Re: [ATM] Central hole during polishing

[Balaji Ramani <balaji@netapp.com>]

Thanks for the quick reply, Mike.

On Tuesday 31 January 2006 12:01, Mike Lockwood wrote:
> Balaji,
>
> Balaji Ramani wrote:
> > I have been working on a 10" f/5 Pyrex glass mirror (1.5 inches thick). 
> > I have been polishing it with a 1/3 W stroke, both MOT and TOT and
> > frequent turnings.  When I put the mirror on the foucault stand, I
> > noticed that there is a huge hole in the center.  The hole is about  4.5"
> > in diameter.  To null out the hole, I had to move the KE towards the
> > mirror about an inch!
>
> An inch is a lot.
>
> If you are switching MOT and TOT and doing a W-stroke, a hole should
> not get that deep.  There is probably some other factor at work.
>
> > So, I am wondering how I might have gotten into this state and how to fix
> > it? I would like to get to a good sphere before I start figuring.  Any
> > suggestions, please?
>
> First, look at the mirror.  Is the polish even?  Look for residual
> pits with a laser or strong flashlight.  If there are more pits in the
> center than elsewhere, chances are that hole was ground in.  If there
> are fewer pits there, it was likely polished in.  If there is no
> difference, then it is also likely it was polished in.

When I previously looked at the mirror, the whole surface seemed evenly 
polished out.  But I will take a look at it in the evening again.

>
> Second, look at the lap.  (I assume you are using a pitch lap and not
> polishing pads.  I also assume you are pressing the lap before use.)
> The nastiest holes I have seen were from pitch lap facets that bulged
> due to bubbles in the pitch.  The pitch heats up during polishing, the
> air in the bubble expands, and that facet rises up and polishes away
> more glass than the others.  You might try gently warming the lap and
> looking at it from the side, and seeing if there are any facets that
> are higher than others.  If you find a bulge, pop the bubble with a
> hot soldering pencil.  Then continue, and hope there are no more bubbles!

Yes, I am using a pitch lap.  Now that you mention bubbles in the lap, I 
remember seeing quite a lot of bubbles when I poured my lap.  Give that, I 
think I am going to remake the lap and give it a shot.

>
> Another possibility is that the lap has more area in the center, due
> to missing outer facets, etc.
>
> 	Mike Lockwood

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