Re: [ATM] First Timer Polishing Questions

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

It is probably polished well.

Are you using soap when you wash it. I ususually wash my hands and then wash
the surface with soap. Careful! They are slippery that way. The polishing
agent residue and swarf glass might be what you are wiping off the surface.
It is hard to get completely removed without some soap. But then you have to
rinse the soap off good too.

Do you see a difference at the edge or center.  One almost always lags the
other in polish. If it is uniform it is likely just some residue on the
surface.  The usual advice of others (and me too) is that if you can rub it
off it is just a residue.

>From your description the pitch is softer than I would like. I wouldn't put
a lap in the freezer, but sure can't say from experience if it is bad. I
operate on the THEORY that any change in temperature of the lap from working
temperature is not good. For me that is 70F +/- 2 degrees, and the glass and
lap should be stabilized at that temperature before working. Before
widespread air conditioning, using a softer pitch for cool weather or harder
for warm weather was more necessary than now. I like to do everything at the
same temperature and adjust the pitch, not the temperature. Of course that
means adjusting the pitch before pouring the lap. I believe that even the
wrong hardness of pitch still works best within a couple of degrees of 70F.
There is a lot of room for other opinions on all of this.

I rarely ever press with weight in figuring. I feel that the weight can only
bend something and the bend will have to be relieved before working. The
entire idea of pressing is to get the lap the same shape as the mirror.
Bending them in pressing just doesn't seem to be a good idea to me and I get
good contact pressing with only the weight of the mirror or lap. That is
another THEORY of mine. I know others do OK working with a different theory.

I have polished an 8" in about 10 hours with no more than the weight of the
glass and the natural weight of my hands without pressing downward. That was
a 6 to 1 diameter to thickness ratio. Of course some weight would speed up
the work, but you have to work harder to push it.

Take a look at the shape by Foucault and/or Ronchi tests and see what the
shape looks like. Soft pitch can make a real smooth surface. Too soft and it
can be rather hard to control just what kind of smooth shape you have.

If you have it fully polished and you are ready to figure, the question will
be what kind of control you have in figuring. Soft pitch will work rather
fast. In figuring you will be using shorter work sessions, so the pitch will
not have as much time to warm from working and hand heat. It will be cooler
for a larger percentage of the work time than in polishing, so it will
actually be harder in figuring than in polishing. It may turn out to be a
good lap for figuring.

As the lap gets thinner it will behave more like harder pitch. It will have
to flow faster to conform to shape as you work. For a facet to be pushed
down 1/1000 inch it would have to expand more if the facet is thin to start
with. But when you notice that it has started to behave that way (like
harder pitch) it also means the trend will accelerate and you will be at the
end of the useful life of the lap very soon. There is very little time to
take advantage of this.

So all in all I say wait to see how it works. If your mirror is not fully
polished the soft pitch will get it polished faster. If it is putting an
ugly shape on the mirror it is already that way by now so you would have to
fix it any way.

I say wait and see.

 
Jerry




-----Original Message-----
From: Meeks Robert-PT1784
 
Hi -
 
I thought I was done polishing my 8" mirror.  After washing and patting
dry, I lightly wipe the surface with my hand to remove dust and then hit
it with the laser pointer and it appears transparent.
 
However, by chance I tried the laser test after cleaning the mirror
<without> touching it with my hands, and the laser pointer now shows a
readily visible reflection from the entire surface that I can actually
rub away with my thumb.  
 

so I am wondering if anyone could provide a description as to the amount of
pressure required to get a proper polish?
 
I generally have to re-channel after 20-30 minutes of
polishing even with only moderate pressure.  So, I've been pressing with
ligher weights (or else the channels close) to get good contact and then
putting the lap in the freezer for about 10-15 minutes to harden it up.
Seems to be working well.  I can go for an hour or two this way without
re-channeling.  Just curious if anyone has any concerns with doing this?
 
Bob


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