June 2001 Manufacturing Engineering Vol. 126 No. 6

Deburring with Nylon Filament Brushes

Nylon brushes not only get rid of burrs, they can improve surface finish

By John Sockman, Weiler Corp., Cresco, PA

Deburring brushes are commonly made with two main filament types--nylon abrasive filament (NAF) and steel wire. There is some overlap in the applications of these two brush materials, but in general, nylon filaments are used in applications involving smaller burrs, specific edge-radius requirements, and surface-finish improvements.

Wire brushes can provide more deburring action than NAF brushes. Aggressively configured wire brushes, however, offer limited compliance and may roughen as-machined surfaces. NAF brushes can perform many deburring and edge radiusing applications involving complex part geometries and stringent surface finish requirements. They can be used either on manual/off-hand setups like bench and pedestal grinders, drill presses, or in automated setups involving CNC machining centers, robots, and automated workstations. NAF brushes replace tedious hand-deburring operations while providing consistent quality, improved productivity, and reduced direct labor content. They also can generate precise edge radii. NAF brushing processes do not require part preparation or post cleaning.

What do they do? NAF filament brushes are not designed to remove material, just burrs. About 30% of the applications involve surface-finish improvement, but brush deburring/finishing will not change a part's dimensions. For example, a surface finish of 40 µin. R_a on an aluminum part can be reduced to less than 20 µin. R_a using a two-stage brushing operation. Additional cycle time and use of coolant can further reduce this finish.

Because NAF brushes are filamentary in nature, they do not function like grinding wheels or coated abrasive products. During use, sharp new abrasive grains are constantly being exposed as nylon wears against the work surface. This provides consistent brushing action throughout the unit's life. NAF brushes only remove very small amounts of material and improve surface finish. The compliance of the filaments and the manner in which abrasive grains are held in the nylon carrier govern their material removal and surface-finishing capabilities.

For instance, an 80-grit coated-abrasive belt and an 80-grit rectangular NAF wheel brush were run on a surface with a 4 µin. (0.1 µm) R_a finish. The belt generated a 100 µin. (2.5 µm) R_a finish while the NAF brush, with aggressive rectangular filaments, produced a 30 µin. or 0.76 µm R_a finish.

NAF brushes are made of heat-stabilized nylon filaments impregnated with abrasive grain in an extrusion process. Working like flexible files, they conform to part contours, wiping and filing across part edges and surfaces.

Nylon, the brush filament material, has excellent toughness and fatigue

properties as well as moisture, abrasion and chemical resistance. Its good "memory," or ability to return to its original position after being deformed, is critical in brushing operations. Three types of nylon--6, 66, and 612--are commonly used. Of these, 612 offers the most heat resistance and is preferred in industrial applications. Normal percentage of abrasive grit weight to total filament weight is 20 - 40%.

What brush shape? The general types of brush shapes are wheel, disk, tube, and end. Which one to use depends on workpiece configuration and burr location. Disk brushes are used when all burrs are in the same plane, such as in a face-milled housing, for example.

End brushes are similar to disk brushes in the types of burrs they address, but they are commonly used to reach into smaller areas.

Wheel brushes deliver a large amount of mechanical energy to a targeted area. They can be used in a wide variety of applications such as gear deburring and bore deburring where bore size is >1.5" (38 mm).

Tube brushes are suitable for applications involving holes with diameters of <1.5". They can remove light burrs or surface contaminants.

After the correct brush type is selected, brush size and filament type need to be engineered to specific application requirements. Brush diameter depends on the size and shape of the workpiece and on process/equipment constraints such as rpm limitations, guard clearance, and space limitations between brush face and work surface.

Generally, larger-diameter wheel brushes offer better production stability, lower consumable cost/part, and shorter cycle times. The only time small brushes are recommended is when a physical constraint on the part or the existing equipment precludes the use of a large-diameter brush. Large wheel brushes are generally considered to be in the size range of 10 - 14" (255 - 356-mm) diam. Large disk brushes range from 6 - 14" (152 - 356 mm) in diameter.

