Proper Specifying, Testing, Eliminates Ceramic Abrasive Wastefulness.

When coated abrasive products made with ceramic aluminum oxide (CAO) grains were introduced in the 1980s, they were considered one of the greatest innovations in grinding in 50 years. When specified and used correctly, they delivered results up to 10 times the performance and productivity offered by other abrasives used in cleaning and finishing. Today, products made with CAO grains commonly are used in the search for improved grinding results.

However, foundries that purchase these abrasives face a new challenge. Because this advanced abrasive product is widely available, chances increase that the product specified might not be the best choice for cleaning/finishing operations. Ceramic grains, for all their performance, cannot be cost-justified if they are not specified properly. Therefore, a foundry may end up spending more than it should for a product that does less than needed.

Understanding what goes into a CAO product and guidelines on how to specify and test these products will help the foundry make the best use of its abrasive dollars.

A Different Way to Grind

The four types of minerals used in high-pressure grinding operations are:

* aluminum oxide(A/O)--a utility grade mineral that provides a good cut and consistent finish on metals like aluminum;

* silicon carbide--a hard and harp but brittle mineral that provides a consistent finish on stainless steel and titanium;

* alumina zirconia--a mineral that's tougher than A/O or silicon carbide and used in high-pressure grinding applications like gate removal;

* CAO--a mineral with the greatest toughness and hardness that is well suited for high-pressure grinding applications.

When considering the use of CAO, foundry personnel must understand that this advanced abrasive is not a universal solution for all material and operations. It is a coated abrasive for hard-to-grind materials in high-pressure, high-productivity cleaning/finishing applications. It can be used in oil or with water-soluble coolants, and in dry grinding. It will rind ferrous and nonferrous metals equally well. CAO abrasive rains are most effective for medium to heavy stock removal on aerospace alloys, carbon steel, nickel-based alloys and some stainless steels.

CAO grain production process begins with submicron-sized particles that are sintered to provide the abrasive grain. Billions of these particles are found in a single grain (Fig. 1). As it breaks down during grinding , it resharpens itself. As it wears, CAO abrasives yield a sharp cutting edge with a relief-like angle. This allows the abrasive to cut cooler and last longer.

With a conventional A/O abrasive, the raw materials used in the grain are fused in a furnace to create a "pig." The pig is cooled, and then crushed to arrive at a desired particle size. This means that a minute number of crystal particles are in each grain--sometimes only one. In grinding, the grains cleave along plains in the crystal (Fig. 2). Each time a grain fractures, it flattens and dulls, reducing its grinding life and effectiveness.

Therefore, the concept behind CAO abrasives is simple: when grinding longer and cooler, a billion particles are better than one.

CAO Specifications

CAO abrasives are made up of three basic components: a flexible or semi-rigid backing, abrasive grains and an adhesive that bonds the other two together. Selecting the most suitable components from the available choices results in the best coated abrasive specification for a particular cleaning/finishing operation.

All abrasive grains are crushed and sized before being applied to the backing. In a process similar to sand sizing, the grains are separated according to the size of their particles using a series of precisely graded screens. Each abrasive is assigned a grit size (i.e. 80 grit). This represents the number of openings/in. in the final screen--the one the grain cannot fit through. Therefore, the higher the grit size, the finer the grain. Grains may be graded using a system developed by the Coated Abrasives Manufacturers Institute (the CAMI system) or a technique developed by the Federation of European Producers of Abrasives (the FEPA system). Abrasive grade selection is important in achieving optimal cut rate and proper surface finish (Table 1).

The backing on a coated abrasive belt must be smooth enough for uniform coating and grain adhesion, strong enough to withstand grinding pressures, and flexible enough to conform to contours (Fig. 3). Most CAO belts feature a polyester or cotton backing; ounce for ounce, a polyester backing is stronger than a cotton backing, and polyester offers better tear and puncture resistance than cotton. Most backings, which are offered in either an X- or Y-weight, are stronger and more resistant to pressure and heavy stock removal. CAO grains also can be coated on vulcanized fiber backing for heavy-duty disc grinding on right angle grinders, or on paper backings for disc and belt finishing (Fig. 4).

