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Abrasive Filament Brush Deburring of Powdered Metal Components

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Abrasive Filament Brush Deburring of Powdered Metal Components
Abrasive Filament Brush Deburring of Powdered Metal Components
Abrasive Filament Brush Deburring
of
Powdered Metal Components
D. Mark Fultz
Vice President Marketing
Abtex Corporation
Dresden, NY
The deburring of PM parts is commonly performed in a batch process. This typically
involves loading a quantity of parts into a vibratory bowl filled with abrasive media. The
vibration of the bowl causes the abrasive media, and parts, to “flow”. This interaction
between the media and parts gradually abrades all surfaces of the part. Drawbacks to this
process include part impingement upon one another, waste generation, lodging of media
into the part, deburring of unnecessary areas on the part and the inherent vibration and
noise created by the operation of these systems.
This paper discusses an abrasive filament medium which is ideal for deburring and edge
radiusing PM parts. When formatted into a brushing tool, it can be applied on machine
based systems for semiautomated or fully automated deburring of PM components.
Through proper application, it is possible to gain the advantages of productivity, quality,
and economics without the negatives associated with alternative methods.
FIBER ABRASIVES
The term “fiber abrasive” is used to describe an abrasive nylon filament. Developed
approximately 25 years ago, they are commonly employed in brush form for a variety of
industrial applications. These generally involve deburring, edge radiusing and general
surface finishing.
The filament is composed of nylon, which has been coextruded with an abrasive grain.
The resulting monofilament is a homogeneous structure of nylon and abrasive. Nylon is
an ideal material for a brush filament. Compared to other polymers, it excels not only in
its durability, but also in its resistance to moisture, abrasion and chemicals (1). Nylon
types used in the production of fiber abrasives are Type 6, Type 66 and Type 612.
Type 612 is preferred in industrial applications. It offers the greatest heat resistance and
least amount of moisture absorbency.
The grain, or grit, is impregnated throughout the filament as well as exposed on the
external surfaces. A magnified example of this filament is shown in figure 1.
Abrasive action occurs on both
the tip and the sides of the
filament. As the filament is
applied to the workpiece and
begins to wear, new abrasive grit
is exposed. The filament is, in
effect, self sharpening. The
filament is typically applied at
slower speeds in order to allow
it to strike and wipe against the
workpiece surface. It acts much
like a flexible file. This feature
makes it ideal for finishing
irregularly shaped surfaces.
Abrasive options are, for the most part, limited to silicon carbide and aluminum oxide.
Other, more exotic abrasives are available. Diamond abrasives are available, but their
expense limits their use to very specific applications. Grit sizes range from 600 through
46 (mesh number used in abrasive separation). Smaller grit numbers relate to larger grit
particle size, larger grit numbers relate to finer (smaller) grit particle size. Filament
diameters range from .012” - .060”. As seen in Table 1, filament diameter increases as grit
size increases. This relationship is necessary in order to effectively bind the abrasive. By
weight, abrasive loading of the filament ranges from 20% to 40%.
Table 1.
Filament Dia.
.012
.018
.022
.030
.035
.040
.060
Inches (mm)
(.30)
(.46)
(.56)
(.76)
(.089)
(1.02)
(1.52)
Grit Size
600
500
120
240
180
80
46
600
320
120
Silicon carbide is the most widely applied abrasive in fiber abrasive brushing tools.
Silicon carbide combines cost effectiveness with excellent hardness and sharpness, making
it ideal for deburring applications. Aluminum oxide is less likely to fracture and is not as
“sharp” as silicon carbide. These characteristics create a filament that is generally applied
to improve surface finishes.
Regardless of grit size or type, the fiber abrasive is not a heavy material removal tool.
Although a large grit size can be applied (up to 46 mesh), the flexibility of the filament
limits its cutting action. The fiber abrasive does remove some material, but at a minimal
Figure 1: Enlarged View of Nylon Filaments
rate. With this feature, burrs and sharp edges are preferentially abraded away. This
enables the tool to deburr without negatively affecting the size and dimensional tolerances
of the part.
