What is Grinding?

What is Grinding?
Posted by bnui4ui on 2021/09/02
What is Grinding?

    What is Grinding?


    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.

    Types of Grinding


    Grinding operations come in many types, with this article covering six major types and several of the subtypes within.


    Cylindrical grinding is a common type of grinding in which both the wheel and the workpiece rotate. The workpiece is

either fixed and driven between centers, or driven by a revolving chuck or collet while supported in a center. This operation

can take place with either traverse movements, where the wheel traverses axially along the part, or plunge movements, where

the wheel is thrust into the part. Straight wheels are most commonly used in cylindrical grinding, with common cylindrical

grinding machines being plain cylindrical (or roll) grinders, centerless grinders and inside- or outside-diameter grinders.

Internal cylindrical grinding does the internal diameter grinding of bores and holes, generating size and concentricity

within millionths of an inch. The grinding wheels tend to range in diameter from half an inch to three inches. This small

size introduces rapid wear, making CBN and diamond wheels in crush dressable and vitrified form popular for these

applications.

    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.

    Snagging is a rough grinding application that removes unwanted metal with little consideration of surface finish. As

such, it uses durable straight and straight cup wheels in horizontal and straight shaft grinding machines, although flaring

cup wheels are used in right-angle grinders and various round and square-tipped cones and plugs also see use. Typical

applications include removing unwanted metal on castings; removing flaws and cracks; removing gates, risers and parting

lines; rough beveling; grinding down heavy welds; and preparing surfaces for cleaning or painting.

    Cut-off operations use an abrasive wheel as an alternative to the laser, abrasive water jet, metal saw, friction saw and

oxyacetylene or plasma arc torch. A study from Norton Abrasives demonstrated that the abrasive wheel can outperform these

other methods with ferrous materials, and that the abrasive wheel is faster and less expensive for nonferrous materials than

the common metal saw choice. The abrasive wheel provides more cutting points than a saw, and cuts just as thoroughly at a

speed of 2 or 3 miles per minute. Cut-off wheels should run at the highest possible speed, with one horsepower for every inch

of wheel diameter. If this proves impossible, use a softer wheel. Production jobs use non-reinforced wheels, with non-

reinforced shellac wheels for applications requiring extreme versatility and quality of cut. Reinforced wheels are compatible

with portable cut-off, swingframe, locked head push-through and foundry chop stroke operations.




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