Chassis Connections
 

So, I'm on a team that does a lot of welding. And by a lot, I mean that every robot since I've been on the team has been welded- the arms, the frames and -the title of the thread- the chassis. The other three robots that our team has made have been kit frames. So, to incite your responses:

How do you keep your 'bot -but more specifically, your chassis- together?

Re: Chassis Connections
 

From as far back as any of the mentors and alumni have remembered, we've used Bosch-Rexroth's 3-way gussets and 2-way gussets to keep the chassis together. We used to be able to weld part of it but our welder got stolen 5 years or so back. Usually we don't find any problems with these and our chassis never really fails.

Re: Chassis Connections
 

Most of the robots I've had a hand in were held together with bolts--then again, most of those robots have used the kit frame.

What 1618 did in 2008 (and 2815/1398 in 2010) was use rivets. 1618 used the IFI kit frame that year and simply substituted a rivet in each place where a bolt was used originally. The result (click for the big image) was rock-solid, with no problems the entire Chesapeake Regional. (Bumpers likely aided that.)

2815 and 1398 used a custom sheetmetal frame, which naturally I can't find a good photo of this morning. (The next best thing is this photo.) We had about five 3/16" rivets on each edge of the frames, along with support from running dead axles. There was then a back plate and a belly pan, also riveted together. While I wasn't in the pits as much this year, I can't think of a single report of frame trouble across two teams and five events. And just as a bonus, we were able to rip apart both drivetrains to shake out drivetrain bugs by drilling out ten rivets and undoing three bolts holding the axles in place. (Okay, fixing the drivetrain issues didn't really seem like a bonus at the time...)

I wouldn't consider rivets an automatic decision on the frame, but they're certainly a strong choice each year.

Re: Chassis Connections
 

In the past couple of years I've developed a pretty cool method for putting our frames (chassis) together.

Basically we have our own gussets made out of ~.060" aluminum and make them in two varieties, 'T' or 'L' (this is usually done by waterjet, and we can get the parts for around ~$1/piece). The Ts have 4, 1/8" holes and are 3" long while being 2" tall. The Ls are 3 holed around 2" X 2".

We pretty much use 1" box beam for all of our framing needs. This then allows us to put a simple hole pattern of 1/8" holes 1" apart, and 1/2" from any edge to rivet our frames together. For most applications the riveted frame is strong enough as is. However, the nice thing about doing it this way is that you have created a welding jig once riveted. If you then want to go back and beef up certain or all joints, you simply just start welding. The gussets will hold the frame extremely square, especially if you put the holes in the frame members on a bridgeport or mill.

I'll try to dig up some pictures to make life easier, but in the meantime if you have any questions feel free to ask.

EDIT: Picture found




-Brando

Re: Chassis Connections
 

We welded a custom chassis in our second year, but have stuck with the kit-bot frame bolted together ever since. For us, welding the frame together ended up taking too long and was too mentor-intensive (and less student participation) than we want.

Re: Chassis Connections
 

My team has used bolt-together frames, typically plywood with angle-aluminum brackets or the kit frame. For 2010 we welded the kit frame together (saves several pounds of weight over fasteners) with some custom wheel brackets. Took 1 mentor (me) less than two hours to do at work (we had no welder at our work space). We then bolted on everything else, including some 3/8" plywood for a deck.

I would highly recommend welding the kit-frame together for the weight lost and stiffness/strength gained. It's 5052 aluminum, so it has no temper to lose through welding. If you are just welding the kit frame together it should take less than 1 hour to assemble, prep, and weld. The kit hardware holds it in place very nicely.

Re: Chassis Connections
 

Our team uses bolts. What we have is a "modular" system of cross supports and drive "modules."

http://www.chiefdelphi.com/forums/sh...ad.php?t=85712

Each of the side plates is .188 aluminum (6061 T6 I believe) despite my best efforts to use .125...

The round spacers between the side plates are half inch aluminum round stock tapped with 1/4-20 on each end and secured with 3/4 inch panheads.

