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

Подождите немного. Документ загружается.

7 New Developments in Lathes and Turning Centres 273

Fig. 7.15 Laser welding in-

tegrated in a turning centre,

by Boehringer

The laser head can be manipulated and controlled in the same way as a milling

tool, with two displacement axes and two rotaries as needed; thus, it is possible to

apply them to complex shaped parts. The advantage of laser hardening against

other more productive methods such as induction hardening is its flexibility, and

the possibility of applying to details in one workpiece, likewise difficult to reach

points.

Laser welding has also been incorporated by some turning centre manufacturers

(Fig. 7.15). In this case laser is applied for welding parts in an intermediate opera-

tion; furthermore, it can be used for hardening and even for finishing some part

details.

Laser-assisted turning has been also investigated in the last ten to fifteen years

for hard to machine materials. This application is based on the idea that continuous

laser beam induced heating of the workpiece provides a local softening of the mate-

rial, enabling the material to be machined using a defined cutting edge.

Several research works have reported positive results from laser-assisted ma-

chining for machining ceramics, whereas in the case of metal alloys such as nickel

and cobalt based stainless steel or titanium alloys, the results are quite contradic-

tory [2,

8]. In industrial terms, the application of laser-assisted machining is almost

negligible.

7.4.2 Roller Burnishing and Deep Rolling

Roller burnishing is a cold rolling process without actual metal removal. It is a new

concept in finishing components.

The surface of metal parts worked through turning, reaming or boring operations

is a succession of peaks and valleys when microscopically examined. The roller

burnishing operation (Fig. 7.16) compresses the peaks into valleys (Fig. 7.17), thus

forming a smooth mirror finished surface [4].

274 R. Lizarralde, A. Azkarate and O. Zelaieta

Fig. 7.16 a Roller burnishing. b Lathe ware rolling is applied to train axles, by Lealde

Danobat

One step ahead, deep rolling is one of the most valuable mechanical process-

es to improve the fatigue strength of dynamically loaded components. It elimin-

ates or at least reduces fatigue, especially on notches like fillets and shoulders

which can lead to fatigue cracks. The deep rolling process works similarly to

roller burnishing; however, the task is different. To ensure equal component

quality all process parameters, in particular the burnishing force, must be con-

trolled during the process. This control is performed mechanically or, in more

advanced systems, hydraulically, controlling the pressure applied to the roller or

ball tool.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

2.2

2.4

2.6

2.8

Turning direction

14 μm

Burnishing

direction

)

(

)

(

Fig. 7.17 a Workpiece topography after turning. b Workpiece topography after turning and

burnishing

7 New Developments in Lathes and Turning Centres 275

Fig. 7.18 Deep rolling of turbine blades, by Ecoroll

Deep rolling is based on the combination of three simultaneously working

physical effects:

• A deep layer of residual compressive stress on a component surface.

• A strength increase through cold working.

• The elimination of micro notches and the improvement of surface roughness

quality.

Compressive stress, generated on the surface layer during the rolling process,

remains to a high extent after the rolling process is finished. The compressive

stresses in the axial direction are most important for the improved fatigue strength.

In the past, deep rolling was only applied to special machines; however,

nowadays, due to the evolution in the design and configuration of deep rolling

devices, it can be incorporated in machines such as lathes and turning centres.

The deep rolling device can be incorporated in the lathe turret as with any

other tool.

Several successful applications of combined turning and deep rolling processes

have been addressed by researchers and industrial manufacturers, from automotive

engine parts, such as shafts of valves, through railway applications, such as wheel

axes, to aeronautic parts such as turbine discs, blades (Fig. 7.18) and shafts or

landing gears.

7.4.3 Ultrasonic Assisted Turning

In difficult-to-cut and brittle materials several research approaches have been

carried out for the purpose of modifying the mechanics of chip formation to im-

prove process capabilities, in terms of chip breakage, decrease in forces and fric-

tion, and thus an increased tool life.

