Re: ATM [Fwd: Side with tool at 0.70r zone]

[Ken Hunter <kb7h@ccaonline.com>]

At 03:15 AM 2/28/00 +0000, you wrote:
>Hello All:

>However when the tool (tool's centre) is moved to the 70% zone of the
>mirror the overhang becomes huge. I don't know how much because I
>haven't physically moved it to this position, nor have I made scaled
>down drawings to determine the amount.

The overhang will not be a "problem" as long as you don't get the center
of the tool (assuming TOT) outboard of the mirror's edge. As long as you
are able to keep the tool's center of gravity on the mirror's surface it
will be OK... BUT, keep in mind, that if the inside of the tool is not
covering the center of the mirror you will end up with a plateau, mesa,
rasised central area or whatever you choose to call it. Typically you
can offset the tool until the edge of the tool reaches the center of the 
mirror unless you are using a really small diameter tool for figuring
out ring zones which would (should) be done by hand.

>
>My question then becomes, with the 24" dia. tool's position centred on
>the 70% zone of the mirror how much lateral overhang should I aim for so
>that 1) mirror remains spherical 2) mirror maintains same radius.
>
>If there are any other variables I should be aware of (such as turntable
>speed, # number of strokes (complete movement of tool across mirror's
>surface), please let me know.
>

Typically you will find that 6-10 strokes per revolution is desirable, 
this keeps the action on the glass to that caused by "stroke action" as 
opposed to "rotational grinding". A slow speed allows you to keep all
actions under control and the speed reduction is usually accompanied with 
an increase in torque which is needed on the larger optics especially
around polishing time. Make sure that for each revolution of the turntable 
that the tool doesn't exactly trace over the last path made by the strokes. 
ie. not EXACTLY 6:1 or 10:1 ratio... you will be able to see this in the 
pattern made on the glass (in the mud or polish compound) as it completes a 
turn and begins a new revolution.

We've found that the diameter, speed, temperature, polishing compound, weight,
humidity etc. all effect the action on the glass. As you play with the settings 
of your machine (Remember, yours is EXACTLY DIFFERENT from all other machines), 
you'll find out what works under your circumstances. No one can give you a set 
of instructions that will cover every possible combination of parameters so the 
best advise you can get here is from learning your own grinding machine.


>Not yet ready to grind. Tool has been cast. Two spherometers to be made,
>a wet/dry shop vac to be purchased, a bevelled 42" long, 20" wide 0.5"
>thick piece of plate glass, two pieces of melamine 0.5" thick with cut
>edges sealed, astroturf yet to be purchased.
>
>Hopefully by May the grinding should begin...
>
>Dan Kahraman
>
>

Have fun and keep us posted.

Happy Grinding,

Ken Hunter
http://www.geocities.com/atm_ken_hunter/

ICQ 1189536