Abrasives for Various Applications
Abrasives are commonly composed of hard minerals used in a variety of industrial and domestic applications for woodworking and metal work to shape or finish a piece of work. Abrasives can be used by hand or machine to rub away pieces of wood or metal for a smooth, polished, or finished look or to wear away a piece of material gradually until the desired shape is achieved.[[wysiwyg_imageupload:144:]]
Abrasives are classified into two main categories: natural and synthetic. Synthetic abrasives are produced to mimic organic material and are manufactured rather than mined. Both types of abrasives are used in a wide range of shapes and are most often coated or bonded. Abrasives come in the form of grinding wheels, belts, rods, discs, blocks, loose grain, sanding sponges, and sheets.
Bonded abrasives are those that are formed into grinding wheels, cut off wheels, segments, cones, and other such formed or “bonded” types used for different aspects of wood working and metal finishing or cutting applications. Bonded abrasives are commonly used with a drill or rotary tool. The reason for bonding abrasive material (natural or synthetic) together is so the grain particles hold together to provide a hard material for the purposes of cutting or grinding (Carborundum Industrial).
Coated abrasives are minerals that can be used similarly to bonded abrasives (Cumi Mugugappa). Open and closed coat are the two types of abrasive coatings. In an open coat abrasive, 50% or 75% of the surface is covered with an abrasive grain. These grains include materials such as emery, garnet, silicon carbide, light brown aluminum oxide, brown aluminum oxide, heat treated aluminum oxide, zirconia lumina, and ceramic alumina (Carborundum Industrial). Coated abrasives have even spaced voids between grain particles to help reduce loading caused by the abrasive rubbing against the material it Is used on such as metal shavings or wood dust. Closed coat abrasives do not have any voids and are completely covered with grain. This type of coating is the most typical and has the largest degree of removal and has a longer life than open coat.
Abrasive backing types are paper, cloth, film, or fiber. Coated abrasive construction consists of the backing, adhesive, and the minerals. The minerals or grains are typically “glued” to the backing and most often include both a make and size coat. A make coat is applied to the backing then the mineral or grain is applied, followed by a size (or top) coat of adhesive.
Paper backings come in six weights or thicknesses: A, B, C, D, E, and F. A and B weight are lightweight papers used for fine to medium grade grains. These are commonly used for hand sanding and handheld electric sanders. B is slightly thicker and more expensive than A weight paper.
C and D weight paper backings are an intermediate weight thickness that are mainly used for medium grade grains and are typically used for sheets, discs, and light duty sanding belts. E and F weight papers are thicker and more durable and are able to support the heavy sanding required for heavier grains. E and F are used mainly for mechanical sanding and take the form of file board paper, belts, and discs. F weight is also used for drums and cones.
Cloth backings include cotton, rayon, and polyester. A backings thickness and stiffness are measured in weights J, X, Y, M, S, T and Z. J weight is the lightest and most flexible of the cotton backing. X weight is medium and handles the widest application range such as drills. Y weight is used for heavy drills designed for heavy duty applications. M weight is one of the heavier cotton cloth backings used for more extreme heavy duty applications. A and T weights are used in extra wide cloth belts and offer the optimal strength in both weave directions with T weight being used with high horsepower equipment. Rayon is strong and flexible and resistant to tearing and fraying at the edges. Polyester is an extremely strong and tough backing used in applications that require additional strength and a water resistant cloth backing. Fibre backing is hard and stiff but flexible enough for drum and disc sanding operations made from rag stock. Polyester film is another yard and flexible backing that is used for very fine to medium grain discs rolls, and belts. Film products are often used in conjunction with water based solutions as the backing is completely waterproof.
Woodworkers do not typically need heavy duty industrial abrasives for projects and most often use sandpaper for use in orbital or palm sanders. Aluminum oxide, silicon carbide, ceramic, and garnet are the typical abrasive materials in fine to heavy coats. Open coat sandpaper typically with d weight paper backing is ideal for most woodworking applications.
Aluminum oxide is a sharp grain that contains the hard, tough, wear resistant material woodworkers most commonly use. This is the only abrasive mineral that “fragments under the heat and pressure generated by sanding wood” (Fine Woodworking). This is known as friability and is desirable to woodworkers as it is not designed to destroy or grind down on wood while remaining sharp and lasts longer than other minerals. Although this material fragments under heat it does not dull very much. This material is best for smoothing wood as well as removing paint. A special zinc treatment can be applied to make it less susceptible to getting clogged with debris when using it for paint removal.
3M is the largest manufacturer of aluminum oxide sandpaper and sanding products. 3M manufactures 26 various kinds of aluminum oxide products that vary in friability and toughness. Their toughest grades are a light nearly white color while their more softer grades are dark brown and are used more for sandblasting rather than sanding applications.[[wysiwyg_imageupload:143:]]
Silicon carbide is a black and iridescent mineral that is shard shaped. This is a synthetic mineral that is brittle and fractures into sharp slivers as it is broken down during use. This abrasive is used for sanding paints and primers as well as non-ferrous metals such as brass, stainless steel, and titanium. This abrasive is good for wood sealer and final finish sanding. These abrasives are most commonly found in paint and auto-body stores for their use on paint.
Ceramics are not friable and their edges to not renew when working with wood (Fine Woodworking). These abrasive materials are not converted with high heat which requires it to be manufactured with fewer impurities that can weaken the grain. Ceramic structure tends to be more uniform than minerals fused with high heat. Cubicut is the trade name for ceramic abrasives and most commonly consist of Cubitron. When cubitron is mixed with aluminum oxide it is retailed as Regalite. Other ceramic trade names include alumina circonia, AZ, and Norzon. The reason behind these various trade names is copyright protection of the manufacturer. Ceramic is becoming increasingly popular and most major coated abrasives manufacturers have developed ceramic based products.
Garnet is a natural abrasive material that is the only one widely used in the woodworking industry. Its reddish brown color consists of a medium hardness that fractures well so it can form new cutting edges for use in light to medium woodworking applications. This mineral is optimal for edge-grain wood sanding. Garnet dulls quickly which is not a defect but a trait that makes it excellent for producing a very smooth finish on wood and does not leave pigtail-like scratches like aluminum oxide can when using a random orbital sander (Fine Woodworking). Garnet is a great choice for final sanding as it has a tendency to burnish wood allowing a stain to penetrate the surface evenly instead of deep which can leave darker patches in some areas. A fine grit paper such as 150 or finer with paper weight a works best for final hand sanding with garnet.
Several abrasives exist for various applications in woodworking. Selecting the right one will ease frustration and ensure you achieve the optimal results. Choosing to sand by hand or electric sander depends on the surface of the wood you are working with. Whether removing paint, stain, or smoothing down wood always consult the manufacturer’s recommended safety tips when working with an electric sander.
Carborundum Industrial. (n.d.). Bonded Abrasive Basics. Retrieved from http://www.carborundumindustrial.com/BondedTechnicalInformation.aspx
Cumi Murugappa. (2008). Applications of Abrasives. Retrieved from http://www.cumi-murugappa.com/useful-articles/abrasives-applications.html
Fine Woodworking. (n.d.). Four abrasive minerals. Retrieved from http://www.finewoodworking.com/pages/w00006_sb1.asp