One Small Step For Magnification, One Giant Leap For Home Lens Manufacturing

There are some types of projects that we see quite often here on Hackaday; 3D Printers, Development Boards and Video Game Hardware to name a few. Once in a while we see an optics-based project but those use pre-made lenses. [Peter] felt it was time to give home lens manufacturing a shot and sent in a tip about his experience.

The typical lens manufacturing process starts off by taking a piece of glass and manipulating it into a rough lens shape, either by removing material or heating the glass and forming it in a mold. These lens blanks are then lapped using progressively finer grits of abrasives until the final lens shape and surface finish are achieved. The tool used to lap the lens is very specialized and specific to one lens contour shape. This lapping process can be very time consuming (and therefore expensive) depending on the quality and size of the lens being made.

Optic Lens Manufactured at Home using CNC Milling Machine

Instead of using very specific tools to make his lens [Peter] wanted to use standard equipment so it was possible to make different lens sizes and shapes in the future. He did this by writing a parametric g-code file that can be used for any basic lens. The desired lens parameters are manually entered as variables in one location of the g-code file after which the machine control software, LinuxCNC, takes the g-code and drives a 3-axis CNC Machine to mill out a rough shape of a lens.

Three millimeter thick acrylic was used in place of glass for this experiment because it is easier to machine than glass. That is not to say there weren’t any problems during the milling process. [Peter] quickly learned that coolant was extremely important in the process. Without it, the acrylic would melt and fill up the flutes of the milling bit resulting in the bit pushing its way through the material rather than cutting through it.

The milling process did not leave a clear finish and required a lot of polishing. After becoming bored of polishing by hand [Peter] tried using a rotor tool… and then burnt a portion of the lens. Lesson learned!

The final lens is not anything special in comparison to commercial lenses but for a first DIY Lens attempt, it is amazing. If you are interested in making your own lenses the g-code file is available at the above link.

31 thoughts on “One Small Step For Magnification, One Giant Leap For Home Lens Manufacturing

    1. I’m actually thinking of polishing it automatically by making a tool with a curved surface. Not sure if it’ll work, though; also not quite sure how to make it balanced–simply gluing sandpaper would make the surface quite non-uniform.

      1. a circular tool (like a ring) with some give and sandpaper/polishing mit is what i recall seing commercial lenses polished with. may be more forgiving than a curved surface, but will have a much greater polishing time (tho i suppose not an issue is a machine is doing it!)

    2. He really needs to make the decision to go with glass or not before proceeding, obviously the difference in approach and setup is significant.

      But if you go with plastics isn’t it easier to just melt the stuff? And with glass for some large lenses they just slowly rotate a container of liquid glass to make a shape, so isn’t the forming plastic by melting it into a shape a much better approach too? Or is that ‘cheating’?

  1. The epoxy he used is not optically matched with the acrylic. Some of the defects in that step (no degassing) could have also resulted in a few of the defects he mentioned in that stage. Epoxy also has piss-poor bond strength on acrylic; CA, solvent bonding, or for a large flat bond like this a two-part acrylate would have been a better choice.

    Either way, this is pretty cool. I hate polishing and tedious steps, and given that this was just an acrylic lens and that he had a CNC mill, it might be worth doing it again, except spending that manual polishing time on a mold. The 3020-like mills seem to be only questionably good enough to do metal (though if you were going to sand it and polish down the tooling marks, I guess it’d be good enough anyway), but milling a metal mold and then casting with clear epoxy or preferably some PU would probably get better results with the same amount of work. I’m pretty sure there’s a few examples of this on the Internet, and a few of the resin suppliers certainly enjoy showing cast plastic lenses as an application for their clear resins.

    1. CA = cyanoacrylate? I think I’ll try it again using cyanoacrylate (or just a thicker piece of acrylic, though those are kind of hard to come by), a ballnose single-flute endmill and a properly working Z axis… I just found out that the chinese forgot to lubricate the linear bearings and they destroyed the rails, which I believe is another reason why the finish is so unimpressive.

        1. Given that they apparently block my (russian) IP at the firewall, I somehow doubt they’ll be willing to have business with me :p Nevertheless, this is a very interesting source. Thanks for bringing it up!

