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24" x 4" thick Pyrex blank...what to do?

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#1 perfcomp

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Posted 18 September 2019 - 02:24 AM

I acquired a truly massive piece of glass.  24" diameter by 4" thick Pyrex.  It weighs 146 pounds.  Both sides have been polished, one with a very slight sagitta.  What can I do to lighten it?  

 

Is there a service that can maybe water jet cut honeycombs in the back? (There is a water jet cutting service near me, can those machines work there way down in predictable depths?)

 

Slice it in 1/2 and get two blanks?  Who does that?  

 

Grind it down with an angle grinder while spinning on the table (under a water bath)?  There's a video where Waite says he does that and averages 1/4" per two hours, so that could be like a 16 hour solution, but what a waste of time and material.

 

Is there a service that could melt it down and re-cast the material into useable blanks?

 

Leave it full thickness and use a car jack to install it in the tube?  The weight would make the rest of the build (I've designed a fork mount, but we'll see) very expensive.

 

Maybe it's a white elephant?  Like, it's cool, but what the heck can I do with it?  

 

It's got a 20 meter radius of curvature polished into it.  32 foot dob?  (just kidding on the long tube, I want ~f3.3).

 

What should I, can I, do to put this super cool blank on a diet?

 

thank you,
Michael

 


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#2 Mike I. Jones

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Posted 18 September 2019 - 02:51 AM

One thought: contact Dan Saltzmann at GlassFab in Rochester NY, (585)262-4000.  Run these ideas by him, including sawing the blank into two 2" thick blanks (minus the diamond wire thickness), or lightweighting it, both of which they can do.

 

Have you strain-tested it?  Flat black between crossed polarizers is what you want to see.  Rainbows are not what you want to see.


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#3 DLuders

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Posted 18 September 2019 - 02:53 AM

Waterjets are used to completely cut through bulletproof glass, so there's a risk that it could ruin your mirror blank if the holes are drilled vertically.  You could drill a series of radial holes through the SIDES of the blank, and if they go all the way through to the other side, it's no big deal.  You could try for a honeycombe effect (along the bottom), or drill holes through the middle of the blank (say, 1.5"-2" from the bottom). 

 

How much is the blank worth to you, if it gets ruined?  Maybe you could experiment with an inexpensive glass block bought at a local Home Improvement store....   



#4 TOMDEY

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Posted 18 September 2019 - 04:32 AM

One thought: contact Dan Saltzmann at GlassFab in Rochester NY, (585)262-4000.  Run these ideas by him, including sawing the blank into two 2" thick blanks (minus the diamond wire thickness), or lightweighting it, both of which they can do.

 

Have you strain-tested it?  Flat black between crossed polarizers is what you want to see.  Rainbows are not what you want to see.

Yes, good shop! Bob and Dan did Pyrex blanks for me a long time ago. And they do have an auto-feed saw that can cut it in half. Whether it would be worth doing that... who knows. At one time, I had 24 ea blanks almost the same size as yours. They were all 25 inches across and 4-6 inches thick. I made a perforated 24-inch optical test flat out of one of them and used it for an autocollimation optical test tower in my barn. Thick solids are great for such applications, because you must support just around the edges (using hydraulic hose and peripheral whiffle-tree). That might be the best use of what you have... if it is already fine-annealed. A smooth (no zones) tenth-wave flat (power removed) would be good.  For a telescope mirror... probably borderline white elephantish... might as well just start with a fresh blank... Dan can quote on that and also do the annealing and center perforation! His shop does wonderful work! Couple pics of my old AC test tower >>> Tom

Attached Thumbnails

  • 133 Toms 24-inch AC-COC Vertical Test Tower 80.jpg
  • 134 Toms 24-inch AC-COC Vertical Test Tower 80.jpg

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#5 OleCuss

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Posted 18 September 2019 - 05:30 AM

OK, I've a 20" mirror which is 4" thick so it is really interesting to read that people on the East Coast are able and will cut such a mirror down to size.

 

If there were someone on the West Coast who did that I might explore the idea further.  But hearing that this might not be a good idea is a bit disappointing (although not a huge surprise).

 

My mirror is probably beautifully done.  It's an F/5 out of a NASA LIDAR unit so I'm betting it had to be high quality and the glass is likely also very good quality (but I don't know what type).  I don't think it is worth taking it somewhere to be tested until I know that it can be cut down thin enough to where it just might cool down enough to be usable before I have to go to bed. . .

