JBee

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again not looking it up at present, but the information I referenced above was from a source that basically described how what your are hypothesizing is not possible at some basic level of materials science - the person described it well enough that I could follow while reading, but not that I can here recount the specifics

but my (rough) take-away being as follows: the SS being a “30x” brand of SS (eg, akin to a 304 but with some secret sauce making for the substituting “X”) requires the SS to have certain qualities that mean that “strengthening” it to the levels indicated by EM (eg inferred their specific bullet-proof was claims, etc.) essentially requires that it become at some proportion less mailable (absent failures/weak-points).

and in any event, isn’t it EM himself that psuggested this was exactly the case: he said they truck had to be so angular precisely because they couldn’t bend the SS

then again, who knows what to believe since EM said it would break presses which this same article described as being wildly incorrect/impossible *shrug*
Maybe he actually meant that the material would break in a press because the hardening makes it so brittle? I'm sure he knows that there are presses that can bend those small panel sizes. I'm wondering why he's calling them presses. Are they rollers or CNC type to get the fabrication speed? Maybe someone should ask him.
 

cvalue13

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Maybe he actually meant that the material would break in a press because the hardening makes it so brittle?
yes, this is what the article i read (from the SS forming modeler) was saying: it’s absolutely false that the SS would break any appropriate press, but instead that at a certain strength rating the SS approaches its tensile strength and so attempts to bend instead produce cracks/failures.

with all this I went ahead and re-dig up the articles (for what it’s worth):

From ‘StampingSimulation.com,’ which as the name suggests is a company that appears to perform theoretical metal fabrication modeling so as to virtually proof-of-concept various such manufacturing processes, or in their words: “StampingSimulation has been in the forming simulation business for over a decade. Our experience enables us to quickly and efficiently clear any roadblocks and provide you with sound and effective products and tools, according to your specifications. We are engineers serving engineers…”

Now, these are probably posts that some or most here have already seen and somehow discredited ping ago and I’ll get flamed for retaining them in ignorance, but until then I’ll repost in full for posterity (but go to the links to see associated graphs:

Elon Musk – You Are Wrong About Forming Stainless Steel – Part 1

Unless your head has been buried in the sand, the recent reveal of Tesla’s Cybertruck unveiled “Ultra-Hard 30X Cold-Rolled stainless-steel” as the primary material for the complete “exoskeleton” of the entire vehicle. Although the details of the entire bodywork is not clear, Musk stated that the panel thickness was 3mm and bulletproof to a 9mm handgun. Musk went on further to also state that the new stainless alloy (developed by Tesla for their starship rocket) could not be formed into any shapes because “it would break the stamping press”.

We were able to make the skin out of thick ultra-hard stainless steel…..it’s really hard….we’re going to show you just how hard” Elon Musk

cybertruck-stainless-steel-body-review-1024x769.jpg

The Cybertruck uses what Tesla calls “Ultra-Hard 30X Cold-Rolled stainless-steel.” This material is supposed to be superior to stainless 304. Source: Business Insider
Is Forming “Hard” Stainless Steel Possible?
The first issue a sheet metal engineer has with this statement is the constant use of the word “hard”. The correct engineering term would be “strong”, as in strength or yield strength, to be more exact. A material that is “hard” is not necessarily strong, and a “hard” material may indeed have lower tensile strength than a “soft” material. The point is, the word “hard” (or soft) is the wrong way to describe the strength of a material.

Hardness, in fact, is a separate property on its own and is measured on a hardness scale, which is entirely different from strength.

Granted, Elon Musk is a highly skilled engineer and probably entirely aware of these engineering facts, and perhaps is simply complying with common industry nomenclature, in the same way, non-engineering folk interchange lbs-ft with ft-lbs when referring to torque (the former is correct, the latter is technically wrong).

So let’s give Elon the benefit of the doubt with respect to the incorrect use of the word “hard”, technically speaking.

Reason Cybertruck is so planar is that you can’t stamp ultra-hard 30X steel, because it breaks the stamping press.” Elon Musk

Are you sure Elon? Various automotive components have been stamped from “ordinary” stainless steel for decades. These include exhaust components (usually 400 series stainless steel), as well as some cosmetic components such as bumpers and aftermarket accessories (usually 300 series stainless steel).

