Grinding

A wide variety of abrasive finishing processes termed grinding are practiced in the finishing of industrial components. They offer the following advantages (compared to machining processes that use defined cutting edges):

• The range of surface geometries generated (e.g., contours, profiles, etc.) is frequently more complex than is economically possible in single-point machining processes.

• Grinding wheels can be used for high stock removal (i.e., rough grinding). The same wheel is often modified in situ to achieve an extremely fine finish or close tolerance during finish grinding processes. In situ modifications of cutting tools are not possible or practical.

• Grinding wheels can be designed so that the abrasive, bond, and pore distribution are carefully controlled to meet unique requirements of surface generation efficiency and quality of the generated surface.

• The material removal rate per cutting edge is often controlled by abrasive grain size and content, and the total volume of material removed per unit of time can be modified by the design of the wheel geometry and the kinematics of the wheel/work motions. The separation often allows for control of surface features somewhat independently of production economics. This separation is not possible in the processes described in the article "Finishing Methods Using Defined Cutting Edges" in this Volume.

Typical grinding processes are shown in Fig. 5. Each of these applications can range from rough grinding to precision grinding to high-precision processes. Many of these operations in rough grinding are carried out using handheld grinders or manual effort. However, a large number of rough grinding processes and almost all precision grinding processes use automated equipment. They occasionally involve manual loading or unloading of parts and wheels. Large-volume production operations are often fully automated. Recent advancements in CNC grinding machines have extended the capability of conventional grinders by allowing for grinding of multiple surfaces in a single setup, using more than one wheel at a time as well as grinding multiple surfaces simultaneously.

Fig. 5 Grinding processes

The grinding machine features that frequently determine the production rate and/or part quality are: • Rigidity/stiffness

• Vibration level

• Coolant systems

• Precision movements and positioning accuracy

• On-machine dynamic balancing capability

• Truing and dressing systems

• Multiaxis CNC capability

• Spindle horsepower

• Speed range and variability

• Torque/speed capability of the motor

• Materials handling systems

• Work space and its accessibility

• Grinding cycle design or capability

Grinding Cycle Design. One of the key elements in the grinding process is the design of the grinding cycle. This involves a programmed application of the grinding wheel against the work material. It can be measured or monitored in terms of the position of the slide in which the wheel or part is mounted with respect to the time.

Initially the slide is moved rapidly, thus removing a large volume of work material rapidly. This is the rough grinding portion of the cycle, as illustrated in Fig. 6(a). This is usually followed by a short dwell, during which the motion of the slide is sometimes stopped to allow the elastic deflections produced during rough grinding to be eliminated. The need for this dwell and its duration are often determined by the stiffness of the wheel, the work material, the work fixture, or the machine tool.

Fig. 6 Grinding cycle design. See text for details.

The rough grinding cycle has now affected the surface topography of the grinding wheel. This topography is modified by processes called truing and dressing, shown as "dressing cycle" in Fig. 6(a) and described in more detail later in this section. These processes are followed by a slower rate of grinding (judged by the smaller slope of the displacement/time curve), resulting in semifinish and finishing grinding cycles. The last portion of the cycle, akin to the dwell described earlier, is often called sparkout.

The suitable combination of rough, semifinish, and finishing grinding steps, with dwell, dress, and sparkout included as required, is called the grinding cycle. The design of the grinding cycle varies, depending on the machine tool characteristics and capabilities, grinding wheel characteristics and capabilities, work material properties and requirements, and operational conditions such as coolant applications, fixturing, and truing methods. Because of the range of variables that can influence the grinding cycle design, the process is often perceived to be an art. However, it is far from an art and can be managed as an engineered system with defined causal relations, as described in the section "Systems Approach" in this article.

The potential to alter the wheel face using the dress cycle is unique to precision grinding processes using grinding wheels. With coated abrasive belts containing a single layer of abrasives, this is often not possible. In those processes, individual coated abrasive product grades are assigned to rough, semifinish, and finish steps.

