Sheet Metal Fabrication vs. Using Channel
 

I have seen both used before, I just want to know if one is in any way better than the other. Or just the benefits of using one over the other.

Re: Sheet Metal Fabrication vs. Using Channel
 

We've used sheet metal for a couple small parts.
It lets you cheesehole like crazy to get rid of material that doesn't really help strength anyway if a sponsor does it. Channel is faster to source though.

Re: Sheet Metal Fabrication vs. Using Channel
 

It really is dependent on many factors: resources, time, designing capabilities, experience, etc. WildStang has recently worked with two sheet metal manufactures to make parts for our robots. This year, our bot was almost entirely sheet metal, apart from most parts on our lift. Channel is useful for many things, you just need to use it right. I'm not going to get into much detail about this, or start a war. You may want to look at older threads that explain pros and cons of both.

Re: Sheet Metal Fabrication vs. Using Channel
 

The best way is using the resources at your disposal. Both require different design methodology, but both will get the job done.

Re: Sheet Metal Fabrication vs. Using Channel
 

For the last 6 years our drive train was c-channel. We have done one and two level chassises for complex designs.
The channel is quick to put together snap together, easy to mount to(just need to make spacers for the inside so it clod rant get pinched), and not all to heavy.(can always still wide holes to take out weight)

We may switch to sheet metal or tube design chassis next year if we can get access to the cnc machine.

Re: Sheet Metal Fabrication vs. Using Channel
 

The biggest difference is that most teams using C-channel or other extrusions are typically doing most of the individual part fabrication inhouse, where most teams that do primarily sheet metal construction have their parts produced in partnership with an external sponsor. This is a very different approach to fabrication, and it works very well with certain resource sets (and not well at all with others). I wrote at length about the process, and why our team uses it, here.

Re: Sheet Metal Fabrication vs. Using Channel
 

Your post that you linked is really great. On a similar note, how do you get your students interested in CAD? It's one thing that I think could help us greatly but no one wants to implement it.

Re: Sheet Metal Fabrication vs. Using Channel
 

A 3D printer really helps here. There is just something about drawing something in CAD, then holding it in your hand a short time later.

Do you do large scale 3-D printing? Or just small stuff? I know of a team that 3-D printed a whole chassis with carbon fiber and fiberglass infused filament on an 8 foot by 8 foot printer platform.

Re: Sheet Metal Fabrication vs. Using Channel
 

Do you know what team that was? Because that sounds awesome and I want to figure out how they managed to get the resources for that haha.

Re: Sheet Metal Fabrication vs. Using Channel
 

The team is 3824. They discuss the process in this thread. Here are their robots from 2012 and 2013. They are really awesome.

Re: Sheet Metal Fabrication vs. Using Channel
 

Sheet metal frames allow you have a much lighter but equally strong chassy/robot compared to slotted tube, however sheet metal is harder to fabricate (bending it correctly) and can be dented easer, though i would definitely use sheet metal if i ever get the chance.

also note that by making the base out of sheet metal you have also move the robot/s center of mass much higher, thus making it more flip prone.

Re: Sheet Metal Fabrication vs. Using Channel
 

Why would you have to move the COM higher?

Re: Sheet Metal Fabrication vs. Using Channel
 

when reducing weight of your foundation you cause it's COM to be to a heavier side, so if i take weight off the bottom and add it to the top i now have a pendulum

it's due to that fact that your weight lemit is 120 so by reduce your chassie weight by 20 pound and now you added it to the top of the robot, you will be more likely to have more mass at the top of the robot and is more likely to tip/flip

if this needs further explanation I'll reply when it's not 2:14 AM lol :)

Re: Sheet Metal Fabrication vs. Using Channel
 

I can only see this being a problem if you have a tall robot (above 60" or so)

Re: Sheet Metal Fabrication vs. Using Channel
 

All he is saying is that if you take weight from the chassis and put it into a system that is even half an inch higher, you have raised your COM. Depending on your robots design, this may not be an issue.

Re: Sheet Metal Fabrication vs. Using Channel
 

Oh, okay.

Re: Sheet Metal Fabrication vs. Using Channel
 

It would depend on the game more or less. Sheet metal allows much more flexibility in shape to allow the robot to better fit the game piece(s). If it's a game heavy in defense, I would want the sturdiest bot on the field to withstand pushes of other bots and push others with ease, not having to worry about any weak points whatsoever.

Re: Sheet Metal Fabrication vs. Using Channel
 

:rolleyes: Seeing lots of generalizations here such as "sheet metal is lighter than tube" and "sheet metal is weaker than tube" ect. ect.

