Davim J. Paulo (editor). Machining. Fundamentals and Recent Advances

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Grinding Technology and New Grinding Wheels 251

files, guide tracks, spline shaft profiles and gear tooth profiles. Operations that are

carried out using high-performance conventional grinding wheels as a matter of

routine are the grinding of:

• Auto engine: crankshaft main and connecting-rod bearings, camshaft bear-

ings, piston ring grooves, valve rocker guides, valve head and stems, head

profile, grooves, expansion bolts

• Auto gearbox: gear wheel seats on shafts, pinion gears, splined shafts,

clutch bearings, grooves in gear shafts, synchromesh rings, oil-pump worm

wheels

• Auto chassis: steering knuckle, universal shafts and pivots, ball tracks,

ball cages, screw threads, universal joints, bearing races, cross pins

• Auto steering: ball joint pivots, steering columns, steering worms, servo

steering pistons and valves

• Aerospace industry: turbine blades, root and tip profiles, fir-tree root pro-

files [1,

9.2.2 Grinding Wheel Selection

The selection of grinding wheels for high-performance grinding applications is

focussed on three basic grinding regimes: rough, finish and fine grinding. The

grain size of the grinding wheel is critical in achieving a specified workpiece

surface roughness. The grain size is specified by the mesh size of a screen

through which the grain can just pass while being retained by the next smaller

size. The general guidelines for high-performance grinding require a 40–60 mesh

for rough grinding; a 60–100 mesh for finish grinding; and a 100–320 mesh

grain for fine grinding. When selecting a particular grain size one must consider

that large grains allow the user to remove material economically by making the

material easier to machine by producing longer chips. However, finer grain sizes

allow the user to achieve lower surface roughness, and achieve greater accuracy

by producing shorter chip sizes with a greater number of sharp cutting points.

Table 9.1 shows the relationship between abrasive grain size and workpiece

surface roughness for aluminium oxide grains. Grinding wheel specifications are

Table 9.1. Relationship between abrasive grain size and workpiece surface roughness

Surface roughness, R

(μm)

Abrasive grain size (US mesh size)

0.7–1.1 46

0.35–0.7 60

0.2–0.4 80

0.17–0.25 100

0.14–0.2 120

0.12–0.17 150

0.–0.14 180

0.08–0.12 220

252 M.J. Jackson

specific to a particular operation, and grinding wheels are formulated to account

for the differences in grinding operations. Figure 9.1 shows a micrograph of a stan-

dard structure grinding wheel that is used for applications used in the automotive

industry. Figure 9.2 shows a grinding wheel that is formulated for use in creep

feed grinding operations that are specific to the grinding of aerospace components.

Figure 9.1. Micrograph of conventional grinding wheel with standard porosity

Figure 9.2. Micrograph of conventional grinding wheel with distributed porosity

Grinding Technology and New Grinding Wheels 253

The wheel has a specially developed structure that is used for increased metal

removal rates. The suggested operating conditions are also supplied by applica-

tions’ engineers who spend their time optimizing grinding specifications for a

variety of tasks that are supported by case studies of similar operations [2]. A list

of web sites for those companies who supply grinding wheels and expertise is

shown at the end of this chapter.

9.2.3 Grinding Machine Requirements for High-efficiency Dressing

Diamond dressing wheels require relative motion between the dressing wheel and

the grinding wheel. Dressing form wheels require relative motion generated by the

path of the profile tool for the generation of the grinding wheel form, and the rela-

tive movement in the peripheral direction. Therefore, there must be a separate

drive for the dressing wheel. The specification of the drive depends on the follow-

ing factors: grinding wheel specification and type, dressing roller type and specifi-

cation, dressing feed, dressing speed, dressing direction and the dressing speed

ratio. A general guide for drive power is that 20W per mm of grinding wheel con-

tact is required for medium to hard vitrified aluminium oxide grinding wheels.