The square tubes run from outside plate to outside plate and are 1/8 wall (I keep pushing for 1/16...) with 2 inch long inserts tapped one inch deep 1/4-20 and secured with 3/4 inch panheads.

We have found this system to be fairly lightweight, especially if you use your spacers for axles. In addition to weight, the outermost module can be quickly detached from the rest of the chassis by removing the bolts in square tubes. For wheel/chain repairs, you can simply remove the outermost side plate.

Re: Chassis Connections
 

we have always used the pancake design with aluminum plate. typically this takes around 150 metal screws. however when designing the robot this year we didn't really do the math and ended up with around 1400 holes. the box of screws, (2000 8-32 5/8 long button head) weighed almost 10 LBS. lock tight application for the whole robot took 5 people around 10 hours.

due to this, we are going for angle gussets with 1 inch box tube this year, hopefully it will be easier.

Re: Chassis Connections
 

We have welded our chassis all of our years as a team. For a while our rookie year we were going to use the kit chassis, so we bolted it together, welded, then removed all the bolts. I don't remember the specific weight of the bolts and supports, but it was well over 5 lbs. We ran into trouble with the kit frame warping during the welding, so we made our own, and have done so every year so far.
If you are trying to decide what to do, I would definitely weld to save some weight. The next best option (imho) are aluminum rivets. I don't know enough about them or have enough experience to give a big recomendation, but I have seen them work very well.

Re: Chassis Connections
 

I'd love to see a picture of this "pancake design" that used 10lbs of fasteners...

Re: Chassis Connections
 

This year, we used lots and lots of bolts. Our whole robot was basically various sizes of aluminum angle, so we just had angle inside angle bolted together with at least 2 bolts.

In the past, we have welded (only drivetrains), used gussets and rivets, and used 80/20 extrusion.

Re: Chassis Connections
 

Through the years, we have built a bolted chassis, a riveted chassis, a glued and nailed chassis, and a welded chassis. All have held up to the stress of 2+ regionals without issues.

You can build a chassis that is strong and light in countless ways if you put enough design work into it. The thing you need to ask yourself is how you can best utilize your resources to build a strong/light chassis as quickly as possible. The faster you can finish fabricating it during the build season, the better.

I would also encourage you to take a look at the big picture when you are designing your chassis. Robot functions change drastically from year to year and greatly influence the design of the drivetrain (hence why we have gone through so many different forms). Remember that you are designing and building a complete robot; the chassis and manipulators are not always independent systems.

Re: Chassis Connections
 

1350 uses bolts and nyloc nuts everywhere. It makes prototyping easy since we can easily build, test, and rebuild any part (easy until you find you built an integral structural or functional component directly over the bolt head you need to get to which can't be moved since EVERYTHING is bolted onto it, but thats OK, it builds dexterity). The real issue comes when everyone is fighting to use one drill bit or wrench that is the only one of its size and happens to be the correct one for EVERY bolt on the robot.

We also like zip-ties, but not so much for the chassis.:D

Re: Chassis Connections
 

This isn't strictly true. http://www.chiefdelphi.com/media/photos/24740 :P

Anyway, Shaker used a pretty simple - if non optimized - chassis with some 1/4" AL plate, standoffs, and 80/20 extrusion to connect the two wheel modules. Standoffs were made on a lathe and secured with 1/4-20 bolts (1/2 bolts for the combination standoffs and axles), and the 80-20 was fastened by tapping the extrusion's holes and bolting them onto the chassis. It weighed a fairly reasonable weight: I think a fully driving base with compressor and everything was 65-70 pounds, with no attempts at all to reduce weight. (We wanted a low CG and we were consistently underweight the entire year)



The chassis worked just fine all year, but it really isn't that well designed. Look at the cross section and you'll see it's a lot more C-shaped than we wanted. Prechargning a pneumatic kicker caused it to noticeably deflect. We also had the electronics mounted several inches higher than we needed to which took room away from a hanger. If we did it again I would take the 80-20 structure and mount it much lower on the chassis.

