Re: [ATM] Fine Grind already Blanchard ground mirror blank back?

[Tom Wicker <tomofreno2000@yahoo.com>]

Please be cautious of advising anyone to use
hydrofluoric (HF) acid.  It is extremely nasty.  There
is no pain felt immediately if you get some on you. 
It diffuses readily through tissues and goes to your
bones where it reacts with the calcium causing
excrutiating pain, at which point there is nothing
that can be done.  The only intervention is to inject
calcium in the exposed tissues within hours after
contact. It is not something to mess, with.  Use
extreme caution, and always use double gloves.
Tom W.

--- Mike Lockwood <melockwo@uiuc.edu> wrote:

> Hi,
> 
> Michael Fremont wrote:
> > I just received a 16" 1.625" thick pyrex blank
> from Newport Glass.  It
> > is pregenerated to f4.5 and Blanchard ground on
> the back.
> > 
> > Having looked through the archive, I'm still
> unsure whether I should
> > fine grind the back at 220 grit even though it's
> Blanchard ground; and
> > am further unsure that I can even do it, since the
> front already has a
> > curve.
> 
> If Blanchard ground, the back should be quite flat. 
> You should always 
> check this, though.  When I have spent the money on
> quality blanks, 
> I've never seen a bad Blanchard grind..... yet.
> 
> If the back is indeed flat right out of the box,
> then there's no real 
> measurable benefit to grinding it, unless you desire
> a smoother 
> surface finish in order to allow it to slide more
> easily on the mirror 
> cell support points.  That's a good thing, in my
> opinion, but 
> inconvenient if you have to make or buy a flat tool.
> 
> Check the back with a known good straightedge across
> several 
> diameters.  You should get EXACTLY the same result
> across all 
> diameters that you check, using the same side and
> portion of the 
> straightedge.
> 
> If the back of the mirror is slightly concave or
> convex, you might 
> consider grinding it, but it's not necessary if the
> back is a figure 
> of revolution.  If it measures differently across
> different diameters, 
> you should DEFINITELY grind it flat, because the
> back of the mirror is 
> astigmatic, and that will tend to make the front
> astigmatic.
> 
> Once you start grinding the back and you get it
> flat, there really is 
> little extra work involved in taking it down to 25u
> and really getting 
> it smooth, so you might as well do it.
> 
> Some may argue that the subsurface damage from the
> generating of the 
> back should be removed by grinding or by other means
> (Bob Goff used to 
> dip pieces in HF acid to etch away any damage). 
> However, in 
> well-annealed, reasonably thick glass where accuracy
> beyond 1/50th 
> wave is not important, this is really not worth it,
> in my opinion.
> 
> 	Mike Lockwood
> 
> 
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> 



 
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