Re: [ATM] mirror grinding

["David Harbour" <scarab2@cox.net>]

Ahh, Bob-

I just show what a dumb Okrie I am by asking such a question. I figured out
what you meant before you replied. By the way, I agree with your analysis of
tools and grinding, below. You have an intuition for practicality here, on
grinding, that I have seen in not too man other atms. All of your logic now
jibes- I understand. Thanks for taking the time to makie this clear to me.

Two points:

Have you read about the boys who hogged out their mirror in only T;WENTY
minutes with A BEAD BLASTER? You know, one of those gizmos that drives tiny
silica beads at high speed on a jet of air? Kind of like a "sand blaaster".
Automotive paint shops have them, for stripping away old paint quickly. The
boys made a template of the curve, and held the bead blasterr nozzle at a
low angle, moving it in circles, checking it with the pasteboard template
frequently, and in 20 minutes they had the f/5 curve roughed out! Then they
made a tile tool and finished up. They rented time on the shop owners bead
blaster. Now, another quickie:

A lathe grinder, (tool post mounted type) mounted on a length of pipe the
same radius that they wanted for their mirror (to rough out), anchored
securely (with provisions to shim it closer to the grinder as the curve
developed, as they passed the grinder back and forth across the face of the
blank! (plenty of water on the affair as it progressed, of course). I think
an ordinary bench grinder could be set up to do this).

Both of these methods seem clever and elegant to me, for roughing out.

Thanks, Bob for your extremely educational treatise, below. Helped me
understand some things I hadn't, before. I always make sure to "mine" your
posts deeply. WHEN ARE YOU GOING TO WRITE THAT BIG BOOK? YOU HAVE TO DO IT
BEFORE THEY THROW DIRT ON YOUR FACE. Bet to crackin'- we need that book-

Dave
----- Original Message ----- 
From: "Bob May" <bobmay@nethere.com>
To: <atm@atmlist.net>
Sent: Wednesday, June 29, 2005 2:10 PM
Subject: Re: [ATM] mirror grinding


> Dave, maybe I'm using the word "hole" a bit off from what some understand
> it.  What I mean is that I'm digging the majority of the glass out with an
> iron (or other material that doesn't abrade fast like a glass or tile
piece
> would) tool so that you end up with a shallow hole which is approximately
> the shape that you want the finished surface to be.  You can also make a
> nice hill if you want a convex surface by grinding the edges of the glass.
> Remember that a hole is a very relative term as testing a polished surface
> with the Foucault tester can show a "hole" in the middle of the surface
> which may be less than 1/4 of a wave deeper than what it should be.
> My whole approach to tile tools is to make them cheaply.  Why make a tool
> that is going to be as expensive as a piece of glass and takes a fair bit
of
> time to make in addition?  For a small lens or mirror, the difference in
> cost isn't much but when you get to a larger piece of glass, the cost
> difference becomes quite important.  Why buy another $100 piece of plate
> glass when the tile tool of equivalent size is more like $10?  Right now,
> one of the grinders in the group is doing a 17+" F4.5 mirror and his sost
> right now for the stuff other than the mirror glass has been $10 for a
> diamond saw that he has been using to grind the hole into the mirror's
> surfface.  He's ready for his tile tool now and that is going to cost
about
> $6 for the materials (12" tool) and that plus about $3 in grit is going to
> be the cost in finishing the mirror down to the point where it will be
ready
> for polishing.   If he didn't have the availability of the tile tool
method,
> he would have had to pay about $80 or more for a piece of glass to grind
his
> mirror against.  Stacking tiles on edge and doing the coarse grinding
> (digging the hole) is a bit more expensive than doing the other way as you
> need a lot more tiles, probably about 5 times the cost for the tiles
alone.
> Epoxying tiles to a tool is also time consuming (got to let that eoxy cure
> for a start) and you end up with big channels where the grit that you put
on
> the tool gets wasted in.  I will note that some shallow groves at the last
> stages of grinding do make the job easier as you tend not to vacuum lock
the
> tool and mirror together but those are just a few lines that you dig into
> the tool that are a few thousandths of an inch deep and they can be dug in
> the plaster matrix rather than the hard tiles.
> A terra cotta tile would make a nice substrate for a tile tool but I'd
worry
> about the flaking off of the terra cotta with my hands (contamination of
the
> grinding surface problem here, especially when you get to the polishing
> operation unless yo expend more time and money for more expensive epoxy to
> cover it) and that would be bad for a good surface.  I'll also note that
40
> grit really has no use in an optical shop unless you are doing very large
> surfaces and don't want to get the hole dug out with diamond tooling.
> Something that coarse digs very big pits and the cracks that end up with
> such large grit also go deep into the surface and you then need to dig
them
> out for a very significant distance or they will come back to bite you
later
> on.
> Basically, the tile tool process is part of a search for making mirrors
> cheaper than the old process that has been in usage for centuries and,
when
> done right, ends up being a lot cheaper to do.  I'll note that I'm so
cheap
> that I wash the 80 grit from the coarse grinding and find that abou 50% of
> the grit never gets used by many of the grinders because it gets washed
off
> of the grinding area before being used.  Using channeled tools probably
ends
> up with more than 80% of the grit never being used.
> My first mirror (8" F7.5) ended up using so little grit because I reused
the
> grit for the larger sizes that I've been doing all kinds of other optics
> with the same stock of grit and still have about half of the 80 grit that
I
> got in that kit. - I am cheap!
> Bob May
> bobmay@nethere.com
> http://nav.to/bobmay
> http://bobmay.astronomy.net
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>


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