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

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5 Advanced Controls for New Machining Processes 213

The SERCOS interface (IEC 1491) is a digital interface for communication be-

tween systems which have to exchange information cyclically at short, fixed inter-

vals. It is appropriate for the synchronous operation of distributed control or test

equipment, e.g., connection between drives and numerical controls [31].

A SERCOS interface communication system consists of one master and several

slaves. These units are connected by an optical fibre ring. This ring starts and ends

at the master. The slaves regenerate and repeat their received data or send their

own telegrams. By this method, the telegrams sent by the master are received by

all slaves, while the master receives data telegrams from the slaves. The optical

fibre ensures reliable high speed data transmission with excellent noise immunity.

The SERCOS interface controller SERCON816 is an integrated circuit for

SERCOS interface communication systems. It contains all the hardware-related

functions of the SERCOS interface and considerably reduces the hardware costs

and the computing time requirements of the microprocessor. It is the direct link

between the electro-optical receiver and the transmitter and the microprocessor

that executes the control algorithms. The SERCON816 can be used for SERCOS

interface masters and slaves alike.

With the SERCOS bus, the motor controllers can be connected in an optical fi-

bre ring, usually at 16

MHz. The SERCOS communications ring offers the follow-

ing functions [4], 1) It carries the speed instruction from the CNC to the controller

in digital format, with greater precision and no possibility of external disturbance;

2) it carries the feedback signal from the controller to the CNC; 3) it provides

information on any errors and manages the basic “controller enable” signals; and

4) it allows parameters to be adjusted, monitored and diagnosed from the CNC

using simple, standardised processes. This minimises the amount of hardware

required in the controller and affords greater reliability. Its open, standard struc-

ture affords compatibility between controls and drives from different manufactur-

ers within the same machine. Work is currently underway to boost the velocity to

100

MHz via Ethernet.

Fig. 5.21 Hardware architecture scheme III

214 J. Ramón Alique and R. Haber

The ISO 11898 CAN interface is an international standard for digital commu-

nication between controllers and machine drives using CNC. The communications

protocol is CanOpen, standardised as EN 50325-4. The CAN communications bus

covers the same functions as SERCOS, the only difference being that the latter is

a ring connection while CAN uses a tree topology.

Position controllers can be located in the CNC or in the external regulators. The

tendency with current CNCs is to use them externally, with the position loop being

closed by the regulators. In this case, position references are sent to the regulator

every two to four milliseconds. The regulator is equipped with an interpolator

(which is cubic for the Fagor CNC 8070), which generates intermediate position

references every 250 microseconds. The regulator returns the real position of the

axis to the CNC purely for visualisation and diagnostic purposes, because the

position loop is closed by the regulator.

In software-based solution systems, the operating system plays an important

role. Real-time control applications like interpolating and servo control require

RTOS to have properties of real-time determinism and low latency. For non-real-

time tasks, Windows NT or Linux can be used, the latter being free-access soft-

ware with open source codes that offers full protocol support, stability and low

latency (with RT-Linux). Real-time tasks can be carried out using RT-Linux

or RTX, a technology for Windows™ NT that offers RTOS performance and

determinism.

5.8.5 Fully Digital Architectures: Towards the Intelligent

Machine Tool

There is now a clear tendency for all system components to be interconnected to

one another using purely digital interfaces. In some cases this involves the use of

standard fieldbuses like SERCOS or CAN. In others it means using special inter-

faces developed by the manufacturers of the controllers themselves. For example,

Heidenhain

uses its own interface, which it calls HSCI (Heidenhain serial con-

troller interface), to connect the main computer (MC) with the machine operating

panel and the machine operating panel with the CC controller units [13]. Digital

interfaces are also used for connection to the PLC. The encoders are connected to

the CC controllers via a bidirectional interface, also developed by Heidenhain,

known as EnDat 2.2. Up to two CC controller units can be connected to control up

to 14 machine axes (see Fig. 5.22).

In addition to developments in fully digital architectures, advances are also being

made in the area of “intelligent machines”, meaning machines that incorporate

a number of “intelligent functions”. Mazak

’s Integrex i-150 incorporates five

intelligent functions, namely:

5 Advanced Controls for New Machining Processes 215

Fig. 5.22 Digital connections of the various control components (courtesy of Heidenhain

)

1. Active vibration control, which reduces the vibration caused by the movement

of the axes and thus gives high accuracy positioning. This makes for high sur-

face quality and better tool protection during machining.