Could the new SpaceX “Ultra-Hard 30X Cold-Rolled stainless-steel” be so strong (not hard) that it would literally break the stamping press? I don’t think so.

Why it Wouldn’t Break the Stamping Press
“30X” most probably refers to a 300 series grade of stainless steel alloy eg: 304 is common. The “X” in “30X” probably denotes the new alloy that SpaceX has developed. Another possibility is that the “30X” actually means 30 times, as in, the new alloy is cold rolled 30 times over, to work strengthen the material (also known as work “hardening”) to improve it’s mechanical properties. It’s not clear which meaning is correct, without input from Elon Musk or his SpaceX engineers.

How Stainless Steel Forming is Achieved
Let’s examine both assumptions.

If the new material is indeed a 300 series stainless steel, that has been cold rolled, then the maximum “benefit” the cold rolling can give the material is to increase its Yield Strength to almost equal its Tensile Strength. Thus, in the case of 300 series stainless steel, typical figures look like this:

tesla-cybertruck-review-1024x719.jpg

Typical mechanical properties for stainless steel 304 – uniaxial tensile test from a real-world sample
In summary, we can see that Yield Strength is about 275 M Pa and Tensile Strength is about 611 MPa. If the material were to be cold worked to improve its Yield Strength, the maximum possible strength is 611 MPa. Cold working or cold rolling cannot improve the base material beyond its ultimate tensile strength. To do this would require a significant amount of cold work to be done on the material. Perhaps this is why the “30X” could refer to cold rolling the material 30 times over, and not to a 300 series alloy.

After all, it’s a “new” stainless steel alloy developed by SpaceX, right?

Thus the real question is (if the above assumptions are correct), will cold-rolled stainless steel with YS = TS = 611 MPa break the stamping press?

Answer: No, assuming the press is correctly sized for the part. The part will fail (split or crack), not the press.

The part cracks or split during the attempted forming operation, because the material that is cold worked to the point of its Tensile Strength, has no ability to stretch or form. If the material is cold rolled or worked to its maximum, any additional cold work (ie: forming in a press) pushes the material past its Tensile limit and it fails (cracks).

successful-stainless-steel-304-stamping-1024x513.jpg

With the help of StampingSimulation, these two parts were formed successfully from stainless steel 304. Press requirements were 150 tons. The press did not break. The resulting strength of the panel is much greater than the original material, due to cold work during forming.
It is not uncommon for large automotive stamping presses to be in the 1000s of tons. Even 5000 tons. That’s a lot of tonnage to stamp even the strongest materials. In fact, the automotive industry already stamps very high strength materials, some with Tensile Strengths greater than 1200 MPa. At even higher strengths, heating is used to form ANY strength of material into ANY shape. This is already common practice, although it comes at an increased cost.

In Conclusion
So Elon is wrong about forming stainless steel. In addition, Elon has specified 3mm thick stainless steel, which GREATLY increases forming tonnages required. Typical automotive skin panels are just 0.65mm thick, so 3mm thick is a huge increase in material mass and means the press has to be many times larger to stamp thicker 3mm material successfully.

One must wonder, would a 3mm thick high strength steel door perform the same as the 3mm “Ultra Hard Stainless Steel” vs the sledgehammer test? Probably yes, given that high strength steel during forming is also cold worked and strengthened in the exact same way as cold rolling stainless steel.

Thus, Elon should have stated “forming a complete exoskeleton using our new 3mm 30X Ultra Strong Stainless Steel will break our stamping press……so we decided not to form or shape the panels”

Clever marketing? OR genius engineering?

More to follow in part 2 next month…….



Elon Musk – You Are Wrong About Forming Stainless Steel – Part 2


We will continue this topic on the Cybertruck, by investigating additional statements made about the Tesla Cybertruck and forming stainless steel. It has been stated by Tesla, that the new Cybertruck will be “incredibly cheap” to bring to market. Let’s also note that the expected initial planned volume of Cybertruck is 50,000 units, as some sources are estimating, although pre-orders are beyond this number. StampingSimulation recently wrote a Part 1 of this topic, that you should go check-out before continuing.

n-Musk-You-are-wrong-about-forming-stainless-steel.png


Let’s quickly recap what was written last month in “Part 1” of this article.

