Friction is the most reliable force on the planet.
And a different pattern will give you a spherical hollow, and then even a finally,
slightly different pattern will approach a pure parabola within an 1/8th wavelength
of visible light. I made a telescope mirror this way. One pattern for roughing to
a spherical shape, coarse, fine, finer, finest. Then 'figureing' to take off a few millionths
extra in the right places to go from spherical to parabolic. Simple tools can do some
truly amazing things, read Ian's signature line one more time.
Bill