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Help Needed with Tiny Mirror crack

ATM beginner mirror making
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#1 Sam Danigelis

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Posted 23 April 2018 - 11:43 AM

I've enjoyed reading and learning from CN forums lately.  I just resurrected a 6" mirror project that I started 43 (that's right) years ago.  It's from a Jaeger's kit, Pyrex blank.  I was doing a bit of fine grinding just the other day for the sharpie test when I discovered what looks like a hairline fracture in the shape of a small crescent about 1/8" long at the surface of the mirror (see photo).  The crack is much smaller than it appears unless illuminated from the side by a strong light source.  There is no discernible pit or crease in the surface of the mirror.  But it does appear that the crack reaches the surface.   I'm heartbroken that my "baby" from so many years ago, lovingly stored, has a defect.  I'm wondering a couple things.  1)  Can the mirror be aluminized safely?  2)  Will this degrade the image?  3)  Does this sort of thing spread over time?  Again, thanks to you experts.  I really am learning a lot reading this forum.

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  • scratch.gif


#2 Pinbout

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Posted 23 April 2018 - 11:48 AM

fine grinding? ...your not even polished out? ...go back to 120 grit and make it a smaller crack if not get rid of it. once your polished out and near final figuring that's when I'd question is it worth it.

 

it won't hurt anything but your ego.


Edited by Pinbout, 23 April 2018 - 11:48 AM.

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#3 Pinbout

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Posted 23 April 2018 - 11:50 AM

look at it with one of these handheld microscopes and you may think OMG! its the grand canyon... lol.gif

 

gallery_106859_4364_67934.jpg


Edited by Pinbout, 23 April 2018 - 11:50 AM.


#4 ed_turco

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Posted 23 April 2018 - 11:53 AM

I wouldn't worrry about it one darned bit.  Been there, done that!

 

 

ed



#5 desert_woodworker

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Posted 23 April 2018 - 12:51 PM

Welcome, Sam!,

 

If the crack reaches (has reached... ) to the surface, and starts shedding bits of glass in finest grinding or in polishing and parabolizing, it may be very ugly, and could prevent satisfactory completion of the project.

 

Sometimes when the disks stick together, in fine-grinding or in polishing, and a disk needs to be whacked in a punctilious way to help separate them, a lot of force could be felt at the surfaces, and if any glass is loose, or very thin (and might break off), then this would be something to be aware of, should "sticking" happen, and the crack not have been ground all the way out.

 

I agree that if the crack does not extend too deep, and seems capable of being ground-out completely with rough abrasive (depends on one's patience and stamina, ...and maybe the family's tolerance of the noise), it may then be worth considering doing it.  Of course, the worker would, in alternate wets, reverse the position of tool and mirror, first one on top, then the other, if you're already fixed on a radius of curvature (and hence focal length) of the desired mirror.  Or, maybe every three, wets, say. 

 

There's a chance that it's OK to proceed just "as-is" though.  It could be worth a gamble!  But even if it should ultimately get ugly, at least you will have learned something, which counts for a lot.  And we'll have learned, too, if you'll please continue to share the joy or grief with us! wink.gif  Thanks... .

 

Wishing you all the best,

 

--Joe (glass-pusher since 1965)


Edited by desert_woodworker, 23 April 2018 - 12:56 PM.


#6 Sam Danigelis

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Posted 23 April 2018 - 01:10 PM

Thanks for these helpful replies. I'm pretty sure it's too deep to grind out. I'm gonna say it extends 1/16" down or more. Hard to say because it's not near the edge. Would the aluminizing process be okay with this? I can't actually find any surface disruption, even under about 20X magnification.

#7 Sam Danigelis

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Posted 23 April 2018 - 02:07 PM

I've included 2 new pictures to show the crack in better detail.  One with strong LED side lighting.  One without.  Also, I've included a ruler for measure.