An additional factor when selecting brush diameter is filament length. Brushes with longer filaments are more compliant and capable of absorbing abrupt changes in part geometry. Brushes with shorter filaments are used in applications requiring maximum aggression and filament density.

What filament size and shape? Brush selection depends on the "aggressiveness" and "conformability" needed. Aggressiveness is controlled by grit size of the filament as well as the filament stiffness as it impacts the part surface. Conformability relates to how readily the filament bends or conforms to the geometry of a part's surface.

Brush conformability is needed to accommodate part contours. Smaller-diameter filaments offer more compliance and are required to reach burrs in tight areas with poor accessibility. However, they have less rigidity and therefore less aggression.

Filament shapes available are round crimped, round straight, or rectangular. Rectangular filaments, having a larger cross section, are stiffer than round filaments and therefore more aggressive. Rectangular-filament brushes are used when brush conformability to parts is not an issue. A rectangular filament with 80-grit abrasive grains provides the most aggressive brushing. Applications on cast iron and steel normally require this type of filament.

Round straight filaments are more compliant than rectangular filaments, and are intended for lighter deburring applications on complex parts with elevation changes, small holes or slots, or other intricate features. They are effective on softer metals such as aluminum and brass.

Round crimped filaments offer the highest conformability. Brushes with 120-grit crimped filaments are a good starting point in applications involving aluminum. Finer grits can be used in applications with more stringent surface-finish requirements.

What type and size of grit? Abrasive grits commonly used in nylon filaments are silicon carbide and aluminum oxide. Another alternative, cubic boron nitride (CBN), is rarely used because of its high cost and the fact that the fiber wears out before the grit.

Silicon carbide is used for all general-purpose applications--about 90% of all NAF jobs. Silicon carbide has excellent hardness, toughness, and sharpness, and contains <0.1% iron oxide and no free iron. Therefore, silicon carbide abrasive filaments can be used on nonferrous metals such as aluminum.

Aluminum oxide is used only in cases where silicon carbide causes part discoloration or raises contamination concerns in certain nonferrous brushing applications. It is tougher than silicon carbide, less likely to fracture, and is used for finishing softer metals. This abrasive is also used when carbon contamination may be a problem, as in aerospace and biomedical applications.

Abrasive grits used in round filaments generally range in size from 46 to 600-grit; in rectangular filaments, sizes vary from 80 to 320-grit. An 80-grit is recommended for all applications except when:

• Processing delicate parts and softer metals such as aluminum and brass,
• Deburring parts with edge radius specifications below 0.003" (0.076 mm), and
• Producing a specific surface finish.

Dry or wet? Brushes can be run dry, but in most CNC applications the brushes use the same coolant as conventional machining. Use of coolant will normally result in more surface-finish enhancement than dry brushing. Oil-based coolants, however, can detract from brush aggression by lubricating the filament/part interface.

What machines? NAF brushes can be used in manual systems, but the trend now is to incorporate the brushes in CNC machines. With the CNC the brush is just another tool in the magazine, one called into action at the end of the conventional machining cycle.

In high-volume production, it may be practical to use brushes on dedicated brushing equipment. These machines usually have movable, multiposition tables that move the parts under shaft-mounted brushes.

Robotic applications are also popular. Either the robot moves the brush over a fixed part or the robot moves the part into a brush that's in a fixed position.

Operating parameters. As with most machining processes, the three main operating parameters are speed, feed, and depth of interference (DOI), which is analogous to depth of cut when machining.

Brushes should be operated at surface speeds below 3500 sfm (1067 m/min) to prevent overheating and smearing of the nylon onto the work surface. The single most common error in the use of NAF brushes is running them too fast. A 6" (152-mm) diam brush running at 1200 rpm, for example, is much more effective than one running at 2500 rpm. Excessive speed causes the filaments to bounce off the workpiece instead of wiping and filing across the part surfaces and edges. Low surface speeds are a requirement when brush conformability is important.

Suggested spindle speed for a tube-type brush is not to exceed 2000 rpm. End brushes can handle spindle speeds to 10,000 rpm. Generally, higher spindle speeds improve brush aggression while lower speeds enhance conformability.

