Okay, I’m a total newbie at this but our library has a nomad 883 that I can use and I want to use it machine out some custom acrylic lenses for some optical experiments. Does anyone foresee any potential problems? They’ll be fairly small and light (15mm thickest at the most, about 40 mm wide). Mostly some concave lenses. Thoughts?
The Nomad only has 3 axis of movement meaning the cutting tool only approaches the work piece from one direction. With appropriate techniques to ensure accuracy, you can do multiple sided machining, but a smooth curve like a lens will be approximated within the confines of your cutting tool and step over. It might work, but I suspect you’re gonna need to do significant work on the lenses after the Nomad is done with them.
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Using the Nomad for the rough shaping is very do-able. I don not think you will be able to get a finished lens. You will need to do final figuring and polishing after machining. I would guess you can get to within 0.05mm (50 microns) or better using fine finish cuts and a good ball end tool, but the surface will not be optical quality and will have scalloping (waves due to the tool end shape). Doing final figuring by hand is not terrible, but is definitely not for the faint of heart the first time. The tools to support this process (surfacing and measuring) are likely more money that it would cost to purchase pretty much any set of lenses unless you already have them, though you can make some of them yourself.
Commercial manufacturing is done by casting and final polish with a machine for lower end, and grinding and polishing for higher quality. These days the polishing is done on specialty CNC machines that avoid the artifacts you get with an x-y-z coordinate design like the Nomad and hold a micron or less positional accuracy.
I would not discourage you from trying (I ground a few lenses and mirrors back when I was young and was into telescopes–technology has come a long way since then), but this machine will not do the hardest/most time consuming part for you.
If you try, let us all know how it goes. I would love it if you can prove me wrong here.
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Just curious, would it be possible to use the Nomad to make a mold, which you could then sand and finish, and use that to cast the acrylic?
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Machineable wax is your friend! You could make a lens blank at the library, flame polish it, use it to make a silicon mold and probably be ahead of the game with not nearly as much hand polishing to do.
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That sounds super fun. Does anyone have an STL of a lens that I could use to test this? I know absolutely nothing about lenses, but I’m not bad at 3d machining, and I can make a decent mold.
I have a vacuum chamber, but I don’t have a pressure pot, so the final acrylic cast might end up with bubbles. Still, I bet I could come up with something good enough for a kind of rudimentary monocular or even just a kaleidoscope.
@davea0511 I don’t know how experienced with mold making and casting you are, but if you’re new to it, you might want to take a look at this amazing guide:
http://lcamtuf.coredump.cx/gcnc/
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Lens math isn’t too bad. See the Edmund Optics website for a good primer. For lenses that aren’t too strong (focal length is fairly long compared to the diameter, say ten times), the surfaces are generally segments of a sphere. For higher power lenses, the aberrations become significant, and the final figure needs a curvature that decreases toward the periphery.
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Here’s an . stl example from Grabcad. https://grabcad.com/library/aspheric-condenser-lens Not the concave example you could get the most from, but hopefully useable… You can degass in a vacuum chamber right before you pour…with silicone you would not even need parting materials.
As a surprise bonus, a free mold making class from Instructables
The classes are sometimes rudimentary, and most of us won’t necessarily want to make copies in chocolate or soap, but it is easy enough to get to where your comfort zone ends and start!
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Thanks for all this. @enl_public I went to the Edmund Optics website, but I’m so unfamiliar with this subject I couldn’t find the primer you mentioned. Undeterred, I did a little searching around and found this Instructable:
https://www.instructables.com/id/3D-Printed-Magnifying-Glass/
I downloaded the stl for his magnifier, and have modified it in Fusion to I can carve a mold out of wax. I’ll report back if I manage any further progress.
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I really like the machinable wax idea since I already have silicone mold casting experience and equipment. I’m skittish about flame polishing though. Is it very difficult?
Wondering about the cost compared to buying a block of acrylic.
Machining out a negative too also a possibility but don’t think the library would let me machine aluminum with it. In this case again the wax seems attractive option … But it is more work … Not as easy as all the YouTube videos make it seem.
These will be some fairly unusual lenses (for AR experiments) or I’d just but them from Edmunds or whomever make them.
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Flame polishing machinable wax just takes a candle and some practice…it is a pretty easy skill to develop…and you can practice on the lump that will become the lens form, or if you clean the machine really well before you start on the lens, the shavings! Using machinable wax means your cutters don’t dull, and you get serious material savings from collecting and melting the shavings…
I use commercial wax(only because I have been reusing it for 10 years for various modeling projects), but there are several formulas for making your own, you just need some HDPE or LDPE and some parafin for many of them…
I still had my recycled billet blank from one of the mirrors I made
Takes a little while to get the hang of melting it down so as not to get a frozen vortex in the middle, but it is a fun challenge to beat…I bought a small Fry Daddy(non stick and with a sealing lid) and a hand vac just for reclaiming the shavings… just happens to make a perfect blank for a 6" mirror! One note if you didnt catch the frozen vortex thing, Machinable wax as a pretty good coefficient of thermal expansion-shrinks a good bit on cooling and expands on heating-but then it does have a pretty good temperature change between free flowing and machinable! Tends to fall right out of the pig mold because of it!
Seems funny now that all the shavings I have made to this point have been by hand…
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That is super cool. I really want to try this! Can you please describe the flame polishing process a bit more? What does one do with the candle? Just lightly move the flame around?
You pretty much have it! Get your shape as close as you can with machining, then use the candle to liquefy just the surface-by moving the surface over the flame just enough to make it glossy where the heat is! you can use a heat gun as well, but keep it moving, and when using a candle avoid getting the soot on the wax(easy to sort out when you actually do it) The wax will expand when liquefied, and contract as it cools-making the molten skin thin as it solidifies, polishing surface…it is one of those things that is pretty cool to do, and takes actually doing it to get the hang of it…but the supplies are cheap, the practice quick, and the results dramatic!
I have better luck with a candle holding the piece to be polished above the candle, and with a heat gun with the piece below the gun. Both work, though with practice the heat gun does polish more area quicker-but it can spoil more area faster too…as with most things, practice makes all the difference. Give it a shot with something unimportant, it is seriously easy to get the hang of! You just have to balance getting the heat in and out of the wax with duration and distance relative to the heat source.
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