Re: ATM 21" f/6 - Rough Grinding

["Bill T." <twentiethwave@hotmail.com>]

Ken,

Yikes!  Are you hogging out the curve on a 21"?  Man, what a lot of work.
You should seriously consider sending the thing off to be pre-generated.

But, if you want to persist, the classic way to kickstart the curve is to be
pretty aggressive and use strokes that just wouldn't be acceptable in any
other grinding or polishing phase.  You need show that blank who's boss.
It's way to early for that sissy W stuff.

With a 21" mirror and 14.5" tool, my choice would be mirror on top and
really long strokes.  For maybe the first half hour bring the center of the
mirror to within about 3 inches of the edge of the tool.  Be sure all the
tiles at the edge get in contact, at least at the edges the tiles facing
outside the tool -- no need to really wait for each tile to show full
contact, just partial start of contact at the perimeter facing the edge of
the tool is ok.  Then gradually (over 1/2 hour) work up to the point where
the center of mirror is within about 1 inch of the edge of the tool. Use you
judgement here, too keep the mirror under control.

Once you have the gouge about 10" wide, start analyzing the sagitta to see
if you're getting roughly the right proportion of diameter to sagitta for
your focal length -- remember to go by the diameter of of the gouge only,
not the whole blank diameter.  If the gouge looks proportionately too deep,
reduce the length of the  strokes.  These measurements will be ballpark
grade at best, so don't try to be too accurate.

One of the biggest goofs is to slam the edges of the mirror and tool
together -- makes a nice big chip or fracture in the edge of the mirror.
Keep an eye on those tiles and their glue.  MOT will wear the tiles at the
edge of the tool very fast.

Once you have the gouge ("curve" is too gentle a word for this stage of
grinding) up to almost  the mirror edge, you can switch over to W patterns,
tool on top.  Broad W's with lots of side to side (past the edges on all
sides) will flatten the curve.  Reducing the side to side (but still hanging
the tool over the edge at the top and bottom of the mirror) will deepen the
curve.  Hang the edge of the tool about 3" over the edge of the mirror.
When you push the curve out to the edge, be sure to maintain the chamfer.
Don't try to apply polishing rules too directly to grinding.  Grinding
involves hard surface contact on both tool and blank.  It's pretty hard to
systematically grind those two hard surfaces together and not come up with
something close enough to a sphere (but it can be done!).  If you settle
into mostly TOT, you will probably won't be as perfectly speherical as if
you alternated positions, but you'll probably still be in the ballpark for
polishing.  Start doing the "Sharpie Test' towards the end of 60 grit, when
the curve is fully established and the tool gives a very even feel over the
entire curve.  I like to use a big, 1", very wet black marker since if fills
the pits and scratches and makes them easy to see even after several minutes
of grinding away the rest of the marker.  Under normal circumstances, the
marks should wear away starting from the center, moving very quickly to the
edge.  If you have isolated areas or rings of persistant marker, you've got
a bad curve.   Some very slight marker irregularity is likely at 60 grit.
By the time you are an hour into 220, the marks should be coming off very
evenly.

All this assumes you are manually grinding, a machine work would be
different story!

You probably are pushing most of the grit off the mirror.  Fact of life,
that's all.  It won't be as bad with the finer grits.  If your tool is
sticking, it may be too dry.  All the grit should be dark colored from being
wet.  Grinding is a pretty wet process.  For heavy grits water acts like
cutting oil

Good luck.

Bill T.