster until we discovered the joy of local surplus stores. If you think you might be missing something, introduce yourself to a local machining company or two (or ten). They'll know. Different profiles definitely offer a lot more design flexibility. Certainly I'd think 1/8" C-channel on the tower is over-designed. Maybe angle if you want to stay with 1/8".

Re: Tube Versus Channel
 

The chassis could use 1/8", but on stuff that doesn't need to be that thick, like the upper structure, 1/16" is usually fine. But it depends on how you design it! We like making upper stuff from 1/16" tube and riveting 1/16" sheet or angle brackets to hold it together, and attach stuff to it.

It's all in the design....

Re: Tube Versus Channel
 

We use almost all 1"x1" square tube. In some places we use 1"x2", but very little of it.

If you look at our 2012 robot, the lower level of the frame is 1/8" wall, mainly because we have our drive wheel bolts mounted through it. The front and rear pieces where the shooter mounts are 1/8" wall. All of the rest is 1/16".

The robot frame is almost all welded by students. They practice a lot in the fall with welding scraps of the 1/16" wall to get good at it. It is easy to burn through.

Re: Tube Versus Channel
 

I was about to write about 2815's construction methods of recent years, but this really is what we use--the last two robots (with three blue banners between them) have been constructed of 1"x1" square tubing acquired from the Lowe's near where I work (which is not as cheap as Kevin's aluminum--I want your source!), attached with gussets of various sizes and shapes (we use some angle as well) with rivets and attached to the AndyMark C-Base kit frame. For all the nail-biter moments these two robots have given us, the structure has been rock solid.

It's been many years since I worked with 80/20 (and then as part of an elevator rather than a frame); the stuff can definitely get heavy. If I were building a sliding mechanism, or a part that I knew would require a lot of quick adjustments, I would consider it...but I doubt I'd want to build a whole robot out of it.

We build our structures so the 1/16" tubing is inboard--the only exception was our original roller claw in 2011 (which broke when our drivers smacked it into the player station glass at speed). From there, it's just been judicious application of 1/8" rivets (sometimes by overzealous freshmen) to hold it together. If you get to St. Louis, look us up--there's no secret sauce to our framing setup, and we'd be happy to show it.

We haven't tried welding any of our parts--call it a bit of paranoia about breaking parts in the heat of competition. Granted, we could duct tape it these days...but the idea of just throwing rivets into a new piece of metal has its appeal.

Re: Tube Versus Channel
 

Where to buy? 80/20 has other shapes. Price is better than local hardware store. Most places have metal supply companies where you will get the best price. They typically have large minimums so you need to buy in bulk. If you explain what are doing they will often waive the minimum charge, but you will still need to buy all you need at once.

Re: Tube Versus Channel
 

Erm. Well I seem to have underestimated that price slightly. I'm told the 20' lengths were around $20. I'm obviously only nominally in charge of the robot's budget, heh. Our local source is SSS-Steel. You mileage may vary with a local ferrous/non-ferrous supplier. I'll admit the Port of Houston gives us a bit of an advantage on raw material prices like this. Still, if you can find yourself an actual bulk metal supplier that's local to you, you're probably going to get better prices (and better material) than you will at Lowe's. Just do a google map search for "aluminum" or "non-ferrous metal" near you. For instance, Billfred might give a call to Loxcreen or Charleston Aluminum (who apparently has 2024 1" x .062" sq tube in stock. I'm envious.). If they won't sell in small lots to you or only sell to businesses, they might still be able to point you to a local distributor that can help you. Just think twice about buying from Metal Supermarkets. They specialize in selling small cut lengths to whoever, and they have a markup to prove it.

At any rate, comparing costs of my 1/16 wall tube to an AM C-Channel, you're talking $20 to $18+shipping. But you're getting 20' of tube to 3' of channel. And I can get more tube on a single day notice. So yeah, I think there's significant advantages there.

In other news, while we're getting pretty good at the 1/16 tube and plates and 3/16 rivets construction, I'm always looking to simplify things. Especially if it means I don't have to shear and break a bunch of corner brackets at work in my copious free time. So has anyone looked at or tried these tubing connectors yet? I swear I saw a team using something like this for their tube based frame, so I started hunting. They look a bit heavier than the plates we're using, but I think things would come out straighter and less complicated. And you could assemble your frame and test things before you put a screw or rivet into the connector to secure it.

Re: Tube Versus Channel
 

wow, those are big rivets. We usually use 1/8 or 5/32" rivets on thin aluminum. We save the big ones for heavy stuff.

