216 Agricultural Mechanics and Technology Systems
Copyright Goodheart-Willcox Co., Inc.
The most important factor in
selecting an abrasive type is its hardness.
Hardness is the resistance of a material to
scratching or deformation. Abrasives
must be harder than the material to be
shaped or fi nished. If not, the work
material quickly wears away the abrasive.
See Figure 8-51.
The second important factor to
consider is the size of particles in the
abrasives. The grain size indicates the
coarseness of the abrasive side of the
paper. The grain size ranges from very
coarse (#8) to extremely fi ne (#1200). A
higher grit number indicates a smaller
abrasive grain and a fi ner abrasive product,
Figure 8-52. Abrasives of lower grades have
larger particles that produce a coarse cut
and remove material at a faster rate than
those of a fi ner grade. Coarse abrasives
leave more noticeable marks or lines on
the fi nished surface that may have to be
removed, but they remove more material
while creating less friction and less heat
buildup in the stock.
Friction and Heat Buildup
Grinders and sanders create friction
as they work. This friction results in a buildup of heat that can damage the
fi nished material being shaped. Friction can be reduced by selecting the
largest grit size that can be used effectively for the job and by using slower
sanding speeds. Following the general rule of letting the weight of the
machine or part do the work can also greatly reduce the amount of friction
generated.
When it is an option, spreading the abrasive work over a larger surface
allows the heat generated by friction to dissipate faster. For example, if a
stationary belt sander with a 48″ × 6″ belt is used instead of a 21″ × 3″ portable
belt sander, the heat is spread out over four and a half times the surface area.
When using a bench grinder to shape small metal parts, excess heat can be
removed from the work by periodically dipping the part in a container of
cool water.
Lodging
As a grinder or sander is used, the abrasive particles are broken down
with particles of the material being worked and may become lodged between
the grains of the abrasive. The result, called lodging, reduces the effective
cutting action of the abrasive and increases the heat buildup from friction.
Material Mohs Hardness
Diamond 10
Boron carbide 9–10
Silicon carbide 9–10
Aluminum oxide 9
Tungsten carbide 9
Zirconia 8
Emery 7–9
Steel 4–8
Glass 7
Silica sand 6–7
Pumice 6
Titanium dioxide 5.5
Aluminum 2.5–3
Gold 2.5–3
Steatite (Soapstone) 1–2.5
Goodheart-Willcox Publisher
Figure 8-51. The Mohs hardness scale is a measurement of
a material’s relative hardness and resistance to scratching. It
was developed to compare different minerals and is the typical
comparison method for categorizing abrasives.