Lopez de Lacalle L.N., Lamikiz A. Machine Tools for High Performance Machining

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8 High Performance Grinding Machines 293

One recent tendency in cylindrical grinding is the use of the headstock spin-

dle to hold the dresser disc, as presented in Fig. 8.13b.

• Some high removal rate processes, such as creep feed grinding, or processes

with complex wheel profiles require the use of diamond roller dressers. In these

devices, the diamond roller profile is shaped with the profile that must be ob-

tained in the grinding wheel and the dressing is performed in a plunge process,

as shown in Fig. 8.14.

• A special process that applies the diamond roller is continuous dressing, per-

formed in very high material removal rate grinding processes, that requires

a continuous sharpening of the grinding wheel surface to ensure the mainte-

nance of abrasive capabilities. Continuous dressing maintains constant pressure

between the dressing roll and the grinding wheel, which is adjustable to ensure

the proper area of the wheel is exposed during each revolution. This is a proc-

ess called microprofiling, as only a micron or less of wheel is removed with

each revolution. This ensures that fresh, sharp abrasive crystals are always

properly supported in the bonding material of the wheel.

8.2.4.2 Non-conventional Dressing Processes

The development of new grinding wheel compounds with superabrasives to grind

hard and difficult to cut materials has led to the development of new dressing

technologies that substitute the conventional diamond dresser by alternate princi-

ples, by means of electrochemical and laser tools. Most of these processes are still

in research stage, but some few applications are present in industry.

ELID. Electrolytic in-process Dressing

This is an electrolytic method that was introduced in 1985 by Murata [8] and op-

timised to its current configuration by Ohmori [9]. Grinding wheel bonding re-

moval is produced by means of a chemical reaction chain that occurs due to the

Fig. 8.14 Creep feed grinding with

dressing roll tool

294 R. Lizarralde et al.

application of a voltage between the grinding wheel and one electrode, following

the scheme presented in Fig. 8.15.

The applied voltage between wheel and electrode generates chemical reactions

between them that depend on the bonding material that must be conductive. The

fine control of the electric parameters allows a very accurate control of the mate-

rial removed from the wheel and, consequently, a very precise dressing. On the

other hand, it is a low removal rate method, limiting the application to fine grain

wheels or to be combined with other dressing methods.

These two characteristics, high precision and low removal capabilities, have

oriented the application of ELID method to superfinishing operations, obtaining

mirror-finishing surfaces. Industrial examples of this technology are superfinish-

ing and ultraprecision grinders of manufacturers such as Jung

, Toyoda Koki

and

Nachi Fujikoshi

. Reportedly [12], very good results can be obtained in the grind-

ing of brittle materials, such as glass, ceramics, hardened steels, and in some cases

substituting final lapping or polishing operations.

EDD, Elecro-discharge Dressing

This is a widely known non-conventional dressing process for superabrasive

wheels. Wheel material removal occurs very much as in the EDM process: electri-

cal discharges occur within a locally ionised dielectric medium between the elec-

trode and the conductive material on the wheel surface (either a metallic bond

material, or a work metal loaded into wheel pores), as shown in Fig. 8.16. Local

temperatures may rise up to several thousand degrees, removing material in the

form of craters. Dielectric ionisation is achieved using a generator that applies

a voltage (known as open-circuit voltage) between the wheel and workpiece [4].

Discharge duration is in the order of tenths or hundredths of microseconds. An

electrode can be made from copper or graphite and the dielectric can be the same

emulsion used as the grinding fluid, simplifying the application of the technology.

Grindin

fluid

Electrode

Wheel

Workpiece

Power

Fig. 8.15 Principle of ELID grinding

8 High Performance Grinding Machines 295

Fig. 8.16 Principle of EDD dressing

The material removing capability is limited and the process is being used for

fine grain wheels and superfinishing processes.

The latest research activities [10,

11] are oriented to the truing-dressing of me-

tallic bonded CBN wheels, providing high removal rates and improving the wheel

life of conventional high removal wheels such as electroplated CBN ones. In this

application, the EDD process is also used to prepare the grinding wheel for its use,

eliminating the run-out of the wheel, providing a very accurate surface.

ECDD, Electro-contact Discharge Dressing

This is a truing-dressing method for metallic bonded wheels, introduced by Y. A.

