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Equipment Discussions >> ATM, Optics and DIY Forum

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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Problems Polishing new
      #5531633 - 11/21/12 07:54 AM

Friends, Romans and Mirror Making Experts,

I'm currently polishing an 8 inch F6 pyrex mirror blank. I've been at it for a while (possibly as much as 20 hours!!!). I'm still not fully polished and it has occurred to me that I'm doing something wrong. So, maybe you can help.

I'm using Gugloz pitch #73. I realized this was a mistake when summer turned into winter. I heat the lap on the boiler of my heating system, then I put it under a heat lamp for a few minutes, then I press it for 15 to 20 minutes and then I start working. I think I'm making good contact, but at this point I'm not sure of anything.

I'm also questioning my cerium oxide mixture. I'm using a 1 to 8 formula as per the Stellaphane website.

I'm putting a decent amount of pressure on it when I polish and I"m alternating the glass on top and on the bottom. I also have the Texereau book and I'm reading it religiously. Yet, I'm still stumped.

Help!!!!!! When will it end?

Bill


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kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Problems Polishing new [Re: Bill Whitehead]
      #5531642 - 11/21/12 08:05 AM

Hi Is it pits that are not going away? or one area ? Press the pitch with screen might help .What does the knife edge show?

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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Re: Problems Polishing new [Re: kfrederick]
      #5531656 - 11/21/12 08:14 AM

I have large pits and general micro pitting all over the surface, that I can see with a laser. I have a sphere as per a ronchi test, but I haven't done a foucault test yet.

Bill


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kfrederick
Carpal Tunnel
*****

Reged: 02/01/08

Re: Problems Polishing new [Re: Bill Whitehead]
      #5531731 - 11/21/12 08:59 AM

Some pits are not going to effect the image On a machine you can polish out a deep scratch . If you have any pads i would put then on the top of your pitch lap .I can send you some . Works some faster . All else fails re grind 10 micron For at least one hour.

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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Problems Polishing new [Re: Bill Whitehead]
      #5531741 - 11/21/12 09:05 AM Attachment (31 downloads)

it should take about 4 hours to polish out a mirror to be optically black if you use polishing pads.

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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Problems Polishing new [Re: Pinbout]
      #5531829 - 11/21/12 09:57 AM

Have you tried cleaning the mirror surface very thoroughly with acetone or alcohol prior to the laser test? You may see that some areas are clear and others apparently still a bit speckly. This can be an indication of dirt on the mirror rather than a lack of polish. Frankly, after 20 hours it's hard to imagine that you are not polished out.

Of course the other possibility is that your fine grinding was not successful in eliminating the pits. What was your final grit size?

JimC


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DAVIDG
Post Laureate
*****

Reged: 12/02/04

Loc: Hockessin, De
Re: Problems Polishing new [Re: Mirzam]
      #5531861 - 11/21/12 10:15 AM

It sounds to me that you did not grind out the pits from the coarser grits. It will take less time to go back to 400 grit and regrind the surface from there back thru 5 micron then to continue to polish. It might take another 20 hours of polishing to remove the remaining pits and in that time you can have the mirror reground, fully polished and be well into final figuring.

- Dave


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Problems Polishing new [Re: DAVIDG]
      #5532435 - 11/21/12 03:02 PM

2nd that, Dave- sounds like a coarse grind wasn't fully ground with the finer grits.
Hey Danny, on my pads, I don't let anything but water and compound, or a clean paper towel touch them. (to assure no scratches)
M.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Problems Polishing new [Re: Mark Harry]
      #5532464 - 11/21/12 03:16 PM

Quote:

Hey Danny, on my pads, I don't let anything but water and compound, or a clean paper towel touch them. (to assure no scratches)





good to know. that's a pic before I knew I don't know what I doin'.


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ed_turco
Pooh-Bah
*****

Reged: 08/29/09

Loc: Lincoln, RI
Re: Problems Polishing new [Re: DAVIDG]
      #5532659 - 11/21/12 05:06 PM

Quote:

It sounds to me that you did not grind out the pits from the coarser grits. It will take less time to go back to 400 grit and regrind the surface from there back thru 5 micron then to continue to polish. It might take another 20 hours of polishing to remove the remaining pits and in that time you can have the mirror reground, fully polished and be well into final figuring.

- Dave




+1


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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Re: Problems Polishing new [Re: ed_turco]
      #5533488 - 11/22/12 07:53 AM

Well, looks like I'm going to go back to grinding.
Thanks for the help.

Happy Thanksgiving!
Bill


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KnightWeaver
newbie


Reged: 11/25/12

Re: Problems Polishing new [Re: Bill Whitehead]
      #5540298 - 11/26/12 01:23 PM

This may sound like a really silly issue but my first mirror (3/4" blank given to me) I was getting frustrating because I was having the similar issue as you but my problem was with scratching during polishing. As it turns out my polish was going so fast I was looking through the first surface when using the magnifier and focusing on the back surface of the mirror and picking up scratches from the back of the mirror. After find grinding and polishing 3 times I finally realized my error and boy was I embarrassed. Wasted about 15 hours in grinding / polishing.

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Bill Whitehead
sage
*****

Reged: 07/11/09

Loc: Exit 135 - Garden St Parkway
Re: Problems Polishing new [Re: KnightWeaver]
      #5540695 - 11/26/12 05:23 PM

Thanks for the info. However, I'm seeing fine pits with a laser pointer held at a 45 degree angle in the dark of my basement. The beam should go right through the surface of the glass if it is fully polished. Instead it reflects off of the micro pitting. It seems I have to go back to grinding.

Peace, Bill


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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Problems Polishing new [Re: Bill Whitehead]
      #5540767 - 11/26/12 06:22 PM

Quote:

Thanks for the info. However, I'm seeing fine pits with a laser pointer held at a 45 degree angle in the dark of my basement. The beam should go right through the surface of the glass if it is fully polished. Instead it reflects off of the micro pitting. It seems I have to go back to grinding.
Peace, Bill



Bill, you need to visually inspect the surface of the mirror with a loupe/magnifier during grind and polish. Strong illumination from the side (bright mag-lite, etc., experiment with the angle) will show more than a narrow laser beam. A low-power microscope is nice, too. Black velvet or posterboard below the mirror will make pits easier to see.

Do this type of inspection before taking the drastic step of going back to grinding.

If you do need to go back to grind, do the same inspection, looking for very large pits.

A laser is a good tool to judge polishing progress with, but it requires clean glass and practice. Use it only after you are sure that you are grinding properly.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Problems Polishing new [Re: Mike Lockwood]
      #5541357 - 11/26/12 11:50 PM

Quote:

A laser is a good tool to judge polishing progress with, but it requires clean glass and practice.




+1 on that. I've often done the test on a section of the mirror and seen the laser reflection only to have it disappear after a careful clean.

Dave


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John Carruthers
Skiprat
*****

Reged: 02/02/07

Loc: Kent, UK
Re: Problems Polishing [Re: dave brock]
      #5541503 - 11/27/12 04:04 AM

going back a grit can be a wise move,
#1 it ensures a good sphere to start figuring from (a good 'bottom').
#2 it removes pits and scratches.
#3 it prevents you from dying of old age before it's polished.


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