Hi komatias, thanks for the info, just had a look at that website and this is the kind of thing im after

Link

I should be more specific in that rather than an actual thread i want to use the bit to create rings inside a routed shape, sometimes it might be just one ring other times several, it would be nice to have the option of continuous threads but being able to just make the rings is the most important part. I could possibly use a very small keyhole cutter but it would need to be only about 2mm in depth of the cutting edge