Cast Iron Alloys

[Commander_Chaos]

Toying with the idea of having a part cast for my lathe. The foundry I have in mind told me that they only pour "gray iron". My dad had them cast wheels for carts and they did a really nice job but is that an appropriate metal for this use?

[Mcgyver]

Not knowing the part, probably. Ductile would be other common cast iron. The huge majority of lathe castings would be grey. Ductile is. well, more ductile (you can a curly when drilling, its that ductile) and a bit stronger. Grey has better vibration damping properties. Both are great to machine. All the ductile I've machined (some tonight actually) arrived as bar stock (as in durabar), not sure how common it is with casting work, i.e. foundries and patterns.

[mikey553]

ASTM A278 standard for pressure-containing castings specifies these classes of gray iron: 20, 25, 30, 35, 40, 45, 50, 55, 60, where the class number represents the minimum tensile strength of the material. We have used class 40 castings at work for many years, they are very good.

I am sure there are other material standards for gray iron, but I don't have them. Ask the foundry what classes of gray iron do they pour and decide for yourself what you need.

[J Tiers]

Not knowing the part, probably. Ductile would be other common cast iron. The huge majority of lathe castings would be grey. Ductile is. well, more ductile (you can a curly when drilling, its that ductile) and a bit stronger. Grey has better vibration damping properties. Both are great to machine. All the ductile I've machined (some tonight actually) arrived as bar stock (as in durabar), not sure how common it is with casting work, i.e. foundries and patterns.

Is not "ductile", actually white iron of a certain type, that has been heat treated? You could not pour "ductile", but you can pour the material it is processed from.

[boslab]

Also known as SG iron, bendy, machines ok, is ductile about 60000 psi ( though seen higher and lower) ideal for what you propose
mark

[Richard P Wilson]

I some of the castings for my home build planer done in SG (spheroidal graphite) iron. The bridge and the bridge standards, which are subject to bending forces. As Mark says, its more ductile than ordinary grey iron. My foundry poured SG on a regular basis, and the cost was roughly the same. It machined well, the only disadvantage was that it produced a lot more very fine graphite dust which put a light covering on the machine and sometimes the operator! When drilling, the cuttings came off as curly, not the usual iron chips.

[Richard P Wilson]

Is not "ductile", actually white iron of a certain type, that has been heat treated? You could not pour "ductile", but you can pour the material it is processed from.

I think that type of heat treated iron was known as 'malleable iron', at least here in the UK. As far as I know, 'ductile iron' is castable, because I've used large quantities of ductile iron water and sewage pipes, up to 36" dia, and I once was present in the foundry when they were being made, cast centrifugaly in a rotating mould.

[mikey553]

The foundry OP mentioned can only pour gray iron. Why are you talking about nodular iron? It may or may not be better for his part, but it is unavailable.

[Commander_Chaos]

The foundry OP mentioned can only pour gray iron. Why are you talking about nodular iron? It may or may not be better for his part, but it is unavailable.

My real concern about this part is that the machined surface of the casting will be in contact with the ways. It clamps, rather than rides on it, but I've heard cast is more abrasive than steel. I thought possibly that quality is greater in some alloys.

[mikey553]

My real concern about this part is that the machined surface of the casting will be in contact with the ways. It clamps, rather than rides on it, but I've heard cast is more abrasive than steel. I thought possibly that quality is greater in some alloys.

You do not make any sense. Why don't you provide full details? What part of the lathe is it? Can you post any pictures? If you want a real help, provide all the information.

[Mcgyver]

My real concern about this part is that the machined surface of the casting will be in contact with the ways. It clamps, rather than rides on it, but I've heard cast is more abrasive than steel. I thought possibly that quality is greater in some alloys.

The idea of cast iron being is abrasive is false. It comes from the fact that can be somewhat abrasive to machine because of the bits of sand and scale on the outer shell (hot rolled for example also has scale which is abarsive) Once you are through that, its easy to machine and is a far better material than unhardened steel as bearing surface.

[Willy]

The idea of cast iron being is abrasive is false. It comes from the fact that can be somewhat abrasive to machine because of the bits of sand and scale on the outer shell (hot rolled for example also has scale which is abarsive) Once you are through that, its easy to machine and is a far better material than unhardened steel as bearing surface.

Precisely!
This is why engine blocks have been cast for decades from gray iron castings, it's natural built in lubricity due to it's graphite content.

[J Tiers]

I think that type of heat treated iron was known as 'malleable iron', at least here in the UK. As far as I know, 'ductile iron' is castable, because I've used large quantities of ductile iron water and sewage pipes, up to 36" dia, and I once was present in the foundry when they were being made, cast centrifugaly in a rotating mould.

You are correct. Ductile is also known as "nodular", and is achieved by adding certain things to the iron. I had them mixed up.

[Baz]

Can you get small one off castings at even approaching sensible prices in the USA? Very rare in the UK and cheaper to machine from solid. There is currently a thread on the model engineer forum about CNC machined blocks to not quite finished position to simulate castings for hobbyists to 'finish off' as it is becoming more practical & available with no local foundries left.

[mikey553]

As far as I know USA still has functional gray and nodular iron foundries taking orders on one off castings, but I only have experience with castings weighing 500-5000 lb. You need to provide a pattern of course. If I remember correctly, we were recently charged about $1 per pound of finished weight.

Cannot speak about small castings. I suspect there is a problem placing orders for very small quantities or singular castings. I had to convert some of the compressor parts from cast iron to solid steel just for that reason alone.