Re: [ATM] Snap, crackle, pop, this is the pits

["Jerry" <wa4guu@verizon.net>]

I know this is long, but I think there are a number of possibilities here
and all worth your consideration. I might jump around a bit on ideas.

IF they are different pits, meaning that you are removing some while
producing others, I would think that the thing to do would be make sure you
have them spherical as possible and then move on to polishing. 

Since you mention "BK7 lenses" I am thinking you are making something like a
Haughton. I don't know how much thickness figures into the design of the
curves but if thickness matters much to you for optical or mechanical
reasons, you might want to see if you can polish the pits out before the
glass is thinner than you want. You might find that it polishes out well. If
there was some earlier stage that had not been completed well, it will be
more obvious after and hour or so of polishing. If it is only the 9 micron
that you have not done well enough you will still be able to polish fully in
a reasonable amount of time.  You can go back to grinding if it doesn't
polish well.

When I look at the surfaces I grind, I can always find some out sized pits
in the surface. I map them and when I determine that I am not looking at the
same ones as last time, I move on.

What I would be particularly concerned with is that you say it binds to a
stop. Grinding with 9 micron, I can't imagine that grit is doing that
without leaving serious scratches or tracks of pits from large grit rolling
its corners into the glass. These should be easily seen. It would take some
large grit to bind the work to a stop.

That makes me believe that there is a fair chance that the reason for the
binding is that the lens and/or tool are not spherical. As you adjust the
radius of curve at each grade of grit, you can either dig a hole in the
middle or work the edge to change radius. Depending on whether the surface
is convex or concave determines which of those lengthens or shortens radius.
Some time needs to be spent making sure to get the curve smoothly spherical
across the surface after you have changed the radius. If you don't, you can
have binding. Usually the binding will be with the disks close to centered.

Consider other sources of outsized particles than the grit you are working
with. Check your bevel and see if you see tiny clamshells at the edge. If
the bevel is too small or ground too coarse, as you grind some tiny chips of
glass can come off and that can cause problems. Can you filter your water or
otherwise verify that there are no particles of significant size in it? In
some environments, airborne particles are not out of the question.

Have you ground all of the surfaces with each grit, or are you working one
surface through all grit grades and then working another surface. If one
side of a disk is rough it can hold large grains of grit that can come out
and contaminate the surface you are working with fine grit. I would work
both sides of a disk with each grit grade (preferably all surfaces if more
than one glass) and then move to the next. That way the two sides are never
more than one grit size different in fineness. 
 
Are you grinding with a tile tool or a continuous surface such as another
glass disk? Tile is less likely to bind, but then it is not as easy to tell
if you have non-spherical surfaces. Tile can chip along the edges too, and
tile tools have much more edge length for chipping to occur.
 
What size lenses are you making?

That is enough for now. 

Jerry 

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Chris Dalla Piazza
Sent: Thursday, February 08, 2007 11:48 AM
To: atm@atmlist.net
Subject: [ATM] Snap, crackle, pop, this is the pits

I am working on the last 9 micron grade of aluminum oxide with my BK7 lenses
and keep on getting one or two pits.  These vary from small specks that
appear dark backlit and grind out after 4 wets to larger ones that are just
big enough to sparkle when backlit.

I've tried two different sources for grit and about every other wet, I still
start out with a few grains that are above average.  You can hear them snap
a few times before they are thrown out to the side or grind down.  Sometimes
they are enough to bind and stop the motion of the lenses.  I'm pretty sure
it is the grit because I've tried a few test wets without grit and don't
hear the popping.

 

That said, I finished a Pyrex mirror fine with the same grit so I am
thinking that the BK7 is just soft enough to be sensitive to this problem.

 

I know, a few pits don't mean much, I just want to be really aggressive with
this problem because it will result in 4 optical surfaces with the same
problem.

 

I am trying to mitigate the problem by starting out with a wide circular
stroke to encourage the larger particles to rub themselves out to the sides
of the lenses quickly.

 

I am just curious, anyone else have this problem?  Any suggestions?

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