Re: [ATM] What a wonderful sound!

["Bob May" <rmay@nethere.com>]

Sounds like you're near the end of hogging time.  Use the pipe
cap to get that curve a bit smoother and you'll be ready to make
the tile tool.  Don't obsess over it or you'll never get to the
next step tho.  This is the point where it is very difficult to
do anything wrong at all other than breaking the glass in two.
I generally tell people not to worry because they always worry
too much about whether they are doing things right and when you
go and look back at where things have been for making fine
surfaces, you'll note that the methods are almost continously
changing.  There was a time when you hung a long pole from a
flagpole or peak of a barn to do the grinding and polishing of a
lens surface to make sure that the surface was the right
curvature!  We tend not to do that sort of thing anymore.
I'll also note that you've already seen the best part of a
spherometer in that it is quite sensitive to changes in
curvature.  You may not have the exact radius of the disk and
thus not be getting the right ROC of the glass surface, but you
definitely see where it is different than another area of that
surface with no problems.
For a more exact ROC, wait until you get to the 25 micron grit
and do the flashlight reflection test to find the ROC of the
glass and that you can do to a fraction of an inch and at that
grit, you won't be that far off for the finished ROC.
In the words of the Stardrek announcere, "Boldly go where
everyone has gone before!" as it is a common easy path you are
following.  I will leave you with this one note tho, the pros are
doing mirrors from start to finish for the amateurs with about an
hour's worth of time these days.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Bill Wheaton <junkwheaton@gmail.com>
To: David Harbour <stainless_steel@suddenlink.net>
Cc: Bob May <rmay@nethere.com>; <atm@atmlist.net>
Sent: Monday, May 05, 2008 7:21 PM
Subject: Re: [ATM] What a wonderful sound!


> So, I'll split the difference between "just do it" and "detail
nut"
> for now, and come up with my own style eventually :)
>
> In many ways, I am both of your camps.... especially after
tonight.  I
> spent about 45 minutes metal on glass time, trying to do
smaller, more
> even steps around the barrel, and more even rounder strokes,
and got
> into my "groove" so to speak (or lack of one, I hope!).  I
certainly
> don't want to be sloppy about it, but I've also read that
randomness
> is an essencial ingredient too.
>
> So, when I did my rough sagitta measurement it said 0.1230
inch. I
> thought I was getting close enough to the target that I needed
a
> better measurement method, so I made a sphereometer.
>
> I saw a suggestion (perhaps on your site Bob?) about making one
from a
> pulley.  I happened to have a 6" steel pulley from an old
furnace fan
> with a fairly sharp edge, so I ground it down slightly right on
my
> mirror until it had a <1/64" flat ring on it and put my digital
dial
> indicator in the middle of it with some bronze bushings, and
some
> masking tape to make up the difference.
>
> I zeroed it on the flat back of the old glass tool I have, and
tested
> the top of the tool in several places and got .021 to.022,
which is
> just about at a 100" focal length, so I figure that I have a
fairly
> good ring type sphereometer.
>
> Am I correct in how I am using it?  The measurements should
give the
> sagitta of a 6" portion of the sphere, so looking at Bob May's
quick
> sagitta reference page
http://bobmay.astronomy.net/misc/sagchart.htm,
> in the 6" column, and found that .022 is 100" FL.  Does that
make
> sense?  Also, I find that I am able to take readings right up
to the
> edge, since its a ring.  I read somewhere that is one of its
> advantages.  Is that right?
>
> Anyway, so applying that principle to the mirror then, I got
> measurements of .026 at the edges, .030 half way in, and .036
at the
> center. Its pretty consistant all around like that.  In other
words It
> looks to me that I dug a hole, and now I will have to widen it
by
> grinding the glass around the edges a lot more than I was
before.
>
> Now, Bob, you have said several times, grind where the glass is
(to
> paraphrase).  I take that to mean in this case, to use the two
inch
> pipe cap instead of the 3" pipe flange, and stay away from the
center
> for now.  I suspect that the best course of action is to grind
it like
> with short strokes around the edge, but measure often.  Do you
agree?
>
> But at what point do I say "hogging is done, time to pour the
tool"?
> Should I wait until the sphereometer readings are all .032 +-
.001"?
>
> I find it amazing how fast this is going, I figured that I
would be
> doing this a lot longer.  This is a blast.  Sure hope the cool
weather
> holds!
>
> -Bill
>

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