I have a unit that is very similar to the Graves you mention.
Pressed wood spools used with diamond pastes in graded sizes to 50K.
Having cabbed with both this unit and various horizontal shaft units,
I can say a bit about it. While the heat wasn’t excessive for me,
actually I think it generates more heat than a water cooled unit used
gently. Remember, there is only the diamond’s paste carrier for
lubrication. The unit is somewhat cool because the stone contacts
the wheel in only one spot at a time. If you press hard and keep the
stone on the wheel constantly, you can generate some heat, but I know
people have successfully polished opal and other sensitive gems on
this type of machine by paying attention and cooling the stone as
needed. Press it to your lip from time to time and use a water glass
to cool the stone, but don’t shock it. If it does get hot, blow on
it till it’s cool enough to put in the water or to start again.
Heat with a horizontal shaft unit gets excessive when you push hard
on a rubber backed wheel and get much of the stone in contact with
the wheel at one time and keep it there. Copious water will help
some, but not obviate the need for a somewhat light touch and
checking as above.
The advantage of the Graves type unit is that it is small,
inexpensive (at least mine was years ago) and could be used on the
kitchen table. It can also be used on star sapphire and other hard
stones with good results. The problem with these units, for me, is
that they contact the stone only at one point, and so require endless
quick movement to keep from getting flat spots and to produce a
decent smooth rounded surface. The same result is forthcoming with
less time and less effort with a rubber backed wheel which conforms
to the shape of the cab to some extent and tends to produce a convex
surface. One needs to sweep the stone across, but one needn’t be
scrupulous in hitting every spot evenly as on the wood spool unit.
Also, the vertical shaft units include a grinder to rough out the
stone, and the wood spool units begin with about 325 grit, I think,
so roughing out the stone first on a rough grinding wheel or lap is
necessary. The early steps on the wood spool unit take forever as
one has to hit each flat spot numerous times from various angles to
produce a curve.
If money is an issue and that is why the wood spool unit is chosen,
one might want to look at a vertical shaft unit, some home made
wooden laps and silicon carbide disks with stick on cement, and a
felt and a leather polishing disk. Sinkankas’ book, Gem Cutting,
covers how to use these units for cabbing. A little less convenient
than the bigger horizontal shaft units, but cheaper and much more
portable and smaller. It is easy enough to build a vertical shaft
lap with a wooden box to fit, a washing machine motor and a couple of
pulleys and a vertical arbor.