Volume 16 - Machining Part 16 pps

Table 4 Advantages of diamond abrasive bond typesResin bond • Readily available • Easy to true and dress • Moderate freeness of cut • Applicable for a range of operations • First se

Fig 12 Superabrasive wheel configurations and their designations

Construction. Because of their long wheel life and the higher cost of the superabrasives, these wheels are generally used in a rim-type construction Exceptions are extremely small inside diameter or very thin grinding wheels

The annular region of the wheel containing the superabrasives, called the rim, is integrally bonded to the core or structural part of the superabrasive grinding wheel (Fig 13) The core is generally made of composites, aluminum, bronze, steel, or ceramic, depending on such performance requirements as strength, stiffness, and dimensional stability

Fig 13 Construction of a typical superabrasive wheel

Concentration. The rim, or grinding face, consists of a bond, or matrix, that contains the superabrasive grains The volume fraction of the abrasive grains in the rim is known as the concentration This often determines the performance or behavior of superabrasive wheels

Bond Systems. Four bond systems are typically used in superabrasive wheels:

Table 4 Advantages of diamond abrasive bond types

Resin bond

• Readily available

• Easy to true and dress

• Moderate freeness of cut

• Applicable for a range of operations

• First selection for learning the use of diamond wheels

Vitrified bond

• Free cutting

• Easy to true

• Does 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 bond

• 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 requires high grinding forces and power

• Difficult to true and dress

Layered products

• 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

• High abrasive density

• Generally not truable

• Generally poorer surface finish than bonded abrasive wheels

Resin bond wheels provide good resilience and vibration-absorbing characteristics, which reduce chatter at the grinding zone Wheels with resin bonds are easy to true and dress and are commonly selected for a wide range of applications

Vitrified bond wheels offer controlled porosity, which facilitates chip removal and coolant flow to the grinding zone They generally last longer than resin bond wheels and are suitable for producing accurate and complex forms

Metal bond wheels, although difficult to true and dress, offer long life, good form-holding characteristics, and good thermal conductivity They are excellent for simple form grinding, but usually require greater grinding forces and more power than resin or vitrified bond wheels

Layered product wheels utilize a single abrasive layer plated or brazed to a premachined preform They usually produce a poorer surface finish than bonded abrasive wheels Layered product wheels are used for small production runs

or where tolerance and surface finish are not very critical

Superabrasive Wheel Applications

The proper use of superabrasive wheels often requires careful evaluation of all factors of the grinding system, such as:

• Machine tool

• Work material

• Wheel selection

• Operational factors

Table 5 lists some of the key variables that affect each of these factors

Table 5 Variables influencing grinding operations with superabrasives

• Truing, dressing, and conditioning techniques and devices

• Grinding cycle optimization

maximize the abrasive/work interactions leading to chip generation and grinding efficiency and to minimize the rubbing

or interaction at the bond/work, chip/bond, or chip/work interfaces

Fig 14 Schematic illustrating interactions in the grinding zone of a grinding wheel/workpiece interface 1,

abrasive/work interface; 2, chip/bond interface; 3, chip/work interface; 4, bond/work interface

Machine Tool Variables

Machine tool developments in the past 20 years have contributed to innovative superabrasive applications Precision spindles and slides, rigid machine frames, accurate positioning methods, multiaxis computer numerical control (CNC) movement to achieve complex geometries with a high degree of accuracy, high-speed spindles, and high-pressure flow coolant systems are some of the features incorporated into grinding machines using superabrasive wheels

High-speed tool steel end mills are produced in the conventional method by milling the flutes on a cylindrical rod and then heat treating (Fig 15a) During heat treating, the flutes become distorted and require finish grinding to restore flute geometry and to generate other features of the cutting geometry It is also expensive to maintain an in-process inventory

of the premachined blanks Capital and labor costs for this process are high The conventional process has been significantly improved upon by heat treating the rod and then grinding in the flutes with a CBN wheel (Fig 15b), thus eliminating the milling operation used in the conventional process The improved process utilizes the high material removal rate capability of CBN superabrasive wheels while maintaining the form or geometry of the wheel face for a relatively long time However, the improved process requires the following:

• Multiaxial CNC machines of high rigidity that have the flexibility to be programmed for a range of part geometries

