Any opticians or glass-makers on the Dope?
I’ve just had my eyes tested and need a new prescription, so I’m changing my glasses. I’ve previously had frameless glasses which have plastic lenses. Which scratch. So I’m switching to glass lenses, which do not - at least not so easily. But I can’t have frameless glass lenses because drilling holes is impossible. I asked the optician why they couldn’t melt or burn a hole instead and he didn’t know. So I ask the Dope.
They could, but the glasses would be more expensive than most customers would be willing to pay for.
I reject the idea that drilling a hole into a piece of glass is impossible. Here’s a video showing someone doing that very thing.
Are you sure that that’s glass? The description just says ‘optical lens’.
What makes the process so expensive?
My guess is that it’s not impossible to drill holes in glass eyeglass lenses, but that the failure rate (number of times the glass cracks) means that it’s not financially viable. As for the suggestion in the OP of melting or burning the hole, we are talking about glass, so that’s going to be difficult or impossible as well.
I speculate that it is a numbers game. Putting holes in the glass results in a sizable number of broken lenses, which means a lot of wasted material.
With framed lenses, the glass rarely breaks, and 90+ percent of the lenses are usable, and can be sold for a profit.
The standard product may or may not be more profitable than what you want, but they know that they can sell it. There is not enough demand for your item to make them willing to take the risk.
I was thinking along the same lines. I’d imagine whatever makes drilling lenses difficult would be even worse if melting them. Applying heat also might deform the lens and ruin the curvature, which I’d imagine would make it no longer function as it should.
Also, here is a machine that can drill holes in a glass lens:
https://www.framedisplays.com/dmluledrma11.html
And yes, glass, not just plastic or polycarbonate. The assertion that it’s “impossible” is belied by machinery that does it.
Has glass lens technology completely changed over the years? Glass used to scratch a thousand times easier than plastic. When did this switch?
The whole process is highly automated. Anything out of the ordinary is going to cost MUCH more.
The other problem is the strength of the glass. You are assuming that the glass will be strong enough to hold the bridge and sides. It probably isn’t, and will just snap when you stress the glasses much at all. The choice of glass characteristics - low Abbe number, ability to be cut and ground will dictate much of the formulation. Mechanical properties will come second. Toughening the glass could in principle be done after the lenses are fully cut and any holes drilled. It would add to the cost, and almost certainly cause deformation of the lens, compromising its optical performance. Once toughened you can’t do anything to the lens, or it will instantly shatter. This may be another reason not to toughen lenses for glasses. You don’t want to have the lenses explosively shatter whilst you are wearing them just because you managed to stress a mounting hole past its limit.
I was advised that the standard plastic lenses - CR39 - can’t take the stress of a frame less design, and you need to go for either polycarbonate (which has a rotten Abbe number, prone to shattering, and is attacked by solvents) or Trivex, which is stronger, has a good Abbe number and is impervious to just about any solvent (to the point you can wash them in acetone).
I currently wear Trivex frameless - with pretty much the whole Monty of coatings, and they are great. You can scratch them, but you need to be pretty careless. The low weight and good optical characteristics make them a no-brainer choice for me.
What, really? I think glass is much, much less prone to scratches than plastic, and has always been. There is new stuff that may have changed that but, no, it’s plastic technology that has changed, not glass.
I was told, when I got my frameless plastic lenses, that I could have them in glass, but they would be three times thicker, much heavier, and prone to cracking where the temple piece was attached. So I went with plastic, and then of course I was given a whole list of things to do so as not to scratch them (wipe them with a special wiping material not with a towel, use warm water and no soap, and so on). You don’t scratch your glass windows when you wash them with Windex and paper towels, or wadded up newspaper, or an old terrycloth towel, but if you apply any of those to your plastic spectacles you will scratch and dull them pretty quickly.
I’ve drilled holes through glass. Everything from 0.5mm to 3/4".
Diamond bits and holes saws with grit work. Underwater is the easiest way to go as it provides constant cooling.
It’s not something I’d want to do for glasses, as the stress of banging them around will build at the hardware going through the holes.
I like Acrylic myself, with no coatings. It holds up to paper towels pretty well, just avoid acetone.
Melting small holes in glass would likely cause shattering, unless the stff is annealed carefully, it can have large internal stresses.
What stress?
Do tell.
Francis Vaughan, what’s Abbe number?
Quartz, do you ever take off your glasses by holding them by one stem? Or bump them when they’re on your face? Or put them down heavier than you should? Any of those will cause stresses in the structure of the glasses, which will be especially significant at any place there’s a join of any sort.
Ah right. I wasn’t aware that those actions would cause significant stresses. Ignorance fought.
I know nothing about eyeglass glass but I too have had occasion to drill through glass. Unless it’s tempered it’s very doable.
Chronos,
I’m surprised. I expect you would have guessed that it is a measure of dispersion. It is somewhat arbitrarily defined in terms of refractive index at several atomic emission lines.
I was also wondering about the Abbe number business. ISTM a low Abbe number indicates high dispersion, which is bad (it causes chromatic aberration), but is also correlated to a high refractive index, which is good. What are the trade-offs and design criteria here?
As for cutting holes, would a water jet be able to handle it?
Or a laser?