A few years ago, one of our local industrial sales representatives was giving his best pitch. He told me we should use his brand of coated abrasives, and I joked that he was trying to sell sandpaper to me.
“We sell a million dollars’ worth of sandpaper in a year,” he replied, “so we refer to them as coated abrasives.”
This lighthearted conversation made me think about the significance of abrasives in metalworking and how the application of abrasive products often is not given the same considerations as other metalworking processes.
Metalworking professionals use abrasives to clean parts, shape geometries, prepare surfaces, improve appearances, size features and cut parts. Today’s metalworking would not be possible without modern abrasives.
The grinding attachment for the 200 metric ton (220 ton) lathe at Mitsubishi Hitachi Power Systems Americas uses a large aluminum oxide wheel. Image courtesy of C. Tate
Abrasive grains like pumice, calcite, walnut shells and garnet, which is the most common grain in sandpaper, occur naturally and have been used by humankind for thousands of years. Technological advances have resulted in human-made materials, such as silicon carbide, synthetic diamond and aluminum oxide, which is the most frequent of these in modern metalworking. Human-made abrasive grains are the most prevalent material in contemporary manufacturing.
Types of Abrasives
Abrasive products are manufactured in various forms and presented to a workpiece in numerous ways to achieve the desired results.
Bonded abrasives are made by combining abrasive material with a bonding agent. The mixture then can be formed into many different shapes, creating grinding wheels, cutoff wheels, grinding points, dressing sticks and many other formed abrasive products.
Welding superalloys requires a lot of cleaning with abrasives to ensure weld quality. Image courtesy of C. Tate
Coated abrasives are probably the most typical abrasive product as this is the group of goods we generically refer to as sandpaper. As my salesman friend pointed out, this family of abrasive materials is far more significant than we sometimes realize. Coated abrasives are constructed by applying a binding agent like glue or resin to a backing material, such as paper, cloth or something similar. The abrasive grains are applied to the binding agent and coated again to create a flexible product that has a single layer of abrasive material.
Loose or bulk abrasives are also routine. Sand, walnut shells, glass beads, garnet, dry ice and aluminum oxide are some of the abrasive materials that commonly are used loose. Delivering the abrasive at high velocity with compressed air, otherwise known as blasting, is the most standard method of application. Loose abrasives are used as well in processes like vibratory finishing and waterjet cutting. Loose abrasive grains are combined with fluids to form lapping compounds or combined with waxes or greases to make buffing compounds.
All abrasive materials are categorized by grain, or grit, size. Grit size is determined by sifting an abrasive through calibrated screens, using air-blown separation or — in some cases — employing water separation, all of which segregate material by particle size. Smaller numbers indicate larger particle sizes, and larger numbers represent smaller particles, so a 24-grit sanding belt is rougher than a 120-grit belt.
Like endmills and turning tools, abrasives must be used under proper conditions, and correct cutting parameters are critical. In most cases, abrasives are used at much higher cutting speeds than tools like endmills. Typical machining speeds for milling and turning are less than 305 m/min. (1,000 sfm), yet the usual abrasive is used between 1,372 m/min. (4,500 sfm) and 2,134 m/min. (7,000 sfm). Abrasive materials can function at these speeds because they are not affected by high temperatures. Also, unlike the edge of an endmill, the cutting edge is expected to fracture during use, which helps keep the grain sharp.
The sanding machine (left) at Mitsubishi Hitachi Power Systems Americas uses 914 mm × 1,829 mm (36"×72") sanding belts (right) to polish parts made of sheet metal. Image courtesy of C. Tate
Operating abrasive products at optimal parameters ensures safe, efficient processes. When high-volume grinding, obtaining cutting parameters for a bonded abrasive product, such as a grinding wheel, is usually an iterative approach involving calculations and testing. However, cutting parameters for operations that utilize coated or loose abrasives are calculated less easily, so they rarely receive any consideration unless something goes wrong. This is unfortunate because substantial guidance is available from manufacturers. Poor tool life, burning, bad surface finish, excessive cycle time and other problems can be solved through research and advice from abrasive experts.
Safe use of abrasives also is not given proper consideration. It is easy for people to understand how milling or turning tools can be dangerous, but a sanding disc or pile of glass beads on the floor can seem benign.
At operating speed, the edge of a flexible disc acts like a knife, easily slicing a finger, and the abrasive surface quickly removes skin. Glass beads from a blast cabinet on a smooth concrete floor create a slipping hazard similar to ice. Compressed air transports abrasive grains around safety glasses, exposing a user to potential eye injury. Excess speed can cause grinding wheels to explode. Gloves, hair, clothes and jewelry can become tangled in machines and air tools used to power abrasives.
Abrasives and the tools that drive them should be given the same safety considerations as every other tool at a shop. Getting proper safety information is easy as all reputable abrasive manufacturers provide very detailed instructions.
Abrasives play a significant part at a modern metalworking shop, but process design, optimization and safety aspects frequently are overlooked until something bad happens. Profitable abrasive processes require the same level of planning and research as other machining operations.
Related Glossary Terms
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- aluminum oxide
Aluminum oxide, also known as corundum, is used in grinding wheels. The chemical formula is Al2O3. Aluminum oxide is the base for ceramics, which are used in cutting tools for high-speed machining with light chip removal. Aluminum oxide is widely used as coating material applied to carbide substrates by chemical vapor deposition. Coated carbide inserts with Al2O3 layers withstand high cutting speeds, as well as abrasive and crater wear.
1. Flexible portion of a bandsaw blade. 2. Support material behind the cutting edge of a tool. 3. Base material for coated abrasives.
- bonded abrasive
Abrasive grains mixed with a bonding agent. The mixture is pressed to shape and then fired in a kiln or cured. Forms include wheels, segments and cup wheels. Bond types include oxychloride, vitrified, silicate, metal, resin, plastic, rubber and shellac. Another type of bond is electroplated, wherein the abrasive grains are attached to a backing by a thick layer of electroplated material.
Use of rapidly spinning wires or fibers to effectively and economically remove burrs, scratches and similar mechanical imperfections from precision and highly stressed components. The greatest application is in the manufacture of gears and bearing races where the removal of sharp edges and stress risers by power methods has increased the speed of the operation.
Rotary tool that removes hard or soft materials similar to a rotary file. A bur’s teeth, or flutes, have a negative rake.
Step that prepares a slug, blank or other workpiece for machining or other processing by separating it from the original stock. Performed on lathes, chucking machines, automatic screw machines and other turning machines. Also performed on milling machines, machining centers with slitting saws and sawing machines with cold (circular) saws, hacksaws, bandsaws or abrasive cutoff saws. See saw, sawing machine; turning.
Removal of undesirable materials from “loaded” grinding wheels using a single- or multi-point diamond or other tool. The process also exposes unused, sharp abrasive points. See loading; truing.
Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.
- grinding wheel
Wheel formed from abrasive material mixed in a suitable matrix. Takes a variety of shapes but falls into two basic categories: one that cuts on its periphery, as in reciprocating grinding, and one that cuts on its side or face, as in tool and cutter grinding.
- grit size
Specified size of the abrasive particles in grinding wheels and other abrasive tools. Determines metal-removal capability and quality of finish.
Finishing operation in which a loose, fine-grain abrasive in a liquid medium abrades material. Extremely accurate process that corrects minor shape imperfections, refines surface finishes and produces a close fit between mating surfaces.
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
Tough, difficult-to-machine alloys; includes Hastelloy, Inconel and Monel. Many are nickel-base metals.
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
- waterjet cutting
Fine, high-pressure (up to 50,000 psi or greater), high-velocity jet of water directed by a small nozzle to cut material. Velocity of the stream can exceed twice the speed of sound. Nozzle opening ranges from between 0.004" to 0.016" (0.l0mm to 0.41mm), producing a very narrow kerf. See AWJ, abrasive waterjet.