 

If you all think it is not worth pursuing I'd appreciate the assessment.  I've concerns that all the attendant issues of crating it to send to the East Coast and also paying to have it cut down to size and then shipped back may be a substantial portion of the value of the mirror and then I worry about stress being introduced into the mirror as it is cut down to size.  I do consider it possible that given the costs and risks it would be better to just buy a mirror which is properly optimized for astronomical use.

 

Does it sound like it is worth pursuing?  I'm guessing it would pencil out a lot better if there were a facility in Northern California which could do this but I've also family in Southern California that need a visit every now and again so I could deliver and pick the thing up again myself.


Edited by OleCuss, 18 September 2019 - 05:32 AM.

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#6 PETER DREW

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Posted 18 September 2019 - 05:39 AM

I don't think doing anything to a finished mirror other than mounting and using it is worth the risks.
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#7 davidc135

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Posted 18 September 2019 - 07:12 AM

Would cutting an unfinished blank with a diamond wire be too difficult for an amateur? Or too expensive?  David


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#8 OleCuss

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Posted 18 September 2019 - 08:57 AM

Would cutting an unfinished blank with a diamond wire be too difficult for an amateur? Or too expensive?  David

I've done a little looking around and I think the answer is mostly in the "too expensive" category.  I'm not trying to minimize the difficulty it's just that it looks like you'd have to spend a lot of money and time to get the needed gear set up and tuned to make it reasonably possible and then you'd have to consider going slowly and worry about mis-steps.  This would be especially true for a larger mirror such as my 20" or the OP's 24".

 

Even if one were to build their own wire saw it looks like it might cost more than just finding an expert with a good machine.

 

You'd have to make sure that you had a way to very precisely position the mirror/blank for the cut and then have the saw tuned and positioned to very smoothly move through the glass while keeping everything cool as well.  And I'd still be concerned that you might introduce stress and might have to anneal things.

 

An expert with great machinery is more likely to have a good outcome.  So while I'll probably still scheme a little about how I could try to do it myself, I'd probably be better off tossing my mirror than to try to rig up a saw and cut it myself.


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#9 PeteDCard81

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Posted 18 September 2019 - 09:54 AM

The disk can be sliced into two blanks. You could keep one and sell the other.

 

I read recently that Glass Fab may no longer deal with amateurs. I do not know first hand if that is true.

 

Advanced Glass Industries, also in Rochester New York, can do it.

Ask to speak to Tony Marino.

Advanced Glass Industries
1335 Emerson Street
Rochester, NY 14606
585-458-8040

 

Maybe a road trip to the East Coast to attend Stellafane or NEAF, with a stop in Rochester.


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#10 MitchAlsup

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Posted 18 September 2019 - 10:03 AM

Modern thin mirror support structures allow a 24" mirror to be only 1.5" thick.

 

At 4" thick and "only" 24" in diameter, I might be tempted to cut it into 3 blanks.


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#11 Chuck Hards

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Posted 18 September 2019 - 10:28 AM

I saw stacks of these at a friend's place recently.  I wonder what they were all poured for, in the first place?


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#12 TOMDEY

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Posted 18 September 2019 - 10:46 AM

I don't think doing anything to a finished mirror other than mounting and using it is worth the risks.

Yes, I agree; if you start whittling on it... then all you have is 146 pounds of vanilla Pyrex... nothing more. If it truly is a finished, superb F/5 Paraboloid... dicing, whittling, regrinding it... is analogous to... >>>

 

Melting down a perfectly good Rolls-Royce, because you want the materials for building a nice Yugo.    Tom


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#13 Bob4BVM

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Posted 18 September 2019 - 01:26 PM

I saw stacks of these at a friend's place recently.  I wonder what they were all poured for, in the first place?

Stacks of 24x4 blanks ??? really

Whats he doing with them ??

CS

Bob



#14 Bob4BVM

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Posted 18 September 2019 - 01:28 PM

Modern thin mirror support structures allow a 24" mirror to be only 1.5" thick.

 

At 4" thick and "only" 24" in diameter, I might be tempted to cut it into 3 blanks.

 

Definitely 3 blanks in there...

Sell one, keep two for the 24" bino-scope you'll be building !

:)

 CS
Bob



#15 OleCuss

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Posted 18 September 2019 - 01:41 PM

1.5" x3 should come out to 4.5".  Might be difficult to get 4.5" of blanks out of a 4" piece of glass.

 

But maybe a really good mirror support would allow a 1.25" mirror?  Then you might be able to get 3 of the 1.25" blanks out of a 4" hunk of glass if your kerf is not more than about 1/8"?


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#16 Chuck Hards

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Posted 18 September 2019 - 01:59 PM

Stacks of 24x4 blanks ??? really

Whats he doing with them ??

CS

Bob

 

Check your PMs, Bob.



