Finishing System for End-Deburring Extruded Aluminum Profiles
The expanding use of aluminum extruded shapes is placing higher quality demands on extruders and fabricators. Electronics related applications such as heat dissipation components (heat sinks) require that there be no loose flakes or burrs. Other applications with functional considerations, as well as safety and esthetic, are forcing the industry to supply saw cut ends which are burr free and slightly radiused. Fiber abrasive systems are ideal for end deburring aluminum extrusions; when formatted into disc brushing tools, they can be applied on a machine-based system for manual, semi-automated, or fully automated end deburring of aluminum extrusion ends. Benefits include increased productivity, safety, and consistently high quality levels.
Off-hand deburring is inconsistent
End deburring aluminum extrusions
has typically been an off-hand process. An abrasive media such as a wire,
abrasive filament, buffing, or non-woven wheel is rotated and the extrusion is
presented by hand for deburring as shown in Figure 1.
In order to
completely deburr the profile, the operator must present it in such a way as to
allow the abrasive to strike and wipe against each inside and outside
dimensional edge. This necessitates repositioning the part and varying the
angle of presentation. As this entire process is dependent on operator skill,
inconsistent finishing is often the result. Variables such as depth of penetration,
dwell time, and angle of attack are all uncontrolled. Furthermore, because this
is generally a high volume process, operators are subjected to prolonged
(Figure 1 here. Cut: Off-Hand
Deburring of an Aluminum Extrusion)
In 1980, a process was developed to improve the operation of end deburring aluminum extrusions using abrasive nylon filaments formatted into a disc brushing tool.
Understanding fiber abrasives
The term “fiber abrasive” is used to describe an abrasive nylon filament. They have been used in brush form for a variety of industrial applications, generally involving deburring, edge radiusing and surface finishing. The filament is composed of heat stabilized nylon which has been co-extruded with a mineral abrasive grit. The grit is impregnated throughout the filament as well as exposed on the external surfaces as shown in Figure 2.
(figure 2 here. Cut: Close-up of magnified
As the filament is applied to the work piece and begins to wear, new abrasive grit is exposed. The filament is, in effect, self-sharpening. Abrasive action occurs on both the tip and the sides of the filament. Slower R.P.M.’s are employed to allow the fiber to strike and wipe against the surface. This, combined with the flexibility of the fibers, makes it ideal for finishing irregularly shaped objects. Abrasive options are, for the most part, limited to silicon carbide and aluminum oxide. Other, more exotic abrasives are available, however their expense limits their use to very specific applications. Grit sizes range from 600 through 46 (grit is specified as the mesh number used in abrasive separation). Filament diameters range from .018” - .060”. 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%. In both ferrous and non-ferrous applications, silicon carbide is the most widely used fiber abrasive. For aluminum applications specifically, there is no threat of corrosion from iron contamination.
The fiber abrasive is not considered
a material removal tool. Even though a large grit size can be applied (up to 46
mesh), the flexibility of the filament limits its cutting action. The fiber
abrasive will remove some material, but at a minimal rate. For this reason,
burrs and sharp edges are preferentially abraded away. This enables the tool to
deburr without affecting the 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 therefore include these familiar types seen in Figure 3.
Image 3 here. Cut:A selection of
brushing tools using abrasive filaments.
Brushes of these types are commonly applied
with hand tools, manual stationary equipment (drill press, pedestal grinder),
as well as semi-automated (CNC, NC, robotics), and dedicated finishing systems.
Aluminum Extrusion End Deburring
For end deburring aluminum extrusions, two brush formats are generally used; the radial wheel or the disc.
The radial wheel, as the name
implies, employs fibers extending radially from a hub. The brush is commonly
mounted on a horizontal shaft and rotated in a direction which causes the
fibers to strike the part in a downward motion. End deburring with a radial
wheel is predominately an off-hand procedure. For a rectangular profile, for
example, the operator presents the profile to the wheel in roughly a
perpendicular angle. The brush tips contact the upper horizontal edges of the
profile, deburring the upper outside edge and the lower inside edge. The part
would then need to be rotated 90 degrees a total of three additional times and
presented to the wheel in a similar manner in order to completely deburr the
part. Although use of the fiber abrasive radial wheel is an ideal choice for
off-hand deburring of aluminum extrusions, the process limits productivity and quality
is subject to operator skill.
