Make Sure Abrasives Rub Customers the Right Way

Pete Nofel, Editor

Grinding discs and abrasives can be nearly as important in creating a good weld as a power source and proper filler metal. Each step in the life of a weld involves abrasives: joint preparation; slag and spatter cleaning from the completed weldment; bead smoothing; or defect removal found in welds by nondestructive testing.

       Here is a basic summary of the nomenclature associated with abrasives.

They're just your type
       The Grinding Wheel Institute, Cleveland, working with the American National Standards Institute, New York, created the standards by which grinding wheel geometries are categorized. Manufacturers designate grinding wheels by type numbers, based on their shape. The wheels most commonly used in welding are Types 1, 11, 16, 17, 18, 27, and 28.

       Type 1 wheels are flat wheels and are designed for use only on outside diameter of the wheel. In the past, welders used this type of wheel for weldment smoothing, but that has been all but displaced by Type 27 discs. Today, Type 1 usually designates cut-off wheels used in applications such as chop saws, though they may also designate thicker wheels used in horizontal/straight shaft grinders.

       Type 11 wheels are cup-shaped and are used for aggressive material removal, typically in snagging.

       Types 16, 17, and 18 designate cones and plugs of different geometries. Type 16 have an elliptical-shaped cross-section. Type 17 are cone-shaped with either square or rounded tips. Type 18 are cylindrical plugs with either square or round tips.

       Type 27 discs are usually referred to as depressed-center wheels or discs and are the most common abrasives discs in welding.

       Type 28 wheels have saucer-shaped grinding rims and are designed for corner and side grinding. They should not be used for cut-off or notching.

True grit
      Matching the correct abrasive material to the work increases both economy and efficiency. Commonly-used abrasives include aluminum oxide, zirconia alumina, and silicon carbide. There are several types of patented ceramic abrasives used in the manufacturing of wheels, discs, and coated abrasives products. Ceramics are a relatively new innovation in the abrasives field.

       Aluminum oxide is the workhorse abrasive in welding. It is usually used on plain steels and weld beads.

       For stainless steels and harder metals, zirconia alumina cuts quicker and faster. The abrasives industry saw the introduction of zirconia in the 1970s. Demands of the nuclear-power industry prompted the development of an abrasive material that could grind stainless steel and minimize heat to prevent distortions, as well as leave no residues.

      Zirconia alumina products are about 50 percent more expensive than their aluminum oxide counterparts. However, on harder metals, zirconia abrasives can last two to four times longer. Zirconia cuts cooler than aluminum oxide, a consideration when grinding heat-sensitive materials. It is suitable for grinding applications on plain steels, but users should justify the economics of its increased cost in those types of applications.

       Silicon carbide also costs more than aluminum oxide, but not as much as zirconia and is good for non-steel applications such as aluminum grinding.

       Manufactures not only provide wheels of specific types of abrasives, they also blend abrasives to create the optimum mix between performance and cost. Mixes of aluminum oxide, zirconia alumina, or ceramic abrasives, for instance, provide greater metal removal rates and longer life that increase the cost of the wheel, but lower the real cost of metal removal.

Speed, too
      Another factor in the proper use of abrasives is using them at their correct speeds. Running the abrasive at the correct rotation rate maximizes its efficiency.

       For best results, grinders should run at the highest possible speed marked on the wheel. Running an abrasive at a lower speed drops its efficiency while running it above its rated speed is dangerous. Excessive wheel and disc speed is a major cause of wheel breakage that can lead to serious injury or death.

       Each grain in an abrasive product provides a sharp face which cuts away the target material.

       Abrasives maintain a sharp cutting face by fracturing the grains to create new cutting edges as the fractured material is carried away.

       A rule of thumb for checking the correct match-up of abrasive, hardness, grain-size, and speed, is the spark produced while grinding. If an abrasive wheel carries a spark all around the perimeter, it indicates that some of the chips are adhering to the wheel as they should.

Creating a flap
       Abrasives used in welding are now moving beyond the solid material disc. Innovations such as the flap disc and the see-through grinding disc offer distributors greater choices for their customers.

       Flap discs – staggered, overlapping layers of fiber-based coated abrasives – are finding greater use. They provide light stock removal, blending, and finishing in one operation. They grind like a depressed center wheel and finish like a fiber disc.

       The AVOS – Allows View of Surface – disc from Norton Co., Worcester, Mass., is a triangularly-shaped disc with rounded-off points and three holes in the disc. As the disc spins, the user gets a stroboscopic view of the grinding zone. An added advantage is that these discs also increase airflow for cooler grinding.

Staying state-of-the-art
       As welding continues to evolve, grinding changes with it. Where it was once permissible to grind a weld to a rough state or skip grinding completely, today's welding specifications often call for cleaner weldments and more finished surfaces. Abrasive materials have kept pace, so must distributors. Where once a 24-grit aluminum oxide would take care of most welding needs, the distributor now must offer customers a wider range of products to fit closer tolerances and finer needs.

Contributors: Donald W. Churchill account executive - welding, Norton Co., and Jim Ballou, inside sales, Pferd, Inc., Leominster, Mass., both contributed information to this article.

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