Re: Chassis Connections
 

2007 we used kit chassis with LOTS of loose bolts (I wasn't around then, but many alumni have memories of the robot dropping things everywhere during matches and in the pits)

2008 was a little different. We used kit chassis rails that we bent around a bunch a bunch to make a cool looking but structurally weak octagonal frame with an indent in front for the trackball, and then bolted a bunch of 80/20 up off of that. The 80/20 was quite strong, and fortunately the chassis rail didn't have any problems.

2009 (yay for the basement!) we had kitbot chassis on the bottom and then two sheets of plywood up the sides. Everything inside the bot was then bolted to this plywood, with a nice plywood roof to top it off, making for a rather sturdy setup.

2010 we welded for the first time thanks to a local prototyping company who did it for free :) . It was definitely the strongest we had ever made. We used the chassis for wheel mounting convenience and then aluminum box tube for a superstructure around that. Initially it was all riveted together, and then we sent it in to be welded, with gusset plates at the corners.

Attached are pictures from the years I could find.

Re: Chassis Connections
 

Here is an FSAE car chassis that I welded together my senior year of college:



Frame weight of 58lbs with all brackets included, welded from 4130 steel. I could imagine a FIRST chassis made in this manor weighing less than 15lbs. Just food for thought for you welding-intensive teams.

Re: Chassis Connections
 

If I recall correctly Kettering's Formula Zero team (hydrogen powered car) is using a similar chassis. Ours is a little smaller due to race restrictions though. Something tells me 1501 used welded 4130 in 2007 for their chassis.

397 has used the KOP frame the last couple years, we just bolt it together. There is something to be said about having a simple chassis together in one day. We have welding capabilities but only use them in situations that require it. No reason to over complicate things.

2337 used box tubing and some custom made blocks. These blocks were basically caps on the tube. Personally I feel this was heavy and labor intensive.

Re: Chassis Connections
 

Based on Billfred's experience and recommendation with 1/4" pop rivets, we've used them for 2 years with excellent results.

Re: Chassis Connections
 

15 pounds is a bit much for a drive chassis, we usually end up with a welded aluminum frame around 7 pounds.

Re: Chassis Connections
 

A very easy method for creating a chassis would be to use 1"x1"x1/16" aluminum tubing with Brunner Connectors. In order to secure the pieces together, we use 1/8" thick aluminum gussets with sheet metal screws.

This method allows easy modification of your chassis without having unweld or cut away pieces.

Re: Chassis Connections
 

...which is less than 15lbs last time I checked ;)

Do you have a picture of this 7lb aluminum frame?

Re: Chassis Connections
 

A 7-lb Al chassis versus a 15-lb steel chassis--that sounds about right, steel being roughly 2-3x the weight of aluminum. (And stronger, so you can use thinner wall thicknesses, further reducing weight, such that for equivalent strength, 2x is closer.)

Re: Chassis Connections
 

2009 robot, 6.9 lbs.

2010 robot, 7.2 lbs.

2010 preseason prototype, 7.0 lbs.

Weights are from memory, but I'm sure they're accurate within .5 if I go back and Check the CAD. Weights not including bellypan, which varied from 1.5-2.5 lbs each year iirc.

I didn't point out the weight difference to be harsh or brag, but to get people thinking. We don't do anything new or innovative with our frames (still ripping off Glenn Thoroughmen's fantastic designs) and we've never failed one.

Re: Chassis Connections
 

A bit off topic, but is the 2010 preseason prototype a 254/968 style with cams/diamond bearing blocks?

- Sunny

Re: Chassis Connections
 

All three posted bases are of that style, except the 09 didn't use cams.

Re: Chassis Connections
 

They look good. If you've got access to the CAD file would you mind switching the material to 4130 steel and making the wall thickness either 0.035 or 0.045? If that's easy to do I would be curious about the weight.

Re: Chassis Connections
 

That's impressive, thanks for sharing. Our first aluminum welded chassis (last year) was 12.365lbm in Inventor. This year's is 19.647, and we've bent it pretty noticeably, though it still works just fine. Do we just put more on the central weldment, or is there something else to it? Exactly what kind of stock are you using?

