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A_P_O_L_L_O

Polishing glass plates

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I am starting to learn that almost all optical companies refuse to polish large pieces of glass  (200mm+) to anything beyond 1/4-lambda because they have deemed it impossible for themselves and too expensive for humans that want to buy things.

 

Now I would like to attempt doing this myself because I have this little bit of OCD where I believe nothing is impossible..

 

Now,  I have had  a quartz Van  Keuren 1/20-lambda 150mm interference flat sitting under my bed for the last 10 years.    Is it at all possible,  feasible, or even, worth trying of using this as the "lapping apparatus" on a piece of optical glass?

 

My thought process is, if my reference flat is made of quartz, which is way harder than any bit of optical glass,  can I rig'a'tron this to a lapping machine on an optical grade leveling table?

 

Then using cerium oxide slurry that is constantly digitally re-flowed and refilling,  over time would my piece of soft glass eventually match the 1/20lamda reference quartz? 

 

My goal here,  is somewhat secretive because I am an inventor developing new things. But I can say, That,  I want 400mm diameter pieces of glass, 10mm thick  that are plane-optically polished on both sides to a minimum of 1/10-lambda  

 

Am I nuts?  Or do I just need to develop the proper orbital rotary lapping process to become automated with an arduino and some linear actuators on timers..    

 

It just seems like as long as I have just the right amount of weight on the van keuren and the right slurry mixture between the glass plate and first keuren surface than its going to liquid level to the flattest surface.   

 

Just like a rock tumbler,  it might take 30 days, but hey,  if thats what it takes then thats what it takes?  Right?   people have hand ground optical lenses for 100's hundreds of years, so why is it so difficult to use motors and digital controllers to make sure everything is automated on a simple flattening process?

 

 

Call me crazy! Call me a genius!  Just dont call me.

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I'm a great believer in experimentation but my gut feeling is that this project isunlikely to be successful, not impossible but unlikely. The thicknesss to diameter ratio of the 400mm work piece to the 150mm diameter of the quartz flat suggests that even orbital polihing action could result in zones. Although quartz is harder than glass, all cutting devices are harder than the materials that they cut but the softer materials always end up blunting them. Still, what do I really know about this?. Good luck.  😀

Edited by Peter Drew
typo

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Polishing glass surfaces is normally done with a soft polishing material ( felt, polyurethane polishing pads, pitch etc ) and even then the Cerium Oxide can cause some scratches ( called sleeks ) when it has some agglomerates in the slurry.  I am pretty sure that putting Cerium Oxide between two hard surfaces will produce more of a grinding action. Try it with scrap pieces of glass.

The last thing I would do with your 1/20 wave reference flat is to try to use it as any sort of a polishing tool. You will need it to check the flatness of local areas of your larger surface after polishing stints. To check the whole surface as one will require a different technique.

Your main problem will be in holding a 400mm dia piece of 10mm thick glass without allowing it to flex during polishing. I have had a 500mm dia mirror 40mm thick flex producing triangular astigmatism when placed on a layer of bubble wrap with small ( 6mm ) bubbles. The bubble wrap has, of course, triangular symmetry.

You could use a technique sometimes used for Schmidt windows where the sheet of glass is placed on a cell supporting only the edges and a slight vacuum is applied to pull the glass into a concave shape. The surface is then polished to a sphere which then reverts to the Schmidt profile required when the vacuum is released. You would, of course, polish to  your required parabola (using normal mirror testing techniques) and it would then be flat when the vacuum was released.

Nigel

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21 hours ago, Peter Drew said:

I'm a great believer in experimentation but my gut feeling is that this project isunlikely to be successful, not impossible but unlikely. The thicknesss to diameter ratio of the 400mm work piece to the 150mm diameter of the quartz flat suggests that even orbital polihing action could result in zones. Although quartz is harder than glass, all cutting devices are harder than the materials that they cut but the softer materials always end up blunting them. Still, what do I really know about this?. Good luck.  😀

 

19 hours ago, Astrobits said:

Polishing glass surfaces is normally done with a soft polishing material ( felt, polyurethane polishing pads, pitch etc ) and even then the Cerium Oxide can cause some scratches ( called sleeks ) when it has some agglomerates in the slurry.  I am pretty sure that putting Cerium Oxide between two hard surfaces will produce more of a grinding action. Try it with scrap pieces of glass.

