Re: ATM Grinding machine arm pressure

[Dennis Rech <dennisrech@home.com>]

At 02:54 PM 4/25/01 -0700, Adam Perkins wrote:
>
>Hi all,
>
>It recently occured to me that every over-arm type
>grinding machine I've seen uses weights to provide
>additional pressure on the back of the tool.
>
>Why not use a spring-loaded arm and reduce the load on
>the drive train?  That way, the eccentric only has to
>work against the drag of the tool across the mirror,
>and not the inertia of waving 40-50 pounds around on
>the end of an arm.  
>
>Extra pressure does not have to imply extra mass.
>
>Has anyone explored this?  It may just be an extra
>layer of complexity added to a machine for not much
>practical gain.
>
>Just wondering,
>
>Adam Perkins
>Covington, Louisiana
>USA
Hello Adam.
On the 22 inch and 28 inch metal Mirror-o-Matics, we use an industrial air bag
to generate downforce. It works wonderfully. I hate stacking weights. I think
that half the scratches generated come from something falling off a weight
that
has been moved back onto the overarm. 
The air bag is good for 200 psi and its not uncommon for me to use a quill
pressure of 125 pounds when rough grinding. Things hog fast at that
pressure. A
12.5 inch f/5 takes about 1-1/2 hours to generate from flat with a thick tile
tool. A steady stream of 60 grit is necessary as a wet lasts about 5 seconds.
Fine grinding pressure drops to 75 pounds and about 15 minutes for each of 4
finer grits. Polishing drops to 50 pounds and things get pretty slick after 2
hours. 
I use about 20 pounds of pressure for cold pressing laps. 
A spring should work just as well for applying down force. 
There is not much need to swing the arm back and forth when grinding and
polishing. An almost stationary arm method works fine. You can see a short
video of the Big-o-Matic polishing at
<http://www.mirror-o-matic.com/>www.mirror-o-matic.com. Its the first video
listed. About 300K. You can see the air bag at the top of the arm towards the
end of the video. 
Dennis

Dennis Rech
Mirror-o-Matic

http://www.oblivion.net/~astro/mirror/