Re: Chassis Connections
 

Just something to think about, regarding steel vs. aluminum: steels are about 2.5 to 3 times as stiff as aluminum alloys, but both vary greatly in strength and in the ways they respond to welding.

For aluminum, all tempered varieties will exhibit reduced mechanical properties in the heat-affected zone of a weld. As a rule of thumb for aluminum, expect it to have the properties of the O temper (as in AA 6061-O) in the region of the weld, and to retain the properties of the original temper (e.g. AA 6061-T6) elsewhere. This won't change the stiffness appreciably, but will change the strength dramatically. Additionally, some varieties of aluminum need to be welded with a dissimilar filler metal that will have different (sometimes greater, sometimes lesser) properties.

In steels, this effect can vary greatly depending on the alloy being welded, the hardening processes already applied to the workpiece, the filler material being used and even the type of welding process. In the case of things like AISI 4130, AISI 4140 or other chromium-molybdenum steels, you'll actually find that they're supplied in an annealed (softest) or normalized (slightly stronger) state. That means that welding them isn't going to diminish the properties of these steels much further.

Failure of most structures can be said to occur when the part is permanently deformed (yielded)—so the yield strength is relevant. Normalized AISI 4130 has about the same yield strength (436 MPa) as tempered AA 7075-T6 aluminum (505 MPa—yes, "stronger than steel"), but if you weld them (producing something near the annealed condition), the steel will only weaken to 361 MPa, while the aluminum will drop all the way down to 105 MPa yield strength. So, if you can fasten the aluminum without welding it, choose the weld locations so they're under less stress, or in rare cases, weld it and then heat-treat the whole structure, you may elect to choose aluminum because of the weight advantage. If you need a welded structure with good strength even at the joints, then an alloy steel is probably a better choice (if you expect to see this sort of loading).

Cheap structural steel (what buildings are made of) is the worst of both worlds, however: low strength and high weight. For example, ASTM A36 is only rated for 250 MPa yield strength. (Actually, structural steels, like the old-fashioned ASTM A36, and the newer ASTM A572 and ASTM A992, have very variable properties—the minimum yield strength is specified, but the steel could actually be significantly stronger. In fact, nowadays, the majority of small-section steel shapes sold as A36 actually meet A572.) It does have low cost going for it, though.

On the other hand, if your structure isn't going to see levels of stress approaching the yield strength, maybe you're interested in designing for sufficient stiffness at minimum weight. In that case, because steels have a stiffness advantage roughly equivalent to their weight penalty (steel is about 2.75 times the density of aluminum), either one can be a good choice. At that point, if welding is possible for your team, you might as well go with the steel—you'll get similar stiffness and weight, but you'll be able to weld it rather than worry about more complicated joints. Indeed, it's rare for unwelded joints to be as stiff as welded ones, so welded steel will come out ahead, especially for complex shapes like that FSAE car pictured above. (Imagine if it was made from unwelded aluminum: as a practical matter, they would have been forced to use a stressed skin to stiffen it, or otherwise add more material. And if they'd welded it from aluminum tubes, it would have been weak at the joints.)

But what if you can handle the assembly of a stressed skin? In that case, you can probably get better results out of aluminum—sufficient stiffness by using thin webs between structural members, and sufficient strength by using high-strength aluminum structure. A steel structure constructed the same way would be infeasible, because of the difficulty of producing, fastening and maintaining paper-thin sheets of steel (compared to three-times-thicker aluminum sheets of the same stiffness and weight). (This is a big consideration with aircraft: you usually see aluminum planes, rather than steel ones. Only when there's another constraint—like air friction heating due to high-speed flight in the XB-70 and MiG-25, or cost as in some kit planes—do you see steel construction in aircraft.) As far as FIRST goes, stressed metal skins aren't typically very impact-resistant, and as a result, I'd be cautious about choosing them for the main structure of the robot. (I did have very good success in 2010 using polycarbonate sheets as the structural skin of a robot—but that was only possible because neither strength nor stiffness were major priorities in that part of the robot.)