Re: Tube Versus Channel
 

We use aluminum interlock structural rivets. They only go down to 3/16". They're probably overkill, but they have good clamp-up and hole filling. And Allen Gregory from 3847 pointed out that they use the same drill as a #10-24 clearance hole.... So if you standardize on 3/16 rivets and #10-24 screws, you only need one drill bit for a lot of your construction. 3847 is doing that, and it sounds like a nifty idea to me.

Re: Tube Versus Channel
 

We love the 3/16 rivets and have also started standardizing on 10/32 after doing 1/4-20 for years.

We also use hysol for the gussets we don't plan on removing.

Our robots are also mostly constructed from 1x1 tube in both 1/16 and 1/8 thicknesses.

1/8 c-channel is also a great choice for many parts.

I love prototyping with 8020 but we rarely use it for anything other than pneumatic supports that need adjustments.

Re: Tube Versus Channel
 

Jumped over to their site. I admit the connectors look good for quick assembly, perhaps rivaling 80/20 in quickness, if you have a bunch of pre-cut "standard" length tubes. building a frame would be quick and you could set your tube pieces up to give useful outside dimensions for the assemblies.

No dimensions given for the connectors - but I infer from the tubing they're selling that there might be a problem with 1/16 1" tube. They list wall thickness for their tube as .065", so the connectors must fit inside there, snugly? Their other option has walls .060 and listed as "slip fit" for things you want to disassemble easily. This makes one wonder how the connectors will fare in the .0625" walls we're thinking about here. Loose-ish but not slippery? I guess I'd have to buy a few for a sample frame to get some hands-on trials.

Esto has quite a few tube profiles with flanges and such. Most are anodized since their target is the construction of static displays and machine guarding frames. They are also rounded edges for a more friendly handling feel.

Re: Tube Versus Channel
 

Precut standard length tubes? I should introduce you to my new best friend, the Diablo Non-Ferrous Circular Saw Blade. Toss this into your favorite 10" miter/chopsaw and you'll be amazed how fast you can put out tube for frames. I CADed the frame for our shooter, wrote out a cut list, and we had all the tube parts cut and ready to go in less than an hour. With accurate bevels and clean square edges. I can't tell you how happy I am the I noticed it on a Lowe's trip a few years ago. It makes it easier to work with just about any long aluminum on the bot. In fact, I think it's so nifty, I'm doing this:

Teams, if you do any significant manual work with long aluminum tube, channel, the kitframe, pretty much anything aluminum, then you need to buy this Diablo Non-Ferrous Circular Saw Blade and a half-decent chop saw like this Hitachi 10" Compound Miter Saw. You will be amazed how much faster and better things go vs. your current vertical/horizontal bandsaw, hacksaw, or dull butter knife. You'll wonder how you ever managed without it.

Ahem. And now that that's out of my system... Yes, I'm a little concerned about the fit of those connectors in various aluminum square tubing. I think I'll be lucky enough to have it work for me, and my terribly cheap square tubing from SSS-Steel just so happens to be nominally 0.065" wall. So I should be fine, but others mileage may vary.

Also, 8020 actually has a similar line of tubing and connectors marketed as their Quick Frame series. No clue if it's better or worse, but it looks like it mates up pretty easy with the standard 8020 profiles for teams that want to mix and match.

Re: Tube Versus Channel
 

If you have a local aluminum supplier, just tell them that you'd like to purchase any 1"x2"x1/8" or 1/16" wall aluminum that they have as scrap. You can cut your costs from about $3.25 per foot to $.40 per foot. Work it year round and you'll have plenty of material come build season.

Re: Tube Versus Channel
 

We used 3*1.5 inch tubing with .125 thick walls and a special hole pattern cut into it this year. 3/16 rivets and grade A 10-32 Button-heads are our fasteners of choice. This year the drive frame weighed 10lbs, and the entire frame weighed less than 16lbs. If you have the machining resources to do it, I would look at perforated tubes as a construction method, I cannot think of a way to make a lighter frame. For some examples check out team 40 or 111's robot from 2011

Re: Tube Versus Channel
 

Of course, that's the part we're trying to figure out! ;) We'll keep at it. And (Billfred) if we make it to Worlds we'll definitely look up 2815. Thank you.


Is the circular saw blade really that much faster than a horizontal? That's impressive. I may make the investment.

Re: Tube Versus Channel
 

Yes. Worth every single penny!

