Andrew, Those scratches are from your very first wheel (I guess that
would be the 175 hard disc right?). You are in too much of a hurry.
Slow down. By way of explanation. Sic is an abrasive but it will
break down into smaller and smaller grit as it is used against a
hard substance such as stones of 7 ( even less but it breaks down
more slowly). So as you cut with the Sic it will tend to remove some
of its own scratches (that we call ‘whiskers’) as the Sic particles
become smaller and more rounded. On the other hand, diamond IS
ALWAYS an abrasive and virtually each grit stays the same size and
each particle only slightly loses its sharp edges. Hard diamond
wheels lose their cutting effectiveness by the particles either
sloughing off or being covered by the metal of the wheel and
sometimes they become more embedded into the metal. As a result,
diamond will not ‘cover its tracks’ by smoothing over the initial
abrasive cuts. Depending on the coarseness of the grit, pressure
used, hardness of the material being cut, etc., these cuts can be
5-20 microns deep. The only way to remove them is to use another
diamond grit that is close to but still finer than the first one
used. For example, if you are doing your first grinding with a 175
grit diamond hard wheel/disc, you should probably go next to a 220
grit. Use high speed on the wheel and a light touch on the stone.
Move the stone very quickly in a circular motion and with each
revolution around the perimeter of the stone, tilt it slightly
towards the crown. After completing a cut from just above the girdle
to the crown, dry the stone and look at the surface with out
magnification. If you see ridges, gently turn the stone against the
wheel on that ridge. Dry and check again. Do this until you see no
significant ridges/flat spots (of course you cannot remove all the
ridges but they should be minor). More importantly, you should see no
’whiskers’ (short white lines like I see in your picture. Instead you
may see some much finer line or areas that have not lines.
Next you need to go to a soft wheel as I explained earlier…a
diamond pad such as a Crystal Pad backed by cork or sponge rubber.
Use a diamond paste (or powder mixed with olive oil for example) at
220 grit. (Don’t use too much paste…go lightly). Continue to use
high speed and medium pressure. Again, start at the girdle and work
towards the crown going in circles. You may want to sort of wobble
the stone a bit as you work up so you will break down any ridges that
remain. After a couple of times doing this, clean and dry the stone
again and look at it under a strong light (no magnification). By now
the stone should be perfectly shaped with a nice crown and good
geometry. There will still probably be a few of the fine lines but
they will not be deep and should be isolated.
Next use a similar wheel (Crystal Pad) with 600 grit diamond. By now
you might want to use a few drops of ‘extender fluid’ which is a
light lube that will keep the diamond particles suspended and active.
Use the same speed, medium pressure and the same process working up
the stone. After two or three times, wash and dry the stone and
inspect (without magnification). By now there should be no ridges or
lines visible and the shape of the stone should be perfect. The stone
should have a semi-gloss to it. Change to a similar pad with 1200
grit and do exactly the same movements, etc. When that is finished,
the stone will be pre-polished.
Now change to a wheel that has a felt pad on it. Use cerium oxide
that you keep damp. Do not get it wet or the stone will slide over it
and nothing will happen. If it becomes dry, the cerium powder will
bunch up and scratch the surface. Keep the wheel slightly damp use
slow speed, and as you press the stone down with heavy pressure until
the motor has to work but not so hard that it breaks off the stick.
Also don’t keep it on so long that it heats up and comes off the dop.
You might feel the wheel ‘tugging’ at the stone. That is perfect and
is the best polishing period. A few times over the stone and when you
clean it off it will be as shiny as possible. Now, if you wish, look
at it under magnification but don’t use 10x…its not necessary. The
human eye will not detect and scratches below 5 microns and should
Stop with all the 3000., 4000, 8000 etc, etc. They are not needed.
If you use the above wheels, I guarantee you will have excellent
results on most stones. Now and then, if you are cutting something
like turquoise or malachite or even marble…the softer stones,
don’t use the coarse wheel…start with the 220grit. When you polish,
use ZAM on a cotton muslin buff as used in polishing metal.
I have been cutting for nearly 40 years and have cut thousands of
stones. Above is the general method I use…modified sometimes to fit
special situations. My advice though, is don’t try too hard and don’t
fight the stones. Just be firm and direct with the proper tools and
you will be successful every time (well, almost every time - there
are always some exceptions).
By the way, the above also applies even if you are using Sic wheels.
But there are slightly different speeds, pressures and hand
Don in SOFL