Now, despite this talk of stiffness, sometimes it doesn't actually matter much. Have we ever known the field in Atlanta to be flat? (No, not a chance—those stupid tiles are worthless.) In that case, for some drivetrains, a bit of flexibility is actually very beneficial. (Holonomic ones that need all wheels on the ground are the best examples.) For the last few drivetrains I've designed (a 4WD wide-base and two 6WD long-base designs), we built them stiff longitudinally (so that they could maintain wheel and gearbox alignment), but flexible laterally, to account for variations in the field and to reduce the amount of structure. They weren't perfect for climbing (a crooked approach to an obstacle caused the frame to flex), but the largely open interior allowed us to keep the robot compactly packaged without worrying about structure.

Realistically, though, if you're willing to overbuild a little, you can ignore the conclusions of this sort of analysis and still make a perfectly serviceable robot. (It's probably for that reason that we see teams being successful with all sorts of designs that aren't necessarily constructed in the most technically optimal way.) Equally, different teams have different priorities when it comes to design. Some favour a rock-solid robot, others want one that can be taken apart easily. There is something to be said for getting the robot done quickly, and building it strongly enough that there's no chance the frame will need repair. That's something else that teams will have to think about—what's the tolerance for risk, and how capable are you of making a field repair to each of the possible designs?

To answer the original question: rivets. Lots of aluminum rivets, inserted into all manner of structural components. I've used aluminum extrusions, kit frame rails (sheet metal), aluminum-plastic composites, polycarbonate and high-density polyethylene as structural materials over the last couple of years. Typically, the way this works is frame rails are positioned at the outermost extents of the frame, and riveted to structural sheets on perpendicular edges. The sheets form the walls of the frame, and stiffen it somewhat, but the principal loads are borne by the frame rails themselves (to avoid drivetrain deflection).

Re: Chassis Connections
 

Last two years we have used welded frames. Mostly 6061 Al MIG welded.

Before that, we used bolts.

Reasons for the shift to welding:
1) Weight! No connecting pieces. Reduced nuts & bolts.
2) Strength - a well-welded frame is nearly as strong as the native aluminum.
3) Durability & Maintenance - Bolts work loose. Our welds no longer break. There's a learning curve here.
4) Design - As head of the Design Team, I am well aware that it is easier and faster to design a welded frame than a bolted frame. Especially if you are paying attention to detail (and the devil always hides in the detail).

The other benefit is that frequent welding makes us better welders, and yields welds less likely to fail. It also becomes easy and fast. We are now able to weld on the fly.

Downside is that when a critical weld fails, you need to work out a field fix. This can be easy. This can be damned near impossible.

Still, for us, the benefits of welding (applied to the right places) overcomes the costs and dangers.

Re: Chassis Connections
 

I would like to add that 4130, when welded with the proper filler material, can be stronger after it is welded. I developed and got AWS certified in a pulsed-TIG process for welding 4130 tube frames. My process used ER-80S-B2 wire, the 80 standing for "80ksi yield strength" in the filler, where annealed 4130 is about 75ksi (I like english units for welding as designations start to make sense: A36 = minimum yield strength of 36ksi, etc). The resulting welded material was stronger than the base material, but still ductile enough to be run through a tube-bender (and anyone who has done it can tell you that bending 4130 is very difficult).

Clem1640: watch out, your 6061T6 is becoming 6061T0 around the welded areas!

Re: Chassis Connections
 

That it is. Luckily, we're [same team] pretty good about design strength & redundant supports. We've yet to fail a critical weld in 2 years* (though we do bow some). Still, I look forward to the the day we win the lottery and can correctly age back to a better temper. ...Then again, I look forward to that winning ticket for other reasons too. Like TIG welding!


*knock on...wood.

Re: Chassis Connections
 

For the past 3 years, our team has had our chassis fabricated out of 3/16" aluminum. The sides are bent at 90 degree angles for us to mount our bearings for the gearboxes. Also, this year we had two machined pieces of aluminum as well as custom brackets for our piston system. These have been helpful for more low to the ground games and our strategies. Here is a picture of the actual chassis alone without electrical wiring and all pnuematic tubing.



