Advances in coated abrasives boost cleaning room options.

This article outlines coated abrasive grinding belts and discs and how to determine if they might enhance your cleaning operation.

Once thought of as useful only for grinding aluminum and other nonferrous castings, coated abrasives have grown up in recent years. As backing materials have become stronger, joint technology has improved and new abrasive groins have become available. In fact, coated abrasives can now compete with grinding wheels (and, in some cases, even milling machines) for ferrous alloys - even the high alloy steels.

These advancements have brought coated abrasives from their first use in foundries in the 1930s to a $55 million foundry market today. In the right circumstances, coated abrasives offer higher metal removal rates with less heat generated, resulting in greater energy efficiency and less operator fatigue.

With finishing and grinding estimated to account for 15-30% of the total cost of casting production, it is imperative that all materials and methods be explored to achieve maximum efficiency. A basic understanding of the advances in technology and materials can help foundrymen determine if coated abrasives will be useful in their operations and which products will provide the best service for [TABULAR DATA FOR TABLE 1 OMITTED] their particular applications.

Coated Abrasives

A coated abrasive is an abrasive grain bonded onto paper or cloth backing. But today's coated abrasive is much more than mere sandpaper. Rolls of abrasives are cut to form products such as cartridge rolls, resin fiber discs, cross pads, flap wheels and grinding belts of all sizes.

Backing Materials - Prior to the 1960s, most grinding belts were backed in cotton cloth. Polyester backings (developed in the 1970s - 80s) are one of the technological advances that has allowed grinding discs and belts to be used today in applications and on metals formerly dominated by wheels and stones.

The stronger polyester materials offer greater tensile strength and are more resistant to splitting, breaking and fraying, so that more pressure can be applied. With this additional pressure, metal is removed faster than previously possible. Today, one of the leading uses of coated abrasive belts is with the heavy duty polyester backings (Y or H weight) and machine fixture pressure-assisted grinding.

The heavy backing should be used anywhere rapid removal of heavy gates and risers is needed and stiff edges are required.

Cotton and cotton/polyester blends are still available and are the choice in applications where flexibility of the edges is desired, such as the blending of parting lines on concave and convex surfaces. New manufacturing procedures have also improved the flexibility of backing materials and have increased the adhesion of the grain to provide better edge wear.

Grinding Aids - Another advancement in coated abrasives is the use of grinding aids. These are usually added to the abrasive in an additional sizing (adhesive) coat after the abrasive has been bonded to the backing material. These grinding aids add lubricity and cooling aids that help inhibit heat buildup.

Abrasive Grains

Zirconia - For years, the grains used in industrial applications were primarily silicon carbide and aluminum oxide. The 1970s introduced zirconia/aluminum oxide grain coated abrasives. The addition of zirconia to the abrasive mix produced belts that removed metal faster (up to 40% faster in some applications) with dramatically increased life.

The structure of the grains make them friable - that is, they are brittle and break off. Coated abrasives can "load" when metal from the casting being ground adheres to the abrasive grains - a big problem in aluminum. When the zirconia grains break off, these metal caps break off also. The removal of the caps and the fresh new edge of the grain are what give these abrasives their sharpness and durability.

Zirconia/alumina abrasives are especially useful in aluminum and brass foundries where loading is a major problem in reducing the life of abrasive products. They should be tested on all nonferrous alloys, as well as stainless steel, cast iron and ductile iron (Table 1).

Zirconia abrasives are adaptable to many operations, grinding methods and metals, and can serve as the all-purpose abrasive that aluminum oxide used to be. They excel at offhand grinding that is typical in foundries, where the castings are ground by operators holding the parts and applying hand pressure.

Grease and soap stick lubricants should be used to help reduce heat buildup and loading.

Ceramics - Ceramic/aluminum oxide grains (introduced in the 1980s) are the technology that perhaps has most expanded the coated abrasive market. Investment casting has especially benefited from these new grains.

Ceramic grains are some of the hardest materials available [ILLUSTRATION FOR FIGURE 1 OMITTED]. The manufacturing process produces grains formed by submicron size crystals [ILLUSTRATION FOR FIGURE 2 OMITTED]. As these crystals break off, they expose fresh, sharp cutting points that resharpen the surface, allowing ceramic abrasives to cut faster and cooler. Most also have an additional sizing coat that includes a cooling aid.

