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Thread: will such a CNC work for making an imaging lens mold?

  1. #11
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    Quote Originally Posted by VDX View Post
    ... my smallest mill-bits have a diameter of 0,3mm ... the thinnest drill is 0,1mm. This was not meant for making optics, but "only" some sensors and parts and tools for micro-assembling them Viktor
    I see. There seem to be more precise ones http://www.pmtnow.com/end-mill/micron "Micro Tool has been able to manufacture end mills as small as 5 microns in diameter" Not sure if this is the brand he is using though, just googled to find one. Of course there's probably more to precision than step precision and mill bit cutter part diameter and I would still love to learn more on what those variables are and what make a micromachining CNC optical grade. Unless the variables are "only" the ones mentioned already (temperature, humidity and vibration isolation). I also understand everyone's examples, but I still don't understand why we seem to treat the requirements of all optics equally. Are cheap chinese M12 lenses for camera modules such as in the below link and DSLR lenses made with the same optics grade CNCs and requiring the same precision? Another project where I need molds, PMMA fresnels with a custom fresnel ring setup Do these fresnels require the same optical grade molds? https://www.alibaba.com/product-deta...63a133e9cPsm82
    Quote Originally Posted by mixedgas View Post
    I built an interpolating interferometer at work that can see 10 nanometer steps. With the case off and the vertical baseplate exposed, body heat causes a measurable bias from 6 meters away. We can detect footsteps 20 meters away. The laser system in use, a heterodye interferometer by HP, has a baseline cost of about 20,000$ for the stabilized laser and another 15,000$ for the motion controller, and is the same model of interferometer used controlling diamond turning machines for custom lenses. Given the cost of just the measuring laser, are you really sure you want your own machine for this?
    if I was a company offering lens and lens mold manufacturing for any kind of lenses then I could probably afford and would probably need such a measuring equipment, but for the lenses I need this example alone doesn't convince that you'd need a measuring equipment with a 10 nanometer accuracy when 1 micron is 1000 nanometers. It comes back to my question of is there a reason to treat all lenses equally in terms of its manufacturing. If this was addressed already and flew over my head then I apologize.
    Last edited by shoujin; 04-19-2018 at 18:09.

  2. #12
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    ... yes, there are finer millbits - but then too you'll need the equipment to handle them -- think about the needed speed and precision to turn/rotate a 5 micron millbit and avoid braking by first contact with the material!

    Common bearings have a runout accuracy (correct wording?) of around 2-3 microns ... high precision ones will give better accuracy (AFAIK 0,1 micron), but actually the best commonly available comercial chucks will give you a runout accuracy/concentricity of 1 micron for the mill-bit -- this is something like 20% of a 5 micron diameter! The mill-bit in this class of chuck will brake instantly, when touching tbe surface!! You'll need special types of bearings, spindles and feeding when working with such small tools ...

    And again - the linear feeding accuracy/precision over the complete lenses size has to be below 1 micron to get it good enough for "optical" use in camera optics or laser collimating.

    Fresnell optics can be made with common tools (mostly by lathe) ... but even the cheap chinese camera optics should have better surface accuracy and curvature precision (in the range of 1 micron or better) to avoid image-distortion ...

    Viktor
    Last edited by VDX; 04-13-2018 at 00:16.

  3. #13
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    Quote Originally Posted by VDX View Post
    The mill-bit in this class of chuck will brake instantly, when touching tbe surface!! You'll need special types of bearings, spindles and feeding when working with such small tools ...
    I see, but I think he is already using those 5 micron bits? I can ask again.

    the linear feeding accuracy/precision over the complete lenses size has to be below 1 micron to get it good enough for "optical" use in camera optics or laser collimating.
    I remember but we have cameras and especially video projectors where the pixels are an order of magnitude less than what DSLRs have. Maybe that creates a different precision requirement compared to DSLRs and lasers?

    Fresnell optics can be made with common tools (mostly by lathe)
    Okay. I just checked and looks like acrylic lens mold insets can also be made with a lathe. He has one too, maybe we should try diamond turning instead?

  4. #14
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    A lot of good info above.
    The only way to be 100% sure is to have a mold and lens
    made and test that lens to see if it does what you want.
    Speculating and arguing on the CNC mold feasibility is just
    that... speculation... IMO

    Jerry
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  5. #15
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    He can definitely get few lens molds machined and injected for free.
    I just wanted to know if its worth even spending time on and even though I agree the info here is very valuable some (not all) seems to be based on opinion or similar, or the info provided is simply not complete, or I am missing some point.

    But VDX gave a good point above. If I can try get a mold made from diamond tip lathe or a 5 micron tip micromachining, which one should I try?

  6. #16
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    You are confusing two different tasks. Your initial plan was to build tooling for injection molding so you could make a custom lens out of plastic. The tooling (the mold) for this type of work needs to be finished with *great* precision, both to ensure the lens is formed correctly *AND* to ensure that you can remove it from the mold once the plastic sets. I am pretty sure you will not be able to do this with the equipment you mentioned.

    Then you start talking about directly grinding and polishing a glass lens, which is a completely different process. Could you achieve sufficient accuracy with your friend's mill to make a crude glass lens? Maybe. I don't know how good you need the lens to be.

    But I do not believe you can make a lens that is as good as the ones you can already buy on the open market for a reasonable price. As a general rule, lenses have exceptional surface quality, because when you're dealing with light it doesn't take much of an imperfection to have a large effect on the image the lens produces.

    Adam

  7. #17
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    Quote Originally Posted by buffo View Post
    You are confusing two different tasks. Your initial plan was to build tooling for injection molding so you could make a custom lens out of plastic. ...Then you start talking about directly grinding and polishing a glass lens, which is a completely different process.
    I didn't start talking about directly grinding and polishing glass, didn't even mention that about acrylic. This is what i said:
    Another project where I need molds, PMMA fresnels with a custom fresnel ring setup (ala https://i.imgur.com/YqGlesK.jpg Do these fresnels require the same optical grade molds?
    looks like acrylic lens mold insets can also be made with a lathe
    He can definitely get few lens molds machined and injected for free.
    If I can try get a mold made from diamond tip lathe or a 5 micron tip micromachining, which one should I try?
    Last edited by shoujin; 04-18-2018 at 11:40.

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