The grinding machine must accommodate the dressing drive unit so that dressing

wheels rotate at a constant speed between itself and the grinding wheel. This means

that grinding machine manufacturers must coordinate the motion of the grinding

wheel motor and dressing wheel motor. For form profiling, the dressing wheel must

also have the ability to control longitudinal feed motion in at least two axes. The

static and dynamic rigidity of the dressing system has a major effect on the dressing

system. Profile rollers are supported by roller bearings in order to absorb rather high

normal forces. Slides and guides on grinding machines are classed as weak points

and should not be used to mount dressing drive units. Therefore, dressing units

should be firmly connected to the bed of the machine tool. Particular importance

must be attached to geometrical run-out accuracy of the roller dresser and its accu-

rate balancing. Tolerances of 2 μm are maintained with high-accuracy profiles with

radial and axial run-out tolerances not exceeding 2 μm. The diameter of the mandrel

should be as large as possible to increase its stiffness; typically, roller dresser bores

are in the range 52–80 mm diameter. The class of fit between the bore and the man-

drel must be H3/h2 with a 3–5 μm clearance. The characteristic vibrations inherent

to roller dresser units are bending vibrations in the radial direction and torsional

vibrations around the base plate. Bending vibrations generate waves in the peri-

pheral direction, while torsional vibrations generate axial waves and distortions in

profile. The vibrations are caused by rotary imbalance and the dressing unit should

be characterized in terms of resonance conditions. A separate cooling system should

also be designed in order to prevent the dressing wheel from losing its profile accu-

racy due to thermal drift [1].

9.2.4 Diamond Dressing Wheels

The full range of diamond dressing wheels is shown in Table 9.2. There are five

basic types of dressing wheels for conventional form dressing of conventional

254 M.J. Jackson

grinding wheels. The types described here are those supplied by Winter Diamond

Tools (www.winter-diamantwerkz-saint-gobain.de). Table 9.2 shows the five types

of roller dressing wheels:

• UZ type (reverse plated – random diamond distribution): The diamond

grains are randomly distributed at the diamond roller dresser surface. The

diamond spacing is determined by the grain size used and the close-packed

diamond layers give a larger diamond content than a hand-set diamond

dressing roll. The manufacturing process is independent of profile shape.

The process permits concave radii to be greater than 0.03 mm and convex

Table 9.2. Types of dressing rolls and wheels

Grinding Technology and New Grinding Wheels 255

radii to be greater than 0.1 mm. The geometrical and dimensional accuracy

of these dressers is achieved by re-working the diamond layer

• US type (reverse plated – hand-set diamond distribution): Unlike the

UZ design, the diamonds are hand set, which means that certain profiles

cannot be produced. However, the diamond spacing can be changed and

profile accuracy can be changed by re-working the diamond layer. Convex

and concave radii greater than 0.3 mm can be achieved;

• TS type (reverse sintered – hand-set diamond distribution): Diamonds

are hand set, which means that certain profiles cannot be produced. How-

ever, the diamond spacing can be changed and profile accuracy can be

changed by re-working the diamond layer. Concave radii greater than 0.3

mm can be produced;

• SG type (direct plated – random diamond distribution, single layer):

The diamonds grains are randomly distributed. Convex and concave radii

greater than 0.5 mm are possible.

• TN type (sintered – random diamond distribution, multi-layer): The

diamond layer is built up in several layers providing a long-life dressing

wheel. The profile accuracy can be changed by re-working the diamond layer.

The minimum tolerances that are attainable using diamond dressing rolls and

wheels are shown in Table 9.3. The charts show tolerances of engineering interest

for each type of dressing wheel.

Table 9.3. Minimum tolerances attainable with dressing wheels

256 M.J. Jackson

Table 9.3. (continued)

9.2.5 Application of Diamond Dressing Wheels

The general guide and limits to the use of diamond dressing wheels are shown in

Table 9.4, which shows the relationship between application and the general speci-

fication of dressing wheels.

Table 9.4. General specification and application of diamond dressing wheels

Application

Wheel spec.,

grain size,

hardness

Surface

roughness,

(μm)

Output

TS type,

diamond

size (mesh)

UZ type,

diamond

size (mesh)

Rough

grinding

40–60,

K–L

0.8–3.2

(hand set)

0.4–1.0

(random)

High 80–100

(hand set)

100–150

(random)

Not

applicable

Transmission,

gear box

60–80,

J–M

0.2–16 High 200–250 200–250

Bearings, CV

joints

80–120, J–M 0.2–1.6 High 200–300 200–300

Creep feed, con-

tinuous dressing

Porous structure 0.8–1.6 Low Limited 250–300

Grinding Technology and New Grinding Wheels 257

9.2.6 Modifications to the Grinding Process

When the grinding wheel and dressing wheel have been specified for a particular

grinding operation, adjustments can be made during the dressing operation, which

affects the surface roughness condition of the grinding wheel. The key factors that

affect the grinding process during dressing are: dressing speed ratio, V

, be-

tween the dressing wheel and grinding wheel; dressing feed rate, a

, per grinding

wheel revolution; and the number of running-out, or dwell, revolutions of the

dressing wheel, n

. By changing the dressing conditions it is possible to rough and

finish grind using the same diamond dressing wheel and the same grinding wheel.

