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.
Hundreds of different variables can affect the interaction between the abrasive and the workpiece, but they generally come down to machine tool, work material, wheel selection and operational factors. Balancing these by setting up a part run that fits within the known parameters of all four categories provides a baseline that gradual parameter adjustment can improve.
Grinding Wheels
Grinding wheels have two major components: the abrasive grains and the bond. The relative percentages of grain and bond, and their spacing on the wheel, determine the wheel’s structure. Different types of grains work better on different projects, as do different types and “grades” (i.e. strengths) of bond. Broad areas of grinding need coarser grits and softer grades, with smaller areas requiring finer grits and harder grades to withstand the greater unit pressure.
Straight wheels are the most traditional type of grinding wheel, with the grinding face on the periphery of the wheel. Recessed wheels are variations on this form, featuring a recessed center to fit on a machine spindle flange assembly. The other major type of wheel shape uses a cutting face on the side of the wheel — names for this type of wheel include cylinder wheels, cup wheels and dish wheels, depending on the particular shape. For these wheels, bonded abrasive sections of various shapes, also known as “segments,” are assembled to form a continuous or intermittent side grinding wheel.
Operational Basics
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.
Traverse distance, or crossfeed, is the distance a workpiece moves across the face of the wheel. Lowering the traverse distance to no more than one-quarter of the wheel width improves surface finish, but slows down productivity. Increasing the crossfeed to one-half the wheel’s width or above boosts productivity, but lowers surface finish.
Different types of grinding use different methodologies to determine the work material removal per unit of width, but one consistently useful metric for shops is the grinding gratio, or g-ratio. This is the ratio of volume of work removed to volume of wheel consumed (or, volume of work removed ÷ volume of wheel worn). From a cost standpoint, a higher g-ratio is better.