Both sheet metal and tube's weight are totally dependant on how you use them.

Re: Sheet Metal Fabrication vs. Using Channel
 

Well, yes, but you could just lose the weight completely and not even put it into a different system. You would just end up with a lighter robot in general.

Re: Sheet Metal Fabrication vs. Using Channel
 

...and then, if you are concerned about being at maximum weight (perhaps you want to maximize traction) you have the ability to put additional weight exactly where you want it to create the best result.

Re: Sheet Metal Fabrication vs. Using Channel
 

I'll challenge you on that.

The vast majority of sheet metal is done with 5052-H32 alloy aluminum due to how easy it is to bend. This ease of bending comes at a strength cost. (Matweb reports 13 ksi for 5052 and 40 ksi for 6061-T6). More material is then required to make the sheet metal design strong. I'd be very surprised if you could make a sheet metal drive base that is as strong and light as a well designed tube stock drive base (and I've designed a number of sheet bases).

Re: Sheet Metal Fabrication vs. Using Channel
 

What about milling/laser cutting lightening patterns into channel stock?

Re: Sheet Metal Fabrication vs. Using Channel
 

Completely doable. When I was on 2220, we did waterjet triangle cutouts on 4" channel in 2011, circular cutouts in 2012, and then we switched to tube for 2013 and 2014, which had rounded rectangular cutouts. Only had strength issues in 2012, and that was because of other stupid design choices. 2014 we had some of the most aggressive drive train lightening I've ever had manufactured and we had precisely zero issues with our drive train.

Re: Sheet Metal Fabrication vs. Using Channel
 

Channel and angle are certainly accessible to more teams; We don't have any real ability to work sheet metal efficiently and effectively.

Most of the places that I have seen sheet metal used effectively that I could not even imagine how to do with angle/channel (short of a whole lot of welding, and reducing strength) were in the form of manipulators, for example some relatively flat claws with rollers between the two sheets such as were used by several ball pickups for Aerial Assist and a number of RC grabbers for Recycle Rush, and long arms with lots of lightening that looked almost like cantilever bridges until you got a bit closer.

Most of the chasses based on sheet metal appear to be executable in angle or channel extrusion for not a whole lot of additional weight, though I will admit that these are appearances, and quite possibly deceiving.

Re: Sheet Metal Fabrication vs. Using Channel
 

NOTE: the COM in my profile picture

I accept you challenge.

you may be correct in the pressure needed to distort the metal (I don't have SolidWorks on this computer so i can't check) though this is when the metal is still in it's flatted form, my Dad is a civil engineer and I've asked him on shapes that would bare loads of weight, if you ever see a 'I' beam holding up a floor or roof you can see that vary little metal is required to hold a lot of weight, this is due to the form it had ( |-| ) <-- shape of an I-Beam) The flat side of this beam give the middle layer it's required strange as it distributes the force being applied to it, the same goes for sheetmetal,


if i have my flat piece of metal and apply said amount of force what i would get would be a traditional bend, how ever if i took that flat piece of metal and Bent bother sides so that it was making an ( [ ) form, and now tried to bend it with the same amount of force, you would see that the metal would no longer bend due to the extra support given off by the two linear walls, this is why support beams are in shapes of U's, I's, T's and L's, also triangles (but thats a completely different level of supports) if i remember correctly 9 so don't take this last part as fact... i believe it requires the for needed to bend the flat plate plus the force required to bend the two walls hight wise. all in all it is definitely a much more rigid way of making a robot base And this time it's 3:27 AM!!

Re: Sheet Metal Fabrication vs. Using Channel
 

The force required to deflect a cantilevered beam a distance X will be given by the formula 3EIX/L^3, where:

• E is the material's modulus of elasticity, which describes the degree to which a material will deform under load.
• I is the beam's cross sectional moment of inertia, which is the inertial resistance to rotation generated by the beam's shape, and in this case, accounts for the bending resistance inherit to the profile of the beam
• L is the distance between the rigid support and the application of load.

This same basic principle also applies to extrusions. Extruded frames also give the advantage of making an even more structurally sound shape, the rectangular box, available in a more weight-efficient manner than sheet metal does, in addition to being made from much stronger material as Austin mentioned. In a pure race to achieve a target strength with minimum weight and optimal design, extrusion will win on paper every time.

