By Imre Karetka, Osborn Product Engineer
The unavoidable and unintentional byproduct of manufacturing practices is the generation of burrs and sharp edges. Besides the associated safety hazards, the presence of these features adversely affects the functionality and performance of the part. Manufacturers have resorted to a variety of methods to remove these unwanted attributes, including grinding, tumbling, sandblasting and brushing. Brushes offer a number of advantages over other methods, such as flexibility, durability and repeatable uniform results. Furthermore, the cost associated with the purchase of equipment is minimized by using power brushes on current equipment.
The industrial applications of brushes vary from paint removal and deburring to honing and polishing. These surface-conditioning tools can be used on practically any material imaginable, including aluminum, steel, carbide, plastic, wood and glass. The physical designs, as well as some of the available adapters, make these tools readily adaptable to machinery that is widely used in today’s manufacturing environment.
Although there are a number of different brush types and configurations available, the two major components that make up a brush are the mounting hardware and the filament type. 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. The most commonly used filament types are abrasive nylon and crimped wire filaments.
Wire filaments vary from steel, stainless steel, bronze, brass and others. The most important trait of wire filament is wire hardness. Generally, carbon steel wire has a hardness of Rc 55-60, and stainless steel wire runs between Rc 30-35. Brass, bronze and beryllium copper filament are softer, and are used for softer materials (e.g. rubber or aluminum).
With wire filament, 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. As the brush wears down in normal use, pieces of wire break away, leaving new sharp edges exposed. The crimps in crimped wire filament help to create clean breaks.
For extra heavy-duty applications, wire filaments are twisted together to form multiple knots. The knots create extra support for the wire tips, resulting in higher aggression for applications such as weld cleaning and rust removal. Tighter knots make a more narrow brush face allowing it to reach into corners and cracks.
A common application problem with wire brushes is over-penetration, when 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, this can often be corrected by decreasing the brush penetration.
Abrasive Nylon Filament
Abrasive filaments use a nylon carrier with silicon carbide, aluminum oxide, or polycrystalline diamond abrasive grit. Depending on the grit size, these filaments can be used for deburring, polishing, blending, and many other surface conditioning applications.
Nylon filament often has additives that aid with heat transfer and moisture absorption. Heat is a significant limiting factor during application. The use of coolant 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 410° F. At 210° F, the abrasive filament looses 70% if its stiffness and will not perform as aggressively and predictably as it had at the start of the application. To counter this, the use of side plates or bridles are suggested.
Due to the shape of the crystals in abrasive grit, a silicon carbide filament is more aggressive than an aluminum oxide filament. The most commonly used abrasive grain sizes vary from 46-grit (coarse) up to 1000-grit (fine). For an aggressive deburring application, the use of coarse abrasive grain size is recommended, whereas the 1000-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 in more jagged, it tends to break off and embed into the work piece. A number of fabricators in the aircraft industry insist on using aluminum oxide rather than silicon carbide to prevent this.
Natural filaments include tampico, sisal and horsehair. The use of these filaments varies according to application. Generally these filaments are not abrasive by themselves but are used for polishing and cleaning applications in conjunction with an abrasive compound.
When selecting a brush for an application, a number of questions need to be addressed. First, what end is being pursued; e.g., deburring, edge breaking, polishing, cleaning, etc? Second, what is the material that needs attention?
For instance, if the goal is to deburr a stainless steel tube that was saw cut, either an abrasive or stainless steel wire brush should be used in order to reduce contamination on the work piece. A carbon wire brush would deburr the part faster, but it would also leave traces of carbon steel that would cause the part to oxidize. If the tube is harder than Rc 35, the only remaining option is the use of abrasive filament. Depending on the equipment used, either a cup or radial brush will perform the job.
Trim length of the brush filaments is another important factor to consider. Brushes with longer trim lengths are less aggressive, but they have the flexibility to reach into curved or recessed areas. Short trim lengths create brushes that are more aggressive but less flexible.
Filament diameter is another consideration. Wire sizes range from 0.003” to 0.020”, and abrasive nylon sizes are from 0.012” to 0.090”. Typically, a thicker filament is more aggressive. However, with wire, sometimes using a thicker filament has the opposite effect because the filament bounces off of the part. Abrasive nylon is also available in rectangular filaments, which can be much more aggressive than round filaments because there is more surface area in contact with the part.
Filament density also varies. Brushes with more dense filaments are more aggressive because there is more weight to support cutting points. This can result in increased heat in nylon abrasive brushes, which should be countered with the use of coolant.
Usually the largest brush the equipment can handle is recommended. The benefits of this are twofold. 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 is that the brush will have to be changed less frequently, therefore causing less downtime for the machine.
As with any power tool, safety is a major concern when it comes to brush 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 high speeds, these pieces have enough velocity to puncture clothing and skin. When using brushes, it is a good idea to wear safety glasses, leather gloves and apron, a full-face shield and work boots.
Because of the wide variety of brush configurations that are available, there can be many answers to production requirements. It can be beneficial to discuss your part finishing needs with an engineer experienced with using power brushes.
Tips of the Trade:
- With wire brushes, let the brush do the work. Too much pressure is inefficient.
- With abrasive nylon brushes, filament sides and tips perform work. Use deeper penetration and slower RPM than wire brushes.
- Maximum Safe Free Speed ratings are not operating speeds. Half of MSFS should be sufficient -- if not, rethink your process.
- Use the largest diameter brush your machine can handle. You’ll get more work per HP with less wear-and-tear.
- Reversing brush rotation can remove burrs the first pass may have missed.
- Keep your cutting tools sharp. Fewer burrs means less deburring.
- Always wear safety equipment!
Information used with permission of Osborn ©2007