Note: Descriptions are shown in the official language in which they were submitted.
33196 CAN 3A
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COATED ABRASIVE SHEET MATERIAL MAGNETICALLY
ATTAC~IED TO A SUPPORT SURFACE ON AN ABRADING TOOL
Technical Field
This invention relates to means for magnetically
attaching coated abrasive sheet material to a support
surface on an abrading toolO
Background Art
Various means have been used for releasably
attaching coated abrasive sheet material to a support
surface on a tool to abrade a workpiece. Such means have
included clamps which engage the ends of rectangular sheet
material or the periphery or center of circular sheet
material. Such clamps can be inconvenient to use, however,
and may have separable parts or re~uire the use of tools
that can be misplaced.
Another approach has been to coat the back of the
coated abrasive sheet material with pressure-sensitive
adhesive and adhere it to a support surface of a tool which
permits the sheet material to be peeled off after use.
While such pressure-sensitive adhesive coated abrasive
sheet material is relatively easy to attach and remove, the
adhesive adds significantly to its cost. Also, adhesion to
the support surface can be adversely affected if dust
(which is normally present in the workplace) or water
(which is used in some abrading processes) comes in contact
with the layer of pressure-sensitive adhesive, so that new
or partially used sheets must be carefully protected from
such contact.
Some prior art attempts have been made to utilize
magnetism for attaching coated abrasive sheet material to a
support surface on an abrading tool; see e.g., U.S. Patents
Nos. 3,226,888; and 4,222,204. The structures for
magnetically attaching described in these patents, however,
apparently did not attach the coated abrasive sheet
material to the support surface so that only the force of
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magnetic attraction and any force applied to the sheet
material through the tool normal to the support surface
would produce sufficient static friction between the coated
abrasive sheet material and the support surface to retain
the sheet material on the support surface while it was
driven by the tool to abrade a workpiece. Instead these
structures included mechanical interlocking rims or lugs to
help retain the coated abrasive sheet material on the
support surface, and portions of the structures
incorporated with the coated abrasive sheet material added
significantly to its cost.
Disclosure of Invention
The present invention provides a cost effective
and efficient means for magnetically attaching disposable
coated abrasive sheet material to a support surface on a
tool so that clamps, adhesives, or interlocking portions
between the sheet material and the tool are not required to
retain the sheet material in place while the tool is used
for abrading a workpiece.
The means according to the present invention for
magnetically attracting the coated abrasive sheet material
to the abrading tool comprises (1) incorporating into the
tool a magnetized pad which defines the support surface and
has more than 6 magnetic poles per inch in at least one
direction along the support surface, and (2) providing a
coated abrasive sheet material that has relatively low
mass, is sufficiently flexible that it can intimately
conform to the support surface, and incorporates sufficient
ferromagnetic material (e.g., an average of more than about
0.007 gram of iron per square centimeter) so that only the
force of magnetic attraction between the magnetized pad and
the ferromagnetic material in the flexible coated abrasive
sheet material and any force applied to the sheet material
through the magnetized pad normal to the support surface
will produce sufficient static friction between the support
surface and the sheet material to retain the coated
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abrasive sheet material on the support surface while it is
driven by the magnetized pad to abrade a workpiece. This
means for maynetically attaching is sufficiently effective
that it can drive the coated abrasive sheet material either
when the magnetized pad is part of a tool that is manually
manipulated, or when it is part of a tool having a drive
motor that reciprocates, oscillates or rotates the pad,
such as an air or electrically operated file, sander or
rotary grinder. Thus, no mechanical clamps, adhesive, or
mechanical interlocking portions are needed to hold the
coated abrasive sheet material, and no tools are needed to
remove or replace it. Dust and water can be wiped off the
coated abrasive sheet material and will not significantly
decrease the magnetic attraction between the sheet material
and the support surface. In fact, water has been found to
increase the degree of attraction between the coated
abrasive sheet material and the support surface, perhaps
because of a surface tension effect therebetween.
Detailed Description of The Invention
While any ferromagnetic material such as cobalt
or nickel or black iron oxide can be incorporated in the
coated abrasive sheet material, the preferred ferromagnetic
material is iron, which is the least expensive and develops
the greatest holding power per unit weight.
It is essential that the magnetized pad has more
than 6 magnetic poles per inch, or at least about 8
magnetic poles per inch in at least one direction along its
support surface to develop the necessary magnetic holding
force between the pad and the ferromagnetic material in the
flexible coated abrasive sheet material. The preferred
material for the magnetized pad is the material
commercially designated "Plastiform'~available from
'~r~ Minnesota Mining and Manufacturing Company of St. Paul,
Minnesota, which is flexible, comprises magnetized
particles within a polymeric matrix, and is available with
4, 6, 8, 11 and 18 magnetic poles per inch (about 1.6, 2.4,
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3.2, 4.3 and 7.1 poles per centimeter respectively) in one
direction along its surfaceO Increasing the number of
poles per inch in the magnetized pad above 8 (e.g., to 11
or 18 poles per inch) will increase this holding force at
the support surface, however, with the "Plastiform"
material it will also decrease the distance that a magnetic
field of sufficient strength to produce this holding force
will extend or reach from the support surface. This
decrease would be of little concern if all of the
ferromagnetic material could be located, and would remain
located, on the support surface during manual or mechanical
manipulation of the magnetized pad to abrade a workpiece.