Spindle speed is generally decreased with increased DOI so the filaments can conform smoothly to part contours. Such a combination ensures that filaments are not hitting the part and bouncing off its surfaces, but are wiping and filing across its surfaces and edges. This brushing action also contributes to longer brush life. Therefore, contoured surfaces are processed at slower speeds and greater DOI than flat surfaces.

Feed rate and DOI are determined by the amount of deburring, edge-radius making, or surface finishing required, as well as type of material processed. Either parameter can be increased to improve the aggression of a brushing application. With disk brushes, 0.100" (2.5-mm) DOI and 20 ipm (508 mm/min) are good starting points. In the case of wheel brushes, 10% of the filament length and 20 ipm provide a good guideline.

Burr size and workpiece material will dictate whether the system needs to be made more or less aggressive. For example, applications involving light aluminum burrs are often processed at 0.080" (2-mm) DOI and 50 ipm (1270 mm/min) feed. The incoming burr condition and the finished part specification will dictate the specific parameters required.

More automation. One of the strongest growth areas for NAF brushes is in automated operations. An increasing need for consistency and precision and a simultaneous drive for reduced direct labor content is creating a boom of automated deburring activity. Brushes have a strong advantage in automated deburring operations because brush conformability essentially eliminates errors which can result in scrap. In addition, brushes are easily implemented into single-part-flow environments, which can eliminate the batch and queue required by many other automated deburring systems.

The compliant nature of brushes also makes them relatively easy to implement because they do not require ultra-precise programming or fixturing. If programming or fixturing varies slightly, the brushes will not damage the parts. For example, it is possible to deburr a gear using a carbide tool that travels all around the teeth. This takes a complex program and careful part fixturing. A brush can be off by 0.020" (0.5 mm) and still remove the burrs in less time than the single tool takes. Brushing also eliminates secondary burrs that can occur in carbide deburring operations.

Another issue is the high value of parts. Most of the value added to a part has occurred by the end of the machining operation with deburring being a last step. Part-destroying errors are more likely to occur if the deburring operation is manual. Automation eliminates the need for inaccurate hand labor. Ergonomic problems associated with repetitive work, such as carpal-tunnel syndrome, also are driving users away from manual deburring.

Air Compressor Problems

The manufacturer of rotary screw air compressors wanted to replace hand deburring with an automated method. Materials to be deburred were high-grade cast iron and alloy steel. The hand deburring operation was labor intensive and dirty, and some parts were damaged by operator error. The unique requirements were:

• The castings were painted before machining, and the deburring operation could not remove the paint, and
• The deburring operation could not produce a large edge break that could create a leakage path in the compressors.

Disk-type brushes in 90 and 130-mm diameters provided deburring of the compressor housings, eliminating handwork and the associated scrap and rework.

The manufacturer also wanted to refine the surface finish of the rotor bores. The company decided to use abrasive nylon filament brushes in various configurations to replace hand deburring.

Compressor housings were mounted to tombstone fixture for automated NAF brush deburring and finishing of bore IDs.

Housings were mounted to a tombstone fixture for machining and deburring. Two sizes of disk brushes were used to access different part surfaces. A 90-mm-diam brush was operated at speed of 1400 rpm, feed of 2250 mm/min, and DOI of 2 mm. Operating parameters for a 130-mm-diam brush were: speed,1200 rpm; feed, 2250 mm/min; and DOI 2 mm.

To finish rotor bore IDs, the manufacturer selected NAF wheel brushes and programmed the machine to interpolate the bore ID. This operation also required two brushes. One was 80 mm in diameter, and was operated at speed of 1500 rpm, feed of 1500 mm/min, and DOI of 2 mm. The second brush, a 63-mm-diam unit, was operated at identical speed, feed, and DOI.

The brush deburring eliminated hand deburring of the housings and the scrap and rework associated with hand work. Painted surfaces were undamaged, and surface finish on the rotor bores was reduced from 64 to 40 µin. (1.6 - 1 µm) R_A. Machining cycle time increased 3%.