Thanks, Jonathan. I'll have a search back through forum history - I hadn't remembered this one being discussed before. My own background is electronics rather than electrics so while I understand the basics, I'm a bit hazy as to exactly what happens inside these motors. However, my thinking had also been to tweak the VF curve at the bottom end - thanks for the suggestion of using motor current to get a bit of insight.

This is the first time I have ever seriously used carbide milling cutters. I've generally used HSS on the mill and carbide for routing wood but I've been driven to carbide cutters because of this minimum spindle speed issue. I bought the ones I am using after a quick bit of web browsing but along with the cutters I was sent an enormous catalogue and looking at that, I see that there are indeed cutters that might be more suitable - at a slightly higher price... My concern about those is breaking them, either due to running them into a clamp (it happens!) or vibration/resonance due to a less than optimally-rigid structure. Ok, my machine is all welded steel with profile rails and ballscrews, but I'm still surprised-but-pleased that I can cut steel at all.

Those APT cutters look like a good deal so I'll do a bit more research down that line as well.