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

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7 New Developments in Lathes and Turning Centres 263

These lathes were specially developed for this purpose, plus the advantage of easier

chip evacuation. Solutions for parts such as brake discs or similar parts were devel-

oped in the 1990s by several leading manufacturers such as Emag

(Fig. 7.1),

Weisser

and Hesapp

and extended later to the rest of the manufacturers.

Recently, some solutions have been developed in vertical lathes for long and

slender parts, such as shafts, which were typical horizontal applications. In

horizontal lathes the loading solution was performed by external elements. Lately,

some remarkable integrated solutions have been developed. Among them, the pat-

ented solutions of Weisser

(Fig. 7.2), that applies the usual pick-up concept of

vertical inverted lathes to a horizontal configuration, and Index

, performing the

pick-up operation via tool turrets, that hold special chucks for picking the work-

piece up in the two tool position.

4. They have a high speed on headstocks and high dynamics on displacement axes

for minimum lead time. For this purpose, the machines are equipped with high-

speed electrospindles in headstocks, linear ball or roller recirculating guides in

displacement axes and, in some cases like Gildemeister

, linear motors for axe

displacement.

5. Basically, they only perform turning operations, thus, with optional simple opera-

tions like drilling or basic milling, they are performed by motorised tools installed

in the turret. Nevertheless, one of the latest evolutions has been the inclusion of

grinding wheel heads to complete the machining processes. This tendency is being

applied both in vertical and horizontal machines, as shown in Fig. 7.3.

Fig. 7.1 Pick-up solution in a vertical lathe

from Emag

Fig. 7.2 Pick-up solution, by Weisser

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

Fig. 7.3 Grinding wheels included in lathes. a Vertical from Index

. b Horizontal from Mazak

7.2.1.1 High Production Processes: Hard Turning Versus Grinding

The high quality finishing of parts is usually a prerequisite in high production lines,

in particular the automotive high production line. Additionally, hardened steel parts

are very common in the automotive industry, especially in engine parts, which are

subject to high precision movements, and high mechanical and contact loads.

The high production machining of these parts brought about the discussion be-

tween grinding and turning as finishing operations, especially after the development

of “hard turning” technology. Basically hard turning is the turning of hardened steel

using CBN (cubic boron nitride) or ceramic tools, in high-stiff and precise lathes.

Lathe manufacturers have defended their solution supported by the higher flexi-

bility and cheaper solution, while grinding manufacturers based their arguments on

the supposedly higher precision, stability and productivity of the grinding process.

Additionally, the application of hard turning has been found over the years as not

suitable for parts with sealing requirements, because of the helix generated in the

surface by any turning process. This problem has recently been overcome with the

application of special tools to eliminate that feature, as shown in Fig. 7.4.

Both lathe and grinding machine manufacturers have directed the discussion to

an intermediate solution, developing mixed solutions in recent years that incorpo-

Fig. 7.4 Lea

-free hard turning, a By Sumitomo

. b By Weisse

7 New Developments in Lathes and Turning Centres 265

rate the “opposite” process, as shown in Fig. 7.3. Nowadays, it is common to com-

bine processes such as hard turning and grinding, in the same machine, depending

on the parts and particular features to be machined.

7.2.2 Turning Centres: Multi-tasking Machines

Multi-tasking machines have become the most important evolution of lathes in the

last ten years, holding a privileged position in the catalogue of many leading

manufacturers.

The origin of turning centres or multi-tasking machines can be found in the ap-

plication of additional motorized tools, which have been used in lathes for many

years. The market tendency towards lower batches, higher flexibility and the re-

configurability of machines has fostered the fast extended development of solu-

tions in this line.

It is difficult to delimit or mark the fields of applications and the configuration

of multi-tasking machines, since nowadays the variations and combinations of

features, devices and processes integrated are almost infinite. The imagination and

development capabilities of machine tool manufacturers have made almost any

combination possible, in vertical and horizontal configurations.

The first steps in turning centres consisted of conventional lathes that included

a milling head instead (or in addition to) of the turret, with one or two additional

axes to perform interpolated milling operations.

Multi-tasking “turning-drilling-milling” centres with the standard B, C, X, Y

and Z-axes allow the complete machining of precise complex workpieces. The

goal to completely machine and measure a wide spectrum of workpieces with

minimal settings in only one machine has been achieved. By the interpolation of

up to five axes even free form surfaces can be machined easily. To summarise, the

claims for these types of machines are:

• The reduction of floor-to-floor time. The number of machines involved in the

machining process is drastically reduced. Queuing up in front of machines or

transport in between individual machines is reduced to a minimum. Work

scheduling and planning is extremely simplified. The cost of stock piling semi-

finished piece parts is reduced.

• Increased flexibility. In multi-tasking machines setup times are usually low

when changing over to another clamping or piece part. Expensive, complicated

fixtures are unnecessary. Due to technological capabilities, these machines of-

fer many creative possibilities in the customer’s design department.

