I want to drill a cone-shaped hole in some thick wood. The hole needs to be about 75mm deep and 45mm diameter at the surface. The *accuracy* of the dimensions isn't so important, but I do want the *finish* to be very smooth inside the walls.

I tried drilling with hole saws and forstner bits to produce a series of "steps" to the bottom, this worked well but I'm now struggling to grind away the steps into a smooth cone.

I was thinking of using a step-drill to finish the hole - but they don't seem to make them this big - and they are very expensive. 

So I'm looking for either a way to drill a cone from scratch - or a way to grind the "steps" to make the walls smooth.

I have a wide array of power tools and hand tools but no lathe or CNC machine!