View Full Version here: : Float Glass for 16"
29-11-2011, 08:38 AM
I am considering making a 16" but the price of mirror blanks for me is prohibitive. I note that some people say 19mm float glass is cheaper and available. Where ? I have done the usuall google search and can only find big plates. Also how does one make it round to start off with ?.
29-11-2011, 03:57 PM
If you go to any large glass supplier they can get for you 19mm float in any diameter,nicely cut (perfectly round), with bevels top and bottom. A 16" blank shouldnt cost more than about A$100-$120. Todays float glass is a lot nicer than the plate glass of earlier days,in that it is of uniform thicknes, bubble free, and pretty much free from strain.:welcome::eyepop:
07-12-2011, 08:47 AM
There is also a new technique for making mirrors (at least I thought it was a new technique) where round sheets of plate glass are joined together by small spacers. Tong Liu of HubbleOptics has an outstanding patent which I managed to find which shows the basic idea but he is not the first person to come up with the idea, the US military had been using it for airborne reflectors since the 1970s. There are actually a number of designs.
While I have never done it myself, I intend to look into it when I start to fabricate bigger mirrors (I am currently finishing off my 1st 6 inch reflector). There are two advantages to this approach. One is a reduction in weight, the other is an increase in surface area. Tong Liu, in his patent cites a study (can't remember where), where it said that there were no intrinsic benefits to using pyrex and that mirror seeing (inability to cool the mirror) was the limiting factor.
Tong Liu claims that performance improves with size rather than degrades for larger mirrors (probably because the way that he scales it, larger mirrors have more glass elements sandwiched together and therefore for spaces between them, which means more surface area to lose heat. It's all fairly new but if you want me to dig up some of the patents, let me know. Unfortunately, I can't give you the benefit of any experience.
07-12-2011, 10:03 AM
I'm not sure what element of the Hubble Optics blanks would be patentable...fusing ribs blocks or discs of glass between face sheets for light weighting of glass blanks has been going on in professional circles since the '60's .
07-12-2011, 10:09 AM
Yes any glass shop who does table tops should be able to supply 19mm.
However I just wouldn't recommend it. If you had the skills to make such a thin mirror you would be wanting to do something thicker anyway - I have a lot of experience and i wouldn't bother doing something that thin. The chances of making something that would work well at high power would be extremely slim. I think Sydney Glass may have some 25mm plate but it won't come cheaply.
I wouldn't recommend anything thinner than 1.5" for an amateur of medium skill level ( assuming has made some successful larger stuff like 12" )
07-12-2011, 04:08 PM
Not sure what you mean by rib blocks. If you mean honeycomb mirrors that is not what he does. What he does is puts very small flat cylinders of glass in the same place as you would put the supports for a mirror cell. The spacers are made of the same type of glass. These spacers are glued or melted to 2 large sheets of glass.
There seems to be less support than a honeycomb but a much greater surface area exposed to the atmosphere, so cooling is apparently never a problem. I understand that they use passive cooling and that the cooling improves with mirror size.
I have downloaded Tong Lieu's (Hubble Optics) provisional patent and another US military patent which is similar to his and which dates back to the 70s.
The US military patent uses hollow cylinders which are much longer than Tong Lieu's. The hollow cylinders have a hole drilled in each one otherwise there would be a cell of trapped air which would expand and contract, creating stress and presumably affecting the figure. The military one was used for aircraft or missiles, I think, so weight and air pressure were more important. Where it also differed from Tong Lieu's is that apart from the primary surfaceof the mirror, all other surfaces were flat.
Tong Lieu, on the other hand used a different strategy. He used much thinner spacers between the plates. His spacers were solid. Tong Lieu's design used multiple sheets (not just 2). Additonally, every surface was curved (including the spacers).
His patent also involved a method of fabrication which involved gluing or otherwise bonding the surfaces which are intially flat. Once they are set, they are placed over a metal template and the whole sandwich is melted and slumped into a curve. The primary surface in then figured. According to Tong Liu, figuring the mirrors is difficult. Not sure if it is difficult to figure because of the heating done to the glass, or if the structure required it to be figured gently.
Another thing in his patent which was interesting was the manufacture of large segmented mirrors. It is hard to describe in words but here goes. Imagine a very thin pizza, cut into four slices only and left on the tray. Then imagine putting some sliced olives in the same place you would put the mirror cell supports. Then put another pizza on top of this, which has been cut into four pieces, but make sure the cut mark don't line up. Keep layering pizza and olives a few more times and baked.
The Hubble Optics/Tong Lieu patent was awarded in China and was lodged in the US in 2007 but has not yet been granted. Not knowing anything about patenting I don't know how to interpret this.
I would like to make a large plate glass blank using this technique, but I have no oven. I wonder if you could get several discs of thickish glass, roughly grind all of them on both sides to the desired shape, get some offcuts of glass from a glass cutter to make the spacers with and glue them using a silicon glue.
I'm not up to that level of skill though. I also have not heard of the opticians who make fast mirrors as to whether or not the claims that are made about these mirrors are true such as-
- the fast cooling (I suspect this is true - surface area to volume ratio indicates they would cool better than a monolith or a honeycomb.
- pyrex being unecessary. This claim hinged on the idea that it is the temperature difference between the air and mirror that was more important than expansion coefficient. He cites a reference in the patent, but apart from this I have seen no confirmation.
To my understanding, very large professional mirrors use honeycomb and not sandwich mirrors. I wonder whether the sandwich mirror is trading structural support for a lightweight quick cooling mirror. I wonder if there is a range of mirror sizes where it is a beneficial design. I've not heard from any ATMs or experience opticians using these techniques.
07-12-2011, 10:42 PM
There is no way they would grant a patent in USA as all these techniques have been used there for years including slumping and holding the parts with epoxy prior to fusing. I have many papers from Applied Optics over the years which show spacers fused between face plates. Given that China shows no respect to US patents I doubt any commercial production of these designs would be any concern in the USA.
Hubble Optics blanks are just the marriage of two low cost techniques- fusing and producing round spacers with a simple core drill rather than traditional ribs. Marrying this with cheap 19mm plate glass for the face plates was a way of making large blanks avoiding the high cost of Pyrex and long anneal times for solid glass - and the cooling aspect is a bonus. A great piece of simple innovation.
08-12-2011, 02:05 PM
Do you think there is any reason that professionals are not using this?
09-12-2011, 04:42 PM
They don't need to use low cost technologies. They generally lightwight blanks by casting them in molds or machining material away. Monolithic blanks are generally regarded as superior to one with fused components.
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