BRUSHING TOOL FORMATS
Fiber abrasives are typically formatted into brushing tools using conventional brush
making machinery. Abrasive brushing tool formats include: disc, wheel, cup, end and tube
as seen in Figure 2. Brushes of this type are applied with portable pneumatic and electric
hand tools, manual stationary equipment (drill press, pedestal grinder, buffing lathe),
semiautomated (CNC, NC, robotics) and fully automated, dedicated finishing systems.
Figure 2: Common brushing tool formats
PM PART DEBURRING
For deburring PM parts, two brush formats are commonly used; the radial wheel and/or
the disc.
1. Radial Wheel
The radial wheel, as the name implies, employs fibers extending radially from a hub. The
brush is mounted on a shaft and rotated in a direction that causes the fibers to strike the
part perpendicular to the surface to be deburred. With a radial wheel, the brushing action
is unidirectional as shown in figure 3.
Deburring with a radial wheel brush
can be either an off-hand process or
machine based. Limitations of off-
hand deburring include low
productivity, irregular quality and
potential for operator injury. Due
to these factors, off-hand deburring
of PM parts is rarely, if ever,
employed.
Machine based deburring systems employing radial wheel brushes are designed to
accommodate their unidirectional brushing action. This inevitably involves rotating the
part to present the targeted surface at a 90 degree angle to the striking action of the
filaments. Radial brushes, and the machinery to drive them, are often employed on parts
which have multiple surfaces, on varying planes, to be deburred. An example would be a
gear with a protruding hub where both the gear teeth and the hub face require deburring.
Radial wheels are produced in diameters ranging from 1 inch through 21 inches.
Figure 4: Disc format offers multidirectional brushing action
Figure 3: A radial wheel offers unidirectional brushing
action
2. Disc
The disc is constructed of a backing into which the filaments are embedded. The fibers
extend perpendicularly from the backing. Unlike the unidirectional brushing action of the
radial wheel, the disc offers multidirectional brushing action as illustrated in figure 4. As a
part traverses across the face of the disc, several surfaces are deburred. Because of this
multidirectional brushing action, the disc brush tends to be a more efficient format for
deburring PM parts. Disc brushes can be employed when the surface to be deburred is
flat with little or no changes in elevation. Disc brushes are available from 2 through 48
inches in diameter.
BRUSH CONSTRUCTION
Variables in brush construction affect their performance in this application. The quality
of the process is dependent upon optimizing each of these variables in relationship to
each other. These variables are:
? Density
? Trim Length
? Filament Diameter
? Grit Size
1. Density
Density refers to the number of individual filaments in the brush. In either format, radial
or disc, maximum density could be achieved by packing the filaments against one another,
offering an almost solid face. This would not be practical in this application.
The individual filaments need to flex in order to provide a wiping action that will follow
the contours of the part. Heat dissipation is also critical in order to avoid a condition
referred to as “nylon smear”. The melting point of the nylon used in the filament is 410°
F. Extreme density can contribute to heat generation sufficient to approach or exceed this
melting point. When this occurs, the melted nylon is transferred onto the part where it
cools and bonds. This process occurs almost instantaneously. This “nylon smear”
appears slightly translucent on the part and can be difficult to remove.
Using a brushing tool with too little density may require prolonged dwell time in order to
effectively deburr. Individual filaments are now required to work harder with less
support. This leads to premature filament breakage and reduced brush life.
The optimal brush density is shown in figure 5. The filaments are distributed evenly
across the face of the brush. Filaments are close enough to support one another yet
spaced to allow flex and heat dissipation.
