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

Подождите немного. Документ загружается.

2 New Concepts for Structural Components 71

Similar to the previous case, the percentage increase in damping is directly trans-

lated to an increase of the same percentage in the critical depth of cut. Therefore, it is

possible to reduce the machine stiffness by a specific percentage, with its subse-

quent associated reduction of mass, maintaining the productivity of the machine.

The three approaches are summarised in Table 2.8, which provides an overview

of the influence of the mass, stiffness and damping variation on the natural fre-

quency, the static deflection and the maximum dynamic amplification.

Given the fact that the damping variation approach increases the productivity of

the machines without a penalisation of mass, the method of active damping will be

analysed thoroughly here.

2.6.2 The Influence of ADDs on Productivity

The influence of material damping is low on the total damping of a machine.

Thus, in the case that a higher amount of damping is required to further increase

the productivity of the machine, ADDs are an interesting option.

Figure 2.16 shows the influence of having placed the two ADD-45 of Micro-

mega

shown in Fig. 2.11 in the direction X of an actual horizontal milling ma-

chine. As can be seen, the additional damping that these two actuators have added

to structural modes has allowed a move in the critical depth of cut in all cutting

directions, (the effect being especially high in the direction in which the actuators

were placed). As a result, it can be seen in the diagram in Fig. 2.16a, that the

critical depth of cut in direction X (0º in the polar diagram) has passed from the

initial 1.7

mm (dashed line) to almost 8

mm (continuous line). The introduction of

damping can be extended to the rest of the directions by placing additional ADDs

on the ram of the machine.

Fig. 2.16 Effect of active damping on stability lobes. a Feed in Y+ direction. b Polar plot of the

critical depth of cut at every direction

72 J. Zulaika and F. J. Campa

2.7 Future Trends in Structural Components for Machines

The main trend in the field of structural components is the de-materialisation of

their structural components, passing from stiffness-aiming machines to lightweight

and robust machines, as seen in Fig. 2.17. This can be tackled mainly by integrat-

ing in the structural components active elements such as active piezo-layers, active

damping devices and magneto-rheological fluids. In fact, these ubiquitous sensors

and actuators will allow the passing from current machine conceptions based on

mechanical stiffness to innovative machine conceptions based on mechatronic

robustness, similar to axes levitating on magnetic bearings, which show a precise

and robust behaviour despite not having any mechanical support.

Fig. 2.17 Prototype with structural parts made with ball-bars frame, by Fatronik

Finally, current machine specifications are defined as a trade-off between pro-

ductivity, accuracy and surface quality. Within a view of sustainable machine

tools and processes, this chapter has introduced a new triangle of specifications

based on productivity, accuracy and above all, eco-efficiency.

Acknowledgements Thanks are addressed to Rubén Ansola for his contributions on the topo-

logical optimisation of structural components. Thanks are also addressed for the financial support

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

References

[1] ADD Catalogue (www.micromega-dynamics.com)

[2] Ansola R, Veguería E, Canales J, Tárrago JA (2007) A simple evolutionary topology opti-

mization procedure for compliant mechanism design, Fin Elem Anal Design, 44:53–62

2 New Concepts for Structural Components 73

[3] López de Lacalle LN, Lamikiz, A, Sánchez. JA, Salgado MA (2007) Toolpath selection

based on the minimum deflection cutting forces in the programming of complex surfaces

milling, Int J Mach Tool Manufact, 47:388–400

[4] Rivin E. (1999) Stiffness and Damping in Mechanical Design, Marcel Dekker Inc.

[5] Slocum A (1992) Precision Machine Design, Society of Manufacturing Engineers

[6] Smolik J (2007) High Speed Machining 2007: Sixth International Conference, 21–22 March

2007, San Sebastian (Spain)

[7] Zulaika J (2006) Project NMP2-CT-2005-013989, Internal Report D1.1

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

Chapter 3

Machine Tool Spindles

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

Abstract This chapter deals with spindle technologies for machine tools. The ma-

chine tool spindle provides the relative motion between the cutting tool and the

workpiece which is necessary to perform a material removal operation. In turning,

it is the physical link between the machine tool structure and the workpiece, while

in processes like milling, drilling or grinding, it links the structure and the cutting

tool. Therefore, the characteristics of the spindle, such as power, speed, stiffness,

bearings, drive methods or thermal properties, amongst others, have a huge impact

on machine tool performance and the quality of the end product. Machining re-

quirements differ greatly from one sector to another in terms of materials, cutting

tools, processes and parameters. Nowadays, the spindle industry provides a large

variety of configurations and options in order to meet the needs of different indus-

tries. Therefore, it is crucial that companies correctly identify their machining

requirements and make well-informed decisions about which spindle to acquire. In

this chapter, some of the main spindle characteristics that are the basis of a well-

informed decision regarding spindles are introduced and discussed.

