What are your tools made of? Carbide will stand more of a chance either the brazed on type of a tool that takes carbide inserts.

You can get chilled areas that are very hard and these usually form on corners or raised parts that cool faster than the rest of the casting which can be a pig to machine though heat treating can help.

Great idea. Thanks for that.

With machining cast iron it is important that the depth of cut is enough to get below the skin when machining castings.

Brian

You need more than "some heat", it needs to be held at around 900deg C for a good time depending on the thickness.

What is the actual casting?

It won't help Garry because he said 'I also don't have a lot of spare metal to machine off'.

Anyway I have a block of cast iron from a night storage heater that has an astonishingly hard thick skin – a couple of millimetres that HSS won't touch and carbide struggles with. Underneath the iron has good grain and no blowholes or other defects, Iit machines beautifully. I skim the surface with an angle-grinder before trying to turn it.

I don't know if the block was chilled accidentally or deliberately. Perhaps someone in the foundry hosed it down to get a non-critical casting off the floor quickly.

Dave

I think Garry has had a hard enough time getting the castings so may not want to send them back.