• Oil coolant systems able to withstand high pressures and high fluid flows

• Suitable enclosure to ensure operator safety

• High-precision truing and dressing equipment that lends itself to automation should production quantities warrant such an investment

Figure 16 shows the setup used to grind in flutes in an end mill and illustrates the array of coolant lines required for the machining operation

Fig 15 Methods of producing high-speed tool steel end mills (a) Conventional process in which flutes are

milled in prior to hardening (b) Improved process in which flutes are ground in with a CBN wheel after hardening The new process proved to be more cost effective and produced an end mill with more exact tool geometry

Fig 16 Schematic of tool setup for grinding in flutes on an end mill (the improved process) showing positions

of coolant lines

Wheel Selection

The shape, size, configuration, and features of superabrasive bond types are described in the section "Bond Systems" in this article A typical example of superabrasive wheel designation is shown in Fig 17 Wheel manufacturers should be consulted on the details of each specification and their influence on grinding results Additional information on wheel selection is available in the article "Grinding Equipment and Processes" in this Volume The principles of superabrasive wheel applications are discussed in the sections "Diamond Grinding Wheels" and "Cubic Boron Nitride Grinding Wheels" in this article

Fig 17 Typical specifications used for superabrasive wheels

Diamond Grinding Wheels. Diamond grinding wheels are used for a wide variety of work materials, such as carbide, glass, industrial ceramics, plastics, electronic ceramics, and composites and high-density/structural ceramics Specification data for the materials are extensive and readily available

Grinding of Carbide Materials. Figure 18 shows the effect of diamond abrasive particle size, concentration, and type

on surface finish, metal removal rate, and G ratio (volume of work removal/volume of wheel worn) These qualitative

curves are based on data compiled from grinding carbide with diamond abrasives

Fig 18 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against particle

size (a), concentration (b), and diamond type (c) to show the relative properties of diamond abrasives in the grinding of carbides

Figure 19 shows the effect of work material structure (toughness), chip size produced, and abrasion resistance of the bond

used on surface finish, metal removal rate, and G ratio Figure 20 shows the effect of wheel speed, Vs, and grinding

pressure on surface finish, metal removal rate, and G ratio Grinding pressure is a control variable commonly used in

toolroom or insert grinding operations

Fig 19 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against material

(a), chip type (b), and bond type (c), to show the relative properties of diamond abrasives in the grinding of carbides

Fig 20 Plots of surface finish (curve A), metal removal rate (curve B), and G ratio (curve C) against wheel

speed (a), and normal force (b), to illustrate the relative effect of operating conditions on carbide grinding

Grinding of Ceramics. The advent of strength structural ceramics and their possible use in a variety of performance applications offer the potential for even wider use of diamond abrasive wheels Some recent results are discussed in this section

high-Figure 21 shows the influence of grit size, bond type, and material removal rate in the grinding of hot-pressed silicon

nitride (HPSN) Figure 21(a) shows the grinding forces measured normal to the workpiece surface (FN), and Fig 21(b)

shows the tangential grinding forces measured parallel to the work surface or in the direction of table traverse (FT) Figures 21(a) and 21(b) show the forces in a normalized scale; therefore, a direct comparison is possible between the bonds used and the grit sizes The normal and tangential forces are generally higher for the finer grit, within the experimental conditions of 50 to 255 mm/min (2 to 10 in./min) of table speed, 2.5 mm (0.100 in.) downfeed, and the corresponding normalized (unit-width) metal removal rate

Fig 21 Effect of bond type and grit size on normal (a) and tangential (b) forces in the grinding of hot-pressed

silicon nitride Wheel speed was 28 m/s (5500 sfm) at both low (2 mm 3 /s, mm; or 0.2 in 3 /min, in.) and high (10 mm 3 /s, mm; or 1.0 in 3 /min, in.) unit-width metal removal rates M2 indicates a modification of the original metal bond (M1) Grit sizes are 180 and 320

Higher forces were generally observed at finer grit sizes for the three bond systems evaluated This conclusion may be more suitable for the grinding of structural components (such as cutting tools), than for electronic applications, in which the normalized metal removal rates used are generally well below the values tested in this example

