Re: ATM Roughing Out

[Anthony Stillman <atmer@flash.net>]

>I'm roughing out a 6" f/6
>...depth about right
>...only the edges are in contact
>...I'm using #60 at the moment


Doug,

60 grit for a 6", this has got to be a first. Time to move on. Switch to
80.  One hour (working time) MOT 1/3 C/C.  You will see the curve flatten
as the sides get dug out so you may need to through in some 1/3 W or wider
to keep the sag (Here I'm assuming you're using a sub-diameter spherometer
to measure sag, if you are measuring the total sag, this won't happen).  If
you go too deep switch to TOT.  After the 80 do an hour of 120 and an hour
of 220.  For a 6" these times are a bit long, but you need to make sure
you've disposed of the sub-surface damage left by the 60. After the 220, 45
minutes (working time) of 1/3 center over center for each grit will be
sufficient.  You can shorten the time per by putting your back into it.  If
you decide on this, use the Sharpie test, gouge test or measure pit size (I
use a 50X hand held microscope) to make sure you've done enough (I vaguely
recall another test recently discussed on the list, anyone?).  Some people
half again or double the time at 220 to make sure the big pits are gone.

Hogging using a chordal stroke produces a deep center with non-spherical
sides.  In theory 1/3 W makes a sphere, but in my experience (with my
overly long strokes) a W tends to drive it oblate.  When I want a sphere I
work C/C and pay close attention to stroke length.  1/3 radius is shorter
than you think, well, than I think.

Anthony