Re[2]: ATM pitch lap question

[Alvaro Cortizo <alvaro@netpandora.com>]

Hello Kenneth,

I  think  is  option  #2...  but im not flexing the tool itself (40 mm
(a  little  over 1 1/2") thick dental stone), but the mirror itself (i
used mirror on top to press too... lately i was using "hot press" with
weights  of  over  16 Kg (32 Lb)... off course i dont do that pressure
when polishing(only the weight of my hands... maybe 1 or 2 Kg (2-4 Lb)
i guess...
So  i guess thats the problem... too much weight in the hot/cold press
:(

I  left the pith lap and the mirror conform themselves for a couple of
hours  before starting to polish... many times... basically to let the
dental  stone to cool down (using the Microwave oven, the dental stone
heats  a  LOT more than the pitch, but letting them cool for that long
time  over  3  or  4  hours) must be some thermal transfer between the
dental  stone  and the pitch... a couple of minutes ago i used the hot
air  pistol,  works  a lot better.. also used the convex glass tool to
press in the center of the mirror.... and the pitch lap looks a lot
better  now...  but  i  used  a  lot of weight (16 Kg as commented), i
already removed te weight as soon as i read your reply, as soon as the
thing cools down, i will hot press again, but only with the glass tool
weight on top (10 mm glass with 35 mm dental stone glued on it)

Yes  i do a sharpie test, and how i see the marks go out, i bet i have
 a  nice  sphere,  after a couple of strokes, the permanent ink erases
 out (with 500 grit, the finer i use b4 going to CeO)

 Thanks again
        Alvaro
        Rosario, SF, Argentina
 

Monday, February 25, 2002, 2:14:47 PM, you wrote:

KH> Hello Alvaro...

KH> A couple oif things could be happening...

KH> 1. You may have not been completely spherical when you started polishing.

KH> 2. You may be flexing your tool by putting too much weight during pressing. If so, and you are not using the same weight to polish, your tool center may be "rebounding" once the weight is
KH> released.

KH> Do not worry about polishing too long wrecking your figure... You do not have a figure until the glass is completely polished anyway. YES... there is a curve that can be called a "figure" but it
KH> is useless to look at it or worry about "figure" until the glass is fully polished. Once that is done, you'll find something very close to a sphere or a plane surface and you can proceed to real
KH> figuring.

KH> Did you do a "Sphere" check (Sharpie test) during the last stages of fine grinding? Are you putting on a lot of weight for pressing and taking it off during polishing? What are the strokes used
KH> for polishing? How much time are you allowing for the lap to conform to the mirror during pressing? How soon after pressing are you going into the polishing and how long are your polishing spells?

KH> All of these things can effect the polishing progress but your description sounds like the first two things I mentioned.

KH> Ken Hunter

-- 
Best regards,
 Alvaro                            mailto:alvaro@netpandora.com