2. Trim Length
The trim length is the length of the visible filament, or the distance from the tip of the
filament to its base. Trim length affects how aggressive the brushing action is. Generally,
with all other variables fixed, the brush becomes more aggressive as the trim length is
shortened. Assuming proper density, the increase in aggression as the brushing tool
wears is generally not detrimental in this application. Longer trim lengths, however, will
reduce aggressiveness. To compensate, longer dwell times are needed. There also is the
tendency to increase part penetration into the brush face. This is counter productive as
cycle times increase and brush life decreases. The ideal trim length is that which offers
adequate aggression and maximizes brush life.
Figure 5: Optimal brush density
3. Filament Diameter and Grit Size
In many applications, it is often most effective to use a smaller diameter filament. This is
true for PM part deburring. The filament is more flexible and, in a given density, more
abrasive surface area can be exposed to the part. Larger diameter filaments may have a
tendency to hit and bounce off the part (1). Larger diameter filaments also lack in
flexibility. They are more apt to fracture and break off instead of wearing consistently,
leading to accelerated brush wear. As was seen in Table 1, grit size and filament diameter
are related. The most commonly applied disc brush for deburring PM parts employs a
filament diameter of .022” and a grit size of 120. Radial wheels, with generally longer trim
lengths, typically employ filament diameters ranging from .030” to .040” and grit sizes of
240 to 120.
BRUSH SPEED
Rotational speed of the brushing tool is also a critical factor in this process. The rule of
thumb for the application of fiber abrasives is for brush speed not to exceed
approximately 3500 surface feet per minute. Optimal speed, however, is determined by
considering the brush construction variables and the parts to be deburred. With abrasive
filaments, increased speeds result in more aggressive cutting action. Slower speeds allow
the filaments to work on all intended surfaces and contributes to extended brush life. The
ideal speed is that which minimizes cycle time and maximizes brush life. Machine
builders, experienced in deburring PM parts, design their equipment to operate at the
most efficient speeds.
EQUIPMENT
As mentioned earlier, both radial and disc brushes are typically applied on a machine
based system. Either brush is capable of deburring the part. The challenge is to present
the part to the brush, or the brush to the part, in a manner which maximizes:
? Productivity
? Quality
? Economics
? Safety
Several machine designs exist to realize these benefits.
1. Rotary Tables
A rotary table is generally designed to apply radial wheel style brushes. The basics of the
system involve an indexing rotary platform with spindles tooled to accept a part or a
closely related family of parts. Parts are typically loaded, deburred and brought back to
the starting point for unloading. The system operates like a merry-go-round. Brushing
heads are positioned around the table to interface with specific surfaces of the part. The
number of heads, or stations, is dictated by the number of surfaces on the part to be
deburred. Generally each station is dedicated to one specific edge plane on the part.
Since unidirectional rotating radial wheels are used, it is necessary to rotate the part while
brushing. As the table indexes to present the part to the brushing station, the spindles
rotate the part. At each station, the parts dwell under the brushing head for a prescribed
length of time or number of rotations. The table then indexes, moving the part to the
subsequent station for further deburring, part turnover or loading/unloading.
These systems can be either run “wet” with coolant, or dry. The effect of coolant on the
brushing action reduces the potential for nylon smear. It also acts as a lubricant, reducing
the cutting action of the abrasive. Systems tend to be designed according to the
preference of the customer. They are successfully applied either wet or dry.
Rotary tables can be designed as manual, semiautomatic, or fully automatic deburring
systems.
A. Manual - Manual rotary tables require an operator to load a part onto the
spindle fixture and activate the indexing sequence. He may also be responsible
for turning the part over and unloading, and monitoring and adjusting the heads
to compensate for brush wear.
B. Semiautomatic - This system may still require an operator, however, tasks
such as part turnover and brush wear compensation may be handled
automatically. The operator may be limited to loading/unloading and
monitoring the overall system.
C. Automatic - Automatic systems are typically incorporated into a production
process, accepting parts conveyed from a prior process. Robotics and/or pick
and place mechanisms are used to load the part, turn the part over, and unload
the part onto an exiting conveyor. Brush wear compensation is done
automatically.
A typical rotary table system is shown in Figures 6 & 7.
