Sheet Metal Fabrication vs. Using Channel

[TheModMaster8]

Originally Posted by Joe G.

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.

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.

[Adrian Clark]

Originally Posted by TheModMaster8

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.

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

[philso]

Originally Posted by TheModMaster8

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.

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.

Originally Posted by Adrian Clark

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

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).

[FrankJ]

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.

[Mike Schreiber]

Originally Posted by philso

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).

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.

[IKE]

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.

[philso]

Originally Posted by Mike Schreiber

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

Thanks, Mike.

Originally Posted by IKE

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

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.

Originally Posted by IKE

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.

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

[FrankJ]

Originally Posted by Mike Schreiber

For thin walled Sections In torsion:

...

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.

[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.

[Mike Schreiber]

Originally Posted by FrankJ

[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 the a monocoque design even if the monocoque is ultimately better.

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).

[TheModMaster8]

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