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

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122 G. Quintana, J. de Ciurana and F. J. Campa

Fig. 3.34 Typical spindle speed vs power in machine tools

The choice of cutting tools, in terms of appropriateness for a workpiece mate-

rial, geometrical characteristics and cutting parameters, is a factor that must be

taken into account in the choice of a suitable spindle.

3.6.2 Tool Selection

The characteristics of the cutting tool used to carry out the machining influence

the performance features required of the spindle. There is a huge variety of cutting

tools in terms of shape, size, number of teeth, surface coating, performance, usage,

manufacturer, etc.

The surface complexity, the accuracy and the surface roughness determine which

operation and which cutting tool is needed. Such factors are considered again and

again when programming roughing, semi-finishing, finishing and super-finishing

operations. For simple geometries that require considerable roughing, large-diameter

tools with several cutting edges are used. On the other hand, small-diameter tools are

needed for complex geometries to ensure high precision requirements or highly

smooth surfaces. For example, the mould and die industry commonly deals with hard

materials, above 50HRC, and considerable precision requirements between 0.01 and

0.05

mm and smooth surfaces with a R

of less than 2

μm.

The cutting parameters can be calculated with the help of the tables usually

provided by manufacturers and by using the well-known semimechanistic formu-

las. The workpiece material conditions the cutting speed Vc which determines the

spindle speed N needed in rpm:

1000

⋅

(3.12)

3 Machine Tool Spindles 123

where D is the diameter of the tool. Then, the feed speed V

, in mm/min is calcu-

lated taking into account the recommended feed per tooth in mm and the number

of teeth of the tool:

VfzN=⋅⋅

(3.13)

Once the radial depth of cut a

and axial depth of cut a

are selected, the vol-

ume of material removed Q in cm

/min can be calculated with the feed speed V

1000

epf

aaV

⋅⋅

(3.14)

3.6.3 The Workpiece Material

The effective power required to remove a certain volume in a specific time de-

pends on the material. That value is easy to estimate from cutting tests for turning,

but for milling it becomes more complex to obtain since the number of tooth in cut

changes and the chip thickness is also variable. To solve that problem, an average

value of the chip thickness h

is calculated in mm and related to the specific cut-

ting force k

measured in turning tests.

ccm

kkh

−

=⋅

where

()

sin 180

arcsin

dad

⋅⋅⋅

⋅⋅

(3.15)

The factor m

is used also to take into account the “size effect” cutting at low

feeds. The second column of Table 3.3 shows the specific cutting force in N/mm

of several materials

Table 3.3 Specific cutting force coefficients and specific power coefficients of several materials

Work material Specific cutting force k

(N/mm

) Specific powercoefficient K

Structural steels 1,600–1,800 4.0–5.7

Alloy steels 1,950–2,900 5.3–7.4

Cast iron ,0900–1,100 2.5–3.7

Titanium alloy 1,300–1,400 4.7–5.1

Aluminium alloy ,0400–700 1.3–2.1

3.6.4 Power and Spindle Speed Requirements

Once the volume of material removed and the specific cutting forces are known,

the required power in kW can be calculated using the following equation, where

is the machine efficiency.

60 1,000

⋅

⋅⋅

(3.16)

124 G. Quintana, J. de Ciurana and F. J. Campa

However, to simplify the estimation of the specific cutting force in milling,

nowadays tool manufacturers provide directly a specific power coefficient K de-

pendent on the feed per tooth and the radial immersion a

/D; see Table 3.3. Hence,

the power required in kW can be simply calculated as:

100,000

pe f

aaVK

⋅⋅ ⋅

(3.17)

Workpiece requirements:

- surface finish

- accuracy

- material (K Table 3. 3, col. 2)

- geometrical complexities

- rough, semifinish, finish, etc.

1, 000



VfzN 

1, 000

epf

aaV



100,000

pe f

aaVK

 

Cutting tool:

- shape

- feed per tooth (f

)

- diameter (D)

- cutting width (a

)

- cutting depth (a

)

- cutting speed (V

)

- number of teeth (z)

if:

f machine

VVd

machine



Machine-tool capabilities

- maximum feed rate (Vf machine)

- conventional/HSM

- material removal rate

if:

f machine

VVt

Fig. 3.35 Flowchart for spindle speed and effective power calculation in milling

3 Machine Tool Spindles 125

The power obtained with this expression is then fine adjusted to include the ef-

fect of the rake angle of the cutting edges. Thus, the spindle rotational speed N and

the effective power P requirements for the machining of a workpiece can be calcu-

lated, once we know which cutting tool will be used, which strategies will be em-

ployed for roughing and finishing in conventional machining or HSM, the material

to be machined and the required precision, smoothness and geometrical complex-

ity. The calculation of these parameters is an iterative process which is shown in

the flow chart of Fig. 3.35.