The rough grinding portion of the cycle can be modified by changing the rate of slide motion (and hence material removal rate), as shown in Fig. 6(b). This figure shows three alternative rough grinding paths, B-1, B-2, and B-3. These in turn require alterations in the semifinish and finish grinding steps, as shown. One reason for using an alternative path may be to reduce the amount of dressing or the number of dressing cycles during a grinding process. Paths B-1, B-2, and B-3 in Fig. 6(b) are less aggressive rough grinding paths that could ensure that wheel topography is not significantly affected during rough grinding, compared to path B-0 in Fig. 6(a). Hence, all these alternative paths can be accomplished and the part can be ground to final tolerances without the need for dwell in some cases and without the need for dress cycle in all cases, as shown in Fig. 6(b).

If the abrasive grain in the grinding wheel is an efficient cutting tool, then the forces or deflection induced during rough grinding may be small. This, combined with the efficient cutting action, may permit further reductions in the semifinish and finish operations and often eliminate the need for sparkout (Fig. 6c). Such optimizing of the cycle reduces the cycle time and hence improves productivity. This is one of the frequent objectives in using premium abrasive wheels, such as CBN grinding wheels for steel grinding.

The total force or power consumed during the rough grinding cycle is generally high, because the associated material removal rate is largest during the rough cycle. Thus, the deflection of the part under the applied forces of the machine tool is generally the limitation during the rough grinding cycle. Sometimes the work material or the coolant system is not able to dissipate the heat generated and the power input must be decreased, thus requiring alterations to the rough grinding cycle.

During the later portion of the grinding cycle (i.e., semifinish, finish, or sparkout), as the size of the chip generated decreases, the forces and power consumed also decrease. However, the chip generation efficiency decreases as the chip size decreases. (This is based on the mechanics of chip formation, described in the article "Finishing Methods Using Defined Cutting Edges" in this Volume.) The result is a large increase in specific energy, or the energy input per unit volume of the material removed. To avoid surface damage, this specific energy needs to be minimized in the later portions of the grinding cycle. The duration of the sparkout portion of the cycle is usually a balance between improved part geometry due to recovery from elastic deflections and the potential for surface damage due to the sliding of the abrasive wheel against the work material.

Truing and Dressing of Grinding Wheels. The tolerances and surface finishes produced on workpiece surfaces and the forces developed during grinding depend to a great extent on the manner in which the grinding wheel was prepared for operation. Preparation of the grinding wheel generally involves two operations: truing and dressing.

Truing refers to the process of generating a geometrically correct wheel surface in order to grind with minimal or no chatter. Successful use of grinding wheels requires that the wheel is concentric and free of lobes (Fig. 7). Conventional abrasives are most commonly trued by feeding a single-point or multipoint diamond dressing tool across the rotating wheel surface. Superabrasive wheels are trued with a vitrified silicon carbide truing wheel mounted on a brake-controlled truing device. Some of the other truing devices used are diamond truing rolls and cutters driven by hydraulic motors and diamond crush rollers.

Fig. 7 Typical examples of conditions that require truing

Dressing is the process of eroding the bond matrix in the wheel surface after a truing operation or after grinding in order to expose the abrasive grains for efficient grinding. After truing, the wheel surface is generally very smooth; the bond adjacent to the grits has to be eroded in order to expose the grits. Occasionally, the chips fill up the pores and clog the wheel during grinding, resulting in an inefficient cutting action. The wheels then have to be dressed in order to remove the chips from the wheel surface. Dressing is usually accomplished by pressing or sliding an abrasive stick against the wheel surface. Recently, in-process dressing of metal bond diamond wheels using electrodischarge machining has been introduced. Electrolytic in-process dressing is another emerging technology for dressing fine abrasive metal bond diamond wheels. Abrasive slurries are also occasionally used for dressing operations.

Wheel Shapes. Grinding wheels are used in a variety of operations, as shown in Fig. 5. The different geometric configurations require that suitable grinding wheels also be available, so grinding wheels are made in several different shapes, including cylinders, cups, cones, and plugs (Fig. 8). In general, conventional abrasive wheels can be readily "machined" to the desired shape, but this is generally not true with superabrasive wheels, which are therefore available in a wider variety of shapes. Superabrasives are almost always made to near-net shape as required prior to their use. (See the article "Superabrasives" in Volume 16 of the ASM Handbook.)