2. An intelligent thermal shield. Changes in environmental temperature and heat

generated during the machining process alter the machine’s own precision. In

order to correct this, the operator is forced to conduct the relevant thermal

compensation. An intelligent thermal shield, however, offers automatic com-

pensation for temperature changes, leading to stable, high precision machining.

3. An intelligent safety shield, which prevents errors arising from interference

with the machine due to operator error. If any machine interference occurs, the

machine motion immediately stops.

4. A Mazak voice adviser, by which the CNC system advises verbally of potential

problems. For example, the function advises that lubricant has to be added or

that the tool life is over for smooth operation.

5. An intelligent performance spindle, which uses sensors housed in the spindle to

provide information on temperature, vibration and spindle displacement. This

can prevent spindle-related trouble in the machine.

Mazak

has also announced that its machines now have intelligent maintenance

support, which is extremely useful for machine maintenance (see Fig. 5.23).

216 J. Ramón Alique and R. Haber

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

Chapter 6

Machine Tool Performance and Precision

A. Lamikiz, L. N. Lopez de Lacalle and A. Celaya

Abstract This chapter introduces main machine tool design, construction and

testing aspects to achieve high precision on machined parts. Not only the machine

tool but also the machining process itself is a source of errors on parts. Therefore,

a holistic view involving machine and machining is required.

The design of high precision machines involves some basic principles and

methodologies which are presented in depth further on. A key factor is the identi-

fication of error sources, studying their physical causes and relevance to the final

uncertainty. Thus, assembly error, thermal growth, component deformations and

control inaccuracy are described.

Errors in components and subassemblies are propagated along the kinematic

chain of the machine, producing larger positional errors at the tool tip position and

tool axis orientation. The use of homogeneous matrices is introduced as a very

useful tool for estimating error propagation.

Tool deflection and machine deformation caused by cutting forces is another

important error source. Unfortunately deflection is impossible to avoid although

some models are now provided with machining toolpath selector implying mini-

mal cutting forces along error sensitive directions.

After construction and just before the machine tool is delivered to the customer,

the verification of precision and performance must be always performed. A de-

scription of the existing standards and procedures for this are described at the end

of this chapter.

_________________________________

A. Lamikiz, L. N. Lopez de Lacalle and A. Celaya

Department of Mechanical Engineering, University of the Basque Country

Faculty of Engineering of Bilbao, c/Alameda de Urquijo s/n, 48013 Bilbao, Spain

{aitzol.lamikiz, norberto.lzlacalle, ainhoa.celaya}@ehu.es

220 A. Lamikiz, L. N. Lopez de Lacalle and A. Celaya

6.1 Introduction and Definitions

On the highly competitive market, machine tool manufacturers must build high

precision machines, while at the same time keeping prices as low as possible.

Machine tool history is irrevocably united to machine tool precision and consequent-

ly to the precision of parts to be produced by them, as shown in Fig. 1.5 in Chap. 1.

The improvement in machine tool precision has greatly helped to increase the

added value of manufactured parts, decreasing the adjusting and finishing opera-

tions when these parts are assembled in complex systems. The final results are

both technical and economic, systems are more and more reliable when machining

tolerances are reduced and at the same time less manual adjusting operations are

required on the final assembly. The explosion motor is a good example. Today,

the reliability of this complex system is much higher than that of car engines of the

past century; their production is absolutely automated using CNC machining cen-

tres, boring and honing machines, automated transfer systems and robotic assisted

assembly. Although, undoubtedly, in twenty years current quality and reliability

will be considered low.

In many applications the need for high precision is simultaneously associated

with its usual contrary requirement, i.e., the need for high productivity rates. And

even low production costs and environmental impact must be considered at the

same time. A good solution where these factors are balanced may be the current

concept of “high performance machining”, where the eco-efficient machine tool is

the basic element. This chapter is mainly focused on the precision concept, how-

ever, it will at all times be considering other aspects.

6.1.1 An Introduction to Precision Machining

Machining cost for a part rises exponentially as the required precision level

increases; this is a widespread belief among many production engineers. However,

this concept of precision, where the cost of the individual part is only considered,

excluding any other considerations, is somewhat obsolete. Following this old idea,

the 20th century criterion for setting production system accuracy level was the

minimum required to avoid part rejection by customers, while at the same time

maintaining item price as low as possible. Extra investments to achieve more

precision were thus considered absolutely superfluous.

