How to Choose-And Use-Non-Woven Abrasives for Best Results in Metal Finishing
Abrasive products used for finishing of metal and non-metallic workpieces
in the production plant are generally categorized into two main groups: bonded
abrasives (usually grinding wheels) and coated abrasives (belts, discs, sheets,
etc.). These are the products used to remove stock during the production
process. But there is a third category of abrasives that serve a specialized and
highly necessary function of preparing and applying the final surface finish,
which can be critical to the performance and appearance of the workpiece. These
are the non-woven abrasives.
These products are manufactured using a "web" of nylon fibers
that are bonded together (not woven together - hence the name) with synthetic
resins. The fibers are impregnated with abrasive grain, producing a cushioned,
three-dimensional material that is pliable, conformable to the workpiece, and
long-lasting.
Non-woven abrasives can be the ideal grinding products for deburring,
cleaning, and imparting the desired finish to a wide variety of materials. A
basic understanding of how these products are made, and how they should be
selected and used to their maximum benefit, can help the metalworking
manufacturer achieve the desired surface finish.
It is often said that the job of non-woven abrasives begins where other
grinding products leave off. The relatively non-aggressive nature of nylon and
the abrasive grit used in non-wovens make them excellent finishing tools. With
their open-mesh construction, non-wovens are waterproof, washable, and
resilient. They resist loading and rusting, and are not conductive.
In the standard non-woven design, abrasive grain is uniformly dispersed
throughout the nylon web, providing a continuous supply of new grain as the old
grain and fibers wear away during use. While 60 grit to 80 grit abrasives are
considered intermediate sizes in other products, they are considered coarse for
non-woven products.
Non-wovens can be used on a range of metals, including aluminum, brass,
copper, nickel, chrome plate, stainless steel, and titanium, as well as other
hard-to-grind materials such as ceramics, glass and plastic. Used wet or dry,
non-wovens enable the user to achieve a consistent, uniform finish, and
conformability to irregular surfaces, with minimal smearing and discoloration of
the workpiece.
While non-wovens are made in a different way than conventional coated
abrasives, both types of products use many of the same abrasive minerals.
Silicon carbide is sharper, cuts faster and produces finer scratch patterns on
most surfaces. Aluminum oxide is more durable and tends to last longer. It is
more aggressive on certain applications (such as hardened steel parts), and
produces less discoloration on aluminum.
When the non-woven supplier talks about grit, he is referring to the size
of the abrasive grain impregnated into the nylon web. The lower the number, the
larger the grains. Use of large particles (coarse grit) results in a more
aggressive cut and a coarser finish. Small size particles result in a non-woven
product that produces fine surface finish, if all other conditions are equal.
Another term used in specification that is probably unfamiliar to the
buyer of conventional abrasives is density. This refers to the amount of bonding
agents and amount of abrasive grain that have been compressed into the non-woven
product. In general, harder density products cut faster, last longer and produce
finer finishes than softer density products. However, softer densities offer
more conformability to the work surface and less tendency to load or burn the
workpiece.
Two other factors in the design and manufacture of non-woven abrasives
are the sizing of the nylon fibers and the bonding agents used. Fiber size has a
significant effect on the product's cutting characteristics, while the type of
waterproof resin used to bond the fibers together (and to anchor the abrasive to
the web) can alter the characteristics of the product. The non-woven
manufacturer selects the appropriate fiber size and bonding agent based on the
intended application for the product; therefore, these components are not
included in the product offering specifications.
A typical specification on a non-woven abrasive product might look like
this: 8 A/O MED.
The first symbol - in this example, the "8"-refers to the
product's density. This can range from 2 (open/most conformable) to 9
(densest/most durable).
The second symbol denotes the abrasive used. "A" or
"A/O" refers to aluminum oxide, while "S" or "SC"
stands for silicon carbide.
The final designation-"MED-denotes the grit size of the product, as
outlined above.
The basic non-woven design lends itself to a variety of product types
that have been developed to meet a range of needs for metalworking and
finishing. In general, non-woven abrasive products can be categorized into hand
pads, rolls, discs, belts and wheels.
Hand pads are designed to provide excellent conformability and
flexibility when hand finishing the work surface. They are generally available
in 6" x 9" sizes, but are easily folded into smaller sizes as needed.
