Hacker News Reader

Comment by kevingadd | Hacker News Reader

Light being able to shine straight through part of your car that's meant to be solid is scary. Does anyone know how common it is for cast metal of this sort to have damage like this? I am kind of surprised their manufacturing would have this issue

View on HN

Normally cast metal parts shouldn’t have issues detectable with optical inspection system. I worked for a company doing welding inspection systems. It is absolutely crazy when such part ends in the final product. This company clearly overstretched cutting corners on this.

Would a part like this get x-ray inspected? I had a summer job working at a machine shop and structural castings would often get sent for x-ray inspection.

It’s the best way and most expensive. For mass production of automotive parts probably too expensive. I mean these guys can’t or don’t want to afford 20000$ camera system for quality assurance.

Definitely not too expensive for automotive parts. Cast wheels are often 100% X-Ray inspected already. This is purely a lack of proper quality control systems.

Cast parts are more brittle than forged parts. Once the cracks are visible you've already lost a great deal of strength. I wouldn't ride in this car.

It's not clear what purpose this cast part serves. I assume the body of a Tesla is formed/stamped from sheets of steel like most modern cars and that this provides most of the structure. It looks like this part is near the top of the hood, so is it structural? Is it just holding a bunch of other parts in place? Hard to tell.

You assume incorrectly.

https://electrek.co/2021/10/05/tesla-building-model-y-bodies...

> In an auto manufacturing first, Tesla has started building Model Y bodies with two giant single casting pieces for the front and back of the electric SUV.

Anyone should be highly skeptical whenever Tesla adopts an "industry first". There may be good reasons the industry doesn't do that. It may be something that an experienced engineer would advise against.

It's mostly just a lack of willingness to spend on large casting machines. Tesla is famous for purchasing the largest presses: https://en.m.wikipedia.org/wiki/Giga_Press

Tesla is partly empowered to do this because electric vehicles are mechanically simpler than ICEs, and the batteries themselves also are made to be structural components. There's nothing fundamentally unsound about pressed aluminum.

The casting flaw in the OP would never have passed through a working quality control system, which would be automated radiography. You don't just buy casting machines, you have to develop the organizational doctrine to put them into practice.

One of the reasons it isn't done is the QC problem. If you have a 10% rejection rate on 20 parts, 18 parts will pass. If they were all one mega-part, the whole casting has to be rejected. You then add in repairability issues and no flexibility to accommodate other types of manufacturing variability (errors, fit) and your savings from a single casting can become a big deficit instead.

Sadly the reason this bad casting was shipped to a customer is probably because they are not achieving the desired pass rate so they knowingly wave through defective castings.

It’s basically the front subframe.

https://www.teslarati.com/wp-content/uploads/2022/10/tesla-m...

https://www.teslarati.com/tesla-model-y-front-megacast-giga-...

Wow, that's indeed a seriously giant cast! How does it get repaired in case of a crash? Can one just cut off a damaged part of it and weld on a replacement, like with sheet metal?

Looking at the picture, it appears there is a boxed steel frame underneath. Probably in case of damage you just take it all apart and replace the entire cast aluminum piece.

Isn't the alternative actually a bunch of smaller cast parts bolted together, not forged parts?

the alternative could also be a forged part, or a cast part that was properly made

I'm not saying there's a better alternative. I'm just saying seeing those types of cracks in cast parts scares the shit out of me. That's the kind of stuff you see after decades of rust damage.

In a well controlled process it should not be common. Bigger parts have been casted before without issue. Tesla cutting corners?

No it's just poor QA. You can live with bad parts coming out of the prices provided you catch them when they do.

This shouldn't ever happen if QA is being done properly.

EDIT: which I recognise is "cutting corners" but the point is, any idiot can make one in-spec part. Making them consistently is the challenge.

> Tesla cutting corners?

Given what we have seen from Twitter, cutting corners and ignoring laws and regulations seems to be an Elon thing.

The guy is even skipping rent.

So why not?

Rumint: Tesla has designed castings that are too big and/or too complex for the casting presses to produce a good yield. Tesla blames IDRA. But another maker of those machines turned down Tesla's RFP.

disclaimer: I have no first-hand experience, just second-hand.

It should be impossible to leave the factory like this. Let's say the issue is a substandard batch from a supplier that cracks from stress at some stage of the frame assembly (like when adding the battery pack, welding or adhesive bonding). The car should be inspected after all structural parts are added, at minimum visually but ideally with some kind of spectroscopy.

Also, if this is a whole batch with this issue it should have been noticed when doing QC on the shipment.

One option is that this happened at some stage during shipping (maybe the car rolled off a trailer and fell half a meter). Still, the dealership should be doing an inspection before handing the car over.

All in all, if this really isn't the result of some accident it is a total failure.

Another option is that workers at the factory are now pushing bad parts knowingly, to get the truck out of the door.

Let's not forget that the truck has been delayed for over two years now. In the meantime, a few competitors have been eating Tesla's lunch, and Musk is notorious for pushing unfinished products.

I have absolutely no knowledge about casting metal but I'm 100% sure you can cast these parts without any cracks. This is a very obvious quality control issue, we've been casting metal for decades..

Reply on news.ycombinator.com