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

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4 New Developments in Drives and Tables 143

Fig. 4.13 Linear motor series 1FN from Siemens

The main characteristic of the linear motor is the absence of mechanical ele-

ments of transmission of movements and/or forces. On eliminating these mechani-

cal elements, wear and tear, gaps, lubrication requirements and noise are also

eliminated. Another important advantage is the absence of natural resonance fre-

quencies of the drive. However, at the same time, the mechanical advantage is

lost, such that the force generated by the motor is directly the force exercised on

the slide, and under no circumstances can it be higher.

In practice, the linear motor is an interesting option provided that the forces of

the machining process are not considerable and high dynamics are required [7].

That is, when a high acceleration substantially increases the productivity of the

machine. For example, in the case of a sharpener.

As has been indicated, one of the main drawbacks of the linear motors is the

impossibility of multiplying the generated force. Consequently, a large amount of

magnetic material and winding is required to provide sufficient force to use these

as slide drives of machine tools. On the one hand, this amount of magnetic mate-

rial noticeably increases the price of the drive and, on the other, the linear motor is

less efficient than a conventional motor used with any kind of reducer, particularly

due to Joule losses.

On the other hand, as has been mentioned, the linear motors used in machine

tools are provided with ironcore coils. Consequently, the magnet plates exercise

a force of attraction over the coil on a continuous basis. This force of attraction is

more or less constant and depends little on the force that the motor is generating

on an instantaneous basis. The force of attraction is approximately three times the

maximum force that the motor can provide, that is, for a motor of 6,000

N of

maximum force; the force of attraction is in the neighbourhood of 2

Tn. This point

should be considered for the appropriate dimensioning of the guidance of the slide

driven by the linear motor.

When a linear motor is used, the thermal aspect should be considered. The heat

generated by a linear motor is considerable, such that it is necessary to carry out

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

a study to determine the impact that this heat may generate, for example expan-

sion, and assess the need to cool the motor.

The rated force that a linear motor can provide depends to a great extent on the

cooling system used. As a reference value, it can be considered that for a motor

cooled by means of natural convection, the rated force is approximate 20% of the

maximum force. This value may be increased up to 40% if water cooled.

The supply of linear motors for machine tools covers from 300

N of maximum

force up to around to 20

kN.

qFF

...+++=

(N) (4.7)

The speed that a drive can reach with a linear motor is comparable or high-

er than the speed provided by rack and pinion drives. It is common to reach

300

m/min in case of medium-sized motors, up to 5,000

N of peak force, and

150–200

m/min in case of larger motors.

4.4.1 Mounting

The mounting tolerances of linear motors are not as narrow as those necessary for

the correct alignment of screws or racks. In spite of this, the gap between the coil

and the magnets should be maintained within a few tenths of a millimetre. Suffi-

cient stiffness of the slide as well as of the guidance should be guaranteed in order

that they do not deform in case of a powerful force of attraction between the mag-

nets and coil.

4.4.2 Configurations

When using a linear motor, the coil may be used as a moving part and the mag-

nets as a stationary part or vice versa (see Fig. 4.14). Each configuration has its

advantages.

When the coil is moving, the method of carrying the supply cables and cooling

lines up to the coil has to be considered. In some cases, the problem of accessibil-

ity to the coil below the mobile slide may be presented.

In case the magnets move, the cables and lines will be on the fixed part, facili-

tating their installation. In addition, the magnets tend to weigh less than the coil,

thus gaining some acceleration. The main drawback of this solution is that the

length in which the magnets should be installed is greater. The slide should have

this length to install the magnets.

An interesting alternative is to have two linear motors in order that the forces of

attraction are mutually cancelled. This point is particularly interesting when

4 New Developments in Drives and Tables 145

a machine which should be lightweight is being designed. Two possible symmet-

rical arrangements which cancel the forces of attraction are shown below.

4.5 Rotary Drives

The tendency of the requirements stipulated for rotary tables is similar to that

required for linear slides. A fast dynamics is desired with a high accuracy of posi-

tioning and interpolation.

Depending on the process for which the rotary table is designed, the speed and

force specifications can be quite different. In case of tables for lathes, the maxi-

mum speed and allowable pairs obtain a greater weight than high dynamics and

accuracy. The case is different in case of five-shaft milling tables.

In case of rotary tables, two large drive groups are distinguished. On the one

hand are the mechanical transmission drives and on the other direct drives.

4.5.1 Mechanical Transmissions

Two of the most common mechanical transmissions for rotary drives are worm-

gear and wheel-gear.

4.5.1.1 Worm-gear Transmission

One possibility is the worm-gear transmission. This type of transmission provides

a very large reduction, a high capacity to transmit force and quite reduced wear.

Fig. 4.14 Configurations with linear motors. a Movable coil, b Movable magnets (by Fanuc

)

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

The main difficulty, in addition to the cutting of the teeth, consists in eliminat-

ing the gap. There are different possibilities for this. It is possible to cut the worm

gear such that the pitch is variable, such that shifting it axially may eliminate the

gap up to the desired level. The wear and tear caused by use can be eliminated in

the same way. Another option for eliminating the gap consists in using a split

worm gear preloaded with a spring against the teeth of the gear.

4.5.1.2 Wheel-gear Transmission

Another option for driving rotary tables consists in using two pinions which act on

a gear. The gap is eliminated in a similar manner to that in the rack and pinion

drive, preloading the pinions by mechanical means or otherwise using two motors.

4.5.2 Direct Rotary Drives

Nowadays, the drive which provides the best dynamic performance for rotary ta-

bles is the direct motor (Fig. 4.15). In addition, there are no gaps or wear and tear.

As occurs in linear motors, in this case special attention should be given to the

thermal aspect. Normally, it is necessary to provide water cooling to the torque

motors.

Torque motors are supplied in two formats. On the one hand, there are com-

plete motors, in which the exit shaft normally tends to be hollow. On the other

hand, in order to be able to provide a more compact solution and better integrated

in the product designed by the machine tool manufacturer, the torque motor sup-

pliers also offer the motor in two units, rotor and stator.

The present supply of torque motