MARCH 2001 / VOLUME 53 / NUMBER 3

¨ BY DR. STUART SALMON

Understanding the fundamentals and possibilities of grinding. Abrasive Erwin Junker Machinery Inc. Lessons PART 1 oday’s manufacturing industry due, in part, to a lack of understanding debris, then it can be dressed or resharp- is desperately searching for al- about the process. Superabrasive and ened on the machine. No other mechan- ternatives to grinding. Among creep-feed (CF) grinding competes, ical process resharpens its T the “new” processes being tried technically as well as economically, tools on the machine. to speed the production of parts are hard with , broaching, planing and, in A grinding wheel also can machine ; dry machining; the use of some cases, turning. There are plenty of surfaces to tolerances on the order of coated, wear-resistant cutting tools; and naysayers at manufacturing engineer- tens of thousandths of an inch, while high-speed machining. ing companies who keep grinding from producing a surface with the finest pos- It should be noted, though, that “high competing with those processes, partic- sible finish and the highest level of speed” isn’t foreign to grinding. An ularly when their expertise lies in tradi- integrity. abrasive wheel typically runs at a pe- tional machining. But when new mate- Unfortunately, grinding has long been ripheral speed of around 6,000 sfm. rials come along—like ceramics, whisk- regarded as an art. Over the past 40 to High-speed superabrasive grinding er-reinforced metals and polymers, and 50 years, however, grinding researchers wheels are used in production opera- multilayer metals with nonmetallic worldwide have studied abrasive ma- tions at speeds of 15,000 sfm to 35,000 laminates—grinding is often the only chining and gained a comprehensive sfm. And, laboratory testing has been process able to do the job. understanding of the process. They carried out on grinding rigs running in Abrasive grains—in suitable bonds— have developed new and improved excess of 60,000 sfm—just shy of the break down or resharpen in a controlled abrasives, bonding systems and grind- speed of sound. manner while in process. If the grinding ing fluids, all of which bring grinding The industry’s dislike of grinding is wheel becomes too dull or loaded with into the realm of science.

Hardened Nickel- Gummy High- Tungsten Aluminum, Rubbers, Copper Plastics Ferrous Ceramics, Steels Based Stainless Temperature Carbide Titanium Polymers Alloys Metals Cermets Superalloys Steels Alloys Aluminum Oxide •• • • (Al2O3)

Ceramic Al2O3 •••• Silicon Carbide ••••• Diamond •• • • Cubic Boron Nitride •• • The types of materials that are suitable for grinding with conventional abrasives and superabrasives. To promote a better understanding of will wear rapidly. How- grinding, this article will examine some ever, diamond is a good of the fundamentals of the process. Part candidate for grinding 1 covers abrasives and abrasive bonds, nonferrous materials, ti- while wheel preparation and grinding tanium and, particularly, fluids and their application will be cov- ceramics and cermets. ered next month in Part 2. xxCubic boron nitride. CBN, like diamond, is Types of Abrasives an expensive abrasive. Abrasives fall into two basic groups: The price of a super- conventional (such as aluminum oxide abrasive wheel may be and silicon carbide) and superabrasive 50, or more, times high- (diamond and cubic boron nitride). er than a conventional CBN and diamond are harder and abrasive wheel. But the

more wear-resistant than conventional Landis Gardner, Company a Unova superabrasive wheel abrasives, but they cost significantly Creep-feed grinding competes, technically as well as eco- might grind more than more. An important additional differ- nomically, with milling, broaching, planing and, in some 100 times the number of ence is that superabrasives are excellent cases, turning. parts. It can grind the conductors of heat (diamond conducts hardest of steels while heat six times better than copper), long and the force on individual grains exhibiting very little wear. whereas conventional abrasives are ce- is very low, such as ID and CF grinding. CBN is ideally suited for grinding ramics. Therefore, they are insulators However, a modified ceramic grain hard, ferrous materials and jobs such as and do not conduct heat. that is “extruded” is nicely suited to grinding ID bearing races, especially Superabrasives also have high ther- grinding gummy stainless steels and when the form will remain on the grind- mal diffusivity, which is the ability to high-temperature alloys, even when the ing wheel for long runs. CBN also is quickly shed heat. This makes super- arc of cut is long. The aspect (length-to- well-suited to operations where the abrasives inherently “cool cutting.” The width) ratio of these grains is around 5. wheel is changed infrequently. Small abrasion resistance of superabrasives is The properties of the SG grain may batches and wheel changes require vastly superior to conventional abra- be complemented by the friability of dressing and redressing during setup— sives, but the properties of superabra- the fused Al2O3 when they are mixed a major expense. Because CBN reacts sives do not necessarily make them can- together to form a composite fused/ce- with water at high temperatures and didates for every grinding operation. ramic abrasive wheel. It is, therefore, wears rapidly, it grinds best with gly- Every abrasive has its niche, so it is necessary to know the length of the arc cols and straight oils. important to fully understand the prop- of cut between the grinding wheel and erties of each. For example, Al2O3 ce- the workpiece to better specify the Bonds ramic abrasive—sometimes called grinding wheel for the job. Conventional-abrasive wheels may “seeded-gel” (SG) or “ceramic abra- Silicon Carbide. A SiC grain has a be made with vitrified bonds, resin sive”—generally exhibits better wear naturally sharp and aggressive shape. It bonds or plastic bonds. In addition, su- resistance and better form-holding ca- is best for grinding hard materials, like perabrasives can be bonded in a sin- pability than fused (conventional) tungsten carbide. Because of its sharp- tered metal matrix or plated to a wheel Al2O3, but not always. Again, ceramic ness, it also is well-suited to machining hub with a layer of nickel. Such wheels abrasive has its niche. very soft materials, like Aluminum Oxide. Al2O3 is the least aluminum, polymers expensive abrasive. It is good for grind- and rubbers, as well as ing hardened steels. Al2O3 also can softer materials, such as grind nickel-based superalloys when low-tensile-strength continuously dressed. Al2O3 is versatile steels, copper alloys in that it can successfully grind soft and and plastics. hard materials at light-to-heavy stock- Diamond. Both nat- removal rates and imparts a superior ural and synthetic dia- surface finish. monds are used for Ceramic Aluminum Oxide. Cer- grinding. Diamond is amic Al2O3 is very tough and best used not a good candidate for where the force on each grain in the arc grinding ferrous materi- of cut is high. Ceramic Al2O3 is good als. Being an extremely for grinding hardened steel when cylin- hard form of carbon, it Landis Gardner, Company a Unova drical grinding or reciprocal grinding has an affinity for the A plated CBN wheel “green-grinds” a crankshaft. CBN is large surfaces. It is not suited for grind- iron (steel being an iron ideally suited for grinding hard, ferrous materials when ing operations where the arc of cut is and carbon alloy) and the form will remain on the wheel for long part runs. Glossary of Grinding Terms

Arc of Cut: The part of the wheel that contacts the workpiece. Aspect Ratio: The ratio of an abrasive’s grain length to its width. Cermet: A heat-resistant alloy formed by compacting and sintering together a metal and a ceramic material. Continuous Dressing: A process where a diamond dressing roll sharpens the grinding wheel by removing dull grits all the time the wheel is machining the workpiece. Norton Co. Pictured are various metal-bonded dia- Creep-Feed Grinding: A process where a very soft and porous grinding wheel mond wheels for ceramic OD and ID takes a deep cut in one slow pass. Well-suited for cylindrical grinding. difficult-to-machine materials, the process offer a high stock-removal rate with accurate form retention. bond is generally reserved for more Cylindrical Grinding: A process for machining round components. open forms with radii greater than Dressing: Removal of undesirable material from a “loaded” grinding wheel by 0.020". Often, plated wheels are ap- using a diamond tool. plied in high-speed grinding applica- tions and sintered wheels are used on Dry Machining: A metal-removal operation where coolant is not applied to ceramics. the cutting tool/workpiece interface. Solid metal wheels have little for- Friability: The ability of an abrasive to fracture. giveness for vibration, runout and fluid Hard Turning: Turning hardened metal on a lathe to eliminate grinding it. flow. If the , part and/or fixture lack rigidity and/or the ID Grinding: A process that removes metal from the inside diameter of holes machine is old with less-than-perfect or profiles by applying a very small, high-rpm grinding wheel. bearings and without an on-machine Reciprocal Grinding: A process where the workpiece is mounted on a table balancer, then the unforgiving plated that moves back and forth beneath the grinding wheel. wheel may cause wheel-life, surface- Swarf: The mass of chips and debris remaining after grinding. finish and surface-integrity problems. A resin bond would be a better choice, Truing: Application of a diamond tool to a grinding wheel to ensure roundness based on the condition of the machine and concentricity. tool and vibrational instability. Resin bonds have excellent vibration-damp- ing capabilities. However, the wheel are impervious and have no porosity. than into the workpiece. would need to be trued and dressed, and Care must be taken to apply the There are two types of metal bond: there are costs associated with dressing grinding fluid properly to plated and plated and sintered. Plated wheels are devices and dressing time. metal bond grinding wheels to prevent never dressed; they are made to the Vitrified bonds are the most popular them from hydroplaning. Significant exact form and grind until spent. Sin- bond, as they make for a porous wheel. hydrodynamic pressures in the arc of tered wheels are generally dressed by Vitrified bonds allow effective applica- cut can lift the wheel from the work- the electrical discharge machining pro- tion of the grinding fluid to the arc of piece surface, resulting in poor surface cess while off the grinder, and then are cut and provide ample chip clearance finish and high wheel wear. mounted like a plated wheel. for the grinding swarf. Vitrified bonds The choice of bond and abrasive go Both sintered and plated wheels need are easily dressed to shape and sharp- hand in hand. For example, CBN might to be set up to minimize spindle runout: ness with diamond dressing tools. be the abrasive of choice if the wheel 0.0005" or less. Minimal spindle runout This concludes the first part of “Abra- form is to remain exactly the same over is especially important for metal bond sive Lessons.” The second installment the life of the wheel and it will stay on wheels. Because the grains protrude a will appear in the April edition. the machine spindle until spent. Since short distance above the bond, runout CBN conducts heat so well, a metal of 0.001" might cause excessive wear to bond is advantageous. This combina- one portion of the wheel while another About the Author tion provides a cool-cutting wheel, section’s grains remain sharp. Stuart Salmon is president of Advanc- since the heat flows into the grain and Some plated applications can form ed Manufacturing Science and Tech- wheel and away with the coolant rather very tight radii (0.005" or so), but this nology, Rossford, Ohio.

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