Hi Matt, I turned some down to make some sample counter weights, it is tough but not so hard. I used a tipped tool but HSS will also cut it, the components I made had a 6m thread as well and the tap was a standard HSS tap but it was tough going. Lube well with a good quality cutting oil and you should be OK.
Many years ago the company I worked for made a lot of counter weights for aircraft and the material was called heavy alloy, it was largely tungsten , not sure of the other components but again there was no particular problem machining these with normal tooling.
regards John
Edited By JohnF on 10/07/2014 18:32:55