Re: Tube Versus Channel
 

We used 3 x 4 x 1/8 rectangular aluminum tube for side rails with wheels and chain runs mostly inside the tube. It wasn't easy to find but Metals Depot (metalsdepot.com) stock it and ship to Canada with no problem. By the time we finished with the tube there wasn't a whole lot left so it was light and very strong. Cross members were 1 x 2 x 1/8 rectangular tube with loads of lightening holes. The chassis is tig welded. 1 x 1 x .090 square tube was used for the superstructure. Here's a photo.

http://www.othsrobotics.ca/images/isabella.jpg

Rivnuts are great but do strip out easily if installed by the inexperienced. They also don't allow for two pieces to be tight together as there's a flange on the nut. Rivnut kits are available from Princess Auto for about $35 (http://www.princessauto.com/pal/prod...ting-Tool-Kit). That gives you the tool and then you can buy more rivets from McMaster Carr. Unfortunately the kit doesn't include 10-32. Making a 10-32 mandrel was straightforward once we bought a 10-32 LH die and much cheaper than buying a kit from McMaster Carr.

Re: Tube Versus Channel
 

It IS that much faster cutting aluminum extrusions. I'd used it on solid aluminum, and it's probably about equal on the cutting speed. But even then, you're saving time on post-processing. The circular saw leaves a cut that's much flatter and smoother, with deburring only necessary to remove the occasional thin burr and round off those sharp cut edges. Even past the processing speed, though, is the fact that it's a compound miter saw. If you maintain it and keep your fences square and in-line, you can pretty accurately cut whatever angle you want.

It's also a lot easier to get cuts to the exact length you want on this saw. Cut your part 1/16" longer and measure it. If it's right, stop. Otherwise, with the saw blade down and off, slide the piece over till it touches the flank of the blade. Hold your part, pull the blade up, turn it on and run it down and you've just taken off the tiny distance between the the flank of the saw blade and the outside of the kerf. It's something like 1/32" or less, so with a little extra effort, your nice square cut tube will be within 1/32" of where you want it.

The only downside is that a standard woodworking miter saw will only be good for cutting aluminum and your kerf is definitely larger than on a bandsaw. But it's still only 0.094", so unless you're cutting a heck of a lot of 0.5" pieces or something, you're not going to notice or care.

Re: Tube Versus Channel
 

I was thinking of the pre-cuts as basis for prototyping frames of superstructure. We use a primarily wood-shop, so prototypes tend to get made of scrap lumber. Cutting and fastening them together securely seems to suffer for the focus on getting right to trying the mechanism. If a reasonably secure framework can be made quickly, then maybe the ideas won't get lost because the prototype was too flimsy to tell if the idea were any good.

Sorry, that seems cloudy even to me. Just think making a sloppy pile of crates to get to the cookie jar. More attention to solidity means better validation of the demonstrated principle.

Re: Tube Versus Channel
 

Our 1/4 plate is .250 ± .001. We got it from McMaster, because we actually had to get it CNC milled by our machine shop. We hadn't tried to get a 2D sponsor before the season, but actually right after the end of build season, we got industrial Metal Supply to sponsor us, and they have a laser cutter that we will be able to use for the offseason and any future games. This is just one of the laser cutters that they have available: http://www.youtube.com/watch?v=-k8JGL8Muq4

We use the 1/4 plate for our ground pickup, shooter hood, and elevator gearbox, which we also made custom. This helped to simplify our frame quite a bit. What I was really talking about though, is it seemed like a lot of teams have way more complicated frames than needed. I like using 1/4 plate, wherever it seems like it would be beneficial to use sheet metal, as it has a lot of similar capabilities, and it adapts to whatever you need it for very well.

Re: Tube Versus Channel
 

Most of our machines are made from 1x1x.063 aluminum tube and 1/8" polycarbonate gussets that are fastened together with 1/4" rivets.

1/4" rivets are definitely overkill, but they make prototyping with 1/4" hardware and swapping in rivets at final assembly a cinch and our laser cutter doesn't always do such a great job when cutting smaller holes into some thicker parts -- a #10 clearance hole in 1/2" material gets pretty oblong by the end.

This year's drive pods were built from aluminum standoffs and 1/4" delrin plate; I was a bit worried about how well the plastic would handle side-loading, but it's been a champ so far.

With respect to tube vs. channel -- in almost all cases, I'd use tube if I could. Tube is much stiffer than channel in torsion.