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Hello everyone, this is my first thread in this forum and I will try to write on tiptoe because I do not have many theoretical aspects regarding mirror processing and self-construction.
Last week they gave me 12 glass plates (probably calcium sodium) 6mm each. 6 of approximately 1m x 0.40m and the rest of approximately 1.60m x 0.50m.
I was thinking about cutting them into Xmm discs and then coupling them with a special compound. Time ago, I discussed this aspect with the teacher Adriano Lolli who had confirmed to me the possibility of “couple” blanks but probably not 6mm but thicker, maybe he knows 10mm, I do not remember. He advised me to use this but this product is no longer available. My questions are therefore:- Is’ 6mm blanks can be coupled?
- If so, in what number it is convenient to couple them to avoid deformations or behaviors “pages” due to thermal expansion and other factors?
- Being my first experience and wanting to pair more blanks, what could be a recommended diameter and focal length?
- Provided that the previous points are satisfactory, what optical scheme could I make?
- Considering that the product that Lolli recommended to me is no longer available, what could be an alternative?
The questions are crowded in my mind but for now I will stop here to understand if this project can actually come to life. In the case of a constructive and stimulating confrontation, I would begin to gather ideas, the perhaps and the materials to be able to start the work.
I would perform the cut with a special tool that I already have.
A few months ago I bought various grains of carborundum from Gemmarum Lapidator and in these months of pandemic I built a small weighted work table for mirror processing.
Waiting for advice on my project I want to thank in advance anyone who decides to give me any indication.
Today I took some photos of the small plates that I link below:
I was thinking of cutting them with this tool:
and to help me in the circular cut with this suction cup:
From BricoIo I found this silicone it transpires that, with the exception of its NON-liquid self-leveling consistency, it seems similar to the one that maestro Lolli advised me.
Referring to this very interesting article to have an arrow that is below 6mm, with a 250mm diameter mirror I should consider, if I'm not mistaken, a focal length of 2750mm. With this estimate I would have a 5.68mm arrow, getting close enough to 6mm and therefore the first question that arises is related to the resistance to deformation and thermal expansion of the hollowed slab glued to the one below.
Then there remains the perplexity for the focal ratio which in this case would be F / 11. Ultimately I would have a probably poorly performing mirror, with an unwieldy focal length and a focal ratio not necessarily desirable.Assuming these considerations of mine are correct, the idea of coupling the slabs (for example 3) doesn't seem to produce a good solution. It would probably have the sole purpose “didactic” to start gaining experience, which I wouldn't mind but ultimately wouldn't allow me to have a usable tool.
I await suggestions and considerations in this regard.
Hi Skylab and welcome again !
I don't think that the problem of laminated glass for optical use has been practically faced by anyone. I think the greatest difficulty is in understanding and evaluating the thermal and mechanical deformations of the plastic material between the two sheets for bonding, usually the PVB.
Deformations that, if for normal use such as in the automobile industry or in construction they are irrelevant, hardly, however small they can be, they would fall within the very narrow optical tolerances of a parabolic mirror.
Moreover, in the case of several layers they would add up. (every single layer of PVB seems to me to be around 0,3 mm thick )
However, this could be interesting to study for large diameters, while for a diameter of 250 mm,a blank obtained from a very normal sheet of float glass from 19 mm thick is fine. It allows you to create any focal length, it is cheap and easy to find.
In any case for an F11 the arrow ( by eye ) it should be just over a millimeter, it would be a very performing mirror but undoubtedly uncomfortable to use with an intubation of almost three meters.
And the more 5 mm of glass to be excavated would be needed for an F3 or so and also in this case a glass from 19 mm is more than enough.Hi Massimo, thank you very much for the exhaustive and valuable reply.
I apologize for the gross error in calculating the arrow, I considered the square of the diameter and not the radius
For an F11 the arrow would be 1.42mm instead for a 3.13mm F5
I don't know if this experiment is worth trying and at this point try to consider a larger diameter as you suggested. However, the question arises spontaneously, bigger than it? For large diameters, what could be the problems of processing for a neophyte like me?
To glue the plates together, it might be valid this product?Anyway, two 19mm blanks of calcium sodium glass to make a 300mm F5 I bought it from a glassmaker a few months ago and I will start processing as soon as possible.
