Working with (machining) glass
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  1. #1
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    Default Working with (machining) glass

    I have an interest (and a moderate, but not immediate need) to make hemisphere's/lenses out of glass. I've searched through the forums and have found some good info, but not a lot. Both of these threads gave me a good starting point:

    Generating spherical surfaces

    Internal Hemisphere

    and I picked up the text "Precision Optical Glassworking" by Zschommler mentioned in one, which provides good detail on fly-cutting hemisphere's with a knee mill.

    So what I am really after are the tools and techniques for how to machine glass in low quantities. My understanding of the normal "grinding" process is that a tool is made (as in the "Generating Spherical surfaces thread) or a hell of a lot of grinding take place (for a lens that has a small radius of curvature relative to its diameter).

    Poking around manufacturers indicates that most are cnc machining the glass to get to the basic shape and then polishing. If I have to make a tool to polish, that acceptable, but I really want to know how to get the glass to its rough shape in a cnc machine. And nto some special glass machining machine - I've seen them and they look like the cats ass, but I need to do this with what I've got. How do I do it with a basic CNC lathe or mill (i.e., the one's I've got).

    Most of what I need would be low quantity - 5-20 units. Poking around on the internet brings up info on single point diamond turning, etc. Some corporations involved with specialized glass also list cnc operators in their careers section, so they must be doing it this way.

    Am I right or worng - can glass even be worked this way?
    Who makes the right diamond tools?
    Feed and speed info?
    Any good papers or books on the subject?

    Absolutely any info would be helpful.

    Brent

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    I've used diamond coated burrs to machine glass. similar to the ones listed at the bottom of this page Diamond Coated Drill Bits- For drilling holes in stones, pearls, ceramics, glass, tile and other hard, brittle items without chipping or cracking the objects.

    the hard part will be polishing it so there is no distortion.

    and the glass dust will wreck your machines.

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    I used to work for this company

    VOGELIN OPTICAL COMPANY

    Believe it or not we used diamond core drills set at an angle to rough out the lens and then they were diamond polished in custom made lap. It's not something that can be done quickly. We made periscope lenses, optical gun sights, wind tunnel windows, and a bunch of other special lenses.
    Constant flood coolant is the key, if the coolant coughs or quits fo just an instant the glass will explode. We had nothing more than Harding tool room lathes, modified B&S hydraulic surface grinders and a Myford ID/OD grinder.
    There was nothing really exotic at all.

    Dave

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    Default Machining glass

    Single point diamond turning requires special equipment, very stiff, laser controlled. I've machined glass parts like these on a Bridgeport with a 100 grit bullnose diamond core drill and the spindle tilted. The part goes in a slow vertical spindle, and you need coolant.

    If you've got CNC you can bypass the tilted spindle and carve it out.

    There will be subsurface microcracking. People who work glass always polish this out. However, they also use curve generators instead of standard machine tools. Curve generators often have air bearing spindles that minimize the damage. If the damage is not removed, the parts will be fragile.

    If I needed these parts I would contract with an optical shop rather than try to make them on a standard machine tool.

    If you want to do this yourself, you'll need formulas and a spherometer or two.

    John Fraser

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    Yes and no. You can machine glass with the tools you have. You can grind your own lenses. What you can't do, or can't do easily, is to mill from block of glass to polish ready lens without making a grinding tool.

    The words polish and grind are not used the same by lens makers as we use them. In the above I used the machinist definitions. I'll try to use them the same way consistently unless the text is clear that I am translating. Most of what a lens maker does to shape his lens he will call cutting ( blank drilling ) or figuring ( rough grind) and polishing ( fine grind, lapping, final polish. ) What a lens maker calls grinding he does to the rim or back of the blank to effect a non-polished surface to minimize reflections ( frosting ). Funny thing is they will call the tool a lap but the using of the tool they call polishing?

    When a lens maker sees your machine set up to cut a sphere he assumes you are going to cut a male master. The male master is used to make soft tools to hold various grits for what we call lapping and they call polishing. If you tell them you are going to cut the glass they will say you're crazy and why would you do that when you have to cut a master anyway.

    See, the thing is, the thing being the problem with cutting class on the mill, the thing is, every imperfection in you mill spindle and rotab is going to be cut into the glass. These imperfections still have to be ground ( polished ) out. To do that you need a lap. To make a lap you need a male master. If you have the male master made then roughing the blank to a hemisphere is not as time consuming as the final polish. So goes the thinking. The big shop will rough blanks to a hemisphere to have better control of the form and to save time on the lapping machines. Their lapping machines are fully utilized and any offloading of work is a plus. But you are asking about low quantities so the need to mill to a fine figure is dubious.

    Some may disagree with my analysis. This is normal and I take no offense. These are just tools and, like all machining, there is more than one way to do it.

    You don't mention the size of the final lens. This is important as the larger the lens gets the harder to get a good figure. Galileo hand ground lenses to about an inch. Amateur lens makers now do spherical lenses to three inches or so and mirrors to ten inches. Past about four inches lens grinding gets much more difficult as the lenses shift from spherical to parabolic in order to correct distortion. Not too many amateurs will attempt a six inch parabolic lens.

    There is nothing magical about diamond turning of glass. Glass requires a negative rake for strength and good abrasion resistance. The hard material and negative rake generates a lot of heat at the cutting face. Diamond's heat and abrasion resistance is a natural fit. I've got old glass books that talk about crushing the wife's diamond ring and holding the shard in a pair of pliers to do turning.
    Of course now we have PCD inserts and stacks of slick tool vendor catalogs but your wife's diamond ring probably costs less per carat ( or would without your jeweler's %400 markup. My jeweler once showed me his 'chip drawer', full of little bits of diamond. He said they were not worth very much. I asked him if he'd sell me one and he replied "Only in a ring.") I think you can see part of the problem if we are talking about taking one thou cuts with a shard of diamond on a six inch workpiece. Could take a long time. Of course, you could buy a bigger insert and push a little harder, up until your workpiece explodes like a small firecracker. We'll leave that image here while we take a small aside; If a lens maker needs five lenses he'll start with eight blanks, one to break because of a flaw, one to drop, and one to be rejected because the polish gave 'orange peel'. And that's a good lens maker. I would have to start with ten blanks to make one lens! What I was trying to say, before I interrupted myself, is it doesn't do any good to cut it if you can't polish it and it doesn't do any good to polish it if you can't test it.

    How are you going to test the final product? How are you planning to determine if the result is spherical or parabolic? Measure the surface RMS roughness? Determine coma, astigmatism, rounded off edge, occlusions, inclusions, orange peel?

    I like the real old books because they used simple techniques and simple setups. I like 'Prism and Lens Making' by F. Twyman, Adam Higler, London, 1942 ( way out of print ) because it talks about testing your creation with some cleverly simple setups and leaves all the math to the optics manuals.

    This guy is and engineer and a professional lens maker yet his setups are dirt simple.
    Mike Lockwood Telescope Making Mirror Making

    Good luck. Are we having fun yet?
    Last edited by starbolin; 10-07-2011 at 11:34 AM. Reason: Harmonized terminology

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  7. #6
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    starbolin -

    Thanks for the lead on the book by Twyman - my copy arrived today and a quick leaf through showed lots of good detail on techniques for low volume glasswork.

    Others - sorry for the delay in my response. All good input. I've probably resurrect this thread periodically as my knowledge grows and I generate more questions and need for details.

    Brent


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