In practice, however, at least some of the ferromagnetic
material in the coated abrasive sheet material (1) always
will be spaced from the support surface either by other
ferromagnetic material or by one or more portions of the
coated abrasive sheet material, such as that portion in
which the ferromagnetic material is contained or that
portion which adheres the ferromagnetic material in place;
(2) can be separated from the support surface by residual
stresses in the coated abrasive sheet material produced
either during manufacture of the sheet material or by
bending the sheet material after it is manufactured, which
residual stresses may not allow the coated abrasive sheet
material to closely conform to the support surface if the
coated abrasive sheet material is not sufficiently
flexible; and (3) can become separated from the support
surface while the coated abrasive sheet material is being
driven by the pad to abrade a workpiece due to flexing of
the coated abrasive sheet material away from the support
surface. Thus, the ferromagnetic material should be
incorporated in the coated abrasive sheet material by means
that allows the ferromagnetic material to be positioned as
close as possible to the support surface while being in a
form that affords sufficient flexibility for the coated
abrasive sheet material that the ferromagnetic material
will reliably be positioned as close as possible to the
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support surface when the coated abrasive sheet material is
placed on the magnetized pad; and the magnetized pad
driving the coated abrasive sheet material should produce a
magnetic field of sufficient strength to hold the coated
abxasive sheet against the support surface that extends or
reaches sufficiently from the support surface that the
coated abrasive sheet material will be returned to the
support surface under the influence of that magnetic field
after any flexing of the sheet material away from the
support surface that will normally result from the sheet
material's intended use. Thus "Plastiform" magnetic pads
having 18 poles per inch (7.1 poles per centimeter) are
preferred for applications such as in rotary grinders where
the coated abrasive sheet material tends to stay flat
against the support surface of the magnetized pad during
use; whereas "Plastiform" magnetic pads having ll poles per
inch (4.3 poles per centimeter) may be preferred for
applications such as reciprocating files where the coated
abrasive sheet material may flex away from the support
surface during use, since, though their magnetic fields
have less holding force at the support surface, the portion
of their magnetic fields that are of sufficient strength to
hold the coated abrasive sheet extend farther from their
support surfaces and thus maintain a stronger magnetic
attachment with the coated abrasive sheet material during
such flexings
Ferromagnetic materials in the form of thin
sheets, strips, screen, fibers or granules (which may be
generally round or in the shape of platelets) can be
incorporated at any location in the coated abrasive sheet
material such as within the layer of abrasive grains,
within or as the backing sheet, or on a surface of the
backing sheet opposite the layer of abrasive grains by any
appropriate dispersing, coating or laminating method.
Laminating a thin sheet of ferromagnetic material
(e.g., steel shim stock) to the side of the backing sheet
opposite the layer of abrasive grains, or using the thin
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sheet of ferromagnetic material as the backing sheet to
which the layer of abrasive grain is attached, places the
ferromagnetic material in the thinnest possible layer on
the support surface and thus will produce the maximum
magnetic force between a given magnetized pad and the
coated abrasive sheet material. Such thin sheets of
ferromagnetic material are relatively expensive, however,
and in thicker forms (e.g., above about 0.005 inch or 0.013
centimeter thick for sheets of steel) become sufficiently
inflexible that the coated abrasive sheet material may not
closely conform to the support surface, particularly if the
coated abrasive sheet material is creased prior to use.
It is preferred to incorporate ferromagnetic
materials in the form of particles or granules in a layer
on a surface of the backing sheet opposite the layer of
abrasive grains because granules afford maximum flexibility
of the sheet material per given amount of the ferromagnetic
material, and are usually the least expensive form of the
ferromagnetic material. The ferromagnetic granules can be
adhered to the surface of the backing sheet opposite the
layer of abrasive grains by a variety of adhesive types
including solvent-based, water-based, or hotmelt adhesive~
The adhesive should wet and cover the granules of
ferromagnetic material to protect them from oxidation, and
should adhere the granules in the thinnest possible layer
to afford placing the granules as close as possible to the
support surface of the magnetized pad. Adhesives that
have been found particularly useful for this purpose
include polyester resins, latices, and vinyl acetate
copolymers. While such adhered layers of granular
ferromagnetic material will not be so thin as a sheet of
the same ferromagnetic material for a given amount of
ferromagnetic material per unit area, an adhered layer of
granular ferromagnetic material can contain a significantly
larger amount of ferromagnetic material per unit area while
still having much more flexibility than a sheet of
ferromagnetic material.
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When adhered in such a layer, commercially
available iron granules that have been screened through
screens having meshes in about the lO0 to 270 range so that
they have maximum sizes in the range of about 50 to 150
microns have been found to produce, per unit weight of
granules, the highest holding force between a given
magnetized pad and coated abrasive sheet material under
most conditions. When the same commercially available iron
granules are screened through a 50 mesh screen and then
adhered to the surface of a backing sheet opposite its
layer of abrasive grains, the larger granules (which may be
up to about 300 microns in diameter) apparently produce
sufficient physical separation between the support surface
of the magnetized pad and the smaller granules that the
magnetic holding force between the magnetized pad and the
coated abrasive sheet material is reduced when compared to
holding force between the same magnetized pad and a coated
abrasive sheet material incorporating granules screened
through screens in the 100 to 270 mesh range. This
reduction is particularly pronounced when the magnetized
pad has 18 poles per inch, apparently because a magnetic
ield of sufficient strength to hold the coated abrasive
sheet against the support surface extends a shorter
distance from the support surface of such a magnetized pad
than from the support surface of a magnetized pad with
fewer poles per inch. Commercially available iron granules
that have been screened through a 325 mesh screen and thus
have maximum sizes of about 44 microns seem to produce
- slightly less holding force than is produced by the same
~weight per area of granules screened through larger screens
in the 100 to 270 mesh range, perhaps because of oxidation
that has occurred on or is included with the smaller
granules.
It has been found useful to process commercially
available iron granules in a ball mill, which removes
projections from the granules and forms them into platelet-
shaped granules, thereby making them more compact so that
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they can be coated in a thinner layer than the same weight
of granules that have not been so processed.
The ferromagnetic granules can be adhered to the
backing sheet before or after the layer of abrasive grains
is applied. There is some indication that the presence of
the ferromagnetic granules on the surface of a backing
sheet opposite that on which abrasive grains are being
coated will reduce the energy required for conventional
electrostatic coating of the abrasive grains and will
improve the sharpness of the coated abrasive. No
conclusive tests have yet been preformed to support this
indication, however.