With a good chance, on that occasion I will certainly refer to a lot of interesting information on this site and draw on the experience of the veterans who populate it by asking questions, I hope not too trivial.Thanks again for the useful comparison.
Ciao Skylab
I agree with the indications that Massimo gave you.
In fact, the multilayer blanks of glass glued with plastic glues have never yet been explored for optical use, due to fears of sealing over time and deformation induced on the glass by the shrinkage of the glue during drying, given the very narrow tolerance of 68 millionths of a millimeter allowed between peak and valley of the maximum defect of the final parabolic curvature, to achieve the famous lambda / 4 quality.It would only be a matter of courageous experimentation, because there is a non-calculable risk of working unnecessarily.
The tube of liquid silicone RS glue you mention in your post, it is a rarity because it is liquid (and therefore it flows positively while keeping the thickness to a minimum); although (as we read on its documents on the same site RS) an acetic silicone, i.e. from the family of common syringe sealants, which, like this, hardens slowly without adding catalyst, absorbing environmental humidity, like that Saratoga you put in the picture.
As for the glass: However, I believe that in the great difficulty of finding a 25mm thick blank at a non-jewelry price, it would be better to orient yourself to the easiest to find 19 mm, rather than paste 4 dishes from 6 mm, for the reason that the more glue there is between the glass, the greater the uncertainty of stability over time, which is precisely the risk already indicated.
As for the calculation of the arrow, you can do it knowing that a mirror diameter 250mm is easy to make if it has a high focal ratio, for example F7, or the latter would have an LF focal length of (250*7) = 1750mm and an RC Curvature Radius of the initial sphere of (2*LF)= 3500mm which would provide a height of the eyepiece from the ground of approximately 1550 mm, and a glass digging arrow of
Arrow = (Mirror radius squared / 2*RC) What for a 250 F7 is 2.2mm
a bit’ the 2.6mm arrow with the eyepiece height from the ground of about 1350mm is less easy for an F6)
And the maximum difficulty for a beginner is an F5 which would have the arrow of 3.12….The processing must not scare, or immediately stimulate to invent new ways , so as not to immediately transform it into an experimentation of a road, complicating the already not simple old one
I say this because for example the processing time of my 250F5 (whose diary is described here in Grattavetro in a series of 4 articles) which excluded the roughing made to me by machine by Reginato, was of 77000 He ran forward back (that last 1 second), then 21 hours of scratching the glass reaching approx 60 total hours of tests included, which could have risen to about 150 if I had to make the initial sphere by hand.Ah…On cutting glass I forgot to mention that it is better to work while wearing glasses, and for cutting discs (by those who do not do it for a living) it is best to do this approaching for easier straight cuts; First by cutting a square with a side equal to the desired diameter, and then it is possible to continue in two ways; to remove the edges according to straight cuts, or vice versa (but I see it more difficult) groped to do it by engraving the curved sectors with a compass.
The important thing is to wet the tip of the glass cutter with petroleum or svitol or similar, which would favor the cut by minimizing the size of the micro splinters, and maintain a continuous cutting pressure and speed which is neither small nor high, but such as to make you feel, with just one step, and by ear the noise “zzzzzzzz” I keep making the tip of the glass cutter when it cuts well.
If it doesn't make that noise, or if you go over some sector, it is easy for the cut to be jagged, or don't come at all.
Any residual glass imperfection exceeding the disc is only aesthetically ugly, but it can be filed with a whetstone (wet so as not to breathe dust) maneuvered in plane with respect to the plane of the edge and along it.
There is also something to be said about this grinding, that in order not to give rise to chips that affect the reflecting surface, you get a good help using a Dremel type diamond grinder with passes without stopping to overheat, to realize a safety groove along the two edges of the disc.
This is because making that groove by maneuvering a whetstone along the thickness, that is, orthogonal to the cut, it's easy “to lift” chips that, however, cannot reach the reflective surface because they are interrupted by the groove that circumscribes them within the edge.
That caulking is also essential that it remains present throughout the processing of the mirror.All this chatter of mine to say that it is better to do some cutting tests to gain practical awareness of what glass is “a misunderstood easy traitor”…and makes unexpected jokes!!
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