  • Elon Musk revealed new Cybertruck to be made from Ultra-Hard 30X Cold-Rolled stainless-steel
  • Panel thickness to be 3mm thick and
  • Cybertruck is so planar because forming stainless would break the stamping press
  • StampingSimulation proved that forming stainless steel is possible, and fairly routine in the automotive industry
  • Clever marketing? OR genius engineering?
Tesla also stated that the process to bend the stainless sheet into the planar shape of the Cybertruck was to score the sheet and bend it, much like origami and then weld it into shape. A very large sheet of stainless would at first be laser cut or water jet cut into a flat pattern. This is how the prototype Cybertruck was made, it was revealed.

Elon-Musk-wrong-about-forming-stainless-steel.png

Laser cutting example, however, this is 3D laser cutting a formed sheet metal part, which is not how the Cybertruck will be made. Laser cutting is a slower process, compared to cutting in a stamping tool.
How the Cybertruck Can be Made
The exact process of bending the exoskeleton into shape is critical to the success (or failure) of the Cybertruck and this also means that “normal” or “traditional” body manufacturing methods cannot be used.

And this is where the problem lies. Sheet metal stamping (including stamping stainless steel) is incredibly efficient when done correctly and in a high-speed production line. It is not uncommon for automotive parts to be produced at a rate of 30 to 50 parts per minute. Sometimes slower, sometimes faster, depending on the size and the exact nature of the part. This is achieved by using mechanical transfer methods, and also with robotic transfer systems. Also, the entire bodysides outer panel is often stamped as one part, using current production methods.

Efficient Tooling vs. Laser Cutting
The cutting of the blank (or the flat layout) is done by “blanking” tools that can cut and trim the entire shape in less than one second and repeats endlessly until an entire roll of steel (or any sheet material, including stainless steel) is consumed. For example, the entire “flat layouts” needed to produce the initial volume of 50,000 units of the Cybertruck could be produced in less than 17 hours, using traditional stamping methods. How long would laser cutting or water jet cutting take to produce the same volume of blanks? Answer: Much, much longer. Perhaps a thousand times longer, if we assume a single laser cutting machine vs a single stamping press.

Similar to the forming process. The forming stages of a well-designed stamping process are always part of the same “gang” of tools, all stages of the stamping tool are subsequent to each other. Thus whether it be a two-stage forming tool or a twenty stage forming tool, once the tool is running, one part is produced with every stroke of the press. The stroke rate (as before) can easily be 50 strokes per minute. That’s the same speed as just the cutting process!

Forming-Stainless-Steel.png

This is an example of STAINLESS STEEL being cut AND formed (stamped) in a high-speed progression tool. This example makes one stainless steel part every 2 seconds.
A bending process can be done in a stamping tool. But the method described by Tesla to bend the laser cut stainless sheet for the Cybertruck implies a CNC bending method or perhaps a brake press method. Either way, this bending method is time-consuming, and even more so for a large part.

The slowest, least efficient way to manufacture a sheet metal part is to laser cut and fold it manually. That’s why this is the method used to create low volume or prototype parts. Granted,
Elon Musk may be set to disrupt the stamping industry with a revolutionary method for high-speed laser cutting and bending, but I doubt it. At least, there is no indication that the manufacturing process for the Cybertruck will be a revolution, at this point.

The reason laser cutting and bending a steel sheet is “cheap” is because there is no special tooling required. No investment upfront. BUT the downside is that this method is SLOW! Elon
Musk forgot to mention that detail.

The Truth About Cybertrucks
More likely, the truth is that the Cybertruck is intended to be a low volume production because the chosen manufacturing method does not lend itself to high volume production. In the automotive industry, high volume production means 300,000+ vehicles per year. Perhaps there could be a limited number of Cybertrucks made in total OR perhaps 50,000 units per year is the maximum product volume possible. Elon Musk may well be planning a “prototype production run” of Cybertrucks, with limited volume.