However, some authors [12] assess that precision machining can be much more

cost effective than conventional machining if other aspects come into play. Thus,

high precision machining eliminates the need for laborious finishing and adjusting

operations, decreasing the final assembly cost when components are inserted. An

illustrative example is a simple plastic injection element for a vehicle dashboard or

bumper. In most cases these parts are designed for both aerodynamic and aesthetic

functions, so high surface finish requirements (in many cases below 0.1

μm Ra)

6 Machine Tool Performance and Precision 221

are usually required. These parts have to be assembled in a more complex system,

which must meet some dimensional requirements without clearances or overlaps.

In addition, these parts are assembled on mass production manufacturing lines, so

roughness and dimensional tolerance must be kept throughout the entire series.

Different sources of error for this simple plastic injection part can be detected:

• Errors from the plastic injection process: These are related to part contractions

due to the solidification and cooling processes, part defects and scratches due to

the flow of the plastic inside the mould, etc.

• Errors of the plastic injection machine: These relate to misalignment in the

mould setup, thermal deviations and machine deformation due to the mould

closing force.

• Errors of the mould itself: All mould machining errors are directly reproduced

in injected parts. Mould machining must ensure a higher precision than an or-

der of magnitude of the required injected part. This accuracy can be in the

range of 0.05–0.01

mm in the entire mould. Mould errors are derived from

those of machine tools and those originated by the machining process; both

types will be explained in the following sections.

Taking into account the added-value of machined parts and relating it to the

cost of production (as shown in Fig. 6.1), one can observe the production of high-

precision parts may be much more profitable than conventional ones, not only in

terms of strictly economic profit but also market consolidation for the future. As

shown, when precision is near the current maximum, the balance is positive for

machine users. Of course, the cost to achieve the current maximum level of accu-

racy is very high, however this is only required in some special applications for

astronomy, lens production, metrology, physics and other instrumentation device

assemblies; in these applications cost is not the key factor.

Precision

Cost

State of Current

Technology

Added Value

Net added value

increment

Ultra-high

precision parts

High precision

parts

Fig. 6.1 Relation between precision, part added-value and cost

222 A. Lamikiz, L. N. Lopez de Lacalle and A. Celaya

A typical definition for high precision machining is the production of complex

components with tolerances below 10

(or even 5)

μm. Below these tolerances, the

phrase ultra precision machining is used. Obviously, the term “high precision” is

continuously changing. Today machine tools are all high precision machines if we

compare them with those of twenty years ago, whereas a lot of them would not be

precise if compared with those constructed ten years from now.

Since the 1990s, high precision machining has become an industry in itself, in-

cluding the manufacture of high-precision components for the optical, electronics

or aeronautical industries. The market was about $US 60 million in 2006 and the

estimated growth by 2009 is in the order of $US 100 million (growing 66% in

three years!). Bearing in mind the sectors involved, the major markets are Asia

(Japan, Taiwan, Korea and China), North America (USA and Canada) and Europe.

Moreover, many industries other than the above are demanding precision levels

which are difficult to obtain by traditional machines [11]. Therefore some of the

precision principles and methods are now spreading to sectors such as the automo-

tive or aeronautical industries.

The greater investment for precise machine tools must be analysed considering

the life span of a machine tool [12]. If we consider the required precision for ma-

chined parts increases yearly and machine precision decreases due to clearances,

surface wear, etc., a high precision machine tool life span is longer than a conven-

tional one. In Fig. 6.2 initial machine precision is presented, plus the usual de-

crease thereof. Therefore, the required accuracy of precision machine tools last for

a longer period than that of conventional machine tools.

Precision is in fact a statistical variable. Real conditions introduce a high num-

ber of sources of error, many of them with random effects, which can affect the

precision value [5,

14]. Therefore, there are three basic concepts, which are

introduced in the next subsection.

Years

Precision

Mach. A: High

Precision Machine

New Machine tool

Start of machine sage

Mach. B: Conventional

Machine tool

Required precision o

machined parts

Mach B, useful time limit Mach A, useful time limit

Life span

difference between

machine tools

Fig. 6.2 Relationship between precision and life span of the machine tool [12]