Author Topic: does polishing glass fill pits?  (Read 443 times)

esrescioripp

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does polishing glass fill pits?
« on: December 31, 2017, 11:38:53 PM »
does polishing glass fill pits or does the glass wear down to the bottom of them? I wonder because there are pin hole size pits that might be deep and polish out slowly on my mirror. I did about 10 hours polishing and still have pits. The ruler is 1/64" lines. The pits are 120 microns across.Attached Thumbnails




Jessie Forbes

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Re: does polishing glass fill pits?
« Reply #1 on: January 04, 2018, 03:42:07 AM »
Yes it does. Eventually. The quality of the area where pits are filled isn't quite as good and causes problems for UV transmission systems because impurities in the silica deposits from the slurry get captured into the fill. High energy laser mirrors will also fail at these points, so there's a whole tech method devoted to avoiding such problems.

hluhsubshoona

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Re: does polishing glass fill pits?
« Reply #2 on: January 08, 2018, 01:45:19 AM »
Is your entire surface like this? That's too many pits, you need to return to fine grind to remove them.

brigtigeartgib

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Re: does polishing glass fill pits?
« Reply #3 on: January 11, 2018, 02:57:19 AM »
Must be 100 pits / square inchat the edge. The middle about 10 pits / square inch.  Left over from rough grinding probably. It sounds bad but the glass looks all polished to the edge.

ceicomfeara

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Re: does polishing glass fill pits?
« Reply #4 on: January 11, 2018, 02:22:53 PM »
It's not polished out if there are pits remaining from fine grinding. You need to go back to fine grinding! Estimate the size of the largest pits and start one grit smaller. Trust me I understand that this is disappointing news! You have undoubtedly put a tremendous amount of time and effort into your mirror. Don't waist that investment. Take the time to make it right, even if you put a decent figure on the mirror it will still perform poorly. You'll be disappointed with the performance and have waisted your time and effort.
Just my opinion. I struggled with the same decision! In the end I was fortunate to have listened to the advice given by many of the very experienced mirror makes here and ended up with a mirror far beyond my expectations and certainly much better in quality than anything I could afford to buy!
-Pete.

freddormasa

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Re: does polishing glass fill pits?
« Reply #5 on: January 12, 2018, 12:28:31 AM »
Judging by the size and number of the pits (and being all the way to the center), if it were my mirror I would go all the way back to a #220 grit.
I have done the same in the past, (on more than one occasion ), even though I may have had only a hand full of similar sized pits over the surface of a 20"+ mirror.

You won't be happy with a final result unless you remove those pits (all of them). If you can't execute a simple grind, how can you expect to get a good polish and figure?

I'm not trying to be unkind, just helpful.

Stephen.(45deg.S.)

Lasaro Tourabi

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Re: does polishing glass fill pits?
« Reply #6 on: January 13, 2018, 12:56:07 AM »
I'll take the pitch off and grind 220 for and hour or two.  Maybe it will get to f/7 .

tersrhythopes

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Re: does polishing glass fill pits?
« Reply #7 on: January 14, 2018, 05:44:28 AM »
There's no need to change your sag unless you want to. If you alternative MOT and TOT you can keep it where it's at. Just take care to measure. Don't get hung up on grinding for a fixed amount of time. Grind each grit until all the pits from the previous grit are gone then move on. When in doubt do 3-5 more wets and check again. I made a reference guide out of a piece of glass from an old picture frame. I framed a series of squares using masking tape then used a sharppe on the masking tape to label the grit size then using a single tile ground each grit from 80 to 25 micron wao. It was extremely helpful to have a reference while deciding if I was ready to move on. It's worth mentioning that I used several layers of heavy duty duct tape on the edges and corners. Window glass is very sharp! 
-Pete.

omunsopoo

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Re: does polishing glass fill pits?
« Reply #8 on: January 18, 2018, 03:30:43 AM »
All the way back to grit #220, really ??

Why not #320 or even 25 micron + a little more time fine grinding?
Time is save in quicker polishing... Or am I totally off here?

brascharnide

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Re: does polishing glass fill pits?
« Reply #9 on: January 18, 2018, 03:45:05 AM »
If one wants a pit-free polish, it's not the number but the size that counts. A pit width of about 120 microns is huge, because when 'fresh' has a depth of about the same. It will take a *very* long time to polish out.

I *feel* that any infilling by glass/slurry is a much slower contribution to their removal than is the bringing down the level of the finished surface to their bottoms. Indeed, I *feel* that infilling becomes of meaningful effect only for the the tiniest of pits, when nearer to achieving a full polish. This is based on the observation of pit bottoms appearing to retain a good deal of ragged sharpness everywhere interior to their peripheries all the way to irresolvability, not long before their disappearance.

migresinli

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Re: does polishing glass fill pits?
« Reply #10 on: January 18, 2018, 07:29:48 AM »
I'd be inclined to go no coarser than 25 micron, or about 400 grit. But I can see why some might recommend a coarser abrasive if the polishing has imparted a notable enough change in the radius; tool and mirror will more quickly conform to each other. And it does not take so long thereafter to grind with subsequent grades, certainly compared to what it would require if you were instead to continue polishing!

Scott Etrheim

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Re: does polishing glass fill pits?
« Reply #11 on: January 30, 2018, 06:40:20 AM »
yeah but you can do 1/2 hour of 25um and see if they all disappear and if not go to 220

lodbelimfo

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Re: does polishing glass fill pits?
« Reply #12 on: February 09, 2018, 11:28:48 AM »
You could get some small pieces of glass and grind them with thevarious grits and compare them to the mirror pits.Sam