To: George Anderson ,atm 
From: Dennis Rech 
Reply-To: Dennis Rech 


At 11:22 PM 12/20/02 -0500, George Anderson wrote:

>A friend is about to start building a mirror-o-matic after I directed
>him to the site, he ordered the manual and while waiting for it to
>arrive he has asked me to post the following two questions.
>1) Has anyone on the list used a glass tool on the mirror-o-matic?
>Normand prefers glass (he has never used tile) and all of the stuff he
>saw only mentioned tile.
>2) Does anyone in the frozen wastelands of canada know of a source or
>part number for the retainer cup used for holding the tool?
>I'll be away for a few days so I probably will not be thanking all for
>their answers in a timely manner.
>Seasons greetings and best wishes to all, may your glass be flawless and
>TDE something on a cheap commercial mirror.
>
>George Anderson
>Montreal Canada
>
>Clear skies and good health

Hi George,
I probably answered the question directly to your friend last week, but
I'll repeat it here so we don't have a dangling question hanging around.
glass tools work about as well as tile. A great way to get rid of that
mis-annealed 18 inch blank is to use it as a tool on your 24 inch mirror.
You will have a problem with holding it. You can machine a small diameter
thick aluminum disk with a hole in the back and pitch it onto the glass
tool. I have some professional glass tools here that have something similar
epoxied to the back.
You should use a diamond wheel to score some grooves across the face of the
tool to aid grit dispersement and keep the tool from sucking itself tight
to the mirror.
The ones that I have here are scored about one inch on center both ways.
There has been some discussion on this group concerning pregenerated
aluminum tools for grinding.
Pregenerated metal tools of all kinds work dandy on fine grinding.
Professional typically use cast iron tools for fine grinding.

Since aluminum is softer than glass (I think), it should wear faster than
the mirror. This allows it to change curvature slightly to make up for
machining errors. Cast iron tools are much harder than glass and will need
to be generated very accurately. Or the mirror curve should be defined as
the same as the tool. It takes a lot of fine grinding to make an iron tool
conform to a glass blank. The glass will definitely be thinner before both
are the same.
The tools can be machined on a lathe by taking stepped increments or on a
milling machine using a tilted rotating turntable and flycutter. The
stepped tool will only be of average accuracy but will wear into the curve
faster because at first the tools and mirror will only be touching on the
ridges. The tilted milling machine method will give a very smooth surface
with no ridges. If you can machine with a tolerance of 0.0001" ,the
tool and mirror will match perfectly. Most people cannot so the curves will
only match within an inch or so of equal ROCs. Don't bother ruining a metal
pregenerated tool by trying to hog a flat mirror blank. Only use them on
matching curved blanks.

When it comes to a good all around do everything tool, I still think that
you can't beat a simple tile, pourstone (or dental plaster ), and plywood
tool. You can lay the tiles on the blank before pouring the plaster and get
an exactly curve match. If you stand the tiles on end, the tool will be one
inch thick and will last through a hogging and fine grinding easily. There
are discussions here in the archives on vertical tile tools and there are
some pictures in the Yahoo Mirror-o-Matic email group.

Best of everything.
Dennis

--- BBBS/NT v4.00 MP