Chuck Harris wrote: > Bruce Griffiths wrote: > >> Bernd T-Online wrote: >> > > >>> Bi-convex contours are more difficult to manufacture, as it is required >>> that the symmetry axis of the upper and lower contour must coincide. >>> Also other parameters become worse. For the BVA the manufacturability >>> would also be much worse as it already is. >>> >>> Regards >>> >>> Bernd >>> DK1AG >>> >>> >> Bernd >> >> Whilst traditional optical edging techniques can easily remove any wedge >> when both surfaces are convex, maintaining alignment of the "lens" axis >> with respect to the crystal axes is another matter, at least when using >> traditional surfacing techniques, with computer controlled surfacing >> techniques even this can be done. >> >> With a planoconvex "lens" bias polishing or an equivalent technique can >> be used to adjust the inclination of the plano surface with respect to >> the crystal axes and this alignment is maintained during edging leaving >> only axial thickness adjustments to be made. The etching process used to >> remove cracks and defects after mechanical polishing is anisotropic >> which may introduce further complications in maintaining alignment and >> shape. >> >> Bruce >> > > So Bruce, what source did you read that led you to change your original > answer from --it's easy--, to --it's difficult--? (reference your quoted > text below: > > No reference (unless you want references to optical manufacturing techniques), its obvious when you stop to think about it, the various crystal cuts (AT, BT, SC etc) require a specific crystallographic orientation, changing the orientation changes the characteristics. When I realised that Bernd hadn't explicitly stated the requirement to maintain crystallographic alignment when contouring the 2 surfaces the real cause of the difficulty of manufacture became obvious.
Introducing wedge when generating the spherical (other contours are much more difficult to produce unless one uses computer controlled equipment together with optical shape measurement) surfaces is geometrically equivalent to changing the crystallographic orientation of the blank. This doesnt arise when imparting a spherical contour to only one surface, as the plane surface defines the crystallographic orientation before and after removing any wedge using optical centering/edging techniques. The original statement merely indicated that well known techniques provide a solution to his actual statement of the problem of centering the 2 surfaces. > > Bi-convex contours are more difficult to manufacture, as it is required > > > that the symmetry axis of the upper and lower contour must coincide. > > > > That problem was solved over a century ago in optical lens manufacture. > If the two surfaces are spherical, then such decentering is equivalent > to adding a wedge, which is easily removed by optical centering and > edging techniques. > > > Also other parameters become worse. For the BVA the manufacturability > > > would also be much worse as it already is. > > > -Chuck Harris > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.