Fig. 8 Standard wheel configurations for conventional grinding wheels

Types of Bonds for Grinding Wheels. In order to effectively grind a large range of materials, a variety of bonding systems are available to hold the abrasives in the wheel. The most common bonding materials are resin (phenolic), ceramic (glass), and metal (bronze). Other bond types, such as shellac, oxychloride, rubber, and silicate are available but are not commonly used. The superabrasive wheels are also available in a metal-single-layer (MSL) specification that consists of a single layer of abrasives held together with the help of a metallic braze. Other superabrasive products, called plated wheels or E-process wheels, have a single layer of abrasives electroplated to the preform. Table 4 lists the characteristics of the four bond types available for grinding wheels.

Table 4 Characteristics of bond types used in abrasive products

Resin bonds

Readily available

Easy to true and dress

Moderate freeness of cut

Applicable for a range of operations

First selection for learning the use of superabrasive wheels

Vitrified bonds

Free-cutting

Easy to true

Do not need dressing (if selected and trued properly)

Controlled porosity to enable coolant flow to the grinding zone and chip removal

Intricate forms can be crush formed on the wheels

Suitable for creep-feed or deep grinding, inside-diameter grinding, or high-conformity grinding

Potential for longer wheel life than resin bond

Excellent under oil as coolant

Metal bonds

(Available with superabrasives only)

Very durable

Excellent for thin-slot,groove,cutoff,simple form,or slot grinding

High stiffness

Good form holding

Good thermal conductivity

Potential for high-speed operation

Generally require high grinding forces and power

Difficult to true and dress

Single abrasive layer plated on a premachined steel preform

Extremely free-cutting

High unit-width metal removal rates

Form wheels easily produced

Form accuracy dependent on preform and plating accuracy

Abrasive density is easily controlled

Generally not truable

Generally produce poorer surface finish than bonded abrasive wheels

Vitrified bonds made of clay, feldspar, and a glass frit are the most commonly used bonds for conventional abrasives and are becoming popular for use with superabrasives. They are rigid, free cutting, and have very good form retention. In addition, the porosity in these bonds can be controlled for more chip clearance and better coolant application at the grinding zone. These bonds do not have high impact resistance, however, and they are not used in heavy-pressure operations such as foundry snagging or steel conditioning.

Resin bonds are made of thermosetting polymers, usually phenol formaldehyde or epoxy resins. These bonds are resilient, have good impact resistance, and are very free cutting. Their rigidity can be varied by adding fillers such as glass fibers. Resin bonds are used in most rough grinding operations, such as snagging, weld grinding, and cutoff. Resin bonds are extremely popular for use with superabrasives and find extensive use in tool and cutter grinding, grinding of ceramics, carbide drill fluting, and glass beveling. The resilience of these bonds results in reduced chippage of brittle workpiece materials. Extremely fine-grit abrasives retained in flexible polyurethane bond material are commercially available and are used in extremely fine finishing processes.

Metal bonds are commonly used with superabrasives and are made from sintered bronze produced by powder metallurgy methods. Cast iron and aluminum bonds have been recently introduced. These bonds are very durable, have excellent form-holding characteristics, and have high stiffness. They require rigid machines, however, in order to withstand the high forces generated. Metal bonds are very popular in geological drilling, in asphalt and concrete cutting, and for cutoff wheels in precision electronic applications. Strong blocky diamonds are generally used as abrasives in metal bonds.

For more information on grinding wheels, see "Grinding Equipment and Processes" in Volume 16 of the ASM Handbook

Quantitative Aspects of Grinding Processes. When the work material is subjected to a grinding process, the interactions can be represented in terms such as equivalent diameter, chip size, specific energy, and grinding force. These parameters are described later in this section. A key parameter is the material removal rate (MRR), the volume of material removed in a unit of time. It is expressed as:

MRR = Work speed (vw) x depth of cut (d) x width of cut (bw)

When grinding takes place uniformly along the entire width of the wheel, the MRR can be normalized using the equation

Equivalent Diameter. Figure 9 is a schematic representation of the concept of equivalent diameter. Grinding processes of various configurations can be normalized for ease of comparison using the following equations:

• Outside diameter grinding: De = Dw x Ds/(Dw + Ds)

Inside diameter grinding: De = Dw x Ds/(Dw - Ds) Surface grinding: De = Ds where De is equivalent diameter, Dw is the work diameter, and Ds is the wheel diameter. Large equivalent diameter represents longer arc of contact between the wheel and the work material. Hence, as De increases, the sliding interactions between wheel and the work material become dominant. This is usually observed as increased grinding power or force for the same grinding condition. In practice, this situation is overcome by using a "softer" grinding wheel as the De is increased.