I know arial views don't simply do justice but I believe you can get the picture from here. If anything, I will take a picture of the chassis we have hanging around at the school from 2 years ago. Let me know!

Re: Chassis Connections
 

Prototype Frame for 2011.

Aluminum (6061-T6) 6.1 Lbs

4130 .045" wall 9.3 Lbs

4130 .035" wall 6.7 lbs

I'd like to point out that a substantial amount of the "weight saved" was in changing the .125" wall 6061 for our siderails to the .045/.035, which would be very impractical for us to do without a redesign of our bearing block system (which would just be more work for a net gain in weight). It's convenient to have that .125 wall. For reference, this is generally our only use of .125 wall on the robot.

I'd also like to point out that for our particular team, switching to a welded 4130 frame would take a significantly longer time to machine and then weld, along with costing us more (in the short term, as we don't stock steel or any filler rod, etc...).

Also, I hear it mentioned all the time that welded reduces the temper of metal. This is true, I do not disagree one bit. I just want to throw a counterpoint out for the inexperienced people reading who might otherwise assume "Welding aluminum is AWFUL! it turns to butter!" (I know I did a few years ago). The weakened material will be in your welded joints obviously, and with a good frame design these won't be loaded to failure. We've never broken a weld in a driveframe before.

I encourage everyone reading who is re-evaluating their frames for steel, to also do so for aluminum; I'm not saying one is better than the other, but if you analyze the steel and fail to do the same effort into aluminum, you aren't really proving anything. For some teams one will be better, and for others, the other; do the math, evaluate your resources, and make a decision.

Re: Chassis Connections
 

I couldn't agree more with this statement.

Often when we're making engineering decisions, we end up choosing between 2 or 3 options and weigh the benefits. Sometimes when making these decisions we all have a tendency to declare one a "safe" bet or automatically declare it as valid, while trying to prove an alternative as feasible. While this is sometimes a good way to explore your options, its important to keep an open mind. Keep all of doors open, and gradually close them one at a time. Closing each door can be based on a variety of circumstances, just make sure you're choosing what works best for your particular situation.

-Brando

Re: Chassis Connections
 

Thanks for the response Adam.

I would add that although steel is harder to machine, the thinner wall thicknesses mean that there is less material to remove, so the machining/cutting times tend to balance out. Using abrasive tools might even expedite cutting time.

I agree that every option should be explored when a team is making a design decision. I remember when I thought "steel? that's too heavy" and dismissed it, oh how wrong I was.

Re: Chassis Connections
 

Sorry for reviving this thread, but I was wondering if there was a major difference between the 6063 Al and 6061 Al tubing? Especially in regards to welding?

Re: Chassis Connections
 

Sorry I can't answer that question seeing how I have little experience in welding (which the most experience I've gotten is Acetylene and Oxygen).

But speaking for 1501 we use aluminum sheet metal and rivets to make most every robot using an engineering style known as monocoque (used in airplanes, vehicles, spacecraft, race cars). A simple effective style requiring primarily hand tools and few larger pieces of equipment such as a bench press.

In 2007 AND 2008 both used welded 4130 Chrome molly. 2008 we used the Chrome Molly as a track for our "slingshot" launching method. We welded tabs on the Chrome Molly frame to attach our monocoque pieces on those 2 robots.

Re: Chassis Connections
 

There isn't an appreciable difference in the welding process for 6063 as compared to 6061, use your 4043 wire, 50Ar/50He shielding gas, and lanthanated tungsten, and you'll be fine. There also isn't much difference in the as-welded strengths of 6063 and 6061. I believe the main difference between the two alloys is that 6063 does not gain as much strength through heat-treatment, but is more resistant to corrosion.

It's chro-moly (sounds like: "crow+molly") abbreviations of the two main alloying elements: chromium and molybdenum. Just a pet peeve :eek: don't take it personally ;)

Re: Chassis Connections
 

I did a double check online last night and that was how they were calling it. I prefer to stick with calling it Chromium Molybdenum. No offense taken.