3.1 Introduction

Machining is applied to a wide range of materials to create a great variety of ge-

ometries and shapes, practically without restrictions on complexity. Typical work-

_________________________________

G. Quintana and J. de Ciurana

Department of Mechanical Engineering and Civil Construction, University of Girona,

Escola Politècnica Superior, Av/Lluís Santaló s/n, 17003 Girona, Spain,

{guillem.quintana, quim.ciurana}@udg.edu

F. J. Campa

Department of Mechanical Engineering, University of the Basque Country,

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

fran.campa@ehu.es

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

piece materials are: aluminium alloys, cast iron, titanium, austenitic stainless or

hardened steels, copper, carbon graphite and also plastics, woods and plastic com-

posites. Machined components can be either simple forms with planes and round

shapes or complex shapes. Two types of surfaces are usually defined: ruled sur-

faces, (e.g., for blades) and sculptured surfaces or free-form surfaces (e.g., for

moulds and dies). This enormous number of combinations is able to meet the

specific manufacturing requirements of a wide range of manufacturing industries,

such as automotive, aerospace or the die and mould industry.

The machine tool spindle plays an important role in machining operations be-

cause it provides the cutting speed of the tool and is part of the force chain be-

tween the machine tool structure and the tool or the workpiece. The finished prod-

uct is created by removing material from a blank workpiece with a cutting tool

through the relative motion between the tool and the workpiece. The relative mo-

tion can be divided into a feed motion, provided by the drives of the machine, and

Fig. 3.1 Main parts of a spindle, by Edel

3 Machine Tool Spindles 77

a rotating motion provided by the spindle, which is responsible for the cutting

speed that permits the material to be removed.

Each type of machining process (e.g., drilling, turning, milling, grinding, bor-

ing, etc.) has specific characteristics regarding feed rate and cutting speed. In

basic turning processes, the spindle rotates the workpiece to provide the cutting

speed and the cutting tool is fed by the drives to remove material. In drilling and

milling, the spindle rotates a cutting tool with several cutting edges to provide the

cutting speed. In drilling the feed motion moves in the direction of the spindle

axis, and in milling, it generally moves in a perpendicular direction. In grinding

processes, the spindle is also the element that provides the cutting speed to the

grinding wheel.

The characteristic elements of a spindle are the tool interface, the drawbar, the

shaft, the bearings, the driving system, the cooling system and the housing. There

are several types of driving systems, basically with a motor coupled, directly or

indirectly, to the spindle or with an integrated motor. Figure 3.1 shows the main

elements of an integrated motor spindle.

The spindle industry provides many options and configurations to meet the

requirements of each sector. As an example, Fig. 3.2 shows a comparison of the

maximum speeds of the direct drive spindles and electrospindles typically used

in automotive, mould and die, and aeronautical industries. Hence, the selection

of the most suitable spindle is a complex task. In this chapter, spindle features

such as motors, bearings, speed, power, stiffness and thermal behaviour are

described and the basic criteria for a well-informed decision regarding spindles

are presented.

Fig. 3.2 Speed requirements of different sectors for electrospindles

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

3.2 Types of Spindles

The spindle drive is the mechanism that provides and transmits movement to the

spindle. The drive consists of the motor and the coupling. In this way, the speed of

rotation, the torque and the power are finally transferred to the cutting tool, via the

toolholder.

In general, there are four types of spindle depending on the type of drives used,

belt drive, gear drive, direct drive and integrated (built-in) drive. In Chap. 1,

Fig. 1.17 shows an example of the belt drive, the direct drive and the integrated

drive. Various characteristics have to be taken into account to evaluate the per-

formance of spindles, for example:

• Transmission performance in terms of movement, force, torque, power and speed.

• Heat loss and expansion.

• Vibrations at various speeds.

• Noise.

• Others, such as maintenance and cost.

Below we briefly describe the common features of each type of drive and the

most suitable applications for each one.

3.2.1 Belt-driven Spindles

This spindle setup transfers the movement of an external motor to the main spindle

by means of a cogged or V-belt. It is widely used in conventional machining due

to its low cost and good performance when it comes to transferring the nominal

power of the motor into the useful power of the spindle. The efficiency of belt-

driven spindles, in terms of transmitting motor power to the spindle is around

95%. This is a little less efficient than direct drive spindles (nearly 100%) but

clearly better than gear-driven spindles (less than 90%) [4].

