@Paul_Frederick why do you assume people are using routers in their hobby builds? I run a 3KW water-cooled spindle with full load/speed control in mine It worked out about $250 more expensive than a decent router. The runout is small, it has an ER25 collet, 6 ceramic bearings and can cut all day long.
That being said, I cut plywood, but you never know 
@Mike_Thornbury
Because routers are what most gantry machines use. So it is a safe assumption to make.
An assumption, at any rate.
All the gantry machines I have seen are using vid-powered spindles.
I guess the Shapeoko crowd are using dreamers and routers, but they are toys.
@Mike_Thornbury
Yes, it is an assumption based on having seen just about everything out there. If all of the gantry machines you see are using VFD spindles then that must be because you are looking no further than your own machine. I could say something about your Far Eastern spindle now too, but I wonāt.
While looking for the formulas for cutting speed calculations, I came across these. They seem pretty useful, so I thought Iād share them (apologies if this is 101 level stuff everybody here already knows):
Wikpedia: Speeds and Feeds (Iāve read thisāseems like a good start for a layman like me)
Autodesk: CNC Handbook (looks like lots of useful info here)
http://cam.autodesk.com/docs/cncbook/en/#start
Way beyond the scope of anything I will be using, but this is interesting (tl;dr: they are creating a database of tool harmonics to improve tool cutting efficiency for a specific tool/bit combination):
Machine Tool Genome Project
@SirGeekALot
I use a formula to calculate cutting speed. (Ļ X Dia. X RPM) / 12 = SFPM Plus I made myself a grid chart I have hanging by my mill with some speeds over diameters calculated that I use for rough speed determinations.
@Paul_Frederick Just as well you donāt āsay something about your Far Eastern spindleā - you mean my German spindle, donāt you? Fuck loads of assumptions on your part. Try dropping your ego levels a bit.
@Mike_Thornbury I misunderstood you when you said it was only a bit more than a router then. Who manufactured your spindle? Where did you buy it from?
@SirGeekALot Iāve not used this but looks pretty good from browsing YouTube videos http://www.cnccookbook.com/CCGWizard.html