Among the bond systems evaluated, the vitrified bond generated the lowest normal forces, while the resin bond generated the lowest tangential forces (or lowest grinding power) The metal bonds evaluated required higher normal and tangential forces The modification to the metal bond (M2) to improve the free cutting action of the wheel appeared to be significantly beneficial at the higher normalized metal removal rate

Figure 22 shows the wheel wear measured as G ratio for the test conditions shown in Fig 21 In general, finer-grit diamond that required higher forces exhibited lower G ratios (within the experimental conditions evaluated) However, the metal bond wheels that required higher forces also exhibited higher G ratios High tangential forces and lower G ratios

(despite the lower normal forces) are measured for the vitrified bond wheel This highlights the significance of coolant application in the grinding of ceramic materials, particularly with vitrified bond diamond wheels When the test was conducted in a machine setup with better coolant application conditions, the vitrified bond wheel showed relatively high

G ratios at the higher unit-width metal removal rate (Fig 22)

Fig 22 Effect of bond type and grit size on G ratio in the grinding of hot-pressed silicon nitride for conditions

shown in Fig 21 Grit sizes are 180 and 320 Wheel speed was 28 m/s (5500 sfm) at both low (2 mm 3 /s, mm;

or 0.2 in 3 /min, in.) and high (10 mm 3 /s, mm; or 1.0 in 3 /min, in.) unit-width metal removal rates M2 indicates

a modification of the original metal bond

The normal forces measured are shown in Fig 23 Hot-pressed silicon nitride requires higher grinding forces than any of the other ceramics evaluated, including tungsten carbide Ferrite, Al2O3-TiC, and zirconia require relatively lower forces The relative grinding power required for the ceramics evaluated is shown in Fig 23(b) The wheel wear measurements

indicate that ferrite has the highest G ratio, followed by zirconia, Al2O3-TiC, hot-pressed silicon nitride, and tungsten carbide (in decreasing order)

Fig 23 Relative unit-width normal force (a) and relative unit-width grinding power (b) required to machine

various structural and electronic ceramics Unit-width metal removal rates classified as low (2 mm 3 /s, mm; or 0.2 in 3 /min, in.), medium (5 mm 3 /s, mm; or 0.5 in 3 /min, in.), and high (10 mm 3 /s, mm; or 1.0 in 3 /min, in.)

Cubic boron nitride grinding wheels can be evaluated with regard to wheel life, concentration, equivalent diameter, surface finish, grit size, and coolant application

Wheel Life. The wear of a grinding wheel is measured in terms of the G ratio, which is the ratio of the volume of work material removed divided by the wheel wear volume This G ratio represents a measure of the real life of the abrasive as

well as the useful life of the abrasive wheel itself

The wheel lives of CBN wheels are often 100 to 1000 times longer than those of conventional abrasive wheels Figure 24 compares the properties of a conventional aluminum oxide wheel with those of a CBN wheel for grinding 52100 bearing steel, M7 high-speed tool steel, and M50 and Inconel 718 The long life of the CBN wheel offers the advantages of lower wheel change frequency, lower machine downtime, consistent part geometry, and ease of automation Because of this long life, the frequency of dressing superabrasive wheels is substantially lower than that of conventional abrasive wheels Proper dressing procedures must be followed to obtain the maximum useful life of CBN wheels and to achieve maximum economic benefits

Fig 24 Comparison of CBN and conventional abrasive wheel performance in the grinding of 52100 bearing

steel (a), M50 high-speed tool steel (b), M7 high-speed tool steel (c), and Inconel 718 (d) With the exception

of the M50 material, which used a water-soluble oil and was inside diameter ground, all of the materials were

outside diameter ground using a 100% oil coolant

Concentration denotes the amount of superabrasive present in the wheel and often represents the volume fraction of the abrasive Figure 25 shows the effect of concentration in CBN wheels In general, the higher the concentration, the longer the wheel life and the better the surface finish In electronic ceramics, higher-concentration diamond wheels are used to achieve tighter tolerances in part geometry and to minimize chipping of the parts

Fig 25 Effect of CBN concentration on wheel and workpiece properties The data are for a vitrified bond CBN

wheel used to grind 52100 bearing steel (a) Normalized metal removal rate values for plot of G ratio versus

CBN concentration (b) CBN concentration for plot of surface finish against unit-width metal removal rate