        2. I’ve had a rather thick block (10 mm or zo) of very clear acrylic from an older LCD monitor.

          In recycling this is one of teh parts that fetch decent money; it is clean, claer acrylic.

        3. If you are just experimenting look up your local plastics manufacturer ( of display boxes, sign holders etc. ) and ask them if you can buy a few small chunks. They’ll probably have a variety to choose from and most likely will give you some for free if you aren’t asking for too much.
          I gave up polishing to make a lens.. way too much work. I used an existing glass lens to press the heated plastic into the shape I needed.
          I was interested in doing a fresnel as well.. but quickly realized why fresnel was and still is in many ways a specialty manufacturing area.

          Wait until the winter and experiment with frozen water?

  2. You dont even need that. A stand, some pitch and some ceramic tile plus the right grit and you can make any lens you want to within a few wavelengths of accuracy. Amateur astronomers have been making lenses themselves for centuries.

    1. It is, but I wasn’t confident that I could DIY a nice smooth surface. It doesn’t look very repeatable to me, without at least some rotating fixture and a calibrated torch…

  3. I’ll wager that making an eccentric lapping tool, such as the one in the above diagram, would probably be worth his time if he plans to do this often. The tool can be fitted to the mill and the process (somewhat) automated.

  4. I’ve often wondered if one could print or mill out a decent pair of mould plates with which to cast a custom plastic fresnel lens from a parametric CAD file, but that would pretty much *require* computer control of a lapping/polishing process, otherwise you could easily destroy a lot of work.

    Alternatively, I challenge anyone to hack together one of these beasts for sun-tracking:
    Apparently they are filled with water in order to save material for a large lens volume, which I think would suit them to roto-moulding of something like PET.

    1. The thing is, I don’t really see anything with that product that couldn’t be done using a standard dual-axis tracker and a multi-fresnel lens concentrated PV panel?

      What am I missing, besides the aesthetics of it (and possibly – but it seems more hype than anything – the “moonlight” collection along with the diffuse light collection)?

      A large sphere of water (might have been better to use mineral oil) used as a lens is nothing new; and in fact, spherical lenses make poor concentrators of light (compared to other lens geometries):

      Again – it seems like the only claim here is marketing hype with the aesthetics claim. It certainly looks more pleasing to the eye than a PV flat panel on a dual-axis tracker (and likely much more pleasing than that same panel with concentrator lenses attached). Whether it is truly more efficient seems unlikely to me, since if it was, PV solar companies would have latched onto the idea decades ago.

      It also lacks something that PV panels (even dual-axis tracking ones) can provide: Shade. This can be very important (and desirable) in places with a lot of sunlight and heat (like Spain, where Rawlemon is located).

    1. I’ve thought about the Frensel lenses. The problem is that you essentially have a singularity at the shear point; in other words, you have a complex horizontal profile that is immediately adjacent to a vertical surface. I don’t think you can machine something like that on a 3-axis mill out of a single piece.

      It’s probably possible to use a lathe and a narrow-angle cutter, or make it out of several concentric rings…

  5. A few months ago I made a 400mm diameter planoconvex lens using a similar method. It works reasonably well. For polishing, I first used fine sandpaper to smooth the ridges, and then used a buff mounted on a power drill saturated with a mixture of cerium oxide and mineral oil. I think it’s a viable technique to make rough optics for special purposes, mine was to be used to project the view through a window onto a wall in a darkened room, but it’s not a very practical method to make optics one could otherwise purchase. For my needs, a 400mm diameter lens with a 3500mm focal length was just not the sort of thing you can find available anywhere, so milling it in this method in this case proved to be reasonable.

  6. Don’t know if this helps anyone, but I am a CNC machinist that works now only with plastics, some fairly exotic, like Ultem and PEEK. Maybe this info will help those trying to machine plastic lenses.

    The best surface finishes we get in our shop are with odd number multi insert carbide facing tools with zero rake. Best surface finishes I get are usually with a very specific speed, anywhere from 1000 RPM to 1500 RPM, with a finish pass of no less than 0.005″ to no more than 0.010″ stock removal. More specific than that might be trade secrets I’m afraid :) But that should give you a starting point for many plastics.