Pachalin in 1987. The application of a voltage generates discharges between the

wheel and the chips of the electrode in continuous contact whit the wheel

(Fig. 8.17). These discharges generate a thermal process that removes bonding

material in the wheel.

Several authors [1] have demonstrated the efficiency of the method for small

grain size wheels, obtaining significant reduction in cutting forces and the thermal

Electroconductive

body material

Electroconductive

grinding layer

Electrode

Brush

Copper ring

fads

Fig. 8.17 Principle of ECDD, after [1]

296 R. Lizarralde et al.

affection of the workpiece surface. Ceramic grinding is one of the main fields of

application for this technology.

Laser Assisted Dressing

This is a new technique in which a laser beam is proposed as the non-contact

thermal dressing tools. Several researchers have studied the process applied for

vitrified and resin bonded conventional wheels [5,

13] and also superabrasive

metal-bonded diamond wheels [2,

6].

The basic principle of the process is supported in the melting or vaporisation of

the bonding material by the heat generated by the laser beam (Fig. 8.18), resulting

in the elimination of bonding material.

The use of a pulsed laser with a controlled operation prevents the thermal dam-

age of grains and the remaining wheel bonding, and usually the system is assisted

by an air jet to remove the melted material.

Like other alternative dressing methods, laser is applied only to very specific

processes such as ceramics ultra precision and super finishing grinding.

Bond

Abrasive grain

Laser beam

Laser scanning direction

Fig. 8.18 Principle of laser dressing, after [3]

8.2.5 Process Lubrication and Cooling

The grinding process is the machining process where the cutting fluid is more

important, due to the high heat quantities generated. The cutting fluid must pro-

vide lubrication and cooling to the process for correct process behaviour and to

avoid thermal damages to the workpiece.

Most of the applications and machines are equipped with coolant systems that pro-

vide high volumes of cutting fluid (neat oil or emulsion) applied at low pressures by

very simple nozzles, assuming that the high fluid volume is enough to cool the process.

8 High Performance Grinding Machines 297

Nevertheless, many approaches have been done to design more advanced and effi-

cient systems that optimise the use of fluid, studying the optimum combination of vol-

ume, pressure and nozzle design to reach to the contact area between grinding wheel

and workpiece. Most of these approaches have been induced by grinding processes

with particularly critical coolant requirements, due to the severe process conditions.

This is the case of high speed grinding with CBN wheels. This process collects

three particularities: a high tangential speed, up to 150

m/s in industrial applica-

tions, that creates an air film around the grinding wheel, which is a barrier for the

fluid to reach the contact area, the high heat generated by the grinding process,

and the high cost of the wheels that gives importance to the grinding wheel clean-

ing to minimise the number of dressings, increasing life. Due to these particulari-

ties, high speed CBN grinding machines are usually equipped with high pressure

coolant systems with special very precise nozzles.

With these systems coolant is applied to clean the grinding wheel, penetrating

in the cavities, removing workpiece clogged chips, and also is able to reach the

contact area between the workpiece and the grinding wheel.

Creep feed grinding is also a process with high coolant demands. While most

machines apply high volumes of coolant, Rolls Royce

developed, and patented,

a technology, called “Viper” that optimises coolant by means of a controlled ad-

justable nozzle that ensures the application of the coolant exactly in the correct

point, in each condition and for any grinding wheel and workpiece configuration.

Machine manufacturers such as Makino

and Bridgeport

apply this system, by

means of a license form Rolls Royce

. The system can be seen in Fig. 8.19.

8.2.6 Integrated Measuring Devices

In cylindrical grinding, measurement devices based on touch probes have been

used for a long time to control the process and stop the cycle once the controlled

Fig. 8.19 VIPER grinding set-up, by Makino

298 R. Lizarralde et al.

measure reaches the programmed values. There are many commercial systems

available in the market, manufactured by specialised companies.

Beyond these applications, that usually present some limitations, the most im-

portant is the short range of measurement of each sensor, that forces the system to

use different sensors when different diameters must be measured, grinding ma-

chine manufacturers are developing special devices, for special applications and

for wider range measurements. Market higher production and efficiency require-

ments have also encouraged these developments.

The following are some of the most representative special measurement devices

developed in the last years by grinding machine manufacturers, providing them in

the associated applications:

• Laser based measurement for airplanes turbine blade tip grinding. In this proc-

ess, represented in Fig. 8.20, the workpiece must rotate at operational speed

(around 3,000

rpm) and each blade must be measured in each lap, to control the

outer diameter of the workpiece. For this purpose, machine tool manufacturers,

such as Danobat

, have developed non-contact laser systems, able to perform

this precise and high speed process.

