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

[John Lynch <john_lynch2007@yahoo.com>]

Tom,

When I read the posting I did a double take when I
came across the hydrogen flouride part.  I work at
NASA Goddard Space Flight Center where they use this
for advanced detector development, and I know that
they are required to take extreme precautions when
handling it.  As far as anyone (amateurs) having
access to this stuff and using it at home, I would
consider them to be a fool.  I would also argue that
there's probably some law against having HF in
household/neighborhood environment.  There are tight
regulations even in sanctioned work environments with
all of the required safety equipment.

John

--- Tom Wicker <tomofreno2000@yahoo.com> wrote:

> 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
> > 
> > 
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
> > 
> 
> 
> 
>  
>
____________________________________________________________________________________
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> in the Yahoo! Answers Food & Drink Q&A.
>
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> 



 
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