With the spindle speed and the effective power required the spindle can be cho-

sen. It must be remembered that the calculated feed rate cannot exceed the ma-

chine maximum feed rate. If a higher feed rate is obtained in the calculations, we

will have to recalculate the required spindle speed N using the machine maximum

feed rate V

f machine

In this way, the capabilities of the machine tool and the technological limita-

tions involved in milling a specific material are taken into account. If the milling

machine is capable of offering the rotational speed, feed rate and effective power

that has been calculated, then machining may proceed.

This process can be carried out in a similar way to calculate the parameters and

requirements in other metal removing processes such as those using lathes, drills

or grinding machines.

3.7 Brief Conclusions

The machine tool spindle plays an important role in machining operations because

it provides the relative motion between the tool and the workpiece and the torque

needed to perform remove material. Hence, spindle specifications have an enor-

mous influence on machine tool performance and the production system flexibil-

ity. In this chapter, the basic elements of the spindles and the technology behind

them have been presented as well as several aspects that characterise their behav-

iour in cutting conditions and some criteria to make a proper selection of spindles.

The advances in spindle technology have allowed high speed machining tech-

nologies to be developed. Higher cutting speeds have been possible due to the

advances in bearing technologies, in lubrication systems, in mechatronics, with the

vector control of motors with encoder feedback or the recently developed AMBs,

in drive systems, in modelling to predict the spindle thermal and dynamic behav-

iour, etc. Nevertheless, further work is necessary to increase the present limita-

tions of maximum spindle speed, maximum power available and spindle life. Fur-

thermore, spindles have also become a tool for process monitoring and diagnosis.

The increasing use of sensors is not only a source of information for the machinist

to optimise the cutting process and check the spindle health, but also allows the

use of control techniques and actuators to avoid dynamic and thermal problems

online.

126 G. Quintana, J. de Ciurana and F. J. Campa

Acknowledgements Our thanks go to J. Ocaña from DMG

, J. Roca from Intermaher-Mazak

R. Kortázar from Fidia

, A. Rainer Förster from GMN

, for Correa Anayak

, Zayer

and

ASCAMM Technology Centre. Also, special thanks are addressed to J. Pacheco for his contribu-

tions on stability lobes identification methods and to Juanjo Zulaika and Jon Ander Altamira

from Fatronik

for their valuable contributions on spindles design. Thanks are also addressed to

the financial support of the Ministry of Education and Research of Spain (DPI2007-60624) and

Etortek Program of the Basque Government. Finally, our gratitude to Professor López de Lacalle

for his valuable suggestions.

References

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3 Machine Tool Spindles 127

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with angular contact bearings, Proc Instn Mech Engrs 205:147–154

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

Chapter 4

New Developments in Drives and Tables

A. Olarra, I. Ruiz de Argandoña and L. Uriarte

Abstract In this chapter the different alternatives of machine tool drives are de-

scribed. They show the industrial state-of-the-art in driving and guiding systems.

Both linear and rotary drives are analysed from a descriptive perspective and some

basic design principles are also explained. Examples are taken from well-known

manufacturers. Finally, the latest trends in components for driving systems are

shown, giving us a perspective of the near future machine tools.

4.1 Introduction

The present trends in the manufacture of machine tools claim higher performance

in regard to the speed and accuracy of the machining. In order to fulfil these re-

quirements, in principle antagonists, important efforts are being made in the de-

velopment of new driving and guiding components and configurations, for linear

as well as rotary drives. This “travel without end” centres its main activities on the

following areas:

1. A constant development in servomotors.

2. A tendency toward the modularisation of slides and drives for families of

machines. Perhaps it is not optimal from the technical viewpoint, but it is from

the economic viewpoint. The advantages of this tendency originate from the

advantages intrinsic in all modularisation: the reduction of machine devel-

opment times, the reduction of design and assembly failures or errors and the

_________________________________

A. Olarra, I. Ruiz de Argandoña and L. Uriarte

Department of Mechatronics and Precision Engineering, Foundation Tekniker-IK4

Fundación Tekniker-IK4, Avda. Otaloa 20, 20600 Eibar, Spain

aolarra@tekniker.es

130 A. Olarra, I. Ruiz de Argandoña and L. Uriarte

reduction of part and component references. On the contrary, in an initial stage

it requires an in-depth study of the family of machines to be developed, in

addition to how to “split” them into modules. The specifications for these

modules are even more difficult to obtain than those of the slides for a specific

machine.

