[ATM] ? grinding mirror on top

[Norm Prince]

neat!...thanks Ken...I'll bet dollars to donuts it took less time for you to 
type that than it would take a novice like me to come to that conclusion by 
searching the archives...thanks for taking the time...now some of us can use 
this info to our advantage...I had just chalked it up to gravity, but now 
i'm gonna play with where i put my hands a bit when i think i need to make 
corrections...thanks again!

-norm prince-


----- Original Message ----- 
From: "Ken Hunter" <atm_ken_hunter at yahoo.com>
To: "hermit" <hermit at outofoptions.org>
Cc: <atm at atmlist.net>
Sent: Friday, April 06, 2007 16:07 PM
Subject: Re: [ATM] ? grinding mirror on top


> Hermit,
>
>  It's a matter of localized pressure doing the work in select areas.
>
>  Take 2 similar sized disks and place one on top of the other - centered. 
> Rotate the top disk one turn around the axis of the bottom disk.
>
>  Notice the relationship between the disks and imagine that anywhere the 
> disks overlap that there is work being performed. It's easy to see that 
> both disks will receive the same work and end up identical.
>
>  Now offset the top disk 1/4th it's diameter to any side and rotate the 
> top disk one turn around the axis of the bottom disk while rotating the 
> top disk one turn also.
>
>  Again, notice the relationship as before and again imagine that where 
> they are overlapped there will be work performed. You should see that the 
> center of the bottom disk has an area that is never uncovered while the 
> outside of the bottom disk has an area that is uncovered in various 
> amounts due to the curve of the top disk... OK, the center of the bottom 
> disk is getting more work than the edge. The bottom disk should get 
> deeper!
>
>  NOT SO FAST...
>
>  The top disk has exactly the same situation, the center is never 
> uncovered and the edge is exposed exactly like the bottom disk... OK, the 
> center of the top disk is getting more work than the edge. The top disk 
> should get deeper!
>
>  THIS DOESN'T SEEM RIGHT!...
>
>  So far both disks have exactly the same situation so they should get the 
> exact same curve.
>
>  NOW... Add downward force to the CENTER of the top disk... Remember we 
> were not using any downward force before. And gravity has been suspended 
> for this essay...
>
>  When the disks are centered, they both get equal and uniform work across 
> their surfaces. When the top disk is offset by 1/4 diameter, look where 
> the areas of possible work and the pressure is being applied...
>
>  The top disk has a greater pressure, and thus more efficient work, at the 
> center where the pressure is while the bottom disk is receiving this 
> pressure directly under the center of the bottom disk when the disks are 
> overlapped.
>
>  So, the top disk is made deeper in the center while the bottom disk is 
> made deeper around the edge.
>
>  Now... Let's change things. Let's overhang the disks again but this time, 
> put the pressure on the edge of the top disk.
>  Since there is pressure at the edge of the top disk, the edge of the top 
> disk receives the work while the area 1/4 diameter from the edge of the 
> bottom disk receives the work...
>
>  Hmmm. If we started off flat, it would seem that the bottom disk would be 
> the one getting deeper while the top one becomes convex. This is a fact 
> that you can verify yourself. The curve of the disk is the result of 
> localized pressure.
>
>  It's physically easier to hold and control the top disk while applying 
> pressure at the center so it's usually done this way but... It is often 
> the case during grinding and/or polishing that one wants to adjust the 
> focal length or figure of an optic where the best approach is to apply the 
> pressure at the edge, center or somewhere in between.
>
>  Hope this helps.
>
>  Ken Hunter
>
>  Oh yeah... I just emembered to turn gravity back on. You can let go now! 
> ;0)
>
>
>
>
> hermit <hermit at outofoptions.org> wrote:
>    horace r davis wrote:
>
>  Why does a mirror deepen in the center when it's on top... (sorta)
>
>
>
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