Grinding takes an abrasive — often attached to a grinding wheel — and uses its many grains to cut a workpiece. Variations on this process are useful for a wide variety of applications.
On its surface, grinding seems simple: a machine takes a rotating tool (usually a wheel) with abrasive grains and applies it to a workpiece’s surface to remove material. Each grain is its own miniature cutting tool, and as grains dull, they tear from the tool and make new, sharp grains prominent.
But there are many variations, approaches and considerations for this type of machining, each of which is particularly effective for certain applications with certain materials.
Principles of Grinding
In all forms of grinding, three different interactions occur between the abrasive and the machined material. Cutting occurs where the abrasive grain is sufficiently exposed to penetrate the workpiece material and curl a chip, and sufficient clearance exists between the grain, bond and workpiece to flush the chip with coolant or throw it away by wheel action. Plowing takes place when the grain is unable to get enough penetration to lift a chip, instead pushing the material ahead of the abrasive edge. Sliding happens when a lack of cut depth, insufficient clearance or a grit staying on the wheel after dulling results in rubbing or creating slide marks on the workpiece surface. Grinding process control balances these three interactions to achieve the desired parameters.
These interactions feed into three major commercial grinding processes: rough grinding, precision grinding and ultra-precision grinding. Rough grinding maximizes the metal removed at the cost of surface finish. It primarily sees use in cutting off billets, grinding weld beads smooth and snagging gates and risers from castings. Additional surface finishing passes typically take place afterward — in particular, a “spark-out” pass relieves some of the stress on the machine tool and uses plowing to impart a better surface finish and size tolerance. Precision grinding is a middle-ground between metal removal and part size control, and serves as the basis for creep feed grinding, slot grinding and high-efficiency deep grinding. In ultra-precision grinding, little to no actual cutting occurs, but sliding action from very fine grains rubs the workpiece surface to a high finish. Most surface finishing processes, such as lapping and polishing, are examples of this type of grinding.
Although speeds for grinding wheels and cutting wheels are measured in sfm or smm, wheels are often rated in rpm. It is important never to operate a grinding wheel over its rpm limit — most experts recommend never mounting a wheel on a machine that can exceed the wheel’s limit.
As speeds increase, each grain cuts and wears less. This emulates a harder grade. Vitrified bonds work up to 6,500 sfm, with organic bonds handling up to around 9,500 sfm. Higher speeds will require specially made grains.
Work speed defines the speed at which a grinding wheel passes over a workpiece or rotates around a center. High work speeds lower the heat retention and reduce the risk of thermal damage. Both high work speeds and reducing the diameter of the wheel result in increased grain depth of cut, performing like a softer grade wheel.
Surface grinding, such as stainless steel grinding, involves grinding a plane surface by feeding the workpiece beneath a rotating grinding wheel. Like cylindrical grinding, it operates in two general formats. The workpiece may travel traversely under the wheel and move back and forth beneath a grinding wheel mounted on a horizontal spindle, or it may move in circles on a rotary table beneath a vertical spindle that cuts on the face of the grinding wheel or grinding segment. Applications for this grinding type may grind a surface flat or introduce grooves by grinding straight channels into the workpiece. While milling can complete these tasks, grinding improves surface finish, has less expensive tooling and allows contours to be dressed into the profile of the wheel — making it much more cost-effective for very hard or abrasive surfaces.
Centerless grinding creates cylindrical forms at extremely close tolerances. This type of grinding eliminates the need for center holding by supporting the workpiece at three separate points: the grinding wheel, feed wheel and work support blade. Nothing actually clamps the workpiece in place, so each piece flows freely for continuous production (also known as “throughfeed centerless grinding”). The grinding wheel, during ordinary metal grinding, and the feed wheel rotate in the same direction, while the workpiece rotates in the opposite direction between them. The rotation keeps the workpiece down, while the work support blade (slightly angled to raise the workpiece above the centerline for better cylindricity) holds it up. The work support blade should always be at least as long as the grinding wheel is wide. Centerless grinding also comes in three forms. Throughfeed centerless grinding is used on straight cylindrical workpieces without interfering shoulder or projections, and involves the offset axis feed wheel feed the workpiece past the grinding wheel to a discharge position. Infeed grinding (also called plunge centerless grinding) is best when a workpiece has projections, irregular shapes, varying diameters or shoulders, and works best for profiles and multi-diameter workpieces. In this submethod, feed wheels above the grinding wheel feed the workpiece downward, with no lateral movement during grinding. Endfeed centerless grinding grinds conically tapered cylindrical sections like shanks on A and B taper drill bits. Here, the feed wheel, grinding wheel and work blade are set up in a fixed relationship to each other, then two wheels are dressed to a shape matching the end taper of the workpiece and the workpiece is fed from the front of the grinding machine until it reaches an end stop.
Creep feed grinding is a slow, one-pass operation that makes a deep cut of up to one inch in steel materials at low table speeds between 0.5 and 1 ipm. It is not suitable for conventional grinding machines, but for those which are compatible with it, it offers high productivity and cost effectiveness. Creep feed grinding is a plunge operation with high horsepower requirements, and which also requires a heavy flow of cutting fluid close to the nip to remove chips and cool the work. Continuous dressing at about 20 to 60 millionths per revolution — preferably with a diamond roll — reduces cutting times of fixed machine cutting and keeps the wheel sharp. When a second pass is required, it is typically of no more than 0.002 inch deep to “clean up” the workpiece.