276 R. Lizarralde, A. Azkarate and O. Zelaieta

Fig. 7.19 Principle of ultrasonic vibration, where x(t) is the ultrasonic movement (20–40

kHz)

One of the most researched technologies is the application of ultrasonic vibra-

tions on the tool, overlapped to feed movements. The principle behind this technol-

ogy, as shown in Fig. 7.19, is the application of a high frequency (usually between

20 and 40

kHz) low amplitude (5–20

μm) linear or elliptical vibration on the tool.

Ultrasonic vibration is introduced by means of a generator into an intermediate

component, the so-called “sonotrode,” designed to have a normal mode at the

required frequency and with the required mode shape to generate vibrations on the

defined plane.

Several authors [3,

7] refer to good results in terms of chip breakage, load re-

duction and tool life increase for very diverse materials, such as titanium alloys,

copper, or stainless steel and glass. Nevertheless, industrial applications are mostly

only reported in the machining of brittle materials such as glass for optics, and

ceramics.

7.4.4 Cryogenic Gas Assisted Turning

This process consists of the application of cryogenic gas (nitrogen usually), to

assist the turning process in hard to machine materials such as hardened steels.

The application of this cryogenic gas is claimed to provide some significant ad-

vantages:

• Increasing cutting speeds and material removal rates by as much as 200% and

reducing the overall cycle time.

• Alternatively, increasing tool life up to 250% and reducing machine downtime

for tool changeover.

• Enhancing the removal of chips from the cutting area and eliminating oily resi-

dues on machines, parts and chips.

• Improving the reliability of cutting with ceramic inserts while maintaining

desired dimensional tolerances.

• Improving the quality of produced parts by preventing mechanical and chemi-

cal degradation of the machined surface.

7 New Developments in Lathes and Turning Centres 277

Fig. 7.20 Icefly

industrial application, by Air Products

Some authors [9] have researched the application of this process to hard turn-

ing, allowing the use of ceramic tools instead of CBN, and eliminating the need

for conventional coolant fluid. The advantages, besides the above, are related to an

environmentally lower impact and a better surface integrity achieved by this alter-

native process.

Concerning the application in machine, it must be mentioned that Company Air

Products

have patented the technology, under the Icefly

brand, collaborating

with some machine manufacturers, like Hardinge

, for the development of indus-

trial applications, as shown in Fig. 7.20.

7.4.5 High-pressure Coolant Assisted Machining

High-pressure coolant jet assisted machining is being researched and applied in

industry for difficult-to-machine materials, such as titanium and nickel-based

alloys. It is well known the poor machinability of these materials is due to several

factors: their low conductivity, chemical reactivity with tool materials, shearing

mechanism during chip formation and the strain hardening during machining.

These factors lead to very high tool wear forcing the use of very low cutting pa-

rameters, especially a very low cutting speed.

To overcome these limitations, the application of coolant jet at high pressures

(up to 350

bar), properly aimed at the interface between the tool tip and chip [5]

generates a effect of the hydraulic wedge that significantly eases the chip forma-

tion and cutting process, via these factors:

• A significant increase of cutting speed and feed rate. The coolant jet facilitates

chip breaking, decreasing the friction and forces generated on the tool. Addi-

tionally, the coolant effect is improved, resulting in the possibility of increasing

cutting parameters.

• An increase in tool life, due to the decrease of cutting loads and temperatures.

278 R. Lizarralde, A. Azkarate and O. Zelaieta

• An improvement in surface finishing, via the easiest selection of optimum cut-

ting conditions and elimination of long chips, a decrease in friction and thermal

effects on surface.

• The final effect, i.e., or sum of all the above is the decrease in cycle times and

cost per part.

Aeronautical part manufacturing is the leading sector in the application of this

technology, due to the extensive use of related materials and critical quality and

productivity process parameters.

Acknowledgements Gratitude is expressed to all cited companies for the pictures, and Prof.

Abele for the Fig. 7.10.

