The unavoidable and unintentional by-product of manufacturing practices
is the generation of burrs and sharp edges. Besides the obvious
safety hazards associated with burrs and sharp edges, the presence
of these features adversely affects the functionality and performance
of the manufactured component. Manufacturers have resorted to a
number of methods to remove these unwanted attributes.
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| Cupbrush
with CNC adapter
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| Radial
Brush with CNC adapter
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Deburring practices can be classified into two larger groups: mechanical
and chemical deburring. Some of the most common methods to mechanically
deburr parts include: sanding, tumbling, sandblasting,and brushing.
All of the above-mentioned deburring methods have advantages as
well as disadvantages. One of the biggest disadvantages is the cost
associated with the purchase of specific equipment. Perhaps the
most economical solution to mass deburr manufactured parts using
current equipment is the use of power brushes.
The industrial applications of brushes vary from paint removal
and deburring to honing and polishing. These surface-conditioning
tools can be used on a variety of materials, including aluminum,
steel, carbide, wood, glass and practically any material imaginable.
The advantages of brushes include flexibility, durability and repeatable
uniform results. The physical designs as well as some of the available
adapters make these tools readily adaptable to machinery that is
widely used in toady’s manufacturing environment (Left).
Although many brush types and configurations are available (See
next four photos), the two major components that make up a brush
are the filament type and the mounting hardware.
The purpose of the mounting hardware is to act as the means of
introducing the filament to the work piece. The more important component,
which is responsible for performing the actual deburring, is the
filament. Filament types range from artificial to natural filaments.
Some of the naturally occurring filaments used in brushes are Tampico,
sisal, horsehair and a wide range of natural filaments. The use
of these filaments varies according to the application. Generally,
these filaments are not abrasive by themselves but are used for
polishing and cleaning applications in conjunction with some sort
of abrasive compound. The commonly used filament types are the abrasive
and crimped wire filaments. Examples of crimped wire filaments vary
from steel, stainless steel, bronze, brass, and phosphorous bronze
and in rare cases gold.
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| Cupbrush
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End
Brush |
Radial
Brush |
Internal
Brush |
The selection of abrasive filaments varies from silicon carbide
to aluminum oxide to polycrystalline diamond. Depending on the grit
size, these filaments can be used for deburring, polishing, blending,
and many other surface conditioning applications. The filament is
a mixture of specially formulated nylon, with varying grit sizes
of abrasive crystals homogeneously distributed (See
Fig 1).
In many cases, the filament has additives that aid with heat transfer
and moisture absorption. Because the carrier is nylon, heat is a
significant limiting factor during application. The use of coolant
with a pH level between 2 and 9 is strongly recommended. If the
use of liquid coolant is not an option, the use of forced air is
recommended to keep the brush cool. The melting point of most nylon
filament is 410F. At 210F, the abrasive filament looses 70% if its
stiffness. Due to the loss of beam strength (stiffness), the brush
will not perform as aggressively and predictably as it had at the
start of the application. To counter the loss of stiffness, the
uses of side plates or bridles are suggested.
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| Figure
1. Distribution of Abrasive Crystals
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The crystal structure, in conjunction with grit size, determines
the aggressiveness of the filament. The silicon carbide and polycrystalline
diamond crystals are characterized by sharp, jagged edges where
the aluminum oxide and aluminum silicate have a more rounded crystalline
structure (See Fig 2).
Due to these characteristics, a brush with silicon carbide filament
is more aggressive than a brush with aluminum oxide impregnated
filament. The most commonly used abrasive grain sizes vary from
46-grit (coarse) up to 1,000-grit (fine). For an aggressive deburring
application, the use of coarse abrasive grain size is recommended,
where as the 1,000-grit grain size can be used for polishing applications.
Another consideration to keep in mind while selecting a brush is
the type of industry the part is being used in. Since silicon carbide
filament is more jagged, it tends to break off and embed into the
work piece. Because of this characteristic, a number of fabricators
in the aircraft industry insist on using a brush with aluminum oxide
filament rather than silicon carbide impregnated filament.
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Figure 2. Aluminum Oxide
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Wire Filament
The most important trait of crimped wire filament is the hardness
of the wire. Generally, the carbon steel wire has a hardness of
Rc 55-60. The stainless steel wire on the other hand is between
Rc 30-35. Brass, bronze and beryllium copper filaments are used
where softer materials need work (rubber, aluminum), as well as
in low-sparking applications. The important characteristic of wire
filament is that the deburring takes place only at the tip of the
wire. Since the wire tip is the area where a sharp edge can be found,
this is the only part of the wire that is capable of cutting.
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| Silicone
Carbide
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The majority of brushes sold are made using crimped wire, which
has multiple benefits. Perhaps the most important feature is its
ability to break off and expose new cutting points. If the crimped
wire is compared to a wave, the breaks are generally occurring at
the crest and trough of the wave. These points act as stress risers;
therefore the initial break takes place at these points and exposes
new sharp cutting points.
A common application problem encountered is over the penetration
on wire brushes. If a wire brush is over penetrated, the filaments
act more like small hammers impacting the work piece surface. The
easiest way to determine if a wire brush is over penetrated is to
examine a part after the initial deburring application. If the edge
of the part resembles a shot peened surface, with the sharp edge
still remaining, most likely this can be corrected by decreasing
the brush penetration. The recommended brush penetration can be
seen in Figure 3.
Brush Selection
When selecting a brush for an application, a number of questions
need to be addressed. The two most important questions are what
is being pursued (i.e. deburring, edge break, polishing, cleaning
act.), and what is the material that needs attention. For example,
if the goal is to deburr a stainless steel tube that was saw cut,
the use of either an abrasive or stainless steel wire brush is recommended.
The use of a stainless steel brush is recommended to reduce contamination
on the work piece. Where a carbon wire brush will probably deburr
the part faster, it will also leave traces of carbon steel that
cause the part to oxidize. If the tube is harder than 35 Rc, the
only remaining option is the use of abrasive filament. Depending
on the equipment used and the limitations of the equipment, either
a cup or a radial brush will perform the job. Usually the largest
brush that the equipment can handle is recommended. The benefits
of this are two-fold. Due to the size of the brush, the part can
probably be deburred in one pass as opposed to two or more passes.
The other advantage of using a larger brush is that the brush will
have to be changed less frequently, therefore less downtime for
the machine.
As with any power tool, safety is a paramount concern when it comes
to deburring applications. Just as cutting tools dull and deteriorate
over time, so do brushes. After a number of cycles, the wires fatigue,
break and fall out of the brush. At the various operating speeds,
these loose wire filaments have enough velocity to puncture through
clothing and skin. For offhand deburring, the use of leather gloves
and a thick leather apron is recommended along with full-face shield
and work boots.
