They work very well but they have to be turned slowly (acw) so as not to overheat the tool and spoil its temper. Also the sawdust must be cleared fequently because there is no kerf on the teeth and they soon clog up. To prevent the tool from initially wandering, it is helpful to make a simple guide, ie a piece of scrap ply drilled with a hole the same size as the extractor, which is clamped centrally over the broken screw.
I have one that I bought from, I think The Wealden Tool Company, years ago and another that I made from silver steel round bar, bored out and the teeth filed by hand before hardening and tempering.
Thanks,that’s exactly what I’m looking for. They are on an American site but very expensive.
I was hoping for a UK supplier. The ones show on Google are all quite small,the large size is 10m and a different pattern.