References

[1] Abele E, Wörn A (2005) Economic Production with Reconfigurable Manufacturing Sys-

tems (RMS) Production Engineering, XII/1:189–192

[2] Chryssolouries G, Anifantis N, Karagianni S (1997) Laser Assisted Machining: An Over-

view. Journal of Manufacturing Science & Engineering. Transactions of the ASME, 119:

766

[3] Kumar J, Khamba JS, Mohapatra SK (2008) An investigation into the machining character-

istics of titanium using ultrasonic machining. Internat J Machin Machinabil Mater, 3:1–2

[4] López de Lacalle LN, Lamikiz A, Muñoa J, Sánchez JA (2005) Quality improvement of

ball-end milled sculptured surfaces by ball burnishing, Int J Mach Tool Manufact, 45(15):

1659–1668

[5] López de Lacalle LN, Perez Bilbatúa J, Sánchez J A, Llorente J I, Gutierrez A, Albóniga J

(2000) Using high pressure coolant in the drilling and turning of low machinability alloys.

Internat J Adv Manufact Technol, 16/2:85–91

[6] Moriwaki T, Shamoto E (1995) Ultrasonic Elliptical Vibration Cutting. Annals of the CIRP,

44/1:31–34

[7] Moriwaki T, Shamoto E (1991) Ultraprecision Diamond Turning of Stainless Steel by

Applying Ultrasonic Vibration, Annals of the CIRP, 40/1:559–562

[8] Rajagopal S et al. (1982) Machining aerospace alloys with the aid of a 15

kW laser. J Appl

Metalwork, 2:170

[9] Zurecki Z, Ghosh R, Frey JH (2003) International Conference on Powder Metallurgy and

Particulate Materials

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

Chapter 8

High Performance Grinding Machines

R. Lizarralde, J. A. Marañón, A. Mendikute and H. Urreta

Abstract This chapter deals with the latest developments in technology, and

describes the current state of the art in machine configuration, critical compo-

nents, and control technologies applied specifically in grinding machines. More-

over, this description of latest developments is structured into the different ma-

chine types: cylindrical, surface, vertical, centreless and their variants, and will

also reference special-purpose machines, oriented to particular but technologi-

cally significant applications and markets, such as the aeronautic or automotive

markets. The principal machine components and peripherals will also be revised,

showing the tendencies in the last years and the newest and most innovative

solutions.

8.1 Introduction

Grinding is a machining process that removes material from a workpiece by

means of an abrasive grinding wheel that rotates at high speed. Grinding machines

vary from very simple “surface machines” with a reciprocating table to very so-

phisticated machines incorporating 5 axes, several grinding wheels and complex

devices for dressing, coolant application and workpiece on-line measurement,

complemented with last generation control units that introduce “intelligence” to

the machine.

In the last years, the industry is demanding process performances that were un-

believable 20 years ago. Productivity, reliability and accuracy demands have been

increased 10 and 100 times in some cases. Industry demands shorter cycle times,

_________________________________

R. Lizarralde, J. A. Marañón, A. Mendikute and H. Urreta

Ideko-IK4, c/Arriaga, 2 20560 Elgoibar, Spain

rlizarralde@ideko.es

280 R. Lizarralde et al.

minimum human actuation, minimum set-ups, the minimum number of machines

and workshop space, and finally “intelligent and autonomous” machines. In the

1990s, high production machines for high batches were demanded. Nowadays, the

shorter life time of most products (the automotive industry is a representative

example) has shortened the batch sizes, adding the requirement of flexibility and

reconfigurability to machines.

These industrial demands have been answered by grinding machine manufac-

turers by means of developments in several areas, which can be summarised in the

following list:

• Multi-purpose machines. Machines that include several wheels for external and

internal grinding, combining conventional abrasives with superabrasives.

• High speed and high dynamics machines. Wheel rotation speeds have increased

10 times reaching values up to 300

m/s. Machine dynamics allow the introduc-

tion of new high production strategies.

• New developments in grinding wheels. New abrasives, such as new ceramics

and CBN (cubic boron nitride), in combination with machine advances allow

material removal rates up to 1,000

/mm/s in processes such as “creep feed”

grinding, “high efficiency deep grinding” or “speed stroke” grinding (see

Sect. 8.3).

• New numerical controls, with higher processing speeds, the integration of sen-

sors for monitoring and control of the process, faster and easier communica-

tion, and integration of measuring devices to close the control loop.

