Hi Mats,
That was an awesome project. I shared your video with my fellow optical and opto-mechanical engineers at work and they were impressed as well. Thank you also for filling in the details explanation..I was typing between meetings trying to explain based on educated guess. I'm assuming you have a background in the industry?
Are you familiar with Hank Karow's book, "Fabrication Methods for Precision Optics"? It would be a useful reference if you're not.
You were right to use another glass blank instead of cast iron for fine grinding. In fact you can use the crown and flint as tool and blank to grind the matching inner radius of curvature (I'm guessing you did). The cast iron tooling does pretty much what you discovered...I think it's really only good for rough grinding. Of course, you have to very carefully clean up the work area when shifting to a finer grit. The iron (and brass) tools I've seen are etched like a pitch lap.
Your illustration is perfect and explains blank generation better than I attempted.
Re: centering. Many shops edge grind after the surfaces have been finished in order to minimize wedge (which can be thought of as variation in edge thickness). You can do this on a lathe with a hollow spindle. Align a laser to the lathe mechanical axis, shining through the hollow spindle against a far wall. You have to mechanically center the laser beam (minimize both runout and tilt misalignment) then chuck up the lens on a wax chuck and center until spinning the lens does not cause the laser to scribe an arc by refraction through the lens. The optical axis is now aligned to the mechanical axis of the lathe. Then, spin the lens while you grind the edge down to required diameter. You have eliminated wedge. There are specialized edge grinders that do this in an optical shop, but it can be done on a mini-lathe with a hollow spindle.
Burgundy is ok pitch but a harder Gugolz (like 80) or Acculap pitch will give you better surface contact and thus better accuracy and surface irregularity. I prefer the feel of Gugolz with totanium oxide or red rouge (if the glass is stain resistant). But if you have good luck with Burgundy then use what works.
A glass supplier will depend on your location. I can find out who shops use for, say, New England or the Midwestern U.S. Other than that, check Schott or Ohara website and determine who their distributor is for your region. That is the best way. I would suggest using Ohara equivalent glasses in your designs..they are usually cheaper, just as good quality as Schott, and the distributors always say they have shorter and more reliable lead times.
For Ohara start here:
http://www.oharacorp.com/group.html
If you're in Germany, go to
http://www.ohara-gmbh.com/
-Jason
Thank's Jason!
Good advice and info and tip about the book. I ordered it at once. It is not easy to find book of this kind while there is an abundance of books on lens design. I did centering in a similar way but it is something I need to improve. Good advice!
I will check Ohara and see if a quote isn't to scary. I live in Sweden so I think Germany is where I can expect to get glass. I have not considered making my own glass. It would also include making the instruments for measuring refraction index and dispertion with high accuracy.