Ferromagnetic material may also be incorporated
~ithin a liner sheet or adhered on a liner sheet by an
adhesive such as those indicated above and the liner sheet
applied over the pressure-sensitive adhesive layer on
commercially available coated abrasive sheet material of
the type having a layer of pressure-sensitive adhesive on
the major surface of its backing sheet opposi~e its layer
Of abrasive grains (e.g., the type commercially designated
"STIKIT" which is available from Minnesota Mining and
Manufacturing Company, St. Paul, Minnesota). This allows
the resultant abrasive coated sheet material to be used on
the support surface of the magnetized pad described herein
by leaving the liner sheet in place, and still allows that
commercially available abrasive coated sheet material to be
adhered to a suitable support surface of a non-magnetized
pad by stripping the liner sheet away. Alternatively, such
a liner sheet coated or filled with ferromagnetic material
may be coated with pressure sensitive adhesive and adhered
by that pressure-sensitive adhesive to the back surface of
conventional coated abrasive sheet material so that it may
be used on the magnetized pad.
The ferromagnetic material can be incorporated in
the backing sheet of the coated abrasive sheet material by
mixing it with the wood pulp slurry from which the paper
backing sheet is made. While granules of the ferromagnetic
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material can be used for this purpose, preferably fibers of
the ferromagnetic material are used, since fibers are more
easily and evenly mi~ed with the other fibers from which
the backing sheet is made and have less tendency to settle
out. With either form of the ferromagnetic material the
thickness of the backing sheet will normally separate a
percentage of the incorporated ferromagnetic material a
greater distance from the support surface of the magnetized
pad on which the backing sheet is positioned than the
distance the same percentage of ferromagnetic material
would be separated if it were in a thin layer on the
surface of the backing sheet opposite the layer of abrasive
grains. Thus, generally more ferromagnetic material will
be needed to produce the same holding force between the
magnetized pad and the coated abrasive sheet material when
the ferromagnetic material is incorporated in the backing
sheet than when the ferromagnetic material is in a layer on
the surface of the backing sheet opposite the layer of
abrasive grains.
It is least preferred to incorporate the
ferromagnetic granules in the layer of abrasive grains on
conventional abrasive sheet material because of the
separation between the ferromagnetic particles and the
support surface of the magnetized pad that will then be
caused by the backing sheet. Incorporating ferromagnetic
granules in the layers of abrasive grains is useful,
however, when the backing sheet is coated by layers of
abrasive grains on both sides, particularly when it is
coated with small size abrasive grains that will not
produce much separation between the ferromagnetic particles
and the magnetized pad (e.g., grits of 120 or smaller),
and/or for abrasive sheet material that is coated with
abrasive grain on both sides and is intended for use on a
hand sanding block where a minimum magnetic holding force
is required.
Preferably iron ferromagnetic material is used at
an average density of over about 0.007 gram per square
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centimeter area measured in a plane parallel to the major
surfaces of the backing sheet, which density has been found
to produce ade~uate holding power between coated abrasi~e
sh^et material and magnetized pads of the type described
above. However, using less than about 0.007 gram per
square centimeter of iron may be adequate to hold coated
abrasive sheet material against magnetized pads for some
purposes such as hand sanding, particularly where the
backing sheet is coated with layers of abrasive grains on
both sides so that the layer of abrasive grains adjacent
the magnetized pad increases the effect of friction between
the coated abrasive sheet material and the support surface.
Using greater average densities of iron material in the
coated abrasive sheet material over the range of about
0.007 to 0.1 gram per square centimeter has been found to
increase the holding force between the coated abrasive
sheet material and the magnetized pad. Increasing such
densities above about 0.1 gram per square centimeter when
the iron is in thin sheet form does not appear to
significantly increase the holding force, perhaps because
of the magnetic densities of the multi-pole flexible magnet
being used. Increasing such densities above 0.1 gram per
square centimeter when the iron is in a form other than in
a thin sheet (e.g. granular) may still significantly
increase the holding force, however, since the larger
amount of iron may be needed to overcome the effects of
separation from the support surface on the magnetized pad
caused by the portion of the coated abrasive sheet material
in which it is contained.
Ferromagnetic material in any form (e.g.,
granules, screen, fibers or sheet) can be included in the
coated abrasive sheet material in sufficient quantity to
afford firm magnetic attachment of the sheet material to
the magnetized pad while the sheet material still retains a
relatively low mass so that such magnetic attachment can
overcome the effects of momentum when the sheet material is
oscillated or rapidly reciprocated (e.g., up to about 6,000
2536~
strokes per minute) or the effects of centrifugal forces
due to normal imbalance or slight improper centering of
circular coated abrasive sheet material when it is rapidly
rotated (e.g., up to over 3,000 R.P.M.). Coated abrasive
S sheet material according to the present invention with up
to 16 grit size will typically have an average density of
less than about 0.45 gram per square centimeter measured in
a plane parallel to its major surfaces, and the finer more
commonly used 80 to 36 grit coated abrasive sheet material
will have typically average densities in the range of about
0.10 to 0.15 gram per square centimeter when measured in
that plane.
Incorporating the ferromagnetic material in such
a manner so as to produce a generally uniform distribution
or density across the coated abrasive sheet material is
preferred because it simplifies coating of the ferro-
magnetic material and produces a uniform holding force
across the coated abrasive sheet material. Densities of
the ferromagnetic material could be varied across the
coated abrasive sheet material, however, such as to produce
concentrations in a concentric, circular, stripe, dot or
radial pattern, if they were desired, for example to
accommodate special magnetized pads, help position the
coated abrasive sheet material, or save material costs.
The surface of the coated abrasive sheet material
intended to lie against the support surface of the
magnetized pad and the support surface should be adapted to
provide a suitable coefficient of static friction
- therebetween. One or both of those surfaces should also
provide recesses for dust particles that may become trapped
therebetween so that such particles do not increase
separation and thereby decrease the magnetic attraction
between the magnetic pad and sheet material while acting as
lubrication or ball bearings to facilitate longitudinal
slippage between those surfaces. Layers of ferromagnetic
granules adhered to the backing sheet, backing sheets
filled with ferromagnetic granules or fibers and layers of
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abrasive granules normally provide suitable surfaces for
both purposes. When the ferromagnetic material is in the
form of thin sheets (e.g., shim stock) which normally have
relat;vely smooth surfaces, it is preferred to form such
recesses in the surfaces of the sheets intended to contact
the support surface by abrading or embossing those surfaces
which should also increase their coefficient of friction
with the support surface.
