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32" F/2.5 slumped project (complete :(

Art Mirror Making
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#1 darksky4us

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Posted 26 October 2021 - 05:01 PM

Not Cool ,not cool at all. After about 1 hour of "matching" this slumped blank to the tile tool with 60 grit, it just split on a regular back stroke, CoC (><)

 

There was no unusual force or pressure or strokes being applied, it just "came apart" without a sound, "out of the blue sky". Waaaaaaaaaaaaaaaaaaaaaa. Sob. Sob.

 

 

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#2 OskiBear

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Posted 26 October 2021 - 05:04 PM

look on the bright si ...... wait, there is no bright side. Condolences


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#3 Augustus

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Posted 26 October 2021 - 05:15 PM

Could it have been from drilling that hole?


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#4 darksky4us

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Posted 26 October 2021 - 05:20 PM

Could it have been from drilling that hole?

Possibly, but the break line is actually offset about an inch from the hole. I specifically fit the 7" aluminum disk to match the curve of the glass on the back, to distribute the forces, so something like this should NOT have happen !?



#5 Augustus

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Posted 26 October 2021 - 05:25 PM

Possibly, but the break line is actually offset about an inch from the hole. I specifically fit the 7" aluminum disk to match the curve of the glass on the back, to distribute the forces, so something like this should NOT have happen !?

Looking at this initially I thought it might be the fault of the glass but I am pretty sure the disk is what did it in. I am not an expert but I have never seen a disk like that used to grind a mirror and I can't see how it was a good idea.


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#6 darksky4us

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Posted 26 October 2021 - 05:32 PM

Looking at this initially I thought it might be the fault of the glass but I am pretty sure the disk is what did it in. I am not an expert but I have never seen a disk like that used to grind a mirror and I can't see how it was a good idea.

So how else was I gonna hold the blank while grinding it? It's too slippery to just safely lay my hands on it, and I can't hold it by the edge alone because it has to slide over the edge of the tool constantly. I guess I could have RTV the disk onto the back, rather than attach by the bolt, but not sure if that would have saved it? It appears the glass just had internal weakness that gave away after a little bit of pushing it around on the tool :(



#7 Augustus

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Posted 26 October 2021 - 05:34 PM

So how else was I gonna hold the blank while grinding it? It's too slippery to just safely lay my hands on it, and I can't hold it by the edge alone because it has to slide over the edge of the tool constantly. I guess I could have RTV the disk onto the back, rather than attach by the bolt, but not sure if that would have saved it? It appears the glass just had internal weakness that gave away after a little bit of pushing it around on the tool frown.gif

Mel Bartels uses glass pullers



#8 Sleep Deprived

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Posted 26 October 2021 - 05:40 PM

Drilling the hole may well have introduced internal stresses in the blank.  The various forces acting on it during the grinding process may have finished it off.  Unless the blank was annealed after drilling the hole - I assume that didn't happen.  

 

Either way it happened, it still sucks!


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#9 davidc135

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Posted 26 October 2021 - 05:42 PM

Very sorry to see this disaster.  David


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#10 gregj888

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Posted 26 October 2021 - 06:08 PM

I used blocking pitch to attach a knob to the back of my 20".  That's the old school way.

 

RTV should also work.

 

Did you drill or grind the hole?  If you used a glass drill, that may have done it.  If you put a metal screw through it, that may also have caused the split.


Edited by gregj888, 26 October 2021 - 07:57 PM.

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#11 darksky4us

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Posted 26 October 2021 - 06:28 PM

I used blocking pitch to attach a knob to the back of my 20".  That's the old school way.

 

RTV should also work.

 

Did you drill or grind the whole?  If you used a glass drill, that may have done it.  If you put a metal screw through it, that may also have caused the split.

I have used the same method to drill over a dozen mirrors from 8" to 25", using a 3/8" tile/ceramic bit, and ground into the face side for a flat head bolt using a countersink bit and 60 grit. Used a stainless steel bolt to attach it to whatever mount I need to use. Never had any problems. Though the thinnest mirror I ever drilled before is 15:1, this was about 60:1 ...



#12 TOMDEY

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Posted 26 October 2021 - 06:41 PM

Big cast or slumped mirrors are notorious for catastrophic failure. About fifteen years ago I designed (and patented) a "Reverse Schmidt" ~Scene Generator~ that needed a 72" mirror with slumped sag exceeding anything that had been done in that size previously. This we accomplished... after shattering two in the slumping/annealing oven. The third one was a charm and the entire project came to fruition, performing as intended. The supporting structure for both fabrication (grind/polish/figure) and use is redoubtable. My recollection is that all mechanical to glass interfaces were and are RTV pads --- and many of them... all with intervening flexures, so that unanticipated (local/differential) stresses would not / could not manifest. Flight Simulator mirrors also fall in this category. Piles of those cycled through our coating shop.

 

Is your holding fixture there mechanically-interfacing with a through-hole in the glass itself? But we optics guys are always ~pushing the envelope~. Time to slump another blank, modify something in the process... and try again! Scaling things up is extremely nonlinear. Good for you for trying and sharing; keep up the Good Work!  Tom


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#13 gregj888

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Posted 26 October 2021 - 07:01 PM

darksky4us, interesting...  I use blocking pitch or a "pitch lap" on the back of the mirror (with foil or plastic wrap between it and the mirror and duct tape on the edges.

 

The 20" is cored to close to the mirror surface.  The cut is filled and the post on the back extends well over the edge of the plug.  Tarpened with grit and a tube.

 

Drilling would scare me, but good to know it can work.  Never know when it might come in handy..  I would probably add the grit to the back of the bolt and grind in the countersink but I'm chicken with big glass.


Edited by gregj888, 26 October 2021 - 07:12 PM.

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#14 GLS

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Posted 27 October 2021 - 05:09 PM

Oh, the pain.  As bad as that is, it doesn't approach what Tom Otvos's story entails.  If you haven't seen the thread, his screen name is totvos.  He broke his 14" sub f/3 slumped mirror during star testing.  He dusted off the misery, rolled up his sleeves and got back on it, slumping another mirror which he finished.  As inspirational a story as I've seen here.  Gil


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#15 chipe450

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Posted 28 October 2021 - 03:53 PM

Hold a mirror 32" diameter, 14 mm thick, with a central metal pin with no rubber, is not a recommended. In a glass showcase, all the glass are hold with metal bracket and rubber.
A laminated glass structure is more rigid then a monolitic geometry. The anty bullet glass use in the bank are laminated.
A hollow structure is more rigid then a plain structure.

Andre
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#16 kur3tking

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Posted 30 October 2021 - 02:59 PM

Grinding my 19" F/3.5 plate glass blank, I use rubber gloves(the tight fitting ones) not dishwasher gloves, for 2 reasons

 

1) a better grip on the back of the glass and avoiding the above probable cause of breakage and

 

2) the gloves help insulate the plate glass from being slightly heated up during hand working.


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