Re: [ATM] What a wonderful sound!

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

I tried it with the small 2" hogging tool at the edge tonight.  It
looks like it worked.  The hole is shallower now, and I have it all
within .0055 +- .0015".  A little more grinding like that, and I'll be
in good shape - maybe a half hour or so.  Getting the hang of this.

Then I need to find some dental stone and some tiles.  I need to talk
to my dentist anyway... (oy).

So how do you know if the tiles are the "hard" sort, and not the
"soft" ones.  Is it pretty obvious when you buy them?  I was going to
get the square ones so it would be easier to clear the channels rather
than the hex ones.

I do have a bunch of nickels, could I use them instead?  Would it work
better?  I saw something about that in the archives, but I'm not
certain if it was simply for hogging or not... I think it was.

-Bill

On Mon, May 5, 2008 at 10:21 PM, Bill Wheaton <junkwheaton@gmail.com> wrote:
> 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/