#17 Spartinix

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Posted 19 September 2019 - 12:13 AM

A 24" F4 binoscope project. Might take some years but should be worth the wait.

#18 davidc135

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Posted 19 September 2019 - 02:34 AM

Very likely I'm being too optimistic but I can't help thinking there's a solution somewhere between a metre of diamond wire for 2.50 and an industrial machine costing tens of thousands. Eg chop up a thin circular diamond blade and braze or epoxy onto a strip of steel. Or use the cheap wire. Make a bow saw. Set up the disc on its edge on powered rollers. Add water. Maybe it'll always be far too slow.  David


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#19 OleCuss

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Posted 19 September 2019 - 06:56 AM

In some sense, you're right.  In concept it is simple.

 

But you will need in the end to have a mirror blank in which the top and bottom surfaces are precisely parallel, you'll need a very smooth surface so that you don't have to grind the thing down to smooth with problems maintaining flatness.

 

So you'd have to build a very good frame to hold the glass very precisely on both sides of the cut.  You'd likely really want to have it held vertically so that as you go through the glass it doesn't start to sag.  You'll need to very precisely ensure that the wire will be drawn through the glass in exactly the way you want/need which means precise tolerances to your build of your wire saw and the frame to hold the glass.

 

You'll also need to have the wire very tight to keep it from flopping around even the slightest bit.  This means a wire saw build which is not only precise but robust.  Because you are looking at a 24", it will also have to be big.  So precise, big, and robust - a bit of a challenge!

 

If I were simply wanting to cut up stone for decorative purposes, a DIY system might be practical.  It might also be fun to build one to cut up a 24" x 4" blank but the time and cost to do it well I suspect would justify simply buying a big wire saw.  I suspect the wire saw would be several thousand dollars (USD) at a minimum and it would not surprise me if it were in the tens of thousands of dollars.

 

Even if one were able to cut the glass oneself there is a chance you might find you have too much stress in the resultant blanks.  You might need to build or buy a pretty big kiln and anneal things.

 

I'd really, really, like to see someone do this as a DIY project and post it where I got to see what was done and how it was done.  I think it would be fascinating.  I don't think it would be practical but if someone has the time and money to do what I think is impractical because it could be a lot of fun - more power to them!



#20 Chuck Hards

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Posted 19 September 2019 - 08:10 AM

It might be worthwhile contacting a local maker of headstones for cemeteries.  The lapidary equipment they have might be capable of slicing one of these disks.


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#21 perfcomp

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Posted 20 September 2019 - 11:53 PM

Hello ATM'ers that responded to my question.  I contacted www.glassfab.com and they indeed can slice the blank, and the job is not very expensive at $350/saw cut and $125/face to grind flat.  They are not set up to generate curves etc.  My first quote for shipping UPS from the west coast was for about a grand, so obviously this is an area that needs a bit more research (I hear Greyhound can be an affordable option for palletized loads).

 

Or how 'bout a CNC router and let it run for a few weeks cutting honeycombs out of the back? Something like this maybe: https://www.robotsho...ide-router.html

Michael McKeown
 


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#22 sopticals

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Posted 21 September 2019 - 03:06 AM

This topic takes my mind back to early 1960's when I had a view to one day building a 24" reflector. At that time it was a "no no" to consider a blank less than 6:1 (diameter v thickness).

In fact when I ground my second mirror a 7" by 1" thick (plate glass), I was told by the ATMers at the time that I would probably not be successful in my attempt. I did succeed producing

an optic that performed well even at 50x per inch. However when I considered a 12" in 1" plate I was dissuaded. Unable to obtain a 2" thick blank I'm afraid my mirror making aspirations

were "put to sleep". However I said to myself that "one day" I would somehow procure a 24" x 4" blank and "build that scope". My, how thinking has changed. If those guys from that time

could see the way that ATMing has evolved, they simply would be "shaking their heads" (like impossible). If I had known then what I know now I would have had large aperture much

earlier (like 40 years ago).

 

Stephen.(45deg.S)


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#23 steveastrouk

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Posted 21 September 2019 - 07:47 AM

United lens do my surface generation for the ZeroChromat lenses. They may be able to do your pyrex generation once the slices have been cut.

 

How thick are you going for ? As others have said, the "rules" of mirror thickness have been broken and revised many times !



#24 PeteDCard81

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Posted 21 September 2019 - 08:55 AM

Recommend getting a quote from Advanced Glass Industries.



#25 555aaa

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Posted 21 September 2019 - 10:17 AM

Make an f2 conical primary. I think that's about the right thickness. Or light weight it. There is a well known company that does this with all their primaries on a cnc machine.
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