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Comment by bee_rider | Hacker News Reader

They talk about machining, casting, and pressing. Mostly with casting and pressing in a good light, machining not so good.

As somebody who knows absolutely nothing about this stuff, I wonder—casting, I thought, was generally a lower quality option (like cast iron doesn’t have fantastic high-performance material qualities, and I had some crappy cast pewter toys as a kid). Are there different, higher quality casting processes, and I’ve only seen the bargain bin results? Is there a general ranking of the quality of the result or is it all very complicated and material specific?

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> like cast iron doesn’t have fantastic high-performance material qualities

Cast iron is a material, not a process. It's an unfortunate legacy term for very high carbon steel (>2% by weight, or <11 iron atoms per carbon atom). For reference "standard" steel is ~.08-.18% carbon, and high-carbon steel is ~.8% carbon.

The >1% carbon precipitates out into graphite within the steel, causing it to behave totally differently. Less rusting, but weaker and much more brittle when solid. Less viscous when liquid, so you can cast long and thin parts.

> Are there different, higher quality casting processes, and I’ve only seen the bargain bin results?

There are, but it mostly is independent of the material. Some turbine blades are cast as single crystals for heat stability; you can't really cut a single crystal without introducing cracks and issues. There's also vacuum casting and spin casting (using a centrifuge to force liquid into the mold), which lets you cast metals that react with air or cool too quickly for normal methods.

Most of the variation in process is about the final form you cast into, though. Engine blocks are sometimes cast into a one-off ceramic shell that is sprayed onto a sand form. It's an expensive process but it lets you do the whole thing in one step.

> is it all very complicated and material specific?

It is very material specific. Fundamentally its all about shaping the grains. In many steels you can physically alter grains. In others, like precipitation grains (aluminum alloys, some steels) the structure is determined by the cooling and you can't physically shape them. In that case you may often get a better structure by casting since you can choose how to cool parts down, while a billet will have a homogenous structure that is usually worse towards the center.

Monocrystalline turbine blades are one of those things that make me appreciate how deep the state of the art is for a lot of "simple" things when you look closely.

There's nothing simple about jet engines :)

> The >1% carbon precipitates out into graphite within the steel, causing it to behave totally differently. Less rusting, but weaker and much more brittle when solid.

If one adds some magnesium or cerium to the alloy, the graphite precipitates out as spherical nodules rather than feathery dendrites. The resulting material, called ductile iron, is much less brittle than traditional cast iron.

An advantage of the higher carbon content is a reduction in the melting point (by > 300 C), so the material is easier to cast than low carbon steel.

Castings can achieve shapes that are impractical to machine. A classic example is a spoked wheel. The spokes are really hard to make precisely with machining, but fairly straightforward when casting. Bandsaw wheels are virtually always cast, for example.

Also, the processes are not really independent. It can be much cheaper to do a rough casting, and then machine just the critical faces of it, instead of using an “off the shelf” hunk of metal and machining it all into shape. So it’s not really “casting is superior to machining” or vice versa. More that machining is high precision but expensive. Casting has some up-front cost but once the patterns are made, each item will use material quite efficiently.

Spoked wheels aren't cast because it's hard to machine them. You can easily CNC them if you really want. But that would be insanely expensive and wasteful and not any better.

Yes, I said “impractical”, not “impossible”.

You said

> The spokes are really hard to make precisely with machining

That is not true.

Back in the days of manual machining it was definitely true. CNC, especially modern 5+ axis high speed high rigidity machines are god level cheating.

I've watched enough Chip Foose on TV to know that in the custom car world it is commonplace to start with a solid blank wheel and CNC away 90% of the face to achieve any particular design. It takes only minutes, and unlike casting or forging you do not need to purchase hard tooling for each different design.

https://www.youtube.com/watch?v=nqOlM6-YIIs

Cast iron is a material, it's good for what it's good for but its material properties don't really come from the casting process, just the ratio of iron to carbon.

Casting has problems with thermal expansion, plenty of materials shrink significantly as they cool and complex parts cool unevenly which can cause them to break or deform.