As noted above, the flexibility of the coated
abrasive sheet material should be sufficient to allow the
sheet material to lie in intimate contact with the entire
support surface under the influence of magnetic attraction
between the magnetic poles and ferromagnetic material to
maximize the holding force therebetween rather than being
biased into a shape (e.g., arcuate) with portions spaced
from the support surface by internal stresses in the sheet
material. Coated abrasive sheet material that can be bent
at least 180 degrees around a 3/8 inch or 1 centimeter
diameter rod at a temperature of 70 degrees Fahrenheit and
at a relative humidity of 50%, and will subsequently be
held flat against the flat surface of an ll pole per inch
magnetized pad of the "Plastiform" brand magnetic material
by only the magnetic attraction between the magnetic poles
in the pad and an average of over about 0.007 gram of
ferromagnetic material per square centimeter area measured
in a plane parallel to the surface of the coated abrasive
sheet material adjacent the pad is deemed to be
sufficiently flexible for use in the present invention.
Such flexibility of the coated abrasive sheet material will
be affected by the stiffness of the backing sheet used in
the sheet material, any coatings or layers of adhesive used
to hold the coated abrasive sheet material together, the
size of the abrasive grains, and particularly the physical
form of the ferromagnetic material (i.e., whether it is in
sheet, fiber or granular form). Paper backing sheets of
the type commonly used in coated abrasive sheet material
generally have been found to be sufficiently flexible for
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use in the present invention (i.e., treated or untreated
paper that weighs 30 to 170 pounds per 320 square yards or
0.0~ to 0.29 killograms per square meter).
Brief Description of Drawing
The present invention will be further described
with reference to the accompanying drawing wherein like
numbers refer to like parts in the several views, and
wherein:
Figure 1 shows a first embodiment of a coated
abrasive sheet material according to the present invention
shown magnetically attached to a magnetized pad on a
reciprocating file;
Figure 2 is an enlarged fragmentary sectional
view taken approximately along line 2-2 of Figure 1.
Figure 3 shows a second embodiment of a coated
abrasive sheet material according to the present invention
shown magnetically attached to a magnetized pad on a rotary
grinder;
Figure 4 is an enlarged sectional view taken
approximately along Line 4-4 of Figure 3;
Figure 5 shows a third embodiment of a coated
abrasive sheet material according to the present invention
shown magnetically attached to a magnetized pad on a hand
sanding block;
Figure 6 is an enlarged fragmentary sectional
view taken approximately along Line 6-6 of Figure 5;
-Figures 7, 8, 9 and 10 are fragmentary sectional
views of fourth, fifth sixth, and seventh embodiments of
coated abrasive sheet materials according to the present
invention; and
Figures 11, 12 and 13 are graphs showing results
for tests reported in this application.
Detailed DescriP-tion of Illustrated Em odiments
Referring now to the drawing, there is shown in
Figures 1 and 2 a piece of coated abrasive sheet material
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10 and a flexible magnetized pad 11 according to the
present invention adapted for use on an air file 12 of the
type commonly used in auto body work, such as t,he air file
sold under the trade designation "Atcoa VikinC~Dual Piston
No. 85182 Air File" by Allan Air Products~ Inc., St. Louis,
Missouri. The coated abrasive sheet material 10 is
rectangular and is shown magnetically attached on a
generally planar rectangular support surface 14 of the
flexible magnetized pad 11. The magnetic pad 11 comprises
magnetized particles within a polymeric matrix (e.g., a
0.15 centimeter thick magnetized pad 11 of the magnetized
material commercially designated "Plastiform" sold by
Minnesota Mining and Manufacturing Company of St. Paul,
Minnesota having 11 magnetic poles per inch in one
direction along the support surface 14). The magnetized
pad 11 is adhered on one surface of a layer 15 of foam
rubber that ls adhered on its surface opposite the
magnetized pad 11 on a rigid backing plate 16 of the air
file 12. The air file 12 includes a drive means including
an air motor 20 adapted to longitudinally reciprocate the
backing plate 16 and magnetized pad 11 at up to about 6000
cutting-strokes per minute to abrade a workpiece (not
shown). During such reciprocation only the force of
magnetic attraction between the magnetized pad 11 and the
flexible, relatively low mass coated abrasive sheet
material 10 and any force applied to the sheet material 10
through the magnetized pad 11 normal to the support surface
14 will produce sufficient static friction between the
support surface 14 and the coated abrasive sheet material
10 to retain the coated abrasive sheet material 10 on the
support surface 14 wile it is driven by the magnetized pad
11 to abrade the workpiece.
As is best seen in Figure 2, the coated abrasive
sheet material 10 comprises a flexible backing sheet 22
having opposite major surfaces and a layer of abrasive
grains 26 adhered on one major surface by a conventional
bonding material. Magnetic attachment between the coated
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25369~
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abrasive sheet material lG and the magnetized pad 11 is
provided by a layer 28 of ferromagnetic (e.g., iron)
particles having a uniform density of over about 0.007 gram
per square centimeter of area (and ~referably in the range
of about 0.015 to 0.03 gram per s~uare centimeter of area)
measured in a plane parallel to the major surfaces of the
backing sheet 22 and adhered by an adhesive on the surface
of the backing sheet 22 opposite the layer of abrasive
grains 26.