Equivalent diameter, DE, determines the conformity between the wheel and the workpiece The closer the conformity, the higher the equivalent diameter Internal grinding generally creates higher conformity than external cylindrical grinding Figure 26 shows that external grinding with lower conformity grinds more efficiently (lower power consumption for given material removal rate) but produces a poor surface finish

Fig 26 Graphs demonstrating that external grinding cuts faster than internal grinding but produces a poorer

finish Material is 52100 bearing steel at 60 HRC that is being ground by a B240J150V CBN wheel dressed with

a rotary diamond wheel Curve A (which is external mode), DE is 30.5 mm (1.2 in.) and VS is 55 m/s (11,000

sfm); curve B, DE is 178 mm (7.0 in.) and VS is 60 m/s (12,000 sfm) (a) Workpiece unit-width volumetric removal rate plotted against unit-width normal force (b) Unit-width power plotted against workpiece unit-width

volumetric removal rate to obtain slope, which equals specific power (c) G ratio and average surface finish

plotted against workpiece unit-width volumetric removal rate

The surface finish produced by CBN grinding wheels is determined by the dressing or exposure of the abrasive grains achieved in resin and metal bond wheels In vitrified bond CBN wheels, the truing procedure and wheel specification dictate the surface finish achieved

Unlike conventional abrasive wheels, CBN wheels are generally not dressed within a grinding cycle to achieve a dull wheel face and therefore improve the work surface finish In most cases, the work surface finish is governed by the

highest material removal rate that the CBN wheel is subjected to, which generally occurs during the rough grinding cycle Therefore, setting the cycle to achieve a short cycle time while maintaining the lowest normalized metal removal rate during rough grinding is a desirable strategy when using CBN wheels for production grinding

Grit Size. In any application in which a conventional abrasive such as aluminum oxide is performing successfully, two conditions can be envisioned:

• The abrasive undergoes self-sharpening due to its inherently low thermal fatigue resistance and consequent microfracturing

• The bond system can be designed to self-sharpen and thus shed the low-cost abrasives (when they become dull and require high grinding forces without serious economic consequences

When applying the CBN superabrasive, the conditions discussed below occur

The CBN abrasive has high thermal fatigue resistance and high wear resistance Any flats (flank face wear) generated in the abrasive grain during the truing process or during grinding cannot be easily removed and will cause high grinding forces This is similar to a cutting tool with large flank wear Therefore, selection of the proper grit size is critical for avoiding excessive forces and consequent poor grinding results (such as chatter, burn, and hard grinding action) The bond system cannot be designed to be excessively forgiving (soft), because it is necessary to minimize the loss of expensive CBN superabrasive This information would suggest that the selection of CBN superabrasive grit size should

be the minimum size possible that is consistent with the metal removal rate required Typically, this grit size is in the range of 150 to 320 for replacing aluminum oxide abrasives used at 60 to 120 grit In addition, the truing parameters should be optimized to avoid the generation of excessive flats or large flank face wear (Fig 27) For example, if the influence of the abrasive grit size and truing procedures (leading to flank face rubbing) is not recognized, hard grinding action will be mistakenly attributed to the entire line of CBN abrasives

Fig 27 Selection of superabrasive grit size 60-grit aluminum oxide (a) yields a bond system comparable to

100-grit CBN having wear flats (b) or 150-grit CBN without wear flats (c)

Effect of Coolants. The importance of pressure, flow, and direction of coolant application for superabrasive wheels is covered in the section "Machine Tool Variables" in this article In addition, a 100% oil coolant applied at high pressure and flow rate and properly directed at the grinding zone generally improves CBN wheel performance significantly (Fig 28) However, heat removal from the grinding zone may be the predominant requirement of the coolant in order to maintain part geometry and to avoid burn marks on thin sections or in heat-sensitive materials In such cases, water-soluble oil may be preferable as a coolant The application of high-pressure coolants and the use of 100% oil as a coolant necessitate proper guarding and ventilation of the system to avoid contamination of the grinding room atmosphere with oil mist

Fig 28 Effect of coolant on grinding performance with CBN wheels The operation is the inside diameter

grinding of M7 high-speed tool steel using a B180J100V wheel A, 5% water-soluble oil; B, 100% oil coolant