• Measurement of crankshaft journals by means of a tracking interpolation de-

vice. Most crankshaft grinding specialised manufacturers have opted by grind-

ing journals and pins in the same setup, performing the journals grinding by in-

terpolation rotation with wheel head translation. Measuring of the journals

diameter is a complex operation in this configuration, and has been successfully

solved by manufacturers such as Naxos-Union

by means of a very special de-

vice that follows the journal rotation, maintaining a stable relative position of

the probe on the journal.

• Multi-diameter cylindrical parts measurement. These systems, developed by

several grinding machine manufacturers, allow the measurement of diameters,

roundness and even axial measurement, which permits taper measurement, in

multi-diameter parts, avoiding the use of one element for each diameter, as it

has been done for many years, increasing machine operability, saving space

and gaining flexibility.

Fig. 8.20 Laser measurement

in tip grinding of turbine

blades, by Danobat

8 High Performance Grinding Machines 299

These systems, as presented in Fig. 8.21, are based on two touch probes

whose position is controlled by the relative movement of two axes, obtained by

different solutions, depending on each manufacturer. The control and mechan-

ics of the systems must be very accurate and stable, ensuring the repeatability

of measurements. Latest developments include non-contact technologies based

on linear motors and aerostatic guides, special thermally stable materials and

temperature control, following the rules applied in coordinate measurement

machines.

8.3 Special Grinding Processes

Most of the systems and technologies described in this chapter are applied to any

type of grinding machine and process, except some particularly mentioned cases.

Beyond the general-purpose machines and the evolutionary developments, there

are some processes and machines that present interesting particularities and have

gained an important place in the market, as new technologies that have introduced

productive advantages based on new ideas. In some cases, these new processes

have become the “signature technology” of their inventor companies.

8.3.1 Peel Grinding–Quick Point

Peel grinding can be considered as the answer of grinding manufacturers to the

“interference” of hard turning in historical grinding applications, in particular in

strategic sectors, such as the automotive.

Peel grinders, as shown in Fig. 8.22, basically collect the flexibility of turning

with the precision and quality of grinding. Supported in the use of a very narrow

Fig. 8.21 Multi-diameter measuring device,

by Danobat

300 R. Lizarralde et al.

CBN wheel, working at very high speed (up to 200

m/s), and high workpiece rota-

tion speeds (up to 10,000

rpm), is able to grind, in one clamping, complex shapes,

multi-diameter parts, faces, grooves, tapers, or any other feature.

Due to the small contact length between wheel and workpiece and the high op-

erational speeds, cutting forces and thermal effects are dramatically decreased, the

process is optimum for hardened parts. Besides this, the low forces simplify the

workpiece clamping, eliminating the need of special chucking devices and, spe-

cially, the need of steady rests in slender parts. Coolant application is also simpler,

due to the small contact region to be covered.

Because of these advantages, car transmission shafts and cutting tools (drills,

mills), even carbide ones, are some of the most successful applications of peel

grinding.

Peel grinders must consider some particular features to guarantee their competi-

tiveness. The effectiveness and productivity of the high speed CBN grinding

wheel must be ensured by a proper wheel cleaning, by means of high pressure

nozzles, and the wheel spindle must be very accurately and continuously balanced.

The high workpiece rotation speed requires a precise spindle, preferably an elec-

trospindle. And the machine structure must be dynamically stable.

One step ahead in the development of peel grinding was presented and patented in

1985 by Junker

, under the denomination of Quickpoint™. This process is a variation

of peel grinding based on a tilting wheel head, which converts, by means of a very

small inclination of the wheel, the contact between workpiece and wheel from a line

to a single-point. Therefore, the advantages of peel grinding in terms of flexibility,

reduction of forces and the thermal effects and increase of wheel life are maximised.

8.3.2 Speed Stroke Grinding

The speed stroke grinding process was first investigated by Japanese researchers

(Inasaki, Yuji, and Akinori), in the late 1980s and oriented to the machining of

Fig. 8.22 Peel grinding,

by Danobat

8 High Performance Grinding Machines 301

brittle materials, in particular ceramics, diminishing thermal affection in a high

material removal process.

