Sheet Metal Fabrication vs. Using Channel

[AustinSchuh]

Originally Posted by TheModMaster8

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.

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).

[carpedav000]

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

[cadandcookies]

Originally Posted by carpedav000

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

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.

[GeeTwo]

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.

[TheModMaster8]

NOTE: the COM in my profile picture

Originally Posted by AustinSchuh

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).

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!!

[Joe G.]

Originally Posted by TheModMaster8

if i remember correctly 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.

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.

Originally Posted by TheModMaster8

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)

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.

[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).