JJHerb on wed 31 dec 97
Nicole Simmons would like to hear more about grinders. First, about grinding.
The process of abrasive removal requires that a harder material gouge into and
break up or deform a softer material. A certain amount of power is required
to drive the harder material because the deformation and removal requires
work. Many grinders operate at high RPMs to apply the power necessary to
grind effectively. The combination of high speed and tiny bits flying means
you should always wear eye protection, full face shield much preferred, when
grinding. The discussions about grinding that use the words harder and softer
are related to the Moh s Hardness Scale of scratch hardness. This scale of
minerals (talc is the softest at 1 and diamond is hardest at 10) indicates
which material will scratch or abrade which other materials. So any material
with a higher number in the Moh s scale will scratch any material with a lower
Moh s number. The numbers and minerals in the important grinding range for
pottery are Quartz, 7, Topaz, 8, Corundum, 9, and diamond, 10. Silicon
Carbide and Boron Nitride are manmade compounds that have hardnesses that fall
between corundum and diamond. A final note about the hardness scale, it is
not really linear. While the first nine steps represent more-or-less equal
increases in hardness, the diamond is several times harder than corundum.
Grinding processes use any of several combinations of abrasive grains,
lubricants, and binders. For example, metal cutting disks use loosely bound
hard grains that are shed from the disk about as fast as the metal is removed
from the piece being cut. The loose grains leave the surface to prevent metal
building up on the surface of the disk and covering the grains. The grains
leaving also cool the disk and prevent overheating. These metal cutting disks
operate in a shower of sparks made by the abrasive grains and metal fragments
leaving the site of the cut at white heat. Another example can be seen in Wet
sanding with Wet/Dry silicon carbide sand paper. This black material is seen
in many places and is often used with water as a lubricating and cooling
medium. The grains are strongly bound to the paper and can withstand the
wetting and flexing of the paper without leaving the paper. The grains are
not sacrificed because the heat can be removed by the water and the unwanted
buildup of grinding products can also be prevented by flushing with water. A
third kind of grinding uses loose abrasive grains placed on a lubricated
substrate (like cast iron or plate glass) and renewed as the grinding
proceeds. This kind of grinding is used to facet gemstones, to grind
telescope mirrors, and to polish stone floors.
Grinding wheels, the things we find most often on bench grinders, are composed
of an abrasive and a binder. The grains of abrasive that make up a grinding
wheel are all of a single size or size range and are usually only one kind of
material. These materials are most often silicon carbide or aluminum oxide.
The material that holds the grains together in the shape the manufacture
intends is the binder and may be any of several materials. The amount of the
binder is another variable in the manufacture of the wheel. It may be that
the spaces between the grains is totally filled with binder or it may be that
the binder is only at the contact points between the grains. The amount and
type of binder has a great influence on how a wheel performs in a particular
application.
Binder composition, wheel porosity, abrasive type and grain size, and wheel
shape are all variables in the grinding game. One consistent characteristic
of grinding wheels is a stated RPM limit. No grinding wheel should be run
faster than the rates speed. Overspeed can cause a wheel to explode, flinging
chunks of wheel around the room at high speed. In addition to overspeed,
wheels can be damaged by abuse. A grinding wheel in intended to be mounted
solidly on a mandrel that fits the wheel hole and supported by cupped washers
that bear against blotter paper on the side of the wheel. Any mounting that
ignores one of these requirements is liable to damage the wheel, leading to
those flying bits again. When starting off with a new wheel or changing one
wheel for another, test the new wheel by tapping lightly with a soft faced
mallet. A ringing sound indicates a sound Wheel while a dull sound
indicates trouble ahead, lady in red.
Belt or disk sanders use abrasive grit bound to paper. The abrasive grains
are again of one size and the adhesive (binder) is selected to suit the
intended use. The arrangement of the belt can allow a flex around a curved
edge, as on a pot bottom, that may make a belt sander attractive for pot
grinding. The flex of the paper belt limits the production of unwanted facets
or edges. Furniture (wood sanding) paper is often coated with garnet grains.
The mineral garnet is not hard enough ( less than 7 on the Moh s scale) to
make any sort of impression on pottery but is very good on wood because of its
sharp grains. For ceramic applications, the abrasive needs to be aluminum
oxide or silicon carbide or diamond. Diamond doesn t often show up on paper.
One thing that all grinding processes have in common is the production of
small particles. The pieces that is gouged or broken out of the ground
surface are generally smaller than the grain size of the abrasive. This means
it is possible that some of the particles are small enough to be carried into
the lungs. If there is free silica in the material you are grinding or in the
abrasive (rare) you could be introducing a hazard into your environment by
grinding. The dust problems are, of course, less or non-existent if the
grinding process is wet. The dust is then trapped in the water and disposed
of as a slurry. Much better.
Standard metal working bench grinders are often found in ceramic studios
because they are relatively easy to purchase. They have no other
characteristic to recommend them. The wheels are often very coarse grained
and tightly cemented. Makes for a long lasting metal grinding wheel but is
not well suited for grinding pots. The standard bench grinder is not well
suited for wet grinding so dust is a constant problem. In addition, the fast
wheel and the brittle nature of ceramics means flying shards are common and
eye protection is vital. A bench grinder with a flexible wheel, like rubber
loaded with grit or a rubber wheel in a paper sleeve would be much better.
Cup wheels with diamond surfaces are available and are better for grinding
ceramics, if you avoid overheating the diamond surface. These things take a
little trouble to find but result in significant improvements in the process.
My favorite grinding method is to use a silicon carbide and water slurry on a
raised edge ceramic bat. If one prepares a flat, thick, high-fired stoneware
disk with a slight rim that can be glued to a bat, then you have a grinder.
Attaching the bat to your wheel, wetting the surface of the disk, and
sprinkling on some silicon carbide grit is all that is necessary. Lapidary
supply houses will sell you all sizes of grit from 10 to 1000. Make you pot
bottoms shine if you want. No dust, eye protection not required, and you
already own most of what you need.
Joseph Herbert
JJHerb@aol.com
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