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