Grit sizes range from coarse grit, used for general cleaning, to micro-fine
(1000 grit) for light scuffing and blending. In wet applications, many users
choose a non-woven hand pad as a replacement for steel wool, because non-wovens
are non-metallic and therefore don't rust.
For broader applications, the user may select non-woven rolls. Offering a
more convenient source of the same cleaning and finishing performance as hand
pads, rolls are available in 30-foot lengths and widths of 4 in. and 6 in. The
user simply cuts the roll to the required size for hand sanding or jitterbug
sanding applications.
Non-woven discs come in a variety of designs for many cleaning and
finishing applications. Arbor hole discs are used on high-speed grinders. Extra
coarse grit products may be used for removal of surface rust and residue,
corrosion, light weld splatter and other surface contaminants. These discs can
also be ganged with spacers to form a wide wheel used for sharp edge radiusing
or deburring a decorative finish.
Right-angle discs are used on portable grinders for cleaning, deburring,
blending and finishing, as well as for the removal of light rust, oxidation and
coatings. The latest advance in grinding for these applications is the
development of "see-through" discs for right-angle grinding. These
discs feature a uniquely shaped triangular profile and holes in the discs and
back-up pads. This provides the operator with a clear view of the cutting
surface, resulting in more controlled stock removal and a better surface finish.
The holes also serve to disperse heat and loose grinding material, for a cooler
finish and less power draw.
Non-woven belts are used on portable, bench and pedestal bench machines
as an effective alternative to coated abrasive belts for cleaning, buffing and
polishing. The conformability of the belts result in minimal gouging of the
workpiece, generating a more consistent finish.
There are three types of non-woven wheels:
•
Flap wheels are fabricated by mounting sheets of non-woven material around a
center hub; the tightly packed pieces form the spokes of a wheel. This is the
best choice where conformability to a surface or a long-line brushed finish is
required.
•
Convolute wheels are made by wrapping the non-woven material around a center
core and bonding the layers together; these wheels can easily be shaped to match
preformed parts.
•
Unified wheels are formed by compressing multiple layers of non-woven web
material and bonding them together to form a wheel; they are ideal for general
purpose cleaning and deburring.
Of all the non-woven abrasive product types, the wheel family requires
the most care in set-up and operation. Maximum wheel life and best surface
conditioning can be achieved by following these recommendations:
Wheel direction: Convolute wheels must always run in the direction
indicated by the arrow printed on the side of each wheel. Flap wheels and
unified wheels can be run in either direction.
Wheel speed: The speed at which the wheel is run affects product finish,
rate of cut, and wheel life. Fast wheel speed, in general, gives harder action
and a finer finish; slower speeds give softer action and a coarser finish.
Following are recommended operating speeds for the most common applications.
Pressure: Light to medium pressure is recommended for most operations.
Flap wheels require much lighter pressure to perform properly than other
non-woven wheels; unified wheels can withstand much higher pressures in order to
perform deburring jobs. In all cases, avoid excessive pressure, which may result
in wheel deformation and damage to the work surface.
Feed speed: Feed speeds directly affect the number of pieces completed
over a given time period. Slow feed speed reduces the number of completed
pieces, while producing a shorter scratch pattern. Slow feed speed also allows
for longer dwell time and permits more work to be done on each piece.
Oscillation: Oscillation may be used to break up scratch lines and
produce a more uniform finish. An increase in cut may also be experienced. A
general starting point for oscillation is 3/8 in. amplitude at 200 cycles per
minute.
Lubricants: The use of lubricants such as water, water-soluble oil and
straight oil will decrease the heat generated while running, improve the luster,
and reduce the surface finish. The higher the viscosity of the lubricant, the
lower the surface finish (RMS value) produced.
This information will assist the user in selecting non-woven abrasives to
complement their bonded and coated abrasives arsenal. However, it is important
to remember that changes in any one of the many factors discussed here can
affect the surface finish in the workpiece. Work in partnership with your
abrasives supplier to make the wisest product and application choices by
considering every facet of your operation. By taking this "system
approach," you will maximize the productivity of your non-woven abrasives
applications.
For more information contact:
Norton
Company
One
New Bond Street
Worcester,
MA 01615-0008
508-795-5000
Fax
508-795-4130
www.nortonabrasives.com