.
Figure 6: Rotary table deburring system
Figure 7: Close-up view of rotary table station
2. Disc Brush Systems
Disc brushing systems tend to be the most versatile and productive. By taking advantage
of the multidirectional brushing action of the disc, it is not necessary to rotate the part
during brushing. Part transfer is usually accomplished by means of a magnetic conveyor
system. This elimination of tooling for part fixturing expands the use of the system to
handle a variety of part shapes and sizes with minimal set up between part changes.
Disc brushing systems are designed to present each side of the part to a minimum of two
brushing heads. In passing a part slightly off of the centerline of the disc brush, the part
receives brush work on 270 degrees of its surface. Subjecting it to a second disc, rotating
counter to the first, will deburr the balance of the surface. A third brush can be employed
to ensure complete and thorough deburring.
An alternative method can be employed which involves several brushes on a single head
which operates in a planetary motion. Each brush rotates independently while the entire
head revolves. This provides random brushing action and subjects all surfaces of the part
to fiber contact.
Disc brushing systems lend themselves to full automation. Typically, parts are conveyed
to an accumulator which then drops them onto the magnetic conveyor of the deburring
system. Parts are conveyed under the disc brushing heads, flipped over, then under
another set of disc brushing heads. As the parts exit the system, they are demagnetized
and conveyed onto the next process. These systems typically include automatic brush
wear compensation and are fully programmable for automated set up according to part
numbers. They can be designed to run wet or dry, with the same effect on brushing as the
rotary table systems. Typical disc brush systems are illustrated in Figures 8 and 9.
Figure 8: Automated disc brush deburring system
Figure 9: Close-up of disc brush deburring system
QUALITY, PRODUCTIVITY, ECONOMICS
Quality
Brush deburring of PM parts offers the advantage of only abrading the intended surface of
the part. Unlike vibratory systems where the entire part is abraded, abrasive action can
be concentrated on a specific surface. The light material removal feature of the fiber
abrasive means that sharp edges are radiused with negligible impact on overall part size.
The degree of edge radius can be controlled by brush variables (filament dia./grit size, trim
length, density), and/or system set-up (dwell time under brush, brush speed). With the
proper brush and system, a consistent edge break on all intended surfaces will be attained.
This can be maintained over the life of the brush.
Another quality consideration is surface finish. Again, brush and machine variables will
dictate surface finish. Generally, the right combination will achieve the desired edge break
and maintain or improve surface finish.
Productivity
Productivity is largely dependent on machine design and level of automation. Rotary
tables can range from 150 parts deburred per hour to 600 parts deburred per hour for
fully automated systems. Fully automated disc brush machines attain deburring rates up
to 2000 parts per hour.
Economics
For the purposes of this presentation, cost per part is evaluated strictly from the
standpoint of brush consumption. Brush life is affected by many variables. Severity of
burr, degree of edge radius desired, and brush characteristics play a large role in
determining brush life. Contributing equally, if not more, is the design and operation of
the deburring system. All of these variables have the potential to significantly affect
brush longevity, either positively or negatively.
With this in mind, we have attempted to offer an “average” cost for brushes consumed
per part deburred. This cost is based on a disc brush system deburring 2000 parts per
hour (4000 sides). The average cost is $.02 per part, or $.01 per side.
CONCLUSION
Fiber abrasives are an effective and viable media for deburring PM parts. When formatted
into radial wheel and disc brushing tools, they can be applied on a machine based system.
These systems are designed to present the part to the brush(es) in a controlled and
precise manner. Fiber abrasive brushes, on a machine based deburring system, provide a
productive, high quality and cost effective means for deburring PM parts.
References:
(1.) Watts, J.H., “Abrasive Monofilaments - Factors that Affect Brush Tool
Performance”, SME Deburring and Surface Conditioning Conference, MR89-112, San
Diego, CA, February 13-16, 1989
 
Pub Time : 2014-01-23 20:40:14 >> News list
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