Quote Originally Posted by richard ide View Post
AFAIK lens polishing using a pitch lap is capable of holding or adjusting curvature to approx 1/4 wavelength of light. I do not believe that a CNC generated profile would come close to that although possibly the highest quality machine (read highest cost) might come close. Having ground and tested a telescope mirror; I still marvel at the close tolerances obtained by a hand process. There is a very good reason that lenses are made the way they are.
It would be a matter of having the equivalent of a pitch lap mounted to the CNC bed, to achieve the final curvature.

And you are right about the precision possible with hand work. I worked in a shop that was making a small double side 7075 aluminum turbine impeller with extremely tight tolerance on perpendicularity to axis, and width and parallelism between sides. The brand new Hitachi CNC machine could not hold the tolerances. So they bought a new Sunnen (top name) machine to lap the two sides into parallel while maintaining perpendicularity. It couldn't do it without taking way too long- a serious production bottleneck. We ended up doing the finish work by hand using a surface plate with sandpaper taped to it. After a little experience, it was easy to judge the amount of pressure to use and where to apply it. Measure, sand on 400 grit, repeat as necessary. Switch to 800 grit to refine the finish and bring to final size, then put the part in the Sunnen to establish the final finish, or switch to 1500 grit paper and do it by hand.

The final finish was not necessary to bring the part into surface finish spec., but established the look expected of precision machine finishing.
It was a political thing. They bought brand new machines that "of course" could do anything the salesman said, and if the machines couldn't, it had to be our fault-they would not accept that hand finishing could be more precise than one machine and faster than the other.