The principle of the process is a combination of very high table speed with very

low grinding depth. Table speed reached 80–100

m/min in the early years and up

to 200–250

m/min nowadays, due to the application of high dynamics driving

systems, based on linear motors. The depth of cut is maintained around 1 micron,

or even below.

This combination of very high speed and very low cutting depth generates

a high volume of material removed with very low forces, very specific energy, and

consequently very low thermal affection to the workpiece.

These process characteristics opened the range of application of speed stroke

from ceramics to other parts that required high material removal and were sensible

to thermal affection, presenting an alternative to creep feed grinding. The German

company Blohm

collaborated with RWTH Aachen University to develop a proc-

ess and machine for the speed stroke grinding of titanium alloys for aeronautic

applications. This machine presents some interesting solutions to achieve table

speeds up to 200

m/min and accelerations up to 50

m/s

compensating the high

dynamic loads induced in the machine structure. The avoidance of the effect that

dynamic forces generate on the machine is the major challenge for machine manu-

facturers. For this purpose different speed and acceleration-deceleration profiles

and strategies have been investigated.

8.3.3 Creep Feed Grinding

Creep feed grinding is a surface grinding process characterised by a very high

infeed rate or depth of cut (in the range between 0.5 to 30

mm) and low feed rates

(0.1–50

mm/s). The process is performed in very few passes, even one single pass

in some applications, in contrast to conventional reciprocating grinding, character-

ised by multiple very low infeed passes at higher feed speeds.

With the special combination of parameters of creep feed grinding, the chip

thickness and therefore the cutting force for each grain are smaller which allows

those high removal rates with less affection to the wheel integrity, since the grains

are easily held in the bonding. The contact length is much higher, but the number

of cutting edges (grains) involved simultaneously is much higher and because the

feed rate is much lower, the achieved surface roughness is much better than in

reciprocating grinding.

On the other hand, the total cutting force is much higher and the thermal effects

as well. The thermal negative effect is increased by the large contact length, which

makes difficult the application of the cutting fluid to the whole contact zone. The

chip length is also larger than the generated in reciprocating grinding.

These characteristics define the design of the machines for creep feed grinding.

Higher loads derive from a very stiff machine and components (guides, ball

screws and spindles) design. Thermal effects recommend the measurement and

302 R. Lizarralde et al.

control of the deflections, especially the axial deflection of the wheel spindle. The

coolant requirement is more critical and its application is more complex, requiring

special high-pressure nozzles with very accurate design to cover the maximum of

the contact zone. The correct application of the coolant jet is critical, thus adjust-

able nozzles are also recommended. The previously mentioned Viper is one of

these special systems. Conditioning of the grinding wheel is also very important,

and continuous dressing is very usual in creep feed processes.

Creep feed typical applications are large batch parts with special complex pro-

files. Creep feed provides high productivity, very good surface roughness and very

precise shapes. The aeronautics, energy generation and automotive industry are

target sectors for creep feed machine manufacturers. In aeronautics and energy

generation, blades and rotor parts with complex slots and profiles in low ma-

chinability materials and in automotive parts such as steering racks are typical

parts in which creep feed is much more productive than reciprocating grinding.

8.3.4 High Efficiency Deep Grinding

HEDG, high efficiency deep grinding (or high performance grinding) is an im-

provement of creep feed grinding that introduces high cutting speed and higher

feed rates (closer to reciprocating grinding, see Table 8.2).

HEDG provides the advantages of creep feed grinding, in terms of surface

quality and profile accuracy, and reduces some of its limitations with a better

thermal balance and a significantly increased material removal rate.

Machine requirements and applications are analogous to those related to creep

feed grinding.

Table 8.2 Comparative parameters between conventional surface grinding, creep feed and HEDG

Process Infeed Feed rate Cutting speed

Specific material

removal rate

Reciprocating

grinding

0.001–0.05

mm 1–30

m/min 20–60

m/s 00.1–10

/mm/s

Creep feed

grinding

0.1–30

mm 0.05–0.5

m/min 20–60

m/s 00.1–15

/mm/s

High efficiency

deep grinding

0.1–30

mm 0.5–10

m/min 80–200

m/s 50–2,000

/mm/s

8.4 Machine and Process Monitoring and Control

Monitoring and control strategies become a very powerful tool to ensure an optimum

performance in grinding process both from the machine and process sides. The grind-