3. There is also a tendency not to develop individual components but to rather

select and integrate; this tendency is above all clear in the case of guiding

systems. In addition, for some time the machine tool manufacturers designed

and had the means to manufacture the guiding of these, basically through

friction systems. Presently, the tendency is to use roller guides developed by

a different manufacturer and integrate these in the own designs.

4. The definition of the design specifications of the linear and rotary drives. Still,

except for specific-purpose machines, the definition of forces, times, speeds, etc.,

involves a challenge which totally determines the drive and guiding configuration.

As in all design exercises, the solution selected always results in being a compro-

mise between requirements with a certain degree of incompatibility.

4.1.1 Precision and Dynamics

The continuing tendency to reduce machining times and improve accuracy re-

quires optimising the dimensioning of all the machine components, including the

driving and guiding systems.

The mechanical design of a drive determines the dynamic characteristics of this

and, in turn, these characteristics limit the capacity of the servodrive control to act,

that is, to perform appropriately even at the high gain values of the control loops.

The basic control loop used to position a machine tool carriage is shown in

Fig. 4.1. The main control parameters, Kv, Kp and Ti are shown too. As it can be

seen, the position loop controller is a proportional gain (Kv) while the velocity

loop has proportional (Kp) and integral (Ti) effects.

1/s

1/Ti

Set point

generation

Mechanical

system

Fig. 4.1 Position and velocity control loops

4 New Developments in Drives and Tables 131

On the one hand, a high gain value of the control loops results in a drive which

responds faster in the case of set points and consequently provides a greater inter-

polation accuracy, without considering other CNC functions, such as feed-

forward.

The control parameters allowable by the drive are related to the natural fre-

quencies of the power transmission chain between the motor and the slide. In

order to obtain high dynamics, it is necessary to increase the stiffness of the trans-

mission chain and reduce the masses and inertia of the moving elements.

On the other hand, the high gains provide stiffness against interruptions. When

the slide is submitted to a power stage, due to the friction force in a change of

direction of movement, for example, the amplitude of the maximum drive excur-

sion is inversely proportional to the product of the proportional and integral ac-

tions of the speed loop, while the recovery time is inversely proportional to the

position loop gain. This behaviour can be observed in Fig. 4.2.

The drive bandwidth, directly related to the gains of the control loops, in addi-

tion to being limited by the natural drive frequencies, should be such that it does

not excite the structure modes of the machine tool. Consequently, it is of interest

to dimension the drive considering the natural frequencies of the structure.

It can be concluded that in view of the new accuracy and productivity require-

ments, the design of the drives takes on special importance given the dynamic

character of these. For this, mathematical models are used which allow us to de-

termine the natural frequencies of the drive.

1.2

m/mm.min

rad/s

0.8

m/mm.min

rad/s

Fig. 4.2 Effect of the gains in the drive response against a force step of 1000

132 A. Olarra, I. Ruiz de Argandoña and L. Uriarte

4.2 Linear Drives by Ball Screws

The drive by ball screw is the most widespread in the machine tool field for strokes

not exceeding 4–5

m. The main characteristics which place it in such a favourable

position are the high mechanical reduction provided maintaining a high efficiency

and stiffness, as well as sufficient accuracy for existing machine tools.

A ball screw is a mechanical device which transforms the rotary movement into

linear movement. Its operation is similar to a leadscrew in which the sliding be-

tween the threads of the leadscrew and the nut has been replaced by the rolling of

balls which are recirculated in the nut.

4.2.1 Dimensioning

A correct dimensioning and selection of a ball screw must take into account sev-

eral aspects.

4.2.1.1 The Elimination of the Gap

The accuracy requirements of existing machine tools require the use of drives in

which the gap has been reduced to the largest extent possible. The ball screws

used for the positioning of the slides of the machine tools tend to be preloaded to

eliminate this gap. There are different systems to provide the preload.

The simplest preload system consists in using balls of a slightly larger diameter

than the available space. This type of preload is only valid when the necessary

preload is small; due to that the balls undergo sliding at the contact points, which

generates a high wear and tear.

Fig. 4.3 Detail of the ball recirculation. a Internal. b External (by Shuton

)