Referring now to Figures 3 and 4 there is shown a
piece of coated abrasive sheet material 30 and a flexible
magnetized pad 31 according to the present invention
adapted for use on a rotary grinder 32 of the type commonly
used to finish metal such as the rotary grinder sold under
the trade deisgnation Catalog No. 1200 "Portable Electric
Polisher" by ~ioux Tools, Inc., Sioux City, Iowa. The
coated abrasive sheet material 30 is shown magnetically
attachsd on a circular support surface 34 of the flexible
magnetized pad 31, which support surface is generally
planar which means that it could be flat or could be
slightly convex or concave if desired to facilitate
grinding certain workpieces~ The magnetized pad 31
includes magnetized particles within a polymeric matrix,
(e.g., a 0.15 centimeter thick pad 31 of "Plastiform"
magnetized material having 18 magnetic poles per inch in
one direction along the support surface 34), which
magnetized pad 31 is mounted on a flexible polymeric
circular backing plate 36 of the rotary grinder 32. The
grinder 32 includes a drive means including an electric
motor 40 adapted to rotate the backing plate 36 and
magnetized pad 31 at over 3,000 R.P.M. to abrade a
workpiece (not shown). During such rotation only the force
of magnetic attraction between the magnetized pad 31 and
the flexible, relatively low mass, circular, coated
abrasive sheet material 30 and any force applied to the
sheet material 30 through the magnetized pad 31 normal to
the support surface 34 will produce sufficient static
16~ 36~
friction between the support surface 34 and the coated
abrasive sheet material 30 to retain the coated abrasive
sheet material 30 on the support surface 34 while it is
driven by the magnetized pad 31 to abrade the workpiece.
As is seen in Figure 4, the coated abrasive sheet
material 30 comprises a flexible backing sheet 42 having
opposite major surfaces and a layer of abrasive grains 46
adhered on one major surface of the backing sheet 42 by a
conventional bonding material. Magnetic attachment between
the circular coated abrasive sheat material 30 and the
magnetized pad 31 is provided by a layer 44 of
ferromagnetic (e.g., iron) particles adhered to the major
surface of the backing sheet 42 opposite the layer of
abrasive grains 46, which layer 44 has a uniform density of
over about 0.007 gram (and preferably in the range of about
0.015 to 0.03 gram) of ferromagnetic material per square
centimeter of area measured in a plane parallel to the
major surfaces of the backing sheet 42.
Referring now to Figures 5 and 6 there is shown a
piece of coated abrasive sheet material 50 and a flexible
magnetized pad 51 according to the present invention
adapted for use on a hand sanding block 52 of the type
commonly used by home craftsman such as the hand sanding
block 52 made of flexible foam rubber and sold under the
2\, trade designation "Soft Hand Block'~ part no. 5442, by
~- Minnesota Mining and Manufacturing Company. The coated
abrasive sheet material 50 is rectangular and is shown
magnetically attached on a rectangular support surface 54
of the flexible magnetized pad 51. The support surface 54
is generally planar which means that the surface could be
flat or could be slightly arcuate around a transverse or
longitudinal axis as may be desired to facilitate abrading
certain contoured workpieces. The magnetized pad 51
comprises magnetized particles within a polymeric matrix
(e.g., a 0.15 centimeter thick magnetized pad 51 of the
magnetized material commercially designated "Plastiform"
sold by Minnesota Mining and Manufacturing Company of St.
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Paul, Minnesota and having 18 magnetic poles per inch in
one direction along the support surface 54). The
magnetized pad 51 is adhered on a planar surface of a
molded foamed polypropylene upper portion 56 of the hand
sanding block 52 which includes a projection 58 adapted to
be manually grasped so that the block 52 can be manipulated
to abrade a substrate (not shown). During such
manipulation only the force of magnetic attraction between
the magnetized pad 51 and the flexible, relatively low mass
rectangular coated abrasive sheet material 50 and any force
applied to the sheet material 50 through the magnetized pad
51 normal to the support surface 54 will produce sufficient
static friction between the support surface 54 and the
coated abrasive sheet material 50 to retain the coated
abrasive sheet material 50 on the support surface 54 while
it is driven by the magnetized pad 51 to abrade the
workpiece.
As is best seen in Figure 6, the coated abrasive
sheet material 50 comprises a flexible backing sheet 62
having opposite major surfaces and a layer 66 of abrasive
grains adhered on both major.surfaces by a conventional
: bonding material. Magnetic attachment between the piece 50
of coated abrasive sheet material and the magnetized pad 51
is provided by a layer of ferromagnetic (e.g , iron)
particles havi.ng a uniform density of over about 0.007 gram
(and preferably in the range of about 0.01 to 0~05 gram)
per square centimeter of area measured in a plane parallel
to the major surfaces of the backing sheet 62, which
ferromagnetic particles are incorporated in both layers 66
of abrasive grains 66 so that either layer 66 of abrasive
grain may be positioned on the support surface 54 to permit
use of the other layer 66 of abrasive grain to abrade a
workpiece.
Referring now to Figures 7, 8, 9 and 10 there are
illustrated fourth, fifth, sixth, and seventh alternate
embodiments respectively of flexible coated abrasive sheet
materials 70, 80, 90 and 100 according to the present
~:~536~
....~. ~
-18-
invention, which sheet materials 70, 80, 90 and 100 could
be magnetically attached to and be of an appropriate size
and shape to be used on a support surface of a flexible
magnetized pad including magnetized particles within a
polymeric matrix, such as the magnetized pads 11, 31 and 51
described above.
The embodiment of the flexible coated abrasive
sheet material 70 illustrated in Figure 7 comprises a
backing sheet 72 having opposite major surfaces, a layer of
abrasive grains 74 adhered on one of the major surfaces by
a conventional bonding material, and a uniform density of
over about 0.007 gram (and preferably in the range of about
0.02 to 0.07 gram) of ferromagnetic (e.g., iron) fibers 78
per square centimeter of area measured in a plane parallel
to the major surfaces which fibers 78 were incorporated in
the backing sheet 72 at the time the backing sheet 72 was
made.
The embodiment of the flexible coated abrasive
sheet material 80 illustrated in Figure 8 comprises a
backing sheet 82 having opposite major surfaces, a layer of
abrasive grains 84 adhered on one of the major surfaces by
a conventional bonding material, and a thin sheet 86 of
ferromagnetic material (e.g., shim steel) adhered by an
adhesive layer 87 to the major surface of the backing sheet
82 opposite the layer of abrasive grains 84 and providing a
uniform density of over about 0.007 gram (and preferably in
the range of about 0.01 to 0.05 gram) of the ferromagnetic
material per square centimeter of area measured in a plane
- parallel to the major surfaces of the backing sheet 82.