Now, whether or not this is race to the top is in fact advisable, practical, or even desirable for your robot and team is a whole other matter. Resource sets, integration with the rest of the robot, ability to work the fabrication process into a build season, factors of safety and how far you want to go with them on the most important robot subsystem, ease of sourcing appropriate materials, and so on are all valid considerations, much moreso than squeezing the last couple tenths of pounds out of the drivetrain. My team does sheet metal drives and plans to continue to do so for a number of reasons, but pursuit of absolutely optimal strength/weight ratio is not of of them. I'm sure 971 has similar reasons. If you want to learn more about the complex ways in which sheet metal parts can interact to add strength to a chassis, I suggest checking out some of 971's drive bases. Some very impressive work.

Re: Sheet Metal Fabrication vs. Using Channel
 

I would have to disagree with you on this portion as in every building that bares weight or hold things together is either I beam or [, etc. never have I seen a square metal tube used for holding up a floor or a roof or a a building, why? because it's unnecessary weight when an I beam can bare nearly the same load. also if you have ever seen a tower-crane, they do not use a solid tube rather they use scaled up version of sheetmetal that uses webbing to give it it's strength/rigidity and it's light weight ( for heavy machinery at least) this principle also applies to sheet metal as well. "available in a more weight-efficient manner than sheet metal does," from this i'm gathering that you are saying tubing is lighter then sheet metal? (correct me if I'm wrong on this) if you are indeed saying this then i would have to disagree with you as our robot using extruded tubing this year/every year has a much heaver weight to it, then sheet metal frame (according to CAD)

as for ease of fabricating, I would agree with you in saying that it is much more time consuming if you don't have a laser/water cutter or CNC machine, and even if you do it still would take much more time then extruded tubing, that much i do not disagree with you on.

Re: Sheet Metal Fabrication vs. Using Channel
 

False. I beams in this application are used because they are easier to produce than hollow profiles. It has to do with the extrusion process, a hollow tube requires an extra die for the middle section. Even disregarding price, square or rectangular tubing is much stronger. There is more material, and it's on the periphery instead of the middle yielding a much stronger beam.

-Adrian

Re: Sheet Metal Fabrication vs. Using Channel
 

You might want to ask your father why he uses I-beam and channel and if he (an other Civil Engineers and Architects) uses other profiles. The industrial buildings that I work in (manufacturer of large electrical equipment) have all sorts of square, rectangular and round steel tube, as well as I-beams, being used as pillars to support the roof structures. This complex of buildings has been expanded at least 4-6 times over the last 30 years.

You can only compare the weight of your robot built from tubing to a sheet metal one designed to same specification if both designs have been optimized properly for the stresses that they will experience and to minimize weight. I suspect that this optimization exercise is beyond the capabilities of most FRC teams since it would involve using tools such as finite element analysis and a very thorough modeling of the stresses that will be experienced by the structure. While you know your robot made from tubing was overweight, you do not know if your sheet metal CAD design is strong enough since, I presume, it was never built.


The "strength" of a square, rectangular or round tube is also different from an open profile such as an I-beam or channel. My empirical experience has been that the tubes, in general, resist torque much better than the open profiles. This characteristic may be more important in FRC robots than the ability of a particular profile to support a static load. Perhaps someone with the appropriate background can offer their comments (I am just an EE but I have had to deal with mechanical issues a number of times over the last 30+ years).

Re: Sheet Metal Fabrication vs. Using Channel
 

A couple of reasons for channel & I-beam for construction are the shapes can be rolled rather than extruded. Bolted & riveted connections are easier than with hollow shapes. I-beams are useful when most of the loading comes in one plane.

With structure, a lot of times I end up sizing for stiffness rather than strength making yield strength less important.

All of these shapes exist for the simple reason is that they all are useful for specific situations.

Re: Sheet Metal Fabrication vs. Using Channel
 

Drive bases have a lot of mixed loads. Cantilevered wheels put a torque on the tubestock. Even drive bases which support both sides of the wheel shaft have torques put on them when one corner lifts up on something, or you have high lateral acceleration. I-beams are very strong in the direction of the I, not in the orthogonal direction, and definitely not in torsion. Check the torsional strength of an i-beam. Drive bases aren't their target application.

I challenge you right back to draw up a sheet drive base and a tubestock drive base and use CAD to weigh both of them.

I'm willing to bet you can get the weight of the tube stock drive base significantly lower (a couple pounds) than the sheet base for a comparable strength. I've been comparing our chassis weight against 254 and 973's for years, and they have consistently come up a couple pounds lighter than ours.

With sheet metal, there is a minimum flange length. If you are going to make a structure (like we do) which is a tube holding your wheels, you need 3/8 - 1/2" of material on each side of each bend. That's 4" of material just in the corners, not including any material used to connect those sides together. Overlapping flanges to attach 2 pieces of sheet together just add to that number. Compare that to the 6" of tube perimeter in a 1x2 that is traditionally used on a WCD, and you are going to have a lot of trouble doing better than the 1x2.

