Re: [ATM] A bit of hand polishing advice please................

["Jerry" <wa4guu@verizon.net>]

Hello John and Bernadine,


My 3 cents worth..... Inflation.

The TDE was in all the images. It is not new after the last session of work.
I will echo what Mark said.  Don't ask me how to fix an edge. There are
plenty of people here to tell you how to do that. The edges on my mirrors
seemed to have more to do with other factors than stroke length. I get much
the same edge quality using short, medium and long strokes. Pitch, trimming,
pressing, temperature control and stroke smoothness seem more important to
me. But I have never made a perfect edge. I'm saving that for my second
mirror.

Your edge is not awful. My recommendation on that is to spend a little time
seeing if you can learn to get it better, but don't let it stop you from
finishing you mirror in a reasonable time. Almost every mirror has some
degree of TDE. I say finish the first mirror and learn to make the perfect
edge on the second mirror.  Others will say that you should make it perfect
because you will have to live with it til you die.  I say you can live with
it and your second mirror til you die. If you do what others say you will
die with one unfinished mirror.  If you do what I say you will die with one
finished and one unfinished mirror.  You could discover a comet with that
mirror even if it has a TDE..... if it is in a telescope.

If the lap action is not smooth I think it is best to stop.  You can work
through that when polishing (sometimes), although you should have a low
tolerance. You should think of poor action as doing damage to the surface
(dog biscuit) that will take some time to remove.

Besides the resulting rough surface, poor contact increases the chance of
scratching!

The lap should be most well pressed the moment before you start work. If it
is jerky. Verify that the channels aren't pinching closed, you have fresh
polishing agent on the lap and again, it is pressed. Let it press by its own
weight for 20 minutes or so and then see if it works smoother. Don't take
the lap off of the mirror. Just start pushing right from the press.  

Start with very slow strokes at first. 5 or 6 seconds or more for 1 forward
and back stroke. You can ramp up faster as you feel it working right. 3 or 4
seconds would be a good "full speed" if you just can't stand to go slower.
If it doesn't start out smooth, let it press again but warm the work room a
degree or two and give the mirror and lap time to warm to the new work
temperature. Have a thermometer in the work area. You will probably find 21
or 22 Celsius to be the right temperature. Find the temperature that works
for you and your pitch, then press and work at that temperature. Controlling
temperature is very important to predictibilty in figuring.

Wow!  That might be 4 cents worth of OPINION.


Jerry





-----Original Message-----
From: John and Bernadine Murray
 
Morning All,
	Last night I was able to cast the lap and get in two laps around the
barrel. The mirror is a 200mm 1240mm f/length (f6 ish) This is the first
session after the machine work. We decided to walk the barrel and try and
get rid of the raised central zone. At first the laps action was a bit
uneven but after a 1/2 lap or so it steadied and became a heavy consistent
dragging action. Around 1 - 2 seconds for a full forward and back motion. 12
strokes before rotating the mirror and tool. BTW The tool was on top. I
think a touch of tde is coming in. A slightly shorter stroke called for
perhaps?

Picures are at
http://users.cyberone.com.au/onslow/
Series 3 are the latest ones.


Cheers

John Murray
Whyalla
South Australia.

_______________________________________________
ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/