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Thread: Replacing Scanner Mirrors

  1. #1
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    Default Replacing Scanner Mirrors

    I'm wondering if anyone has done a tutorial or can direct me to a site that will provide a guide to replacing the mirrors on a galvanometer shaft. A few issues come to mind. What are the best ways to remove a mirror and then prepare the shaft. What are the best adhesives and if any surface prep needs to be done to the glass or the steel shaft. How do you assure a balanced glue-up and how tolerant is this regarding the balance (this is the issue that has always deterred me).

  2. #2
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    Its not too hard. I have done it a few times.

    Use a soldering iron to heat the glue directly. Don't heat the shaft like you're trying to "unsolder" the mirror

    Use a sharp knife to remove the glue. Don't use any kind of solvent as the vapors can enter the bearing. It will come off very easy.

    Clean the glass with acetone.

    I use standard 5 min epoxy.

    I have done the center the mirror by eye and hang upside down trick.

    If I were to do it again I would make a jig.

  3. #3
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    If I were to do it again I would make a jig.
    This is where a 3D printer comes in handy

    I've printed up jigs for mounting mirrors to flex mounts to keep them nice and square and allows them to be clamped while the glue dries
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  4. #4
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    Don't use superglue or other CA based adhesives. Put a small amount of epoxy on either side of the shaft on the back of the mirror, it seems to stop the epoxy from fracturing.

    Steve
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  5. #5
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    I've done it using the above methods--soldering iron, jig and epoxy. Afterwards you will have to re-tune the scanners, as they will be wayyy off.

  6. #6
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    I've also done it using the above method, sans jig. I got REALLY REALLY lucky in that the scanner performed acceptably (for my purposes) without re-tuning.

    A locking hemostat is very handy for immobilizing the scanner shaft while simultaneously acting as a heat sink while you're heating the mirror slot for gunk removal.

  7. #7
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    OK, this sounds reasonable. I plan to mill a jig for each mirror. Based on my own experience gluing knife edge prisms and small mirrors a jig makes the result far easier to perfect. My design was to construct a jig with a locking bore to mount the motor and this will be centered in a gap with a width, 20-50um smaller than each mirror. As the mounted and glued (still soft) mirror is slowly rotated by rotating the shaft/motor the clearance will disappear and the mirror will sit slightly canted (in rotation) in the gap, but centered and in line with the shaft. This should allow subsequent mirrors with very slightly different dimensions to use the same jig. Clever, or am I missing something? Aside from the need to re-tune have people found that a lack of perfect balance has caused failure or unacceptable vibration? How forgiving is this?

  8. #8
    mixedgas's Avatar
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    Its actually pretty forgiving. Jigs are almost overkill. All you want perfect is that the mirror is square to the shaft, left-right tolerance is no where as important as square to the shaft. Odds are if the mirror has the same inertia, retuning is pretty much negligible. On most amps you can turn the servo gain down to near zero or zero and slowly bring it up while watching a 30 Hz square wave on the scope, This lets you see the rough tuning and roughly adjust the damping without incurring oscillation.

    On the amps you have Eric, there is one slim region where the right combination of too much servo gain and being well over critical damping might oscillate.
    Drop me a email and I'll send you super-detailed tuning instructions. (Ones I can't post on line, for copyright reasons)


    Steve
    Last edited by mixedgas; 05-28-2014 at 12:59.
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  9. #9
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    Eric;

    I've replaced mirrors on at least 3 scanners that I can think of off the top of my head. I used the soldering-iron trick as mentioned above on all of them, and used 2-part epoxy (I've tried both clear epoxy and QB-quick; either one works) to glue on the new mirrors. I've never used a mounting jig (never had one), and in two cases I actually flipped a broken mirror around and glued the remaining "good" end to the shaft, leaving the broken end protruding.

    Re-tuning was not absolutely required, as even with the change in mirror mass the test pattern still looked pretty good. A little off, perhaps, but good enough to display most artwork without any real problems. Admittedly, to get it PERFECT I did have to do a little tweaking. I was quite surprised at how well they worked without the re-tune though.

    Bottom line: if you are going to use a jig, you'll have no trouble at all. I eye-balled mine, and they still work fine. (Two of the three sets I fixed are still in service today. I sold the third set a couple years ago.)

    Adam

  10. #10
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    I have thought about building a similar jig to what you describe. Machining something like that would not be trivial.

    To simplify things, I thought about making a collar out of nylon or abs that is bored with close tolerance to the galvo body. Then a smaller bore for the mirror. Both holes would have to be very good.

    Check my ghetto paint drawing.

    Click image for larger version. 

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