A belt-driven spindle can reach moderate speeds (15,000

rpm) and perform

well or with high torque at lower speeds (1,000

rpm), depending on the belts and

the transmission ratio. In contrast, at low speeds, gear drives transmit the torque

better and at high speeds, direct drives are better (above all in situations where

dimensional precision and surface quality requirements are high) because they

produce less vibration and noise. However, as belt drives are very versatile, they

are used for a wide variety of jobs whose requirements range between high

torque/low rotation speeds and low torque/high speeds [10]. The main disadvan-

tages of the belt-driven system are:

• They undergo significant thermal expansion, compared to other drive systems

due to the constant contact of the belt.

• They are noisier due to the movement of the belt.

• The tensioning of the belt makes a radial force on the shaft that takes some of

the available loading capacity of the bearings.

3 Machine Tool Spindles 79

As the motor and spindle are separate, the housing and maintenance of this kind

of drive is simple, although on the other hand, more space is required.

3.2.2 Gear-driven Spindles

Gear-driven spindles (see Fig. 3.3) can reach high torque at low revolutions and

they characteristically have multiple speed ranges.

The gears, however, may cause vibrations which have a negative effect on the

finished surface of the workpiece. In addition, as we mentioned above, they are less

efficient when it comes to converting the nominal power of the motor into the cut-

ting power of the tool, due to its constructive nature. Such power is lost as heat, with

all the negative effects that this creates, such as reduced precision due to thermal

expansion. For all these reasons, gear-driven spindles are unsuitable for very high

velocity machining operations although very adequate for heavy-duty work.

Fig. 3.3 Haas EC-630 gear-driven spindle with two-speed gearbox: 610

ft-lb of torque for heavy

machining or 6,000

rpm for finish cuts, by Haas Automation

3.2.3 Direct Drive Spindles

Direct drive spindles have almost 100% efficiency in terms of transmitting power

from the motor to the cutting tool. They can work at high rotation speeds but at lower

torques.

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

Since there is no transmission chain, it is not possible to increase torque me-

chanically in response to reductions in motor speed. This drive system behaves

well in terms of vibrations, which means high speeds can be reached and still

achieve good surface finishing.

3.2.4 Integrated (Built-in) Drive Spindles

In these spindles, also called electrospindles, the motor can be a synchronous or an

asynchronous electric motor that is integrated into the spindle structure between

the front and rear bearings (see Fig. 3.4). In this way, vibrations and noise are

reduced and work can be carried out at higher rotation speeds, from 15,000

rpm.

This is why integrated spindles are very common in high speed machine tools.

Controlling heat transference within the spindle and the subsequent thermal ex-

pansion is a key factor for getting good performance from this kind of drive. With

the motor inside the housing, the auxiliary system for removing heat is the first

priority (see Sect. 3.5.4).

The great precision required for the assembly of these spindles and the neces-

sity of auxiliary systems for cooling and monitoring, makes them very expensive,

despite their excellent mechanical performance.

Fig. 3.4 Integrated spindle with the motor between the front and rear bearings

3.3 Spindle Configurations

The arrangement of the spindle in the machine tool is an important feature that is,

in fact, related to the versatility and the final applications of the machine tool. We

describe here the usual configurations of the spindle and also some alternatives.

3 Machine Tool Spindles 81

3.3.1 Common Configurations: Vertical and Horizontal Spindles

The arrangement of the spindle depends on the purpose of the machine tool but, as a

general rule, horizontal spindle configurations are used in machines with great

power and palletised workpieces for a greater flexibility (see Fig. 1.23 in Chap. 1).

One of the advantages of this configuration is that the removed chip falls away from

the cutting zone. Otherwise, vertical spindles are generally used in machines with

less power which need more accessibility, typically for machining dies and moulds;

see Fig. 1.25 in Chap. 1. Both configurations can gain accessibility through the use

of indexed angular headstocks, as the Huré-type for horizontal spindles, or the

swivel-type for vertical spindles. These solutions are still widely used and they

allow the tool to be orientated in fixed lockable steps to cut inclined surfaces, which

allows the (five faces) of a cube, i.e., the workpiece, to be machined.

3.3.2 Machines with Rotary Headstocks

Many milling machines incorporate rotating axes in the headstock to provide more

operation possibilities, so that the orientation of the tool can be changed continu-

ously as required by the surface being machined; see Fig. 1.7 in Chap. 1. These

machines, with 5 or more axes, can produce more complex shapes although they

require more sophisticated computer numerical control (CNC) systems in order to

control the direction of the tool according to the trajectories required. Figure 3.5

shows two classic examples of bi-rotary headstocks: the twist head and the swivel

head. The twist head allows higher accessibility and positioning possibilities than

the traditional 3 axis machines. However, these heads lack a high stiffness, so they

are not suitable for high power applications due to their limitations in terms of

accuracy and dynamics. In comparison, the swivel head with its closed design is

a much stiffer solution and more indicated for high power machining.

Fig. 3.5 Two rotary headstocks. a Twist head by Zayer

. b Swivel head by Droop

Rein