Well the obvious choice (see my username) would be a lathe. :)
Similar to what is done to make fluted columns, you could mount stock between revolving centers and build a trammel to hold a router that makes cuts angled to the stock.
e.g.
(Except imagine one end of the platform higher than the other to make a cone.
Technically, you wouldn't even need a true lathe, just a couple of sharpened bolts in a frame to create points about which the stock can rotate.
image via: http://customfurnitureandfab.com/how-to-make-fluted-or-beaded-columns