Casting has problems with microstructure, plenty of materials, especially steels develop complex crystal structures with multiple phases of materials as they cool from liquids and even extensively in hot solid phases. It's hard to control this in a cast part.

Casting has problems with precision. The molds just can't be all that precise when in machining, a thousandth of an inch can be a relatively large distance.

However casting gets a bad reputation because most of the time you see it it's because it's actually very cheap, cheap materials, cheap process, minimal post processing. Higher cost things don't necessarily realize the savings from casting as much so they don't use it. And also a lot of higher quality materials have higher melting points which require more advanced tools to melt and handle.

Plenty of things though are cast and then machined, you notice this if you look.

They also talk about forging. Forging can produce higher quality parts because metal has a grain structure. Just like wood, it's easier to break between grains than across the grain. Forging is deforming the metal without melting it. If the temperature isn't too high, it will deform the grain structure along with it. The grain can be aligned to make the metal stronger along the axis where it needs to resist the most force.

Wikipedia has a image of an connecting rod that has been etched to show the grain:

https://upload.wikimedia.org/wikipedia/commons/5/5c/ForgedCo...

You can see the grain has been stretched along the length of the narrow parts. Wrenches are another example of something that's commonly forged for this reason.

Casting isn't necessarily "lower quality," it just has different properties. It's also much older (by thousands of years) than machining or pressing, but you can't get to the pressing or machining step without having an ingot, which comes from the casting process.

The main drawbacks of casting are you get a hard, but brittle product with (generally) uneven quality. There are processes (like annealing, though I don't know how you anneal a massive component) that can solve these problems, but all iron/steel is "cast" at some point.

Until the mid-1800s, basically only the Chinese used cast iron, since it's so brittle. Ingots would come from smelting, not casting.

depends on what you mean by 'machining'. grinding is at least 7000 years old https://en.wikipedia.org/wiki/Shoe-last_celt and roughly contemporary with pottery. casting presumably began with pottery but was definitely in full swing by the bronze age, a mere few thousand years later

Which cast iron? There are many different grades/alloys with different properties. Sometimes cast iron (some specific grade) is better sometimes it is worse. And of course you can cast things other than iron, cast steel does exist (rare outside the feed to a press, roll, or some other process, but you can cast steel into a specific shape if you want to). There are trade offs. The topic is so broad we cannot even start to talk about it. Instead we start with what your application needs and then look at the options to get that.

Cast iron is fantastic for building machine out of - while it isn't as strong, it is stable against vibration. There is a reason engine cylinder selves are often cast iron.

> I had some crappy cast pewter toys as a kid

Those were pot metal, not pewter (pester has many definitions but implies some qualty control). They are made out of whatever melts in a pot - often whatever is cheap at the recycle yard (without trying to identify what is in the metal - including lead which shouldn't be used in toys). Typically no control of the alloy was made and often they start with several different things that are great in isolation but when mixed result in bad behavior. Then the next time the make the toy they use different mix and get different properties. If you spend a little extra to get a known alloy pot metal is a high quality castings with great properties.

> I’ve only seen the bargain bin results

You have likely seen a lot of non bargain bin results. However since the parts are invisible you never thought about it or the alloy used. You see the failures and so casting gets a bad reputation because it is obviously used in the cheapest things with low quality control. (the door knobs in your house are likely cast pot metal plated with brass, but they last for decades)

>Cast iron is fantastic for building machine out of - while it isn't as strong, it is stable against vibration. There is a reason engine cylinder selves are often cast iron.

I thought it was because cast iron had very high resistance to wear.

Machining is fine, but in the context of the article the problem is that it usually starts off from plates or otherwise block-shaped materials. That means that if you have a weirdly shaped part full of holes and/or with many protrusions in weird angles (as vehicle parts tend to have), then you either have to combine many smaller parts into a final part or have to machine away a huge amount of material from a solid block to come to the final shape. It would be much better to cast or forge the part into roughly the right shape straight away, so you'd need only minimal finishing work. That is what these presses enabled.

Other materials can be cast, such as various aluminum alloy. Casting can have a higher cycle time, be cheaper, and require a less expensive equipment.

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