What power of spindle for cutting 4mm phenolic with 3mm cutter?

[Neale]

If that's all you want to do, I doubt that you need more than an 800W spindle. After all, you could probably take a cut like that with a 600W hand-held router. I think a lot of us have spindles that are significantly over-powered for what we ask them to do, but in my own case that is purely because the usual 2.2KW spindle will take an ER20 collet which allows 1/2" shank cutters; if you don't need bigger than ER11 then I'm sure that the smaller spindle would be fine. However, incremental cost in going up a size is not that great, so might be worth doing to give a margin.