[HN Gopher] Making a Concave Mirror Using 15th Century Technolog...
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       Making a Concave Mirror Using 15th Century Technology (2018)
        
       Author : Tomte
       Score  : 21 points
       Date   : 2021-04-17 07:37 UTC (1 days ago)
        
 (HTM) web link (wp.optics.arizona.edu)
 (TXT) w3m dump (wp.optics.arizona.edu)
        
       | [deleted]
        
       | abdullahkhalids wrote:
       | > It turns out that if you rub two bars of metal (or pieces of
       | glass) across each other with grinding paste in between, one
       | surface naturally ends up convex, and the other concave. The
       | reason is that when the pieces aren't directly over each other
       | the same amount of downward force exerts a greater pressure on
       | the portions that are still in contact, causing more material to
       | be ground away at the edges of one piece to produce a convex
       | surface, and from the center of the other to produce a concave
       | surface.
       | 
       | Does someone have a deeper explanation than this?
        
         | DoctorOetker wrote:
         | suppose the 2 plates have the same dimensions, and you start
         | grinding, by rubbing them over each other with dilute paste,
         | 
         | then part of the time the edges are uncovered, while the
         | centers are nearly always covered,
         | 
         | so the centers should grind faster. and both try to go concave,
         | 
         | this is however self-limiting: if the centers are ground
         | slightly more than the edges, then the edges protrude and would
         | grind faster again to catch up.
         | 
         | the used motions translating and rotating in 2 dimensions,
         | result in spherical surfaces being the only solutions, concave
         | and convex, flat and flat, and convex and concave.
         | 
         | its just that because of the initial phenomenon (the centers
         | trying to grind faster than the edges, because the edges are
         | unexposed part of the time) that you get spontaneous symmetry
         | breaking and you get a concave and convex surface pair.
         | 
         | this unstable equilibrium at flat can be made stable by using 3
         | surfaces, A,B,C and going through AB, BC, CA, and repeat, so
         | that the only valid solution becomes an optical flat
        
         | tsomctl wrote:
         | Go read up on the three plate method to manufacture surface
         | plates.
        
       | [deleted]
        
       | tsomctl wrote:
       | Queue the machinists shuddering at the polishing compound sitting
       | on the table of the vertical mill.
        
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       (page generated 2021-04-18 23:01 UTC)