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

[William Marriott <bmarriott@pacbell.net>]

Chris;

In general, a given grit size has a bell curve of
particle size distribution. So your 9u has particles
that are smaller than 9u, and has the problem larger
particles as well.  This may vary by manufacturer, but
by the time you get to 9u, even the bigger pits should
polish out, especially with BK-7.  What is the product
you are grinding with?  There are aluminium oxide
products that have a platelet form to minimixe
subsurface damage.  You can also try grinding with 5u
if your set up is clean, to see if that helps. 

Just some thoughts

Bill Marriott
VA Optical Labs


--- Chris Dalla Piazza <dalchri@hotmail.com> wrote:

> 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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> 

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