Re: ATM [long] Getting the smoothest mirror (no dog biscuit)

[mark cowan <cowanm@open.org>]

Jon Bertrand wrote:

> How do you get a _really_ smooth figure?
>
> I have a sphere on my 10" mirror.  Monster hill is gone, I don't see any TDE.
> All that's left is a tiny hint of zoning and a little dog biscuit.  It reminds
> me of the Tex photo of the "1/20 wave" ripple only its a little worse.

Having actually finished a mirror that looked exactly like Tex's "beautiful bagel"
8" f/6, I'll take a crack at this.  :)   (Be aware that these opinions were fully
packed when sent and the contents may have settled during shipment...)

> "Tool size"
>
>     Some say bigger, some smaller, mine is 100% of the mirror with a nice
> beveled edge.
>

Ditto.  But maybe it doesn't really matter?

> "Use slow strokes"
>
>     How slow?  I'm going pretty slow.

Like one per second near the end on an 8"...  Most rock music has about the right
tempo.

> "Use light pressure"
>
>     How light?  Just my hands, or even less?

Just push it back and forth - don't push it down at all.  You're trying to
smoooooth it.

> "Refine the CeO to make 'the good stuff:'"
>
>     Mix CeO and water, settle for 30 seconds, pour off the good stuff or

Never bothered, but use tiny amounts of whatever you have for the last sessions.

>     Use a cloth square to make a "tea bag" of CeO and kneed
>     it in water until the good stuff leaks out (al la ATM books).

Just don't forget to add ice... ;)

> "Use rouge in place of CeO"

Well, Newport's scratched/sleeked like crazy.  Ruined a perfectly good lap.  Maybe
WB's would work, or what Michael mentioned.

Some people do truly odd things to rouge, like turn it for 6 months in a rock
tumbler with ball bearings.  You'd probably want to do that in the garage, and plan
ahead a bit. :)

> "Press press and press again."

Whatever it takes to get *perfect* contact!

>     Keep good contact - warm with a hair dryer and press, press, press.

Warm lap and mirror with warm tap water, soak don't rinse, full-body press for
about 30 seconds does it for me...

>     Use your net mesh and cold press, press press.

By the time you get to the last polishing phase you should have a well broken in
lap.  Why mess with it like that?  The net thing is to increase the polishing
action, as I understand it.  You're after *less* action, more control,
smooothing...

>     Do short (minute or two sessions) then press press press.

Worthless, IMHO.  Dog biscuit is a large tedious thing and you need to remove it by
dedicated effort.  Underline effort.  Even though you're not pressing *down*,
you're pushing *across* and a lap that's conforming, warmed up, and well charged
with polishing agent takes beaucoup effort to push.  Anything else, you're just
kidding yourself.

> "Keep it wet"    Don't let it dry out and get all screechy.

Absolutely, though the occasional "eeek" is not an indicator of disaster.  If it
starts to stick (this is way beyond screechy, BTW), find the way it *isn't*
sticking and move it off in that direction.  This is probably where bad dog biscuit
*comes* from, the chemical attack that Tex talks about.  I've seen it.  It's as
ugly as a Pomeranian-Pit Bull-Pekinese mix.  But you can polish out anything you
can polish in (see below).

> "Channel my way"     Wide or narrow, seems some people say narrow is best?

>From having used a wide variety of laps and techniques while chasing that final
happy mirror, what will get you in trouble the fastest is having a *centered* tile
pattern of facets.  If there's an axis of symmetry to the lap across the center,
polishing will faithfully imprint a disgusting rendition of the facets upon the
face of the mirror - and it's dog-biscuit time.  Rowf.

It's the reinforcement of the chemical attack from the lap each time you stop at
the end of the stroke that leads to it, and it's one of those things that once it
starts, it keeps getting worse.  The only cure is to avoid it, or use a lap that
doesn't produce it.  Facet spacing doesn't seem to matter much, channel width is
only there for allowing the pitch to flow slowly, you will experience both wide and
narrow as the lap changes.

> "Use my stroke"
>
>     Lots of 1/3 COC but there is some talk of W and zig zag, what's the deal
> here?

I just use what I used for fine grinding - sort of fore-and-aft, back and forth,
sometimes more, sometimes less, mix it up as best you can.  You can tell if it's
too long or too short on the average by what happens to the edge while you're
polishing.  Check it however often your paranoia dictates.  ;)

> "Watch the drag"
>
>     Its supposed to be even and steady.  What about straight
>     strokes where you stop at the end of each stroke?  Should you keep it
> moving?

How can you "keep it moving"?  Go sideways?  Just don't let it hang at the end.

I polish sitting on a pine board that rests on two, uhm, inert supports with three
cleatish-type thingies to keep the lap or mirror from skidding away and breaking.
This allows frequent turning of the bottom piece (every wet) and turning the top
with each back-and-forth fore-and-aft set - turning the top (usually the lap)
*during the stroke* - not at the end.  This is part of the mix-it-up philosophy.

> I plan on spending the time finding what works in my case and I'd like to have
> "the complete list of ideas" from this great pool of experts.

I am no expert...what works in your case will be something you can suggest to
others...very few of who will actually take your advice...wishing instead to find
out for themselves...which will then be knowledge they *own* and will never
forget.  It's a wonderful world!

Oh, and one final thing - *never* let the drying lap sit on the mirror.  When
you're done, wet it down and do some minimal strokes, carry both pieces together to
the sink, run water over them as you separate them and scrub off the CeO with a
clean sponge with some dish soap (Dawn unscented).  If you let the CeO dry on the
mirror it will never go away again until you polish again.  This may seem extreme
but who'd want to have gotten the final perfect figure only to have CeO streaks for
all eternity?

Once you figure out what works for you - add it to "the complete list of ideas"!

Best,

Mark Cowan
Salem, Oregon