Time will tell. Engineering genius OR clever marketing? Can the Cybertruck achieve what the DeLorean could not? Time will tell.”
 

Ogre

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Anyone who can write that many words and concludes the Cybertruck is going to be low volume needs to have their head checked.

The entire point of the Cybertruck from start of their design process was to reduce cost of production. That is by definition not going to be low volume.

What this tells me is only that whatever solution Musk has in mind has not occurred to the author.
 


cvalue13

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Anyone who can write that many words and concludes the Cybertruck is going to be low volume needs to have their head checked.

The entire point of the Cybertruck from start of their design process was to reduce cost of production. That is by definition not going to be low volume.

What this tells me is only that whatever solution Musk has in mind has not occurred to the author.
to be fair, 1: these posts came out days after the initial reveal

and 2: I posted here mainly for the materials science insights in “Part 1” but included the more unrelated bits of Part 2 only for completeness
 

CyberGus

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9_45411339_10156145403124401_1895138556830023680_n.jpg


The stainless panels on the DeLorean were stamped, but they weren't cold-rolled for a space program, nor were they 3mm thick.

See the notch in the hood? That's the fuel-filler flap. Stamping out this piece resulted in too many hoods cracking, so the flap was discontinued.

Another thing to consider for the Cybertruck is that the stamping/bending process cannot leave marks on the metal, unless they intend to grind a grain into the finish as on the DeLorean.
 

firsttruck

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9_45411339_10156145403124401_1895138556830023680_n.jpg


The stainless panels on the DeLorean were stamped, but they weren't cold-rolled for a space program, nor were they 3mm thick.

See the notch in the hood? That's the fuel-filler flap. Stamping out this piece resulted in too many hoods cracking, so the flap was discontinued.
....
Yup, cost and production time delay might be main reason for no door handles on Cybertruck.
 

cvalue13

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And saying stamping won't break the machines is dumb - every time you stamp, the machine itself is made of hardened steel, the die as well, and parts of it bends and is worn down. Most people would consider that 'breaking'.

-Crissa
But then you’re merely describing how every such machine is incrementally “broken” by any use of it.

If that’s all EM meant, it’s uninteresting rather than incorrect.
 


anionic1

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Interesting point.
I was in the clothing business for 20 years, and have watched many cutters go about their job.
Laying out the patterns in order to get most out of the meterage is key to a cutters job.

The side sails are easy enough, just reverse and cut them out of a rectangle.

The width of the SS roll could be just enough to cut out 2 doors with very little waste. Both sides of the bed can be laid out one on top of the other too.

The back gate, the front grill, could also fit length wise to fit the width of the roll.

Close enough. So far so good.

But then we get to the 'V' across the top of the doors, going across the front side seemingly cut out of one piece... and I have put a red arrow where its bent.

This bend at the red arrow needs to be welded, otherwise there is heaps of waste.



1663774508604.png


Apart from that, it looks pretty spectacular.. as far as wastage goes.
I'm going to guess the dimensions /design of pieces were also tweaked to get the most out of a sheet.
Anything that is welded would need to be refinished due to the heating likely causing color changes in the steel so I imagine there won’t be much welding or they will lose the benefit of no paint. My vote is no welding. They will use structural adhesives to combine parts.
 

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But then you’re merely describing how every such machine is incrementally “broken” by any use of it.

If that’s all EM meant, it’s uninteresting rather than incorrect.
Certainly the stamping dies have a limited lifespand and are intended to be regularly replaced. Drill bits and saw blades break far more frequently than the tools that power them.
 

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But then you’re merely describing how every such machine is incrementally “broken” by any use of it.

If that’s all EM meant, it’s uninteresting rather than incorrect.
Well, a machine that can only make a dozen vs hundreds or thousands?

The increment matters.

-Crissa
 

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This video is from 6 years ago, but there are large number of AIDA crates stacked up at the south end of the gigafactory. For Cybertruck, methinks?
 

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This video is from 6 years ago, but there are large number of AIDA crates stacked up at the south end of the gigafactory. For Cybertruck, methinks?
I’m confused. Giga Nevada was I believe the only Gigafactory 6 years ago.

 

 
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