The sheet 86 of ferromagnetic material is embossed on its
surface 88 opposite the backing sheet 82 both to improve
its coefficient of friction with a support surface, and to
provide recesses 89 which can receive dust that may become
trapped between the surface 88 and the support surface of a
magnetic pad on which the sheet material 80 is attached,
thereby restricting the separation and the lubrication or
bearing effect between those surfaces that the dust might
. ~,
253~
--19--
otherwise provide to promote slippage between the sheet
material and the support surface in the plane of the
surface 88.
The embodiment of the flexible coated abrasive
sheet material 90 illustrated in Figure 9 is the result of
modifying a commercially available piece 91 of coated
abrasive sheet material, including a layer 92 of pressure
sensitive adhesive by which the piece 91 may be adhered to
a support surface of a non-magnetized pad (such as the
sheet material sold under the trade designation "STIKIT" by
Minnesota Mining and Manufacturing Company, St. Paul,
Minnesota) by applying a liner sheet 93 comprising
ferromagnetic material so that the combination of the
commercially available piece 91 and liner sheet 93 can be
magnetically attached on the support surface of a
magnetized pad such as the pads 11, 31 and 51 described
above. The commercially available piece 91 comprises a
backing sheet 95 having opposite major surfaces, a layer of
abrasive grains 94 adhered on one of the major surfaces by
a conventional bonding material, and the layer 92 of
pressure-sensitive adhesive on the major surface of the
backing sheet 94 opposite the layer of abrasive grains 95O
The liner sheet 93 (e.g., 0.005 centimeter thick
polyethylene) overlays the surface of the layer 92 of
pressure-sensitive adhesive, and is filled with
ferromagnetic granules 98 which were mixed with the
material from which the liner sheet 93 was extruded to
provide a uniform density of over about 0.007 gram (and
preferably in the range of about 0.02 to 0.04 gram) of the
ferromagnetic material per square centimeter of area
measured in a plane parallel to the major surfaces of the
backing sheet 94. The coated abrasive sheet material 90 as
illustrated can be magnetically attached to a support
surface of a flexible magnetized pad including magnetized
particles within a polymeric matrix, such as the pads 11,
31 and 51 described above, or the liner sheet 93 including
the fer~omagnetic granules 98 can be pealed away, and the
'' ~ . .
: '
.
, .
~ ~2536~
-20- .
remaining commercially available piece 91 of coated
abrasive sheet material can be adhered by the layer 92 of
pressure-sensitive adhesive to the support surface on a
non-magnetized pad.
The embodiment of the flexible coated abrasive
sheet material 100 illustrated in Figure 10 is the result
of modifying a conventional commercially available piece
101 of coated abrasive sheet material (which commercially
available piece 101 includes a backing sheet 102 having
opposite major surfaces, and a layer 104 of abrasive grains
adhered on one of the major surfaces of the backing sheet
102 by a conventional bonding material) by the application
of a laminate 105 comprising ferromagnetic material so that
the commercially available sheet 101 can be magnetically
attached on the support surface of a magnetized pad, such
as the pads 11, 31 and 51 described above. The laminate
105 comprises a liner sheet 106 (e.g., 0.038 centimeter
thick polyethylene) which, like the liner sheet 93, is
filled with ferromagnetic granules 108 to provide a uniform
density of over about 0.007 gram (and preferably in the
range of about 0.02 to 0.04 gram) of the ferromagnetic
material per s~uare centimeter of area measured in a plane
parallel to the major surfaces of the liner sheet 106, and
a layer 107 of pressure-sensitive adhesvie originally
coated on the liner sheet 106 by which layer 107 the
laminate 105 is adhered to the major surface of the backing
sheet 102 opposite the layer of abrasive grains 104. The
commercially available piece 101 of coated abrasive sheet
- material may be modified in the field by applying the
laminate 105 to produce the coated abrasive sheet material
100 that can be magnetically attached to a maganetized pad.
Alternatively, either of the liner sheets 93 or
106 in the embodiments 90 or 100 of the coated abrasive
sheet material shown in Figures 9 and 10 could be replaced
by a liner sheet having a layer of ferromagnetic particles
adhered on its surface opposite the layer of pressure
sensitive adhesive in the manner the layers 28 and 44 of
~ .
-21- ~536~
ferromagnetic particles are adhered on the backing sheets
22 and 42 in Figures 2 and 4; or could be replaced by a
sheet of ferromagnetic material such as the sheet 86 shown
in ~igure 8.
Test Results
The following describes several tests, the
results of which are graphed in Figures 11, 12l and 13 and
shown in the table at the end of this specificationO The
tests were made to determine the force of magnetic
attraction that will result between magnetized pads having
18 (Figure 11), 11 (Figure 12) or 8 (Figure 13) magnetic
poles per inch along their support surfaces, and various
embodiments of coated abrasive sheet material according to
the present invention incorporating varying amounts of iron
ferromagnetic material.
Test No. 1
Pieces of coated abrasive sheet material having
structures generally like the coated abrasive sheet
material 80 illustrated in Figure 8 were made and tested.
Pieces of shim steel commercially designated QQ-S-698
C-10-10 No. 1 temper obtained from Baisdell Manufacturing,
Inc., Buena Park, California, 90622, and nominally 0.0025,
0.0051, 0.0076, 0.0127, and 0.0254 centimeter (0.001,
0.002, 0.003, 0.005 and 0.010 inch) thick, were adhered to
the layers of pressure sensitive adhesive on the surfaces
opposite the layers of abrasive grains on the backing
sheets of commercially available pieces of coated abrasive
sheet material commercially designated "STIKIT" and
manufactured by Minnesota Mining and Manufacturing Company
of St. Paul r Minnesota. Several 7 centimeter (2-3/4 inch)
by 41.9 centimeter (16-1/2 inch) rectangular pieces of the
resulting coated abrasive sheet material were die cut using
a hydralic press, along with several pieces of each
thickness of the shim steel which were weighed on a
laboratory balance to determine the weight of ferromagnetic
~25~
-22-
material in each sample for each thickness of shim steel.