(a) Unit-width power plotted against unit-width metal removal rate (b) G ratio plotted against unit-width metal

removal rate

Effect of Wheel Speed. The optimum wheel speed for CBN grinding is approximately 45 m/s (9000 sfm), but the speeds typically used range from 25 to 35 m/s (5000 to 7000 sfm) Higher grinding wheel speeds produce lower grinding forces, better wheel life, and better surface finish (Fig 29) These benefits are generally utilized in higher-accuracy grinding to achieve better-toleranced parts or in higher-productivity grinding to obtain more parts per hour without sacrificing tolerance requirements However, high-speed grinding is always associated with higher grinding power (for a given unit-width metal removal rate) and therefore requires better coolant systems to prevent workpiece burn or damage

to the part High-speed grinding is also often associated with special machine designs having a high-accuracy spindle and low vibration levels which lead to improved wheel design for safe operation at the higher speeds

Fig 29 High wheel speed cuts faster and improves surface finish Material is 52100 bearing steel at 60 HRC

that is being internally ground by a B240J150V CBN wheel dressed with a rotary diamond wheel Equivalent

diameter is 127 mm (5.0 in.) Curve A, VS is 60 m/s (12,000 sfm); curve B, VS is 30 m/s (6000 sfm) (a)

Workpiece unit-width volumetric removal rate plotted against unit-width normal force (b) Unit-width power

plotted against workpiece unit-width volumetric removal rate to obtain slope, which equals specific power (c) G

ratio and average surface finish plotted against workpiece unit-width volumetric removal rate

The fatigue life of parts ground with CBN wheels is reported to be higher than that of unground case-hardened components such as gears The reason for this improvement is not well understood However, the use of CBN for gear grinding is being pursued to obtain increased gear life, smaller gear size at equal fatigue life or higher efficiency, and lower noise levels for transmission applications

Truing. For the successful use of superabrasive wheels, the wheel surface must be concentric, free of round lobes, and straight across the thickness of the wheel, and it must have the correct profile for form wheels

The process used to generate geometrically correct wheel faces is called truing A properly trued wheel will grind with minimum or no chatter and will generate a straight cylinder, flat surface, or accurate profile on the workpiece, provided it

is also dressed properly (Fig 30)

Fig 30 Typical examples of conditions that require truing

Dressing. After truing, the wheel face is generally very smooth without much exposure of the superabrasive grits For efficient grinding, the bond adjacent to the superabrasive grits will have to be eroded away, exposing the superabrasive grits A properly exposed superabrasive wheel surface contains abrasive grits supported in the trailing side with tails In the leading side, the bond is removed for about 30% of the grit size for free cutting action The grits are connected by shallow grooves for chip clearance and cool flow Exposing the superabrasive grits on the wheel surface for efficient grinding action is called dressing (Fig 31)

Fig 31 Schematic of a wheel that has been trued and dressed (a) After truing Wheel face is smooth and

closed (b) After dressing Wheel face is open with grits exposed and ready for efficient grinding (c) After dressing Bond supports the grit (d) After dressing Note path connecting the tails for efficient coolant and chip flow

A properly trued and dressed wheel produces a workpiece with the required geometry, tolerance, and surface finish; draws minimum grinding power; and produces a workpiece without burn, surface damage, or chatter marks Truing and dressing are required during the first setting up of the wheel Resin and metal bond wheels must be trued and dressed Vitrified bond wheels generally require truing only and do not require dressing Electroplated superabrasive wheels do not require either truing or dressing, except in special situations

Conditioning. When parts of acceptable quality are being ground, the superabrasive wheel may require periodic conditioning to restore the geometry of the part, the surface finish, or both The conditioning may involve minor truing and/or dressing (Fig 32)

Fig 32 Typical conditions that require periodic correction to the wheel face (conditioning) (a) Out-of-form

wheel requires precision truing to restore geometry (b) Poor surface finish caused by bond erosion can be corrected by precision truing of the wheel face (c) Freeness of cut is restored by precision dressing to reduce worn out CBN grit and exposed grit

Conventional abrasive wheels for precision production grinding are generally vitrified bond wheels They do not require dressing In this case, the terms truing and dressing are used synonymously Truing and dressing are not synonymous terms for superabrasive wheels

Truing Methods for Production Grinding

A variety of methods are available for superabrasive wheel truing in production grinding operations These methods can

be broadly divided into three categories:

• Stationary tool truing

Single-point truing can be used for very small vitrified wheels or for small scale infrequent production However, the diamond point of the tool wears rapidly, requires frequent turning, and demands considerable operator attention As a result, the single-point truing of CBN wheels is generally not recommended

Cluster tool truing generates high truing forces This method is not recommended

The nib (Fig 33a), containing many small diamond particles, is the preferred tool for stationary truing The truing forces generated are acceptable

Fig 33 Truing methods used for CBN wheels (a) Stationary tool truing with a nib (b) Powered truing using a

rotary cutter (left) or a rotary cup (right) (c) Form truing using a diamond roll (left), tracing with rotary cutters (center), and a crush roll (right)

Powered Truing Methods. A systematic study of the truing process indicates that powered truing methods utilizing a rotating tool offer the best results (Fig 33b) Figure 34 shows the truing forces generated by a diamond nib and a rotary cutter under identical conditions on a resin bond CBN wheel The truing forces are considerably lower with a rotary tool than with the stationary tool In addition, the rate of increase in truing force with a rotary tool is considerably smaller

Fig 34 Comparison of truing forces in the nib truing and rotary truing of CB100WBA wheel

Commercially available rotary powered truing devices are versatile to use They also generate low truing forces, offer consistent results, and are easily automated Wheels with a straight face or simple forms can be trued with a traversing diamond rotary cutter or a rotary cup; plunge truing with a diamond roll can also be used

Form Truing. Complex form wheels can be trued by plunge truing with diamond rolls or crush truing methods (Fig 33c) Another method of truing form wheels is the use of a thin rotary cutter (considerably thinner than the wheel width) This rotary cutter can be traversed across the wheel profile with a mechanical arrangement that generates the wheel profile, for example, using a form bar or a CNC-controlled form generator Cubic boron nitride form wheels have been successfully trued with this approach in order to generate simple radius or complex profiles

Wheel Truing Objectives for Superabrasives

The essential feature of all CBN wheel truing methods is to provide accurate and precise relative motion between the wheel and the truing tool in order to generate the straight cylinders, tapers, flat surfaces, or forms required on the wheel Successful wheel truing depends on the five factors discussed below

Minimizing Truing Tool Wear. Wear of the truing tool causes loss of form, crowning, or a taper on the wheel face Excessive wear is generally associated with poor tool selection, large truing forces, very small traverse speed, high infeed rates, abusive conditions of wheel truing, or excessive wheel runout that could have been reduced during mounting of the wheel

Ensuring Consistent Wheel Surface Quality. Unlike conventional abrasive wheels, a CBN wheel is not trued after every part or grinding cycle If properly operated, it should be conditioned after typically 20 to 100 grinding cycles and as often as every 3 months Once the conditioning frequency is established, the wheel should be conditioned at regular intervals If this is not done, the production schedule will be disrupted, and increased operator attention will be required

The truing mechanism should be reliable, accurate, and rigid to ensure consistent wheel surface quality The machine should have the capability for precision movement and skip dressing

Roughing/Finishing Using the Same Wheel Face Without Conditioning. Cubic boron nitride wheels can grind at high unit-width metal removal rates and can produce good surface finishes at low unit-width metal removal rates This is possible because the CBN grit resists wear The open condition of the wheel allows for high unit-width metal removal rates, coolant flow, and chip removal without burning The flats generated on the CBN grits during the truing operation determine the surface finish at finish grinding rates Excessive flats on coarse grits will generate a fine surface finish, but burning or smearing can occur

Machine Tool and Truing Device Requirements. The truing of CBN wheels can be satisfactorily accomplished on moderately rigid (18 kN/mm, or 100,000 lbf/in.) machines Excessive rigidity is useful but not necessary However, a machine with worn ways, inconsistent feed rates, stick slip, and lack of fine infeed control (for example, 0.005 mm, or 0.0002 in per step or lower) will degrade any truing effort Similarly, high precision, low runout, and moderate rigidity

are required in the truing devices Good-quality coolant flow, proper diamond selection, and smaller-grit diamonds generally improve the truing

Goals for Successful Truing. In a properly managed truing effort, it is necessary to:

