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Comment by jstanley | Hacker News Reader

In my (admittedly limited) experience metal casting is a waste of time if you want to produce usable parts. It never goes right. It always has porosity. It very rarely even fills the entire mould.

And preparing the mould and heating the metal takes forever, so it doesn't take too many failed attempts before you have sunk days into a project with nothing to show for it.

It's fine if you just want to cast metal shapes as a hobby, knock yourself out. But if you are casting because you want to make parts that you can use, then you're better off machining them out of solid material if you can. You'll save yourself time and trouble.

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So from your reports (porosity, not filling the mold) you were likely running into 2 very common issues, one easily correctible.

For porosity, especially if you were using aluminum, that is very likely from using the wrong alloy. Trying to remelt aluminum cans or extrusions especially will produce very porous castings. Start with a known-castable alloy, the easy way is to start with scrap that was previously cast. Look at the original part for signs it was cast, that doesn't tell you the exact alloy but it does tell you that it can be cast.

For the not filling the mold, that is shrinkage. The metal contracts when freezing, which can cause the part to be undersize or have large voids. This is fixed by proper mold design, which is a surprisingly large topic and a bit of an art as well and can't be covered in a single topic. One thing to look into is adding a riser, a larger chunk that is designed to stay molten after the part has frozen to concentrate shrinkage.

DIY metal casting definitely can be done. Not everything is machinable shape wise, casting allows you to make things that are either impossible or very hard to make out of a single billet.

But like any other skill at this level of complexity it will take substantial dedication to be able to do it at a level where you can do it with a useable degree of (repeat) accuracy. Even real foundries test new molds and more often than not improve them after they learn how a particular mold behaves in practice.

Sand casting (the preferred method for low quantity bronze and cast iron runs) is error prone even in the best of cases but in the end it is very much worth it. Lost foam is another process that can work well for for instance aluminum (and is comparatively easy).

One reason why you really might want to stay from all of this is simply because it is far more dangerous than you might think when looking at the gear. But if you do go for it I would suggest starting with very simple castings and then to cut them open afterwards to check them for integrity and to learn what causes issues and how to avoid them. All the magic and the hard bits are in the prep work, the pouring is the easy part.

Forgive my ignorance, but what is there that's not machinable shape-wise, but is still castable? The more difficult the shape, certainly, the more difficult the casting?

Am I missing something?

You're not necessarily missing anything if you haven't done a lot of machining. But if you have then you probably know what I'm getting at. Tooling requires space to work, far more space than is sometimes available in the workpiece. So complex shapes, especially shapes with a lot of interior work and lots of detailing on all sides of the workpiece are much easier to make as a casting than as a milled or turned piece.

Every time you take a piece off the machine you need to re-register it, perfectly. This is doable but requires a ton of work and can severely constrain your design, after all, you'll have to re-clamp it after changing orientation.

You can easily see this by looking at for instance the output of 3D printers and then to try to think about how you would go about making the exact same pieces using substractive processes. Plenty of times you'll find that this is super hard or even that it can not be done at all.

Of course, the reverse is also true: there are pieces that you can mill and turn but that you can't make using an additive process. For every workpiece there is an optimum set of tools and processes to make it, some workpieces have a lot of options and some have only one.

When the number of solutions is 'zero' or when you have only some tools available and no access to others you have to get creative, maybe split the piece up, use a different material or come up with your own tools or processes.

> what is there that's not machinable shape-wise, but is still castable

Yes. For example imagine a 3d jewellery made of a web of thin filaments. It can be non-machinable for two reasons: The wires can be too thin individually and machining them out of a solid block would break them, while they can support themselves once cast. Or you might have shapes where one of the wires is surrounded by the other wires and you can't get the tool in.

As an example image: https://static.wixstatic.com/media/4f7223_21af8794dc874622a6...

Thank you for the example. It's clearer now. Still, I am puzzled at how you even go about making a mold for that.

Traditionally, hand carve the ring out of wax and use a lost wax casting method. These days you can 3D print in a wax filament or machine the wax in multiple pieces, hand finish, then cast.

The lathe is honestly one of the most underrated inventions of modern civilization.

Though I can't recall of the top of my head, there are certainly more than a few inventions which were simply not practical to build before the invention of the lathe because the tolerances were not good enough with traditional techniques.

The practical kind of steam engine that drove the industrial revolution is one example. Steam engines existed earlier but when they started turning the cylinders, they became much more efficient which allowed them to be used for a much larger variety of tasks.

"The Perfectionists" by Simon Winchester goes into a whole lot of detail about the history of precision and the role it's taken.

Also Machine Thinking's videos "The 1751 Machine that Made Everything" (https://www.youtube.com/watch?v=djB9oK6pkbA), "Origins of Precision" (https://www.youtube.com/watch?v=gNRnrn5DE58) and some others on his channel.

There's an entire books series on this concept, the "Gingery" books where the first in the series is setting up a foundry for casting, while the second is making a lathe from investment castings because a lathe is the only tool in a workshop which can duplicate itself.

https://gingerybookstore.com/

the balance of the books are making the other tools in a machine shop.

Had the second book once upon a time, and will eventually get the entire set.

There's an argument to be made that the rapid advancement of human technology is down to the creation of the metal lathe: https://www.youtube.com/watch?v=djB9oK6pkbA&t=751s

It sounds like you may have gone into casting with light info and unrealistic expectations for the process? Precision parts are always cast in a multi-step process wherein blanks are produced via the casting process and then surface prep and machining is applied as needed.

This post helps calibrate the expectations of those who follow. That's a good thing. It's one thing to know that something is possible, it's another to know the grade of the learning curve.

But casting is also fun and exciting. Mark of a good hobby, the waste of time also protects you from getting caught up in monetization.

> In my (admittedly limited) experience metal casting is a waste of time

Don't you mean pastime?

> It never goes right. It always has porosity. It very rarely even fills the entire mould.

It sounds like a few sets of skills that need to be mastered.

Metal casting is relatively safer and far more scalable than machining. Not to mention, who's going to be paying for all the worn lathes?

eh. I just bought a new mill because of space, but otherwise all my machine tools are from the 60s. yes, the ways are a bit worn. yes the cross-slide and table have a divots. yes, I had to pay too much for a screw because my lathe was broken and I couldn't make a new screw for it.

but they are much better made and with a little tlc are very serviceable machines. and they run less than $1/lb in the US because we shut most of that down.

Porosity can be reduced by using the correct flux.

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