The samples were then sequentially attached along
their abrasives coated surfaces to a 1.27 centimeter (1/2
inch) thick by 7 centimeter (2-3/4 inch) wid9 by 41.9
centimeter (16-1/2 inch) long rectangular block of aluminum
using Acrylic ~oam Tape No. Y 4205 available from Minnesota
Mining and Manufacturing Company of St. Paul, Minnesota
such that the shim steel could be placed in intimate
contact along the support surface of various 0.03 inch
thick magnetized pads of the flexible material including
magnetized particles within a polymeric matrix of the type
commercially designated "Plastiform", of the 8, 11, and 18
poles per inch variety, which magnetized pads ~ere secured
to the load platform of a Type 9281B "Kistler"~testing
device available from Kistler Instrument Corp., Amherst,
N.Y., that was used to electronically measure the force
required to separate the samples from the magnetized pads.
The results of the tests labeled lA, lB, lC, lD an~ lE are
shown in Figures 11, 12 and 13 for the 18, 11, and 8 poles
per inch "Plastiform" material magnetized pads
respectively, and are recorded in the table at the end of
this specification.
As can be seen, the holding force between the
sample coated abrasive materials and the magnetized pads
increased as the thickness of shim steel increased so long
as the shim steel provided less than about 0.1 grams per
square centimeter of ferromagnetic material, and then began
to decrease for shim steel which provided more than about
- 0.1 gram per square centimeter of ferromagnetic material.
Also, it was separately judged that shim steels having
thickness of 0.0127 centimeter and 0.0254 centimer (0.005
inch and 0.010 inch) did not have sufficient flexibility
for use in the present invention as when such shim steels
were bent 180 degrees around a 1 centimeter (3/8 inch)
diameter rod under the conditions indicated above in this
specification they were not held flat against the support
surface of the 11 pole per inch magnetized pad of
~Tra.G~e b~c.~
~s;~6g~
-23-
"Plastiform" material by the magnetic field from that pad.
Shim steels having thickness of 0.0025, 0.0051 and 0.0076
centimeter (0.001, 0.002 and 0.003 inch) were found to have
sufficient flexibility to pass this test.
Test No. 2
Backing sheets for coated abrasive sheet material
having a layer of iron particles adhered on one surface
like the backing sheets 22 and 42 and layers 28 and 44 of
ferromagnetic particles illustrated in Figures 2 and 4 were
made and tested. lO0 pound batches of iron granules
commercially designated MH 100 and obtained from Hoeganaes
~Corporation, Riverton, New Jersey, 08077, were placed in an
11 gallon ball mill with 316 pounds of about 1.5 centimeter
(5/8 inch) diameter steel balls and milled dry for 20 to 22
hours to produce flat platelet-shaped granules of iron.
The milled iron granules were then screened through a 180
mesh screen, and the course iron granules were discarded.
A slurry was then prepared using 57.4% by weight of the
milled screened platelet-shaped iron granules, 25~6% by
weight of a polyester resin having 40~ solids, 9.5% by
weight of a polyester resin having 30% solids, 2.6% by
weight of methyl ethyl keytone, 2.6% by weight of toluene,
0.4% by weight of wetting agent, and 1.9% by weight of
isocyanate crosslinking agent. The slurry was then coated
at various coating weights using a Gravure Roll coater onto
"D" weight paper obtained from James River Corp. of
Virginia, Richmond, Virginia at a slurry viscosity of about
1,000 centipoise and a temperature of about 70 degrees
Fahrenheit and allowed to dry. Samples of the rasultant
coated papers and of the uncoated paper were then die cut
and weighed using the same equipment used in Test No. l,
and the weight of iron in each sample of coated paper was
calculated by subtracting the sample paper weights from the
weights of the coated samples to determine the weight of
the ferromagnetic material containing coating, and by then
calculating the weight of ferromagnetic material in each
: .
~:ZS~G9~
-24-
sample as being the percentage by dry weight that the
ferromagnetic material was in the slurry from which it was
coated. The force to separate the sample coated paper was
determined using the "Kisler" tes'ing device in the manner
described above in Test No. 1. The results are shown on
the graphs of Figures 11, 12 and 13 where they are labeled
as points 2A and 2B, and are recorded in the table at the
end of this specification.
While the magnetic holding force for equivalent
amounts of ferromagnetic material per unit area was not as
great in the samples produced in this Test No. 2 as for the
Samples produced in Test No. 1, the resultant samples were
much more flexible.
Test No. 3
Backing sheets for coated abrasive sheet material
having a layer of iron particles adhered on one surface
like the backing sheets 22 and 42 and layers 28 and 44 of
ferromagnetic particles illustrated in Figures 2 and 4 were
made and tested. 74.1% by weight of milled screened
platelet-shaped iron granules prepared as described in Test
No. 2 were thoroughly mixed with 25.5% by weight of latex
SBR having 45~ solids (e.g., No. 219A from Dow Chemical
Company) and 0.4% by weight of acrylic emulsion thickener
having 13~ solids (e.g., Cruthix No. 46 from Crucible
Chemical Co., Greenville, South Carolina) to produce a
latex water based slurry having a viscosity of about 8,000
centipoise which helped keep the iron granules in
suspension. The slurry was knife coated at different
thicknesses onto "D" weight paper obtained from James River
Corp. of Virginia, Richmond, Virginia at a temperature of
70 degrees Fahrenheit and allowed to dry. Samples of the
resultant coated papers and of the uncoated paper were then
die cut and weighed using the same equipment used in Test
No. 1, and the weight of iron in each sample of coated
paper was calculated by subtracting the sample paper
weights from the weights of the coated samples to determine
6~
.
-25-
the weight of the ferromagnetic material containing
coating, and then calculating the weight of ferromagnetic
material in each sample as being the percentage by dry
weight that the ferromagnetic material was in the slurry
from which it was coated. The force to separate the sample
coated paper was determined using the "Kisler" testing
device in the manner described above in Test No. 1. The
results are shown on the graphs of Figures 11, 12 and 13
where they are lableled as points 3A and 3B, and are
recorded in the table at the end of this specification.