8.2 The Machine Configuration

This section will describe the main general trends for the configuration of grinding

machines. It will cover the principal machine elements from the general architec-

ture, axes configuration, material applied for the construction of the structural

parts, driving units, guideways, main spindles, dressing systems and cooling fluid

application systems.

Although the general configuration of grinders could be considered as “conser-

vative”, with few variations in many years, a very significant number of modifica-

tions and improvements have been performed in the last decades, beyond the in-

troduction of CNC, which is always mentioned as the greatest innovation in

machine tool industry in the last 40 years.

Besides the non-discussed relevance of the electronics and software in the evo-

lution of machine tools, many technologies related to the mechanical parts of the

machine have been developed, and translated from books and laboratories into the

shopfloors and the production facilities. Grinding technology is not an exception

in this tendency, trying to answer to the demands of the 21st century society and

industry, under the target parameters of productivity, accuracy and the reliability

of machines and productive processes.

8 High Performance Grinding Machines 281

8.2.1 The Machine Architecture

The architecture of grinding machines will be analysed separately for the typical

configurations: cylindrical, surface, centreless and vertical machines, since each

one of these configurations presents a complete range of applications and, there-

fore, a complete range of configurations. The separation in the analysis of archi-

tectures will simplify the understanding.

8.2.1.1 Cylindrical Machines

The basic process of cylindrical grinding consists in a grinding wheel rotating in

parallel to the workpiece, which is supported between the working head and the

tailstock.

Concerning the general architecture of the machine and the axes distribution,

there are two main tendencies: the “traverse table” and the “traverse wheel”.

The traverse table is the most conventional and widely applied configuration.

The advantages of this configuration that make it the most used are related to the

accuracy and stability of the process, in particular from the wheel side. This con-

figuration structurally separates the infeed movement, in the grinding wheel side,

from the traverse movement, in the table. The machine is more compact in the

grinding wheel side with only one guideway; this provides better static and dy-

namic behaviour. Additionally, the manufacturing and assembly of this side is

significantly less complex.

On the other side, the major disadvantage on the traverse table is the larger

space required by the table and, consequently, by the machine. The space required

is the sum of the table size (depending on the maximum workpiece to be

grounded) plus twice the traverse stroke. For this reason, limitations in the foot-

print and big workpieces to be machined are situations in which the traverse grind-

ing wheel is selected.

The grinding wheelhead is also subject of several different configurations, de-

pending on the requirements of flexibility, the adaptability to different applications

and workpieces and tendencies towards one set-up machining, to avoid errors

produced by workpiece manipulations.

Figure 8.1 is representative of several configurations that are common among

the grinding wheel manufacturers.

Position 1 presents the most basic straight fixed head. It is the least flexible,

applied in simple machines, extending the principle of ancient manual machines to

general-purpose and training-purposes grinders.

Position 2 shows the oriented head disposition. These heads present a higher

flexibility, since they can not only perform simple plunge operations but also can

grind faces and shoulders. This type of head presents also some variants: from two

fixed positions in minimum and maximum angles, up to a continuous controlled

rotary axe.

282 R. Lizarralde et al.

The increasing productivity requirements have promoted the development of

more complex solutions, implementing several (2,

3 up to 4) wheels in a rotary

head, as presented in positions 3,

5 and 6 in Fig. 8.1. These multi-wheel heads

can include external and internal grinding wheels (positions 4 and 6), or conven-

tional wheels combined with high speed superabrasive wheels, or wheels for spe-

cial processes such as peel grinding. The applications of these configurations are

wide: from general-purpose shopfloors, that manufacture a wide range of parts, to

high production facilities that produce complex parts with several different fea-

tures that require different wheel geometries or process specifications.

Finally, in these multi-wheel configurations, the search of maximum operabil-

ity with minimum floor space required has led to configurations where the wheels

are located in not perpendicular faces, as shown in Fig. 8.2.

Fig. 8.1 Configurations of grinding wheel heads (top row: 1,2,3, bottom row: 4,5,6)

Fig. 8.2 Two non-perpendi-

cular high production wheel

configuration