The last thing I would do with your 1/20 wave reference flat is to try to use it as any sort of a polishing tool. You will need it to check the flatness of local areas of your larger surface after polishing stints. To check the whole surface as one will require a different technique.

Your main problem will be in holding a 400mm dia piece of 10mm thick glass without allowing it to flex during polishing. I have had a 500mm dia mirror 40mm thick flex producing triangular astigmatism when placed on a layer of bubble wrap with small ( 6mm ) bubbles. The bubble wrap has, of course, triangular symmetry.

You could use a technique sometimes used for Schmidt windows where the sheet of glass is placed on a cell supporting only the edges and a slight vacuum is applied to pull the glass into a concave shape. The surface is then polished to a sphere which then reverts to the Schmidt profile required when the vacuum is released. You would, of course, polish to  your required parabola (using normal mirror testing techniques) and it would then be flat when the vacuum was released.

Nigel

 

 

What about a method that combines the technology of  wave soldering machine with that of an air knife?    If the grinding mixture is flowing the slurry at a perfect 90 degree angle that is completely uniform pushing the slurry wave's,   Would the flatness be that of the molecular surface tension of the liquid slurry pushed by the the precise air knife?

Again this brings me back to the theory of a rock tumbler.  The rocks tumble in the aluminum oxide and it just slowly erodes over time from the rotational flow.  Now the same thing happens with a horizontal flow, or circular flow.    It is just a matter of establishing how to predict the flatness of the flow itself.      An air knife is like a laser, and the wave solder machine develops uniform crests with digital control tidal peaks

I dont have to start with 400mm, that is just my long term goal once I establish a method of getting an assitsed grinding DIY technology established.

 

cropped-air-knife-demo.jpg

wave.jpg

 

https://upload.wikimedia.org/wikipedia/commons/transcoded/d/db/Solder_wave.ogv/Solder_wave.ogv.240p.vp9.webm

Edited by A_P_O_L_L_O

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There is no reason why techniques from one discipline cannot be adapted for use in another. However, I have doubts that the air blade/wave system would work to 1/20th wave accuracy. You will just have to try it to see.

Before trying any polishing technique you must be able to determine the accuracy of the surface you produce and for that you will need to build/buy some test equipment ( which I assume that you don't have at the moment ). I suggest that you get a 400mm dia float glass sheet and work on being able to determine the profile of it's surface ( spoiler: it will be horrible on the scales we are talking about ). Once you have done that check the profile of your reference flat. When you are satisfied that you can determine errors as small as 1/20th wave then you can start trying to flatten glass to those tolerances with whatever method you want- just don't try using your reference flat as a tool- you need it as a reference!

Nigel

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Of all the things the amateur optician may attempt then optical flats are one of the most difficult. It assumes some experience with making mirrors and perhaps objective lenses first to gain the vital knowledge to proceed onto flats. The above assumes that three, normal thickness, low expansion blanks are used. Each worked upon another in a strict sequence.

 

After allowing each blank to cool they would be brought to the [crown jewels] reference flat and tested. Plane parallel 16" both sides to at least 1/10? Why not? It would requires a stable temperature and dust free room if quality is desired, long experience at testing against reference flats under monochromatic light source. It requires enormous patience, years of learned optical working skills and great care to achieve even modest results. Don't forget that your plate will distort under gravity and you must allow for the curvature of the earth and gravity waves.

 

The only way I know of to polish a thin flat is to polish it on a proper polishing machine with a suitable ring for driving the thin blank. How big a ring? No idea. NASA might know but I wouldn't bet on it. Polishing takes place very slowly on a pitch lap which is already almost perfectly flat. Or as near to flat as very long experience at polishing perfect flats suggests. The glass may not be weighted or it will distort. It may not be ignored while running without constant attention to the polishing medium. Automatic polishing machines will have a pump to keep the pitch lap and polishing medium from drying out. I doubt that is good enough for your needs.

 

If you have the required qualifications, temperature regulated room, vast optical experience, vast amounts of time, a very large flat to flatten the pitch lap, a decent polishing machine and a monochromatic light source in a proper set-up for testing then why not give it a go? If you lack any of these items, conditions or personal qualities then the cost of buying your finished optical flat will seem excellent value. :thumbsup:

If you want to work your way up more gently to making large, thin, plane parallel, optical flats to 1/10th, in optical glass then perhaps you could turn your inventiveness to [say] reaching Mars, using only the expertise available on astro forums. :biggrin:

 

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