Attached Thumbnails

  • mirror crack with LED sidelight.jpg
  • Mirror crack with no sidelighting.jpg


#8 Billyboy78

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Posted 23 April 2018 - 02:54 PM

Nothing is too deep to grind out. If you're planning on a top shelf mirror you wont be happy until its gone. If its 1/16th like you said, 80 grit will get it out in no time. Just take a big sigh and do it.



#9 ccaissie

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Posted 23 April 2018 - 03:26 PM

Been discussing strain lately.  A chip usually relieves its own strain, but a fracture stores some tension.  If you look in the forum here you'll find several ways to determine if there's strain, and there may just be a bit around the fracture.  If any strain, I'll bet it's insignificant, and in any case I wouldn't worry about it ruining the figure or causing any other difficulty.  You might see it when Foucault testing..might look like a little scratch. 

 

1.  Should coat fine.

2.  Won't affect performance

3.  Won't spread.  


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#10 Mike Lockwood

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Posted 23 April 2018 - 06:23 PM

1/16" is a lot of glass.

 

Chances are, if you grind properly, this will be completely invisible after the mirror is coated.  Estimate how many hours it will take you to get back where you are and consider that before you put in the time to get rid of something you likely won't see after coating.  Would that time be better spent doing something else, or do you have lots of extra time?

 

Glass is not perfect, this is why large refractors are so expensive.


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#11 Steve Dodds

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Posted 23 April 2018 - 06:30 PM

It's ruined.  Send it to me!


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#12 Sam Danigelis

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Posted 23 April 2018 - 08:22 PM

Thanks so much everyone. I finished fine grinding waaay back in '75, got polishing, discovered pits around circumference, went back and re-ground completely. Was in way over my head for a 14 year old with only a book to guide me thru the figuring and testing process, so the mirror went into storage. Now, with such amazing sources of help, I'm starting again! Mirror has 97.5" r.o.c., not bad as I was shooting for f/8. And, it's perfectly spherical, per the sharpie test. Even in its imperfect state, this mirror's surface is what I wrought way back then. So, I'm gonna gamble and go ahead with polishing, figuring, and aluminizing. I will post a follow up on this to let you all know how things go, good, or...
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#13 Sam Danigelis

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Posted 23 April 2018 - 10:53 PM

Pinbout, thanks, I'm getting one of those. Sorry Steve, LOL.

#14 Sam Danigelis

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Posted 26 April 2018 - 12:37 AM

Okay... update.  Pinbout, thanks for the handheld microscope recommendation.  I purchased a 20-40X model, and set to work examining my fine-ground mirror.  At 40X magnification, I believe the crack is below the actual surface.  At least, at this magnification, I am not able to detect any surface irregularity.  If true, that will be good news.

 

The last grade of abrasive used was #800, which I believe was a reference to 1/800".  Now, against the surface of finely ground glass, which appears almost gray under the microscope, I see a few subsurface (and maybe even a few at surface) tiny bubbles (maybe 1/400" dia).  I also see a few larger pits, maybe similar size.  If I were to hazard a guess, I'd say that over the entire mirror surface, there could be from 50-100 of these pits.  Should I go back to maybe 400 grit, and work to remove these?  Would more fine grinding uncover a few more tiny bubbles?  Until now, even using bright light, I was not able to detect tiny pits.  But, use of the microscope has opened up a new world of fine scrutiny which I previously did not have.  Handheld microscopes were either prohibitively expensive or not even made back in 1975!

 

Once again, I thank you all for your input.  I read and consider each entry.  I learn something each time.


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#15 davidc135

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Posted 26 April 2018 - 01:33 AM

I think 800 grit particles are far finer than 1/800th inch. But does anyone have a conversion table? One grade of alumina powder I have is labelled 600/9 which I take to mean 9 microns ie 1/3000 inch approx. At any rate 800 is a fine finish and you'd expect to see a range of pit sizes. Regrinding with 400 grit would leave pits that are massive by comparison so I'd be careful about returning to that stage. Why not get some small squares of window glass? One pair could be ground with 400 grit, another with 800 etc to give a guide to the state of play on your mirror surface.