With new ceramic grains, and the heavy polyester backings, foundries currently using bonded abrasive grinding wheels and stones may now consider testing coated abrasive in their casting cleaning operations.

As shown in Table 1, ceramic abrasives are designed for hard-to-grind metals. The abrasive needs heavy pressure to break the groins and maximize the grinding aids. They perform best in applications that require heavy material removal of hard metals. Ceramics should be used on all steels, and are effective even on the high-nickel and cobalt specialty steels.

Generally, ceramics thrive on pressure and wheel speed [ILLUSTRATION FOR FIGURE 3 OMITTED]. Ceramics can be run at belt speeds of 6500-8000 surface ft per min and pressures as high as 700 psi. They are especially effective in machine fixturing high pressure applications and the stiffest polyester backings. Ceramics also require little or no lubrication.

For most foundries, however, grinding machine speed is not variable. For typical offhand grinding operations on machines with 3-12 hp drive and moderate fixed speeds - where pressure depends on the method and strength of the operator - ceramics may not perform to merit their additional costs. Full testing and cost analysis is required to determine the optimum product.

If used in offhand grinding, ceramic abrasives should be used with harder robber contact wheels (70-95 durometer) with aggressive serration. For maximum cut with all coated abrasives, rubber contact wheels must be kept in top condition.

Cost Factors

When considering coated abrasives, foundries should test various materials and manufacturers brands. Your tests will determine the product or brand that is most cost-effective for your particular application. The best performance, however, may not be the most cost-effective.

As an example, in one foundry application, 36 grit zirconia, ceramic and aluminum oxide grinding belts by several manufacturers were each used to remove the gates and riser connections from aluminum manifolds by offhand grinding. The ceramic belt had the longest production life, grinding 65 castings, as compared to 50 on the zirconia and 23 on the aluminum oxide.

But when foundry officials considered the purchase price vs. production, they felt that the added production of the ceramic belt didn't justify the additional cost and decided that the zirconia belt was more cost effective for their application.

RELATED ARTICLE: New Choices for Surface Conditioning, Too

New technologies have also brought nonwoven abrasive surface conditioning products into foundry applications where coated abrasive products and other materials were previously the only products available. Nonwoven abrasives for U.S. metalworking have grown into an estimated $104 million market.

The nonwoven abrasive is basically a synthetic fiber material impregnated with abrasive grains and bonded with resin. Used for many years in household applications (you probably clean your kitchen sink with one, although with a much gentler grit), recent advances have produced stronger materials that can be used in a widening variety of industrial applications.

The primary industrial use is for surface conditioning of metals, or changing the surface without changing the dimension or shape. Surface conditioning products are useful in many applications where light stock removal and fine surface finish is desired, or where the potential for excessive grinding is a concern. Even in the coarse grades, surface conditioning materials remove very little stock, and undercutting or gouging of the work piece is almost impossible. Surface conditioning products are self renewable, show no decrease in cutting rate and won't load up, as can occur with coated abrasives.

Surface Conditioning Table

Product       Comparable Surface          Removes Coated Abrasive
               Finish Fineness              Scratch Pattern

Coarse            60-80 grit        60 (heavy pressure)-120 (light)
Medium           120-180 grit            80 (heavy)-150 (light)
Very Fine        180-130 grit           180 (heavy)-320 (light)

Surface conditioning products are available as hand pads, discs, grinding wheels, belts, deburring wheels and other shapes and forms. They can be used in many applications where fine grit-coated abrasives used to be the choice. They can also replace wire brushes, steel wool and other products for cleaning and buffing castings. These nonwoven materials excel at removing rust and corrosion from castings, blending machining marks without affecting machined surfaces or tolerances, removing grinding marks and providing cosmetic (buffed, satin etc.) finishes (see above table).

The open web design of nonwoven products allows a controlled contact with the work piece. It is flexible enough to follow the shape of the part around contours, corners and edges. The open nature of the material also gives it a springlike effect, so that the operator can change the surface finish by changing the pressure.

Some current foundry applications include the: deburring of aircraft parts, satin finishing of aluminum bows, and deburring and finishing of roller blade rails. One investment foundry is using 6 in. wheels to blend and polish the hostle of golf heads, while an art foundry is using nonwoven material to deburr (remove the light parting line) and polish their product. A common use is the removal of grinding scratch patterns to provide a uniform finish to castings prior to coating or painting.

Because surface conditioning products don't change a part's dimension, they can also be useful for cleaning of permanent molds.