• Increase in use. Decreasing setup times are equivalent to a significant increase

in production hours. Complete machining means machining processes are not

cut by manual interventions, which eliminate interruptions by breaks.

• Reduction of manpower. The reduction of machines involved in the production

process automatically reduces personnel requirements for operation, inspection,

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

piece part transport and work scheduling. The complete machining process

combines formerly individual processes in just one operation, enabling a multi-

ple machine operation.

• Increase in quality. Due to the lack of multiple clampings as in split manufac-

turing not only costs for fixturing are reduced but also clamping and adjustment

errors are avoided.

These machines have progressed towards higher sizes, power capabilities and

the integration of different solutions and processes. Some of the latest, most sig-

nificant examples will be presented below.

7.2.2.1 Large-size Machines for Heavy-duty Processes

These machines, such as the M150 of WFL

(Fig. 7.5), can hold workpieces of up

to 6 metres long, with a 1 metre diameter, capable of performing heavy-roughing

operations, by means of their high torque and power spindles and milling heads,

reaching values up to 90–100

kW.

Fig. 7.5 Heavy-duty multi-tasking

machine from WFL

7.2.2.2 New Ideas in the Structural Machine Concept

From the original configuration of machines, usually an inclined bed lathe incorpor-

ating the milling head, manufacturers have progressed to the development of opti-

mum configurations for multi-tasking purposes, giving the same importance to mill-

ing and turning operations. One of the best examples is the new NT family by Mori

Seiki

, that integrates the milling centre concept (a vertical “box in box” configurati-

on with the milling head located in a octagonal Z axis ram), together with the horizon-

tal bed of a lathe, as shown in Fig. 7.6 and Fig. 1.3 in Chap. 1. Operability, precision,

mechanical performance and chip evacuation are ensured in this machine concept.

Besides the machine concept, Mori Seiki

has introduced new features in these

series, one of the most innovative being the tool turret that integrates a milling

electrospindle, as shown in Fig. 7.7.

7 New Developments in Lathes and Turning Centres 267

This example of machine configuration shortens the distance between turning

and milling centres, leading to a new discussion in the machine tool industry. This

discussion is more extended in the case of vertical turning and milling centres

equipped with rotary tables. Vertical turning centres are the evolution of original

vertical lathes towards one complete setup for part machining.

For this purpose, vertical lathes incorporate a vertical displacement axis con-

figured with a milling ram or combination of a ram plus a moving portal. This ram

can incorporate a milling head or turning tools. In the latest evolution, analogous

to the horizontal turning centres, turning tools are held by the same milling head,

locking the rotation of the spindle.

These vertical turning centres have found a very important market in the ma-

chining of big parts, energy generation and land transport being some of their

target markets. Figure 7.8 presents one example of this type of vertical turning

centres, by Pietro Carnaghi

(see also Fig. 1.21, Chap. 1). These machines hold

workpieces of diameters up to 6–8 metres and the turning power can reach values

from 200 to 300

kW.

Fig. 7.7 Milling electrospindle integrated into the turning turret, by Mori Seiki

Fig. 7.6 Structural config-

uration of Mori Seiki

series

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

From the technological side, vertical turning centres usually collect two very dif-

ficult to make compatible requirements: big workpiece sizes and narrow tolerances.

Aeronautic turbine components (rings, housings, seals …) and wind energy

generator gearboxes are typical applications of the medium size vertical turning

centres, which lead to very high demands in terms of quality, precision and reliabil-

ity, together with big size and high material rates to be removed. For this reason,

these machines combine high level technologies from the mechanical and control

sides: hydrostatic guides on all axes, including the rotation head, the monitoring

and control of process and machine components, and the compensation of thermal

and dynamic disturbances or online measurement are technologies usually found in

these machines.

Besides the pure machine configuration evolution, these multi-tasking ma-

chines have introduced innovations in the application of processes. One innovative

and significant example is the use of milling to perform operations that were usu-

ally done by turning, in a process called “turn-milling” or “spin milling” by differ-

Fig. 7.8 Heavy-duty vertical turning

centre, AC 46 by Pietro Carnaghi

Fig. 7.9 Turn milling operation. a By WFL

. b By Mori Seiki

7 New Developments in Lathes and Turning Centres 269

ent manufacturers to increases the productivity of turning by a higher material

removal capability. Figure 7.9 shows some examples of this application.

Flexibility and reconfigurability associated to multi-tasking machine has also

been demanded by industry, and several approaches have been developed and are

currently in the process of being developed. Reconfigurability is understood in this

context as the possibility of re-configuring a machine, from one purpose to another

by means of substituting or adding modular components. These approaches, such

as the German project “METEOR” (Fig. 7.10) seek the elimination of lead times

[1], not only during the machining of one particular reference, but also in changing

from one reference to a new one.

