Super-smooth finishing of diamond turned hard X-ray molding dies by combined fluid jet and bonnet polishing

Fabricating super-smooth aspheric optics for future hard X-ray telescopes will require a new process chain. Whilst diamond turning of electroless nickel plating can generate ~100 nm P-V aspheric molding dies, associated turning marks must be removed before replication. An innovative two-step freeform finishing method is presented, that combines fluid jet and precessed bonnet polishing on a common 7-axis CNC platform. The removal rate and surface texture relationship of fluid jet with abrasive type and pressure is documented, and form correction demonstrated down to 27 nm P-V. A novel bonnet tool-pathing method called “continuous precessing” is then applied, which delivers super-smooth anisotropic surface texture of 0.28 nm rms.