While the magnetic holding ~orce for equivalent
amounts of ferromagnetic material per unit area was not as
great in the samples produced in this Test No. 3 as for the
samples produced in Test No. 1, the resultant samples were
much more flexible.
Test No. 4
Backing sheets for coated abrasive sheet material
having a layer of iron particles adhered on one surface
like the backing sheets 22 and 42 and layers 28 and 44 of
ferromagnetic particles illustrated in Figures 2 and 4 were
made and tested. 69.78% by weight of milled screened
platelet-shaped iron granules prepared as described in
Example 2 were thoroughly mixed with 14.96% by weight of
ethylene vinyl acetate, 14096% by weight of polyterpene
tackifier resin and 0.3% by weight of antioxidant (e.g.,
Irganox 1010 available from Ciba-Geigy Corp., Washington,
Pennsylvania) to produce a hotmelt slurry. The slurry was
roll coated onto "D" weight paper obtained from James River
Corp. of Virginia, Richamond, Virginia, at a viscosity of
24,000 centipoise and a temperature of 300 degrees
Fahrenheit and allowed to cool. Samples of the resultant
coated paper and of the uncoated paper were then die cut
and weighed using the same equipment used in Test No. 1,
and the weight of iron in each sample of coated paper was
calculated by subtracting the sample paper weights from the
weights of the coated samples to determine the weight of
9~
-26-
the ferromagnetic material containing coating, and then
calculating the weight of ferromagnetic material in each
sample as being the percentage by weight that the
ferromagnetic material was in the surry from which it was
coated. The force to separate the sample coated paper was
determined using the "Kisler" testing device in the manner
described above in Test No. 1. The results are shown on
the graphs of Figures 11, 12 and 13 where they are labeled
as points 4A, 4B and 4C, and are recorded in the table at
the end of this specification.
While the magnetic holding force for equivalent
amounts of ferromagnetic material per unit area was not as
great in the samples produced in this Test No 4 as for the
samples produced in l`est No. 1, the resultant samples were
much more flexible.
Test No. 5
A steel fiber-filled flexible backing sheet like
the backing sheet 72 shown in Figure 7 was prepared using
mild steel fibers commercially designated Type PF and
obtained from Hoeganaes Corporation, Riverton, New Jersey,
08077. The backing sheet was made on a Fourdinier-type
paper making machine from a slurry of 1 part by weight of
semi-bleached wood pulp (e.g., Prince George Wood Pulp,
available from Canadian Forest Products, Ltd., Van Couver,
British Columbia, Canada), 3.5 parts by weight of the steel
fibers, 0.49 parts by weight of laytex having 50% solids
(e.g., Hycar No. 1562X103 available from B.F. Goodrich,
Akron, Ohio), 0.004 parts by weight of surfactant (e.g.,
Tamol SN available from Rohm and Haas, Philadephia,
Pennsylvania), and 0.028 parts by weight of jet black dye.
Samples of the resultant paper were then die cut and
weighed using the same equipment used in Test No. 1, and
the weight of the steel fibers in each sample of paper was
calculated as being the percentage by dry weight that the
ferromagnetic material was in the slurry from which the
paper was made. The force to separate the samples of steel
53~
-27-
fiber filled paper were determined using the "Kisler"
testing device in the manner described above in Test No. 1.
The results are shown on the graphs of Figures 11, 12 and
13 where they are labeled as points 5A and 5B, and are
recorded in the table at the end of this specification.
The magnetic holding force for equivalent amounts
per unit area of ferromagnetic material was not as great in
the samples produced in this Test No. 5 as for the samples
produced in Test No. 1 or as for the samples produced in
Test No's. 2, 3 and 4. The resultant samples were much
more flexible than the samples from Test No. 1, however,
and at least as flexible as the samples from the other
TestsO
Test No. 6
Backing sheets for coated abrasive sheet material
having a layer of iron particles adhered on one surface
like the backing sheets 22 and 42 and layers 28 and 44 of
ferromagnetic particles illustrated in Figures 2 and 4 were
made and tested. A pre-mix was prepared using 12,027 grams
of iron granules commercially designated MH 100, obtained
from Hoeganaes Corporation, Riverton, New Jersey, 193.1
grams of methyl ethyl keytone, 193.1 grams of toluene, and
5.3 grams of wetting agent. The pre-mix was placed in a
2.3 gallon ceramic ball mill with 12.9 kilograms of about
1.5 centimeter (5/8 inch) diameter steel balls and milled
for about 24 hours to start forming the iron into flat
platelet-shaped granules. 142 grams of a polyester resin
having 40% solids, 383.4 grams of a polyester resin have
30~ solids, 317.2 grams of methyl ethyl keytone, and 317.2
grams of toluene were then added to the pre-mix in the ball
mill. The resultant mixture was again milled for about 20
hours to further form the iron into flat platelet-shaped
granules, and was then screened through a 4 mesh screen to
remove the steel balls. The resultant slurry was then
coated at various coating weights using a knife coater onto
"A" weight paper obtained from James River Corp. of
6 9
-28-
Virginia, Richmond, Virginia at a slurry viscosity of about
66 Kreb units and a temperature of about 70 degrees
Fehrenheit and allowed to dry. Samples of the resultant
coated papers and of the uncoated paper were then die cut
and weighed using the same equipment used in Test No. 1,
and the weight of iron in each sample of coated paper was
calculated by subtracting the sample paper weights from the
weights of the coated samples to determine the weight of
the ferromagnetic material containing coating, and by then
calculating the weight of ferromagnetic material in each
sample as being the percentage by dry weight that the
ferromagnetic material was in the slurry from which it was
coated. The force to separate the ~ample coated paper was
determined using the "Kisler" testing device in the manner
described above in Test No. 1. The results are shown on
the graphs of Figures 11, 12 and 13 where they are labeled
as points 6A and 6B, and are recorded in the table at the
end of this specification.
While the magnetic holding force for e~uivalent
amounts of ferromagnetic material per unit area was not as
great in the samples produced in this Test NOD 6 as for the
Samples produced in Test No. 1, the resultant samples were
much more flexible, and had the best magnetic holding force
for samples including granular iron.
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