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Machining cast iron in a lathe or a mill.


Machining cast iron in a lathe or a mill can be a bit of a problem for anyone new to this metal.

Cast iron, by its nature, can be very awkward to deal with.

In the process of casting, when it is at a very high temperature in a liquid state, as the liquid is poured into a cool mould, then the sudden change in temperature of the contact surface causes the liquid iron to solidify very quickly, while the remainder of the iron cools more slowly.

What in fact this does is to form a very hard skin on the surface of your casting, maybe only a few thou’ thick, and it is this surface that causes most problems to engineers.

It is mainly due to the mould not being hot enough at the start of the pouring stage of metal casting.


High speed steel cutting tools tend to get bluntened very quickly owing to the hardness of the skin, so what most engineers do is to start off with a well used tool and force it under the skin and literally rip it off. Once you are through the skin of the cast iron, then lighter cuts with a sharper tool will keep its edge longer.

Carbide tipped tools tend to work better with cast iron, but need that first layer taking off with a high speed steel tool first, otherwise you carbide tips may well shatter - depends mainly on the hardness of the cast iron surface.


Another problem associated with machining cast iron on a lathe or mill is the speed of the cut.

Cast iron needs a very slow speed to cut it properly – too high and there will be sparks everywhere and your tools will need re-sharpening very quickly. As a general rule of thumb, to get a smoother finish on cast iron, you tend to need a rounder edge to your cutting tool.


We all think of cast iron as being a solid lump of metal, but what it is in fact is a bonding of very fine particles of iron, and for this reason there can be iron dust literally floating around in the air while you are machining cast iron.

Holding a magnet near the cutting tip has been tried as a way of capturing this iron dust, but is very limited in that respect as much as the particles and dust are thrown out in all directions and are seldom close to the magnet’s attractive forces.


An adequate method of protecting yourself from this dust is needed, with a dust mask being the basic method through to a dust extractor near the cutting point being the best option, otherwise you will find yourself covered in black particles after only a few minutes of machining this metal, and that is not counting the dust you will have breathed in without any protection.

All lathe beds and milling ways should have some protection from this dust and the easiest option is to have some newspaper sheet placed over them. Do not us a cloth as if these get caught up in the moving parts, they are a likely distraction from your work and can lead to physical damage done to yourself if you make a grab at it to remove it.


So, all in all, it boils down to low speed cutting, along with using dust protection and getting that first bite under the very hard skin to produce good parts from iron castings.

If you feel you need to have a good surface finish on your cast iron parts, as usually they tend to be a bit rough after machining, then using a very fine emery cloth to smooth it down is the best option.


Generally, if you happen to have a casting that ruins your tooling, as sometimes these castings can have very large parts of ultra hard sections, you can ask your supplier for a replacement casting and they usually oblige.

All they do is drop the hard one back in the pot and recast it again, but hopefully in a better manner next time.

Happy metalworking,


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