RE: ATM - Starting Over - Now metal tools

[Dennis Rech <dennisrech@home.com>]

>> 
>> If you want to do radical re-hogging of a blank, and if you want to
>> remove a lot of glass, your best bet is to grind with #80 carbo on a
>> Jack Schmidling aluminum tool.   Those things cut really fast.
>> 
>> Jack makes them to your specifications, approximating your 
>> curve with a
>> step function.
>
>I have used three of Jack's tool...they work fast and wear slow!  Great for
>making binoc' scopes...the tool won't change radius...also great if you make
>batches of mirrors since you get such long mileage from one aluminum tool.
>
>I'm surprised they aren't mentioned more, especially for those using
>grinding machines, turning out stacks of mirrors.  ;-)

Hi Tom
Certainly not to disparage Jack, his tools or anyone else, but aluminum tools
on a machine wear away just as fast as tile tools. The hardness of aluminum is
less than ceramics or glass. You can do the standard scratch test and the
aluminum will just leave a bright streak of aluminum if you drag it across
rough glass. And you can certainly carve your initials into an aluminum blank
with a shard of glass. 

Metal tools work great on machines, but only for fine grinding where the curve
has already been generated by diamond or ceramic and grit. If you use a
spherical metal tool on a flat glass blank, you will wear the center of the
tool flat very soon ( even with cast iron ). There is not much area of the
tool's contact spot and there is a lot of mirror area.
 
I suspect that aluminum tools would work well for hand grinding because the
tool is constantly being rotated and rocked on it's curvature, which is
constantly regenerating the curve ( much like tile tools). It is the softness
of aluminum that allows it to follow the curve being generated so nicely. The
nice thing about aluminum is that it is thick. The 3/16 inch thick tiles that
we use wear through fairly quickly, a chunk of 2 inch thick aluminum will last
a long time although I find that it needs to have the channels re-machined
rather often. The good news is that I now make the channels 3/4 inch deep to
start so they can fine grind many mirrors.

Many people assume that if you have a curved metal tool and machine stroke it
over a flat blank that the blank will automatically take the shape of the
tool.
However, it is just not true. 
If you try to hog a mirror with a generated curved metal tool, the curves will
not match up and little contact will occure until they eventually wear
together
to form similar radii. This takes a long, long time with cast iron. With
little
surface to surface contact, not much grinding occures. Also, little spinning
occures and it is the differential surface speeds between mirror and tool that
generates the curve.

When using grinding machines, turning out stacks of mirrors, metal tools
(especially cast iron) are invaluable for fine grinding. They clean up
beautifully, never have chipped corners that scratch, and they do last a long
time when running fine grit on equally radiused surfaces. 
However, for generating curves, they aren't worth a hoot.

My appologies for being so long winded on such a mundane subject.


Dennis Rech
Mirror-o-Matic


http://members.home.com/swwas/mirror

http://www.bobrech.com