• Minimize truing forces

• Minimize truing time

• Control the exposure and extent of flatness on CBN grits to obtain the desired surface finish

• Minimize loss of superabrasive grain

• Automate the truing process to achieve consistent wheel surface condition

Truing Methods for Batch Production

For operations in which the wheel is used for short production runs, with each run containing different parts (for example, toolroom grinding), powered rotary truing methods are preferred and should be used whenever possible However, for batch production, the truing operations can be carried out using the methods discussed below

Truing With Abrasive Wheels. Brake-controlled and powered rotary truing devices use conventional abrasive wheels The precision with which a trued wheel surface is generated by these methods is lower than in the methods described earlier In general, a relatively harder wheel with silicon carbide abrasive will true the superabrasive wheel faster at lower levels of abrasive consumption However, an open wheel surface is obtained with a softer wheel containing aluminum oxide abrasive Occasionally, the superabrasive wheels can be finished to the required form or geometry by using a grinder with an optical attachment

Truing With Hard Ceramics. Resin bond CBN wheels can be trued by grinding with hard ceramics such as tungsten carbide or boron carbide In this method, 0.005 mm (0.0002 in.) downfeed per pass is used in a surface grinder Slightly more than half the wheel thickness should be crossfed, and table speed should be moderate The wheel speed is the same

as the grinding speed The grinding power will gradually increase as the wheel becomes dull while being trued When the power exceeds the normal power drawn during workpiece grinding, truing should be halted The wheel face should be stick dressed with an abrasive stick Truing is then continued This cycle is repeated until the wheel is completely trued Diamond nibs and, occasionally, single-point tools can also be used for truing CBN wheels in batch operations

Quantitative Understanding of Truing Parameters

The following variables apply to any truing operation:

• Truing method used: Stationary or powered rotary truing

• Type of truing tool: Stationary (single-point or nib) or rotary (disk, cup, diamond roll, bonded or plated

cutter, or reverse-plated cutter)

• Specification of truing tool: Diamond type, grit size, concentration, and geometry

• CBN wheels to be used: Bond type, grit size, concentration, and wheel size

• Truing process variables: Speed, infeed increments, speed ratio, overlap ratio, relative motion,

crossfeed, coolant flow, and machine rigidity

• Output variables: Truing forces, power, chatter or lobing, total indicator reading, accuracy of form,

wheel surface quality and truing time, and grind quality after truing

Some of the key variables are discussed below

Infeed is an incremental advance of the truing tool or cutter relative to the wheel face The infeed used for superabrasive wheel truing with a diamond tool or cutter should be about one-tenth the values used for conventional abrasive wheels This generally implies an infeed of 0.0025 to 0.005 mm (0.0001 to 0.0002 in.) per increment in traverse truing and 0.050

to 0.10 m (2 to 4 in.) per revolution in plunge truing

The effect of infeed on truing forces is shown in Fig 35 Control of infeed is a key aspect of achieving outstanding performance of superabrasive wheels without chatter or hard grinding action

Fig 35 Effect of truing infeed on truing force A, resin and metal bond CBN wheel; B, vitrified CBN wheel; C,

conventional vitrified wheel

Intermittent Dressing. Figure 36 shows the gradual increase in truing forces as a function of time or number of traversals of the cutter across the wheel face However, if the truing forces exceed the normal grinding forces, deflection

of the spindle, wheel lobing, and chatter will occur Intermittent or periodic dressing of the wheel minimizes these problems

Fig 36 Effect of intermittent dressing of a CBN wheel on truing forces

Parameters for Rotary Truing. The ratio of cutter speed to wheel speed is called the speed ratio Selection of the speed ratio and its relative direction determines truing forces and such grinding results as work surface roughness and wheel wear

Grit Size. The diamond cutter dressing tool for superabrasive wheels generally requires smaller grit sizes than those used for conventional abrasive wheels Large diamond grit in the cutter generates higher truing forces and poor grinding results

Overlap ratio is the ratio of the width or thickness of the cutter to the lead The smaller the overlap ratio, the more open the wheel face, the lower the grinding power, and generally the shorter the wheel life However, high overlap ratios can lead to a dull grinding wheel that results in poor wheel performance

Practical Dressing Methods

Several methods are available for dressing CBN wheels These include:

• Abrasive stick dressing using mechanical devices

• Abrasive-jet dressing

• Slurry dressing

• High-pressure waterjet dressing

Abrasive stick dressing using mechanical devices is the most frequently recommended practice and is suitable for a wide variety of applications This method consists of pushing an abrasive stick (called a conditioning stick) into the wheel face with a constant force or a constant infeed rate (Fig 37) Several variations of the stick dressing arrangement are used in industry

Fig 37 Schematic illustrating abrasive stick dressing Low coolant flow is required during the stick dressing

Fig 38 Variation of relative dressing force with time at a constant infeed rate of the dressing stick (a) Normal

force (b) Tangential force

Figure 39 shows the results of constant-force dressing tests The CBN wheel was trued under identical conditions The wheel was then dressed using abrasive sticks of 600, 320, and 220 grit; the same grade; and two force levels: 65 and 135

N (15 and 30 lbf) In each case, the wheel was dressed using 25, 50, 75, and 102 mm (1, 2, 3, and 4 in.) of stick length The resultant wheel face was used to grind M2 high-speed tool steel under identical conditions The work surface finish was measured to reflect indirectly the wheel face roughness The wheel surface becomes rougher as the abrasive grit in the dressing stick becomes coarser For the same abrasive stick (A32OJ), the increase in dressing force also increases wheel surface roughness

Fig 39 Effect of dressing stick grit size and dressing stick consumption on wheel roughness (as measured by

work surface finish) during constant-force dressing Work material ground was M2 high-speed tool steel using a CBN wheel (CB 220 WBA) measuring 255 × 13 × 3.2 × 32 mm (10 × × × 1MATH OMITTED in.) and a wheel speed of 46 m/s (9000 sfm) The dressing sticks measured 13 × 13 mm ( × in.)

The exposure of CBN is not the only factor influencing the work surface finish; the CBN grit size also has a strong influence Figure 40 shows the grinding performance of two CBN wheels of the same concentration but different grit size that were dressed under the same conditions (constant force, constant volume of abrasive used, and constant grinding conditions) The finer-grit CBN wheel produces workpiece surface roughness that is dependent on the abrasive grit size used for dressing

Fig 40 Effect of dressing stick parameters on CBN wheels for constant infeed rate dressing

Such is not the case with the coarser-grit CBN wheel The coarser-grit CBN produces finer work surface roughness than CBN of smaller grit size The quality of work surface produced by 100-grit CBN is poorer because of smearing or a burnished appearance Therefore, in grinding with CBN, it is preferable to use finer grit sizes without sacrificing wheel life for a given metal removal rate

Abrasive-jet dressing consists of impacting the CBN wheel face with a jet of fine-grit abrasive propelled by pressure air This method is also suitable for dressing a contour or form on the wheel face The system is expected to be portable from machine to machine within a plant

high-Slurry Dressing. Some machines that are specifically designed for CBN use an abrasive slurry (abrasive entrained in water) pumped across a gap between the wheel and a steel roll

High-pressure waterjet dressing consists of a high-pressure waterjet directed at the wheel face to erode the bond in

a controlled manner Preliminary test results show that these methods generally required an abrasive waterjet rather than

an ordinary waterjet

The cost of such units for slurry dressing and waterjet dressing and the specialized nature of the equipment involved do not justify these methods for a wide variety of applications However, the methods may become useful under special machine and production conditions

Dressing Methods for Batch Production

Abrasive Stick Dressing. Manual stick dressing is used in a number of batch production operations However, from the standpoints of safety and consistency, manual stick dressing should be discouraged The stick dressing can be mechanized whenever possible by mounting the stick on the machine table and rapidly feeding it into the wheel with machine controls Proper guarding is essential in such cases The conditioning sticks used for dressing CBN wheels should be distinguished from the soft rubber sticks used for cleaning the wheel face

The dressing block can be used as a work material in surface grinding, and the wheel can be dressed by grinding such blocks Care should be taken to prevent excessive wear of the CBN wheel or abrasive conditions that can lead to uneven wear

Dressing While Grinding. Cubic boron nitride wheels, particularly resin or vitrified bond wheels, can be dressed by erosion of the bond during grinding This invariably results in a loss of work material during dressing due to burn or improper work quality In addition, depending on the grinding conditions, the wheel may not be adequately dressed at any time