Fiber abrasive finishing systems

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.

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 sawn 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 incosistent:

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.

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 repetitive motion.

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. Developed approximately 25 years ago, 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 coextruded with a mineral abrasive grit. The grit is impregnated throughout the filament as well as exposed on the external surfaces.

As the filament is applied to the workpiece 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 to 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 percent to 40 percent. 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. Despite the fact that 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. 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 of 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 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; Face/Band Width; Trim Length; Filament

Diameter; Grit Size

Density:

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 that 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 (410°F – 482°F). 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.

Face/band width:

The face of the disc brushing tool refers to the diameter of the brush which is occupied by filaments. The bandwidth is a term which describes the distance between the inner ring of filaments and the outer diameter of filaments. The bandwidth 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 tool.

Trim length:

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.

Filament 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 .022”/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 2000 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, results in highly productive deburring with minimal dimensional changes and excellent surface finish for downstream finishing processes like anodization or plating.

Mark Fultz is the president of Abtex Corporation, in Dresden, NY.

For more information visit: www.abtex.com