Re: [ATM] What a wonderful sound!

["Bill Wheaton" <junkwheaton@gmail.com>]

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
_______________________________________________
ATM mailing list http://www.atmlist.net/