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grinding glass perfectly flat



 
 
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  #1  
Old November 13th 03, 07:33 AM
Allan Adler
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Default grinding glass perfectly flat


I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler


************************************************** **************************
* *
* Disclaimer: I am a guest and *not* a member of the MIT Artificial *
* Intelligence Lab. My actions and comments do not reflect *
* in any way on MIT. Moreover, I am nowhere near the Boston *
* metropolitan area. *
* *
************************************************** **************************
  #2  
Old November 13th 03, 10:58 AM
Mike Firth
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Default grinding glass perfectly flat

There was a thread on here a while back on this topic after someone asked
about the same question and flat for that person turned out to be
EXTRAORDINARILY flat - like quite an area flat to 0.00001" or some such.
If you are talking about several square inches and flatness measured with
Newton rings - basically you use very fine grit with stuff that is basically
flat to start with and reverse the techniques of mirror grinding - instead
of using force on the edge revolved around to make the basic spherical
shape - you apply even force and use one pretty flat surface to take the
high points off another pretty flat surface.
Float glass is rather flat for the purposes of people who want to grind a
flat surface on steel cutting tools.

--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit45.htm Latest notes
http://users.ticnet.com/mikefirth/NTBowl.htm Once again, Empty Bowls
will collect for the North Texas food banks - donate.
"Allan Adler" wrote in message
...

I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler



************************************************** **************************
*

*
* Disclaimer: I am a guest and *not* a member of the MIT Artificial

*
* Intelligence Lab. My actions and comments do not reflect

*
* in any way on MIT. Moreover, I am nowhere near the Boston

*
* metropolitan area.

*
*

*

************************************************** **************************


  #3  
Old November 13th 03, 03:14 PM
Bill Browne
external usenet poster
 
Posts: n/a
Default grinding glass perfectly flat

You need to decide how flat 'perfect' is. .001"? .0001"? Give yourself a
tolorance. Then you can decide how to get there.

Tool and die makers use something called a surface plate. They are made of
black granite and are relatively cheap (around $20 for 9" by 12") Flatness
depends on the size and grade purchased. Tool room grade is good down to
+/- .0001". Inspection grade down to +/-.00005".

There are also cheap ($20) micrometers that will measure down to .0001"

I wouldn't put the abrasive directly on the surface plate, as you could
unflatten the plate trying to flatten the glass. :-) Maybe some of that
peel and stick lapping paper stuck to a surface plate? I think that would
get you down around .001".

--
Bill Browne
Computer for work http://excalibur-dbf.com
Metal & glass for fun http://w.browne.home.att.net
"Allan Adler" wrote in message
...

I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler



************************************************** **************************
*

*
* Disclaimer: I am a guest and *not* a member of the MIT Artificial

*
* Intelligence Lab. My actions and comments do not reflect

*
* in any way on MIT. Moreover, I am nowhere near the Boston

*
* metropolitan area.

*
*

*

************************************************** **************************


  #4  
Old November 13th 03, 03:34 PM
Moonraker
external usenet poster
 
Posts: n/a
Default grinding glass perfectly flat

You are posting this from MIT? Don't you have a library there?

"Allan Adler" wrote in message
...


  #5  
Old November 13th 03, 04:14 PM
nJb
external usenet poster
 
Posts: n/a
Default grinding glass perfectly flat

Allan Adler wrote:

I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler



We attained flatness on carbon rings in the machine shop by the use of
lapping machines. We also ground the matching hard faces of tungsten
carbide. We measured the flatness with an "optical flat", which is an
extremely flat slab of glass. The optical flats were able to detect
variations of one light wave, a particular light wave, which on our
instrument IIRC was .000016" I assume that if the optical flat could
measure 16 millionths of an inch it must have been at least that flat
itself. I will also assume that the optical flat was ground in the same
manner. They are quite expensive and are stored in velvet lined cases.

Read more he
http://www.surfacefinishes.com/optiflats.html


Jack


http://photos.yahoo.com/bc/xmissionbobo/
  #6  
Old November 13th 03, 05:25 PM
Parallax
external usenet poster
 
Posts: n/a
Default grinding glass perfectly flat

"Mike Firth" wrote in message ...
There was a thread on here a while back on this topic after someone asked
about the same question and flat for that person turned out to be
EXTRAORDINARILY flat - like quite an area flat to 0.00001" or some such.
If you are talking about several square inches and flatness measured with
Newton rings - basically you use very fine grit with stuff that is basically
flat to start with and reverse the techniques of mirror grinding - instead
of using force on the edge revolved around to make the basic spherical
shape - you apply even force and use one pretty flat surface to take the
high points off another pretty flat surface.
Float glass is rather flat for the purposes of people who want to grind a
flat surface on steel cutting tools.


My experience with float glass is that it is noticeably wavy although
amazingly smooth.

--
Mike Firth
Hot Glass Bits Furnace Working Website
http://users.ticnet.com/mikefirth/hotbit45.htm Latest notes
http://users.ticnet.com/mikefirth/NTBowl.htm Once again, Empty Bowls
will collect for the North Texas food banks - donate.
"Allan Adler" wrote in message
...

I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler



************************************************** **************************
*

*
* Disclaimer: I am a guest and *not* a member of the MIT Artificial

*
* Intelligence Lab. My actions and comments do not reflect

*
* in any way on MIT. Moreover, I am nowhere near the Boston

*
* metropolitan area.

*
*

*

************************************************** **************************

  #7  
Old November 13th 03, 08:18 PM
Dave Mundt
external usenet poster
 
Posts: n/a
Default grinding glass perfectly flat

Greetings and Salutations.

On 13 Nov 2003 01:33:58 -0500, Allan Adler
wrote:


I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler


There is a wonderful trilogy of books titled "Amateur
Telescope Making", edited by a fellow named Ingalls that covers
all aspects of optics - including how to make optical flats.
Go to your public library and see if they have a copy. The
articles there will tell you more about the process than you
want to know.
However, in short...take three thickish pieces of glass
and a series of grits, ranging from fairly coarse, to jeweler's
rouge. Starting with the coarsest grit, grind all three plates
on each other, until all three have an even scratch pattern on the
ground face. Clean everything off. Goto the next finest grit.
Repeat above steps for each grit until polished.
It helps if you have a monochromatic light source, say,
a sodium arc lamp, or, a candle burning salt for that bright,
yellow flame. That is how one tests the surfaces to see how
"flat" they are getting. By putting two of the plates together,
face to face, and illuminating with the monochromatic light,
one gets a series of interference fringes. The shape of the
fringes and the number of them give an idea of how parallel
the surfaces are. In a "perfect" world, you would have one,
wide fringe that covers the entire contact surface...that
would mean the plates are "perfectly" parallel, and, if you
have used three plates, perfectly flat.
(remember that it takes three points to define a plane,
mathematically...works with glass too).
Needless to say it is a TAD more tedious than this
explanation, but, this hits the high points. (haw, haw, haw).
Regards
Dave Mundt

 




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