Abrasive grains are adhered to the backings in one of two types of coatings, open coat or closed coat. With an open coat, 50-75% of the coated abrasive surface is covered by grain. Evenly spaced voids between grains reduce the effect of loading caused by metal particles. With a closed coat, the entire surface is covered with abrasive grain, leaving no voids between the particles. This is the most typical coating method, permitting the greatest degree of stock removal and longest product life.

The adhesive used to bond the abrasive grain to the backing creates a significant factor in the performance of the coated abrasive product. Most abrasive-coated products used for cleaning and finishing feature an adhesive made of synthetic heat-hardenable resin.

Some CAO products also include grinding aids designed to improve cut, finish and product durability by reducing heat and loading. Grinding aids are applied to the surface of the product during the manufacturing process. One common treatment, called "supersizing," is designed to reduce extreme heat generated from grinding on the casting and is especially effective on alloyed steels.

Setting Up a Belt Test

Testing is unparalleled in determining whether GAO abrasive can deliver improved results compared to current specifications. A supplier should be able to provide the technical resources needed to help conduct a product test.

Before setting up a test for a CAO product, however, the cleaning and finishing operation should meet three criteria

* the casting being ground should be a "production" item. It should be a part that is ground daily, rather than a short-run or specialty item;

* the productivity can be measured. Measuring the weight of material removed or the number of castings being ground can provide this metric;

* the grinding parameters must be suitable. For example, the contact wheel must be rated between 70 and 90 durometer; the back-up wheel for the more flexible products (X- and J-weight backings) are usually in the hardness range of 40-60 durometer.

An abrasive supplier's technical representative should be able to help determine if the cleaning/finishing operation meets these three criteria for optimum use of CAO products. For greatest accuracy, insist that the tests be conducted on foundry premises, using plant machinery and castings.

Make sure the supplier offers a "no-risk" test policy. This means that if the product tested proves unsatisfactory, a refund will be issued for the cost of the belts used in the test.

Is It Cost Effective?

After testing, the best product for cleaning and finishing can be determined because the measure of any successful test is productivity. In some cases, increased productivity is immediately evident by a dramatic increase in the number of castings ground per belt or disc used.

In other cases, the casting per cost, including labor and overhead, also may need to be measured to obtain an accurate measure of productivity. To quickly figure cost per casting, use this formula: Abrasive cost + labor + overhead = Grinding cost Number of castings ground per casting

In this formula, add the cost of the abrasive that was used in grinding to the cost of labor and overhead for the time that the abrasive was being used. Divide that amount by the number of castings ground. This will provide the casting per cost of grinding.

One Size Does Not Fit All

Because they use a premium abrasive, CAO products should never be dismissed as a commodity, "on -size-fits-all" product. Maximizing the effectiveness of these abrasives can only be achieved by taking a "systems approach"--considering all the factors, including machine parameters, work material and casting quality, that affect the grinding and the way those factors interact to produce the final result.

Correct specification and testing of GAO abrasives can best determine whether the investment will pay off with improved productivity and improved casting quality.

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Table 1

Cleaning/Finishing Operations Appropriate to Abrasive Grades

     Operation        Grade Range               Examples

Heavy Stock Removal      20-40      gate removal, heavy weld removal
Medium Stock Removal     50-80     defect removal, light weld removal,
                                     forge and parting line removal
Light Stock Removal     100-150       blending, scratch refinement
     Finishing          180-240            decorative finishes
     Polishing          280-600       sequence to polished surfaces

Source: "Advancements in Abrasive Technology and Cleaning Room
Automation," N. Orf, Investment Casting Institute, 47th Annual Technical
Meeting (1999).

RELATED ARTICLE: Elements in Choosing the Correct CAO

An understanding of the components of a CAO-coated abrasive product is the first step in correct specification. The next step is knowing which specific grinding operation will be performed. The following information is needed:

Type of metal to be ground--always the first consideration.

Amount of pressure--another way to account for this is to know how much stock needs to be removed;

Present belt or disc spec--a starting point for grit size and backing weight;

Hardness of the back-up pad or wheel--know the type of back-up being used and how aggressively a wheel is serrated;

Machine type and conditions--does the type of machine being used have high horsepower? Are the contract wheel and back-up paid in good condition?

Quality requirements--determine if finish is important. On some operations, CAO yields a higher RMS (root-mean square, a measurement of surface roughness) than conventional abrasives.