The finishing touch—selecting and using nonwoven abrasives
Nonwoven abrasives come in different types and compositions. It’s important to choose the right product for optimal finishing.
Nonwoven abrasives—abrasive grain-infused nylon fibers bonded together with synthetic resins—are used to prepare and apply a product’s final finish. You must choose these abrasives wisely for your specific application and use them appropriately to achieve optimal results.
Constructed with an open mesh, nonwovens are waterproof, washable, and resilient. They are load- and rust-resistant and nonconductive, and can be used wet or dry on most metals to achieve a uniform finish, even on irregular surfaces. Little if any smearing and discoloration are associated with nonwoven abrasives.
The Nitty Gritty
Nonwovens contain many of the same abrasive minerals as conventional coated abrasives, such as silicon carbide, which is sharp, cuts fast, and produces fine scratches on most surfaces, and aluminum oxide, which is durable, aggressive on some materials, and causes less discoloration on aluminum.
Grit refers to the size of the abrasive grain in the nylon web—the lower the number, the larger the grains. A large grain cuts more aggressively and creates a coarser finish. A smaller grain typically produces a finer finish.
While conventional bonded or coated abrasives are chosen by grit size, nonwoven abrasives are specified by grit designations and ranges:
Coarse (C) 50 - 80
Medium (M) 100 - 150
Fine (F) 180 – 220
Very Fine (VF) 240 – 360
Ultra Fine (UF) 600
Micro Fine (SF) 1,000 – 1,200
Density, Sizing, and Bonding Agent
Density—the amount of abrasive grain and bonding material compressed into the product—is another consideration when selecting a nonwoven abrasive.
Higher-density abrasives cut faster, last longer, and produce finer finishes than softer-density types. However, softer densities conform better to the work surface and are less likely to load or burn the workpiece.
Other factors that influence nonwoven abrasive selection are the nylon fiber size and the bonding agents used to join the fibers together. Both help determine the abrasive’s performance.
Specifications on an abrasive product denote its density—from 2 to 9, with 9 being the densest—abrasive component, and grit designation. For example, the specification 8 A/O MED means that the density is 8 (on the high end of the density range); the abrasive is aluminum oxide; and the grit is in the medium range—from 100 to 150.
Different Products for Different Applications
Nonwoven abrasive products are available in hand pads, rolls, discs, belts, and wheels.
Designed for hand finishing, hand pads have good conformability and flexibility. Available in coarse to microfine grit, they usually measure 6 in. by 9 in. and can be folded into a smaller size if necessary. Because they are rustproof, pads are a good alternative to steel wool in wet applications.
For larger sanding applications that can be done by hand or with a jitterbug, nonwoven rolls are suitable. They typically are available in 30-ft. lengths and 4-in. to 6-in. widths. You simply cut the roll as needed.
Nonwoven discs come in various designs. High-speed grinders use arbor hole discs. An extra-coarse arbor hole disc provides an effective means to remove surface contaminants, such as rust, residue, corrosion, and light weld spatter. You can gang these discs together with spacers to create a wide wheel for sharp-edge radiusing or deburring.
Right-angle discs are used on portable grinders for cleaning, deburring, blending, and finishing, as well as removing light rust, oxidation, and coatings. Transparent discs that feature a triangular profile combined with holes in the discs and backup pads provide a clear view of the cutting surface. This enhanced visibility enables better process control. The disc holes also disperse heat and loose grinding material.
Nonwoven belts designed for portable, bench, and pedestal bench machines are suitable for cleaning, buffing, and polishing. Because they are highly conformable, these belts cause minimal gouging and produce a more consistent finish.
Nonwoven wheels fall into three categories: flap, convolute, and unified.
Flap wheels, which are made by mounting sheets of nonwoven material around a center hub, are the best option when surface conformability is a requisite or when a long-line brushed finish is desired.
Convolute wheels, created by wrapping nonwoven material around a center core and bonding the layers together, are easily shaped to match preformed parts.
Suitable for general cleaning and deburring, unified wheels are formed by compressing layers of nonwoven web material and bonding them together to form a wheel.
Wheels require more careful setup and operation than other nonwoven abrasive types. Following these guidelines will help you produce a high-quality finish and extend wheel life.
Direction—Convolute wheels must be run in the direction indicated by the arrows printed on the wheel’s side. You can run flap and unified wheels in either direction.
Speed—Using the correct wheel speed for your application is critical to achieve the desired finish and maximize wheel life. A fast speed typically is more aggressive and creates a finer finish. A slower speed is less aggressive and produces a coarser finish.
Recommended speeds for some common applications are:
- Cleaning and upgrading surface conditions: 2,200 – 6,000 SFPM
- Buffing cuts on metal surfaces: 6,500 – 8,000 SFPM
- Deburring: 5,500 – 8,000 SFPM
- Decorative finishing 500 – 3,000 SFPM
- Removing oxide: 3,500 – 6,500 SFPM
Pressure—Pressure is another critical factor. Applying too much can cause wheel deformation and damage the workpiece.
How much to apply depends somewhat on the wheel type. For example, flap wheels require much lighter pressure than others. Unified wheels can be operated at much higher pressures when deburring.
Feed Speed—Feed speed has an impact on production. A slower speed achieves a shorter scratch pattern and finer finish, but produces fewer parts per hour.
Oscillation—Moving back and forth with a steady rhythm (oscillations) helps obscure scratch lines to produce a more consistent finish. Note that oscillation also can increase the cut. The recommended starting point for oscillation is 3/8-in. amplitude at 200 cycles per minute.
Lubrication—Using lubricants decreases the heat generated by the abrasive process and helps improve the finish. The higher the lubricant’s viscosity, the lower the surface finish, as measured by the root mean square (RMS) value. The lower the surface finish, the smoother the product.
The nonwoven product you choose depends on your specific application. Your abrasive supplier can help you determine which product is right for you.