By controlling the speed of the dressing wheel, or by reversing its rotation, the

effective roughness of the grinding wheel can be varied in the ratio of 1:2.

9.2.7 Selection of Grinding Process Parameters

The aim of every grinding process is to remove the grinding allowance in the

shortest time possible while achieving the required accuracy and surface rough-

ness on the workpiece. During the grinding operation the following phases are

usually present:

• Roughing phase, during which the grinding allowance removed is charac-

terized by large grinding forces, causing the workpiece to become de-

formed

• Finishing phase for improving surface roughness

• Spark-out phase, during which the workpiece distortion is reduced to a

point where form errors are insignificant.

Continuous dressing coupled with one or more of the following phases have con-

tributed to high-efficiency grinding of precision components:

• Single-stage process with a sparking-out regime

• Two-stage process with a defined sparking out regime coupled with a slow

finishing feed rate

• Three- and multi-stage processes with several speeds and reversing points

• Continuous matching of feed rate to the speed of deformation reduction of

the workpiece

For many grinding processes using conventional vitrified grinding wheels, starting

parameters are required that are then optimized. A typical selection of starting

parameters is shown below:

• Metal removal rates for plunge grinding operations: when the diameter of

the workpiece is greater than 20 mm then the following metal removal

rates (Q

’

in mm

/mm.s) are recommended; roughing – 1 to 4, finishing –

0.08 to 0.33. When the workpiece is less than 20 mm, roughing – 0.6 to 2,

finishing – 0.05 to 0.17;

• Speed ratio, q, is the relationship between grinding wheel speed and work-

piece speed. For thin-walled, heat-sensitive parts q should be in the range

258 M.J. Jackson

105–140, for soft and hard steels, q should be 90–135, for high metal re-

moval rates q should be 120–180, and for internal grinding q should be in

the range 65–75;

• Overlap factors should be in the range 3–5 for wide grinding wheels and

1.5–3 for narrow grinding wheels;

• Feed rates should be between 0.002 and 0.006 mm per mm traverse during

finish grinding;

• Number of sparking-out revolutions should be 3–10, depending on the ri-

gidity of the workpiece.

However, it is important that the user ensures that the specification of grinding

wheel and dressing wheel are correct for the grinding task prior to modifying the

grinding performance [1–3].

9.2.8 Selection of Cooling Lubricant Type and Application

The most important aspect of improving the quality of workpieces is the use of

high-quality cooling lubricant. In order to achieve a good surface roughness of less

than 2 μm, a paper-type filtration unit must be used. Air cushion deflection plates

improve the cooling effect. The types of cooling lubricant in use for grinding in-

clude emulsions, synthetic cooling lubricants and neat oils:

• Emulsions: oils emulsified in water are generally mineral based and are

concentrated in the range 1.5–5%. In general, the ‘fattier’ the emulsion the

better the surface finish, but this leads to high normal forces and roundness

is impaired. Also, they may become susceptible to bacteria;

• Synthetic cooling emulsions: chemical substances dissolved in water at

concentrations of 1.5–3%. These are resistant to bacteria and are good wet-

ting agents. They allow grinding wheels to act more aggressively but tend

to foam and destroy seals;

• Neat oil: enabled the highest metal removal rates with a low tendency to

burn the workpiece. Neat oils are difficult to dispose of and present a fire

hazard.

There are general rules concerning the application of cooling lubricants but the

reader is advised to contact experienced grinding applications engineers from

grinding wheel suppliers and lubricant suppliers. A list of suppliers is shown in the

internet resource section of this chapter.