A more efficient format for end
deburring aluminum extrusions is the disc brushing tool. The disc is
constructed of a backing into which the filaments are embedded. The fibers
extend perpendicularly from the backing. Unlike the unidirectional rotation of
the radial wheel, the disc offers multidirectional wiping action. To take
advantage of the disc format, the disc is rotated on the vertical plane. The extrusion
end is presented, in a controlled manner, perpendicular to the face of the brushing
tool as shown in Figure 4.
Image 4 here Cut: An extrusion being
presented to an abrasive filament disc brush for deburring.
The end of the extrusion is then
passed from left to right through the top half of the brush. With a counter
clockwise rotation of the disc, as the extrusion enters into the face, the
fibers are in a downward motion. This deburrs each horizontal upper edge of the
profile. When the part moves to the center point, the fibers are now traveling
from right to left. The filaments impact and deburr the right facing, vertical
surfaces. As the shape moves to exit the disc, the fibers are traveling from
bottom to top. The bottom horizontal surfaces are now deburred. The extrusion
is then brought back through the lower half of the brush. In the center
position, the fibers are wiping from left to right and thus deburr the
remaining left facing vertical edges. This process offers 360 degree deburring
regardless of profile geometry.
Disc Brush Construction
Variables in disc construction
affect its performance in this application. The quality of the process is
dependent upon optimizing each of these variables in relationship to each
other. These are:
Density refers to the number of
individual filaments across the face of the brush. While 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 because individual
filaments need to flex in order to provide a wiping action which will follow
the contours of the profile. Heat dissipation is also critical in order to
avoid a condition referred to as “nylon smear”. The melting point of the nylon
used in these filaments is in the range of 210 degrees Celsius -
250 degrees Celsius. Extreme density, depth of penetration, or dwell time can
generate heat sufficient to melt the nylon. The melted nylon would then be
transferred onto the part where it cools and bonds. Subsequent anodizing will
reveal this phenomenon.
Employing a disc brushing tool with
too little density will require prolonged dwell time. Individual filaments are
required to work harder with less support. This leads to premature filament
breakage and reduced brushing tool life.
The face of the disc brushing tool
refers to the diameter of the brush which is occupied by filaments. The band width
is a term which describes the distance between the inner ring of filaments and
the outer diameter of filaments. The band width determines the overall profile
height which the brushing tool is capable of deburring. A 12 inch disc brush
with a 4.5 inch band width is able to effectively deburr a profile no taller
than 4.5 inches. Taller extrusions would require a larger diameter brushing
The trim length is the length of the
visible filament, or the distance from the tip of the filament to its base.
This affects how aggressive the brushing action is. Generally, with all other
variables fixed, the brush becomes more aggressive as the trim length is shortened.
With proper density, a brush is rarely too aggressive for 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 brushing
tools’ face. This is largely counterproductive.
Diameter and Grit Size
In many applications, it is often
most effective to use a smaller diameter filament. This is true for end
deburring aluminum extrusions. 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. Remember, grit
size and filament diameter are related. The most effective combination for end
deburring aluminum extrusions is .028”/120 grit.
Rotational speed of the brushing
tool also is a critical factor in the process. The rule of thumb for the
application of fiber abrasives is for speed not to exceed approximately 3600
surface feet per minute. Optimal speed however, is determined by considering
the brush construction variables and the parts to be deburred. Experience has
shown that for a 12 inch or 14 inch disc brushing tool, speeds between 1000 RPM
and 1600 RPM are optimal. Speeds in excess of this can result in nylon
smearing. Slower speeds will extend cycle times.
Although simple in concept, abrasive
filament deburring is a complex process controlled by multiple variables.
Selection of the proper product, combined with sound operating procedures or
automation, result in highly productive deburring with minimal dimensional
changes and excellent surface finish for downstream finishing processes like
anodization or plating.