There was an artical in woodwork magazine about two years ago about a man from England who was making helical spirals. Only his were about 20' tall. There is a jig mounted to his lathe with a circular sawblade which rides on helical gears. Definately an engineering feet. The lathe spun extremely slow maybe 1rpm and made the cutter move in the desired direction with exact repeatability. The gear ratios determine the amount of spiral and so forth. My gesstimation is he makes a fat spiral and with the same gear ratio cuts into the middle with a ball mill. Then adjusts the gears and cuts a piece to fit into the hole. Very amazing indeed, it is rare to see something that is actually new being done with wood.