The Nonlinear Library LW - the Giga Press was a mistake by bhauth
Aug 21, 2024
09:01
Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: the Giga Press was a mistake, published by bhauth on August 21, 2024 on LessWrong.
the giga press
Tesla decided to use large aluminum castings ("gigacastings") for the frame of many of its vehicles, including the Model Y and Cybertruck. This approach and the "Giga Press" used for it have been praised by many articles and youtube videos, repeatedly called revolutionary and a key advantage.
Most cars today are made by stamping steel sheets and spot welding them together with robotic arms. Here's video of a Honda factory. But that's outdated: gigacasting is the future! BYD is still welding stamped steel sheets together, and that's why it can't compete on price with Tesla. Hold on, it seems...BYD prices are actually lower than Tesla's? Much lower? Oh, and Tesla is no longer planning single unitary castings for future vehicles?
I remember reading analysis from a couple people with car manufacturing experience, concluding that unitary cast aluminum bodies could have a cost advantage for certain production numbers, like 200k cars, but dies for casting wear out sooner than dies for stamping steel, and as soon as you need to replace them the cost advantage is gone.
Also, robotic arms are flexible and stamped panels can be used for multiple car models, and if you already have robots and panels you can use from discontinued car models, the cost advantage is gone. But Tesla was expanding so they didn't have available robots already. So using aluminum casting would probably be slightly more expensive, but not make a big difference.
"That seems reasonable", I said to myself, "ふむふむ". And I previously pointed that out, eg here. But things are actually worse than that.
aluminum die casting
Die casting of aluminum involves injecting liquid aluminum into a die and letting it cool. Liquid aluminum is less dense than solid aluminum, and aluminum being cast doesn't all solidify at the same time. Bigger castings have aluminum flowing over larger distances. The larger the casting, the less evenly the aluminum cools: there's more space for temperature differences in the die, and the aluminum cools as it's injected.
As a result, bigger castings have more problems with warping and voids. Also, a bigger casting with the same curvature from warping has bigger position changes.
Tesla has been widely criticized for stuff not fitting together properly on the car body. My understanding is that the biggest reason for that is their large aluminum castings being slightly warped.
As for voids, they can create weak points; I think they were the reason the cybertruck hitch broke off in this test. Defects from casting are the only explanation for that cast aluminum breaking apart that way. If you want to inject more aluminum as solidification and shrinkage happens, the distance it has to travel is proportional to casting size - unless you use multi-point injection, which Tesla doesn't, and that has its own challenges.
Somehow I thought Tesla would have only moved to its "Giga Press" after adequately dealing with those issues, but that was silly of me.
One approach being worked on to mitigate warping of large aluminum castings is "rheocasting", where a slurry of solid aluminum in liquid aluminum is injected, reducing the shrinkage from cooling. But that's obviously more viscous and thus requires higher injection pressures which requires high die pressures.
aluminum vs steel
Back when aluminum established its reputation as "the lightweight higher-performance alternative" to steel, 300 MPa was considered a typical (tensile yield) strength for steel.
Typical cast aluminum can almost match that, and high-performance aluminum for aircraft can be >700 MPa. Obviously there are reasons it's not always used: high-strength aluminum requires some more-expensive elements and careful heat-treatment. Any hot welds will r...
the giga press
Tesla decided to use large aluminum castings ("gigacastings") for the frame of many of its vehicles, including the Model Y and Cybertruck. This approach and the "Giga Press" used for it have been praised by many articles and youtube videos, repeatedly called revolutionary and a key advantage.
Most cars today are made by stamping steel sheets and spot welding them together with robotic arms. Here's video of a Honda factory. But that's outdated: gigacasting is the future! BYD is still welding stamped steel sheets together, and that's why it can't compete on price with Tesla. Hold on, it seems...BYD prices are actually lower than Tesla's? Much lower? Oh, and Tesla is no longer planning single unitary castings for future vehicles?
I remember reading analysis from a couple people with car manufacturing experience, concluding that unitary cast aluminum bodies could have a cost advantage for certain production numbers, like 200k cars, but dies for casting wear out sooner than dies for stamping steel, and as soon as you need to replace them the cost advantage is gone.
Also, robotic arms are flexible and stamped panels can be used for multiple car models, and if you already have robots and panels you can use from discontinued car models, the cost advantage is gone. But Tesla was expanding so they didn't have available robots already. So using aluminum casting would probably be slightly more expensive, but not make a big difference.
"That seems reasonable", I said to myself, "ふむふむ". And I previously pointed that out, eg here. But things are actually worse than that.
aluminum die casting
Die casting of aluminum involves injecting liquid aluminum into a die and letting it cool. Liquid aluminum is less dense than solid aluminum, and aluminum being cast doesn't all solidify at the same time. Bigger castings have aluminum flowing over larger distances. The larger the casting, the less evenly the aluminum cools: there's more space for temperature differences in the die, and the aluminum cools as it's injected.
As a result, bigger castings have more problems with warping and voids. Also, a bigger casting with the same curvature from warping has bigger position changes.
Tesla has been widely criticized for stuff not fitting together properly on the car body. My understanding is that the biggest reason for that is their large aluminum castings being slightly warped.
As for voids, they can create weak points; I think they were the reason the cybertruck hitch broke off in this test. Defects from casting are the only explanation for that cast aluminum breaking apart that way. If you want to inject more aluminum as solidification and shrinkage happens, the distance it has to travel is proportional to casting size - unless you use multi-point injection, which Tesla doesn't, and that has its own challenges.
Somehow I thought Tesla would have only moved to its "Giga Press" after adequately dealing with those issues, but that was silly of me.
One approach being worked on to mitigate warping of large aluminum castings is "rheocasting", where a slurry of solid aluminum in liquid aluminum is injected, reducing the shrinkage from cooling. But that's obviously more viscous and thus requires higher injection pressures which requires high die pressures.
aluminum vs steel
Back when aluminum established its reputation as "the lightweight higher-performance alternative" to steel, 300 MPa was considered a typical (tensile yield) strength for steel.
Typical cast aluminum can almost match that, and high-performance aluminum for aircraft can be >700 MPa. Obviously there are reasons it's not always used: high-strength aluminum requires some more-expensive elements and careful heat-treatment. Any hot welds will r...