Single-layered approach: The fundamentals of using coated abrasives in metalworking

Courtesy of Rex-Cut Abrasives

Whether the surface-treatment task involves deburring, blending, grinding, polishing, finishing, dimensioning, patterning or shaping a metal workpiece, there’s a coated-abrasive product available to get the job done. The products include belts, rolls, sheets, pads and flap discs and wheels.

The coated-abrasive name derives from a single layer of abrasive grains being coated, or deposited, onto a flexible or semirigid backing material, using an adhesive, such as resin, to bond the grains to the backing material. This article examines the coating process, various types of grains, or man-made minerals, and backing materials for coated-abrasive products used for several metalworking applications.

Similar to other metal-removal operations, the choice of product depends on the application. “Our salesmen go in, look at the application and decide what the best product to go with is,” said Caitlin Murak at National Abrasives Inc., Lewisberry, Pa. However, operator preference often plays a major role in product selection, she added. “Some people just like a certain brand over another brand and it really doesn’t matter how well it works.” In addition to bonded abrasives and various metalworking products, National distributes an array of coated abrasives, including those from Mirka USA, Norton, Radiac Abrasives and VSM Abrasives.

Applying Abrasives

Two layers of resins create the bonding system for coated abrasives, according to a technical paper from VSM Abrasives Corp., O’Fallon, Mo. The first layer is the make, or base, coat, which anchors grains to the backing. The second layer is the size coat, which is applied over the grains to further anchor and stabilize them.

VSM also stated that grains can be applied via the gravity coating process or the electrostatic coating process. In the gravity method, grains drop from an overhead hopper onto the adhesive-coated backing. In the electrostatic method, the adhesive-coated backing and grains pass through an electrically charged field, which propels the grains upward toward the backing that travels upside down above the grains. The grains are then embedded in the adhesive with the sharpest edge of the minerals exposed to ensure uniform cutting.

With these coating processes, grain coverage can be modified to produce open- or closed-coat products. According to information from Norton/Saint-Gobain, Worcester, Mass., an open coat typically has 75 percent of the backing covered with evenly spaced grains, which is ideal for operations where the grinding debris loads or clogs the surface, reducing cutting efficiency and shortening tool life. When making a closed-coat product, the backing is almost completely covered with grain, which is suitable when loading is not an issue and a fine surface finish is required.

Norton/Saint-Gobain’s Director of Marketing and Strategy David J. Long said the abrasive products manufacturer produces large rolls of coated abrasives, called “jumbo rolls,” then converts them to make the required shapes, such as discs, belts and sheets, in a secondary operation.

Grain Types

The primary abrasive grains for metal-working are aluminum oxide, zirconia alumina and ceramic alumina. Less-frequently applied ones include crocus, a natural abrasive of iron-oxide particles used mainly for cleaning and polishing soft metals, and silicon carbide, a hard and brittle grain for nonferrous metals and hard materials.

“The nice thing about silicon carbide is it breaks down very quickly and easily, producing a sharp edge based on its crystalline properties,” said Jim Schnorr, general manager of Wendt USA LLC, Buffalo, N.Y., and president of the Coated Abrasives and Fabricators Association. “The downside is silicon carbide breaks down very quickly and easily, so, in terms of life, it wears out quickly. However, you can essentially grind or cut anything with it, including difficult materials like titanium and carbide.”

Courtesy of Coated Abrasives and Fabricators Association

Electrostatic coating is the most widely used process for applying abrasive to the backing of coated-abrasive products. The process leaves the abrasive grains standing upright, perpendicular to the backing, with the sharper ends of the grains pointing up and away from the backing.

Long described Al₂O₃ as an entry-level abrasive grain that provides a 20 to 25 percent utilization rate. The blocky grain is tough, meaning it resists fracturing, and is suited to grinding materials that are not considered difficult to machine, such as carbon steels. The next level is zirconia alumina, which he noted Norton invented in 1972 and continues to refine. The self-sharpening grain is well-suited for heavy grinding because the controlled fracturing continually produces sharp, new abrading points.

The top tier, according to Long, is ceramic alumina because it cuts at a higher rate compared to the other abrasives. It is a long-lasting, dense abrasive that produces new, sharp cutting edges as micron-sized particles break off during use. “You get about 80 to 85 percent utilization rate of the grain,” he said.

Ceramic alumina is produced via the seeded-gel process, which grows the grains to specific grit sizes, Long explained, whereas zirconia alumina and Al₂O₃ are “fired,” crushed and screened to achieve the desired grain size. Ceramic alumina and zirconia alumina are sometimes blended, depending on the application. “They work well together for high stock removal,” he said.

Instead of wearing and becoming dull like “old tried-and-true” Al₂O₃, zirconia and ceramic grains break apart to expose sharp, new cutting edges, Schnorr pointed out. “If they’re used properly, a physical reaction takes place that causes microfractures within the abrasive grains. The microfractures are generated by applying pressure to the abrasive media as it is used.”

Backing Options

Used to carry and support the abrasive grains, backing materials come in four types: paper, film, cloth and fiber. Some manufacturers consider a film, or latex, backing to be almost like plastic paper. The order of that list indicates the relative cost of each type, from low to high, Long said. “In many cases, you want to make sure you need the finishing capabilities that film provides. Otherwise, if you use film on a 60-grit disc, for example, you’re not getting a very good finish anyway so why spend the money on a more expensive film backing?”

Long explained that the grit range for film is usually 220 to 3,000, whereas the range for paper is 40 to 2,000, cloth is 24 to 600, and fiber, being the workhorse, is 16 to 80.

Being the lowest-cost option, paper tends to be the least durable, Schnorr noted, adding that a paper backing is often found in hand-use applications, such as sanding.

However, not all paper backings are equal, as they are divided into six weights designated with the letters A to F. A has a weight of 70 g/m², B is 100 g/m², C is 120 g/m², D is 160 g/m², E is 250 g/m² and F is 300 g/m², with E and F generally considered heavy enough for use as belt materials.

Schnorr said a cotton backing is stronger and more durable than paper, but tends break down fairly easily. “The benefit of cotton is it operates very cool and allows heat to dissipate.”

The stronger cloth materials include polyester and polyester-cotton blends. However, Schnorr added that more durable cloth materials tend to hold heat and can cause other problems, such as leaving residue on the work surface or causing heat discoloration.

VSM Abrasives stated that cloth backings are designated by their flexibility, identified as E (extremely flexible), F (very flexible), J (flexible), T (moderate), X (sturdy) and Y (very sturdy). Mechanical flexing of the coated abrasives creates this range of flexibility, and the types of flexes the company employs include (from stiffest to most flexible) single-, double- and full-flex.