9.3 High-efficiency Grinding Using CBN Grinding Wheels

9.3.1 Introduction

High-efficiency grinding practice using super-abrasive CBN grinding wheels has

been successfully applied to grinding external profiles between centres and in the

centreless mode, grinding internal profiles, threaded profiles, flat profiles, guide

Grinding Technology and New Grinding Wheels 259

tracks, spline shaft profiles and gear tooth profiles. These operations require dress-

ing of the grinding wheel with a highly accurate and precise rotating wheel that is

studded with diamond. Operations that are carried out using high-performance

conventional grinding wheels as a matter of routine are the grinding of:

• Auto engine: crankshaft main and connecting-rod bearings, camshaft bear-

ings, piston ring grooves, valve rocker guides, valve head and stems, head

profile, grooves, expansion bolts;

• Auto gearbox: gear wheel seats on shafts, pinion gears, splined shafts,

clutch bearings, grooves in gear shafts, synchromesh rings, oil-pump worm

wheels;

• Auto chassis: steering knuckle, universal shafts and pivots, ball tracks,

ball cages, screw threads, universal joints, bearing races, cross pins;

• Auto steering: ball joint pivots, steering columns, steering worms, servo

steering pistons and valves;

• Aerospace industry: turbine blades, root and tip profiles, fir-tree root pro-

files [3].

9.3.2 Grinding Wheel Selection

The selection of the appropriate grade of vitrified CBN grinding wheel for high-

speed grinding is more complicated than for aluminium oxide grinding wheels.

Here, the CBN abrasive grain size is dependent on the specific metal removal rate,

surface roughness requirement and equivalent grinding wheel diameter. As a start-

ing point when specifying vitrified CBN wheels, Figure 9.3 shows the relationship

between CBN abrasive grain size, equivalent diameter and specific metal removal

rate for cylindrical grinding operations.

However, the choice of abrasive grain is also dependent on the surface rough-

ness requirement and is restricted by the specific metal removal rate. Table 9.5

shows the relationship between CBN grain size and their maximum surface rough-

ness and specific metal removal rates. The workpiece material has a significant

influence on the type and volume of vitrified bond used in the grinding wheel.

Table 9.6 shows the wheel grade required for a variety of workpiece materials that

are based on cylindrical (crankshaft and camshaft) grinding operations [1–3].

Considering the materials shown in Table 9.6, chilled cast iron is not burn sen-

sitive and has a high specific grinding energy owing to its high carbide content. Its

hardness is approximately 50 HRc and the maximum surface roughness achieved

on machined camshafts is 0.5 μm Ra, therefore a standard structure bonding sys-

tem is used that is usually between 23 and 27 vol.% of the wheel. The CBN grain

content is usually 50% by volume, and wheel speeds are usually up to 120 m/s.

Nodular cast iron is softer than chilled cast iron and is not burn sensitive. How-

ever, it does tend to load the grinding wheel. Camshaft lobes can have hardness

values as low as 30 HRc and this tends to control wheel specification. High stiff-

ness crankshafts and camshafts can tolerate a 50 vol.% abrasive structure contain-

ing 25 vol.% bond. High loading conditions and high contact re-entry cam forms

require a slightly softer wheel where the bonding system occupies 20 vol.% of the

260 M.J. Jackson

entire wheel structure. Low-stiffness camshafts and crankshafts require lower

CBN grain concentrations (37.5 vol.%) and a slightly higher bond volume (21

vol.%). Very low-stiffness nodular iron components may even resort to grinding

wheels containing higher strength bonding systems containing sharper CBN abra-

sive grains operating at 80 m/s.

The stiffness of the component being ground has a significant effect on the

workpiece/wheel speed ratio. Figure 9.4 demonstrates the relationship between

this ratio and the stiffness of the component. Steels such as AISI 1050 can be

ground in the hardened and the soft state. Hardened 1050 steels are in the range

68–62 HRc. They are burn sensitive and as such wheels speeds are limited to 60

m/s. The standard structure contains the standard bonding systems up to 23 vol.%.

The abrasive grain volume contained is 37.5 vol.%. Lower-power machine tools

usually have grinding wheels where a part of the standard bonding system con-

tains hollow glass spheres (up to 12 vol.%) exhibiting comparable grinding ratios

to the standard structure system. These specifications also cover most powdered

metal components based on AISI 1050 and AISI 52100 ball bearing steels.

100

120

140

160

180

200

0246810

Specific metal removal rate, Q' (mm3/mm.s)

Equivalent wheel diameter, De, (mm)

B64 B76 B91 B107

B126 B151 B181

Figure 9.3. Chart for selecting CBN abrasive grit size as function of the equivalent

grinding wheel diameter, D

, and the specific metal removal rate, Q

’