Fig. 9 Use of equivalent diameter to relate (a) internal and (b) external cylindrical grinding to surface grinding

There are unique situations, such as grinding a cam lobe profile, where De changes constantly from a small value, during outside diameter grinding of the nose or the base circle, to a large value, during surface grinding of the opening and closing ramps. The grinding wheel and the grinding system should be capable of accommodating such variations within one operation. Disc grinding or grinding of flat surfaces with cup wheels is another unique situation because the De can be perceived to be infinity. In such situations every attempt must be made to eliminate the frictional interactions. This is usually accomplished by using coolant slots in the grinding wheels, interruptions to the wheel face, and improved coolant applications.

Chip Size. The size of the chip produced by the abrasive grain during a grinding process (h) can be estimated as:

h = (Vw/Vs)1/2 x (dDe)1/4 x (KC)-m where vs is wheel speed, K = 1 to 20, and C is the number of grains per square inch of grinding wheel surface. C is larger for fine abrasive grains and smaller for large abrasive grains. Like De, chip thickness can be used to compare grinding operations. In some cases, the larger the chip thickness, the higher the "cutting efficiency" as measured by a decrease in specific energy. However, as chip thickness increases, the force per cutting edge may increase, thus degrading the surface finish.

Grinding force is the force exerted between the grinding wheel and the work material. These forces can be normal to the work surface (Fn), tangential to the wheel (Ft), and occasionally in the transverse direction (Fz).

Specific energy is the ratio of grinding power to the material removal rate. It is a measure of the energy input per unit volume of material removed.

G-ratio is the ratio of the volume of work removed per unit volume of abrasive product consumed. It is a measure of the life or durability of abrasive product for a given application. In many applications, the wheel is trued or dressed frequently or even continuously. In those situations, the G-ratio may need to reflect the abrasive consumed during these nongrinding operations.

Grinding system performance index (GSPI), or "grindability," is one measure of the ease or difficulty in grinding. It is defined as:

GSPI = G-ratio/specific energy

Higher values of GSPI imply lower specific energy, greater performance economy (larger G-ratio), or both. More commonly, G-ratio is equated with grindability, although it is an incomplete description of grinding results.

Effect of Material Removal Rates on Force and Power. Figures 10(a) and (b) show the variation of normal force and power with material removal rate in a typical grinding operation. In general, the force and power increase with an increase in material removal rate, and a minimum force is required to initiate cutting. This minimum value of force or power is termed the threshold force or threshold power. Thus:

Total power = Cutting power + threshold power which when divided by the MRR results in the relation:

Specific energy = Specific power + threshold power/MRR

Fig. 10 (a, b) Effect of material removal rate on force and power. (c) Work removal parameter

Specific power is the slope of the power-versus-MRR curve (Fig. 10b) and is another important parameter used to analyze grinding results. It represents the cutting component of the specific energy and in physical units could be considered the specific cutting energy. The threshold power can be approximated as the power component required to overcome friction and material deformation effects. If these frictional effects are absent, specific energy is equivalent to specific power (or, more precisely, to specific cutting energy).

Figure 10(c) is a plot relating the material removal rate to the normal force. Such data can be obtained on a grinding machine using force-measuring equipment and can be effectively used to differentiate between the "sharpness" or cutting efficiency of grinding wheels. The slope of the MRR-versus-Fn curve is called the work removal parameter (WRP). A steep slope indicates a sharp wheel and low force, and a shallow slope indicates a dull wheel and high force. Like the power relation, force is modeled as follows:

Total force = Cutting force + threshold force which when divided by the MRR results in the relation:

Total force/MRR = (WRP)-1 + threshold force/MRR where (WRP)-1 is the ratio of cutting force and MRR.

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