    Speed and feed sync optimization are supercritical for optical finishing on plastics- most will melt some if you don’t get them perfectly matched to your material, it’s obviously more difficult to finish well than metals. Those little details are how we make better parts than the rest.

    We do do clear acrylic sometimes, but I don’t work with the stuff, so I can’t advise good feeds and speeds to start with that.

    I’m also trained in lapidary work and have used optical lens grinders to custom size pocketwatch crystals, and then hand lap them optically clear. Best results I’ve had lapping glass optical blanks for pocket watch crystals used Alumnium Oxide powder for rapid polishing, and for final polish, use Cerium Oxide- it is by far the best polishing compound for lapping glass. That’s what commercial lens grinders still use, from what I was told. Diamond wheels and constant water coverage are your friend for rough grinding to shape.

    Hope some of that helps someone! Thanks AKA the A for the cool pdf link too.

    1. It’s the same for lathes, manual or CNC. Tool bit geometry and material and speeds and feeds are all important. If you change one of the three, you should count on having to do some test cuts and experiment with the speed and feeds to find the sweetspot.

      We oldtimers scoff at these newfangled carbide insert cutters. Real toolmakers grind their own :-)

      A couple of videos on tool bit development, see Oxtool’s channel on youtube.
      Tool Bit Development 1

      Tool Bit Development 2

      Chip Control to Major Tom

      Now I’m not recommending hogging out chips like he’s doing, heck those chips are bigger than most of the parts I’ve made tooling for, but these methods scale down to small fine detailed work as well. I’ve got to say I respect that man. I’m not sure my skillset scales up to bigger stuff he builds, but I’m not sure his skills scale down to what I did either.

      NASA recommends single point diamond turning for optics to minimize polishing time for aluminum mirrors and plain old draftsmans India Ink for the polishing compound, I wanted to try that out myself but haven’t gotten around to it. On the backburner for nearly two decades.

      Keep in mind for polishing that the tool side of the polishing has to hold the abrasive, ie be charged with the abrasive, and to hold the abrasive it needs to be softer than the material being polished. That’s why optical pitch is used, or why brass barrel laps are used in steel, and cast iron lapping plates for lapping hardened steel and glass and carbide.

      The MILITARY STANDARDIZATION HANDBOOK: OPTICAL DESIGN MIL-HDBK-141 is a useful reference for optics work as well.

      It comes down to experience. Many don’t understand how much time that a lot of us machinists have put into running single machines. I explained it once to the Missus like this, if the machines (Wire EDM in my case) were cars driving at 55 MPH, I’ve driven one particular machine to the Moon and back more than twice. Once you’ve logged 2000 hours (one work year) you’ve probably have a pretty good feel for the machine, 20,000 hours and you’re insanely intuitive of it. We rarely did production work, nearly everything was one of a kind, so it was lots and lots of setup. You know you’ve been working when you lose track of how many carbide indicator points you’ve worn flat.

  7. you know if we work somehting like this down to a simple repeatible proses we could make it easy for aid stations to make Negetive grade lenses for glasses (nearsighted) producing anything higher than -3 or -5 right now is kind of hard.
    I myself am curintly using over -30 do to extream miopea and cataracks bluring things up

    It’s gotten so bad last time i whent to the docs they got the script so wrong i ended up mounting test lenses to my glasses to corect the underpower

    Yes i could force the point but the last time they came up with -25 and it took a -2 and -7 lenses to get a decent enough focus my glasses cost nearly 700 a pair without frames as is. i wonder what a – 34 would hit me for

    i’d love a rig that would allow me to custom grind pollycarb from eather a blank (genril shape lense but not the final script) or near mold ( basickly the script with just a tad more meterial to grind/polish the finished lense.)

    with a combenation of 3d printer to make the premold and a hald desent way to polish and finise the power without going indrustrial would be wonderful

    with such high scripts we dont need opticly pure lenses most of the time. we’ll never notise minor defects in relation to our allready poor view of the world :p

    This has got me thinking now :p

    i need to ffind a sorce for liquid pollycarb (it’s a binery liquid and hardiner for clear )

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