Although they cannot be considered a multi-tasking process or machine, the turn-

milling or turn-broaching processes are remarkable. These are high productivity

processes, mainly orientated to the rough machining of engine crankshafts, espe-

cially end journals by means of big diameter milling tools, as shown in Fig. 7.11.

Fig. 7.11 Turn-milling/turn-

roaching operation, by Boehringe

Fig. 7.10 Reconfigurable machine concept,

after [1]

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

7.3 The Latest Technologies Applied to Lathes

and Turning Centres

This section summarises the most important technologies developed to provide

new machines with the performances required to satisfy increasing market de-

mands.

7.3.1 General Configuration Technologies

Historically, lathes have been the most “conservative” machine tools, being mostly

used as general purpose machines and, until some years ago, to perform pure rough-

ing processes. Finishing operations were performed by grinding, honing or polishing.

Nevertheless, the latest most advanced machines, i.e., both high production

lathes and turning centres, incorporate the same high level mechatronic technol-

ogies as other machines such as grinding machines or machining centres.

• The architecture of machines has been described in the previous section. There

is no other machine tool that has undergone so much evolution, from the con-

ventional lathe configuration the complex turning centres with several head-

stocks, turrets and milling heads, combining other tools such as laser, roller

burnishing or ultrasonic sources. Machine architecture has evolved to adapt all

these innovations.

• Guiding systems have progressed from the friction sliding guides of conven-

tional lathes to a massive use of recirculating ball or roller units, pushed by the

drastic increase in speed and acceleration required for faster machining, loading

and unloading operations.

In heavy-duty and high precision machines, such as vertical lathes and turn-

ing centres orientated to aeronautic and energy applications, hydrostatic guides

are used in rotary and displacement axes, providing a high load capacity, high

damping ratios and positioning accuracy that provide safety, precision, stability

and productivity to the high added value solutions of those sectors.

• Conventional motors for main spindles and transmissions have also been sub-

stituted by integrated electrospindles for headstocks (see Fig. 6.5 in Chap. 6)

and linear motors for displacement of axes when speed and acceleration re-

quirements have become excessive for conventional transmissions.

• Numerical controls must also answer to the high demands the complexity level

of these machines induces: multiple axes to be controlled and synchronized,

different processes and machining cycles (turning, milling, grinding, drilling,

tapping, etc.) to be programmed and controlled, and the high dynamics of axes

and the implementation of measuring devices. Due to all these particularities,

the latest generation controls are being applied and some manufacturers, such

as Mazak

, have developed or customized their own NCs.

7 New Developments in Lathes and Turning Centres 271

7.3.2 Complementary Technologies to Improve

Machine Performance

In recent years, some complementary technologies have been integrated in lathes

to improve the productivity and precision of these machines. Thus:

• The machine behaviour control and compensation of errors, in particular, vibra-

tions and thermal effects. The “thermal friendly” concept is used by most of the

leading manufacturers, with similar approaches that cover, basically, two fields:

structural design, attempting to reach minimum thermal sensitivity designs, and

the measurement of critical temperatures to apply compensation strategies

through the NC.

Concerning vibration prevention, there are several approaches. Some manu-

facturers have developed strategies to suppress chatter vibrations (Sect. 3.5.3),

by the application of mechanical passive dampers or software based strategies

such as the variation of the rotation speed.

More widespread are the developments related to the balancing of parts during

machining. The so-called “adaptive balancer” by Mori Seiki

, shown in

Fig. 7.12, which is able to balance the potential part imbalance during turning,

using two plates with small masses distributed asymmetrically, is remarkable.

Other advanced balancing applications have been recently developed for car

and train wheels machining. In these cases, the imbalance is measured between

roughing and finishing, and balancing is performed via eccentric turning, elimi-

nating more material on the overloaded side.

• Programming and simulation of processes. Three main factors make these tech-

nologies almost necessary for multi-tasking machines: 1) the inclusion of multi-

ple different processes 2) the complex configuration of machines, with many

devices and tools, and 3) the complex shape of the parts and the high number of

different operations combined for a total machining in one setup.

For this reason, specific CAM software has been developed for turning centre

machines, combining features of turning and milling. These CAMs are com-

bined with 3D simulations of the process (Fig. 7.13) to optimise the working cy-

cles, preventing collisions and non-desired cycles. Several machine manufac-

turers have developed their own systems.

Fig. 7.12 “Adaptive balance

” by Mori Seiki

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

Fig. 7.13 Simulation of the multi-tasking process, by Spring Technologies

7.4 Special Machining Processes Applied

in Multi-tasking Machines

Multi-tasking machines are now integrating different machining processes, for

example laser processes, cold forming treatments and others. Current develop-

ments are explained in the following sections.

7.4.1 The Laser Application

Lasers have some potential applications in combination with other processes in

multi-tasking machines. Laser hardening (Fig. 7.14) is the most applied. Several

manufacturers integrate a laser head as an additional tool, in different configurations.

Fig. 7.14 a Laser hardening process integrated in the turning centre. b System by Boehringer