We do a sheet metal drive base because we have sponsors who are sheet metal shops and we know how to work with it. We can design in our superstructure mount points and bumper mounts, and get a bunch of parts back while we continue designing the superstructure that only need to be riveted together to have a robot. I really love our drive bases, but getting the weight down that last little bit is one of those things that we struggle with and end up just giving up on.

Re: Sheet Metal Fabrication vs. Using Channel
 

For thin walled Sections In torsion:

Theta = (TL)/(GJeff)

T = applied torque
L = Length of the section
G = Shear Modulus (material property for 5052 Al this is ~26GPa)
Jeff = Effective Area Moment of Inertia or Torsion Constant for the section (I'm fuzzy on the terminology here)

For closed sections:

Jeff = (4 t (Aenc)^2)/S

t = material thickness
Aenc = Area Enclosed by the section
S = circumference of the section

For open sections:

Jeff = (s t^3)/3

t = material thickness
s = arc length of open section (similar to circumference, but ends don't meet)

Applying a 10 Nm load to a 50 mm diameter circular section x 100 mm long x 2mm thick yields the following:

Closed Section:
Theta = .01 degrees

Open Section:
Theta = 5.26 degrees

There's also stress calculations I could go into, and this gets more complicated with different thickness walls on parts of the section and warping of open sections, but I think you get the idea. If anyone is interested in more detail PM me.

This is why you rarely see open sections in automotive sheet metal. You'll always see a bunch spot welds down the length of a section.

I'll try to post the bending equations for thin walled beams later when I get time.

Re: Sheet Metal Fabrication vs. Using Channel
 

For the record, this debate occurs more than just the realm of FRC. If you walk the pits in FSAE, you can hear similar debates over "monocoque" vs. "spaceframe" designs. and there are very reasonable arguments that can be made eaither direction.
http://www.fsae.com/forums/showthrea...(stressed-skin)

It should be noted though that most Formula Cars, which are often looked at as the pinacle of performance engineering end up going with Carbon fiber "monocoque" designs. But they have evolved into those over many years, and arguably great performance was found using other methods before.

In FRC, I have observed equally awesome chassis design using plate and spacer, sheet metal, and stick/tube frame, and hybrid.

I will say a lot of very good teams use a slowly evolving chassis design from year to year, and thus optimize their design a little bit better. This gives them a "proven" platform to support the most basic need for most games "move". I believe/suspect that this allows them to spend more time/talent on end effector and manipulator development as they are not consistently re-inventing the wheel.

Other teams re-invent the drive base each year, but this does come at a heavy design resources cost.

Ultimately a well thought out XXX design that the team has had success with will usually be out poorly developed "superior construction method" chassis that has little development time on it.

Re: Sheet Metal Fabrication vs. Using Channel
 

Thanks, Mike.


I have noticed many similar (often heated) debates in other fields regarding which particular material or construction method is superior. In the end, it is often the quality of the design and the quality of the execution that makes more difference in the performance of the end product.


I think the Kitbot on Steroids concept was developed for this reason.

Re: Sheet Metal Fabrication vs. Using Channel
 

[Pointless point] Body panels? No you cannot count the plastic inner squirts as closing the section. :) [/pointless point]

OOH Race cars. F1 Oriented strand layup. Carbon/Carbon construction. I think I just blew my FRC budget.

The Maserati Birdcage show hows much sexier space frame construction is than a monocoque design even if the monocoque is ultimately better.

Re: Sheet Metal Fabrication vs. Using Channel
 

I was specifically referring to beam sections such as rocker, roof rail, and A, B, and C pillars.

Body panels don't exactly serve that same purpose. But in a full body having a closed box is very important for torsional stiffness. A great example of this is a shoe box. Twist it while the top is open and it is very easy. Close the lid and twist it again - you'll notice a significant difference. Without the top there is no surface to react the shear load of the 4 walls it contacts. If you don't get what I mean try drawing a free body diagram of a box with a load at one corner. This can often be a problem with FRC chassis (or helpful if you're using a mechanum wheel set up).

Re: Sheet Metal Fabrication vs. Using Channel
 

After consulting me father, I was indeed wrong on I beams being stronger, they are almost as strong, but the main reason the use them is that they are lighter and cheaper (cheap. . . go figure). I will have to do some actual testing to see wether my hypothesis are correct or not, as i am more visual then mathematical, if you understand what I'm saying. though i am currently making a robot in cad that I intend to publish and will post a tubing one as well just to see how much there is of a difference