 

David


Edited by davidc135, 26 April 2018 - 01:42 AM.


#16 mark cowan

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Posted 26 April 2018 - 03:03 AM

https://www.washingt...ilicon-carbide/

 

500 grit is about the same as 25 micron from experience and that chart.  9 micron is around 1500 SiC but the action of AlOx and SiC isn't the same - a 25 micron AlOx grind will be finer then a 500 grit SiC.  


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#17 davidc135

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Posted 26 April 2018 - 06:20 AM

Thanks for that link, Mark. But I see there are three scales. Perhaps Europe and USA follow different standards? Just checked the labels and F precedes the grit size on my containers of 600 and 1000 alumina, so they must be Fepa sizes and maybe different to what the OP has used.

David



#18 Pinbout

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Posted 26 April 2018 - 06:42 AM

I’d go back to 25 alox then 12 then 5 again 

 

it’s a lot better to do it now, since your not polished out, again if you were already polished out, I wouldn’t worry about it.

each grit take about 45min, that’s less than two hours. Polishing a 6” mirror about 1hour/in-diameter.


Edited by Pinbout, 26 April 2018 - 08:33 AM.

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#19 jdupton

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Posted 26 April 2018 - 08:30 AM

Mark,

 

https://www.washingt...ilicon-carbide/

 

500 grit is about the same as 25 micron from experience and that chart.  9 micron is around 1500 SiC but the action of AlOx and SiC isn't the same - a 25 micron AlOx grind will be finer then a 500 grit SiC.  

   Can you give us a short tutorial on how to read that chart?

 

   For ANSI Grit Size of 500 for example, the chart seems to say that the Maximum particle size is 19 and the minimum particle size is 10. How is it that the average of 19 and 10 is 40? What are the units for ANSI grit sizes? Is it still microns?

 

   I am sure I am missing something about how the ANSI sizes are specified but what is it I'm missing?

 

   Sorry for the off-topic question here...

 

 

John


Edited by jdupton, 26 April 2018 - 08:32 AM.


#20 Sam Danigelis

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Posted 26 April 2018 - 02:01 PM

A couple things. 1) The abrasive #800 was from Jaegers in 1970. Is that the a.n.s.i. scale? 2) I'm also confused by the chart, as it looks like the "average" and "max size" columns are switched. How can the "average" particle size be larger than the "largest" size?

#21 davidc135

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Posted 26 April 2018 - 02:43 PM

Must be switched.

 

One issue that others may have come across is likelihood of scratches during 5micron and finer grinding. I don't think it is due to contaminated grits but I may be wrong. So I stop now at 9micron. I read somewhere of bunching of fine particles? Is this common and if so is there a remedy?

David



#22 mark cowan

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Posted 26 April 2018 - 03:30 PM

Thanks for that link, Mark. But I see there are three scales. Perhaps Europe and USA follow different standards? Just checked the labels and F precedes the grit size on my containers of 600 and 1000 alumina, so they must be Fepa sizes and maybe different to what the OP has used.

David

That sounds about right.



#23 mark cowan

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Posted 26 April 2018 - 03:42 PM

Mark,

 

   Can you give us a short tutorial on how to read that chart?

 

 

Not really.  You'd have to find out what the absolute sizes were,  but in practice all you do is grind with finer grits until polishing is effective. ;)

 

Yes it looks like the columns are mislabeled.  I actually only looked at the first one and saw the rough correspondence.  Texereau has a grit size chart in the first part of the book that is perhaps more useful.

 

Of course, for fun, examine specific grits under a loupe.  SiC breaks down into smaller sharp particles, AlOx doesn't do that and has elongated particles.  The latter could be part of the difference in sizing.   What I mean is that the same "size" of AlOx will produce a finer grind.  Also tool pressure influences this, the more weight you pile on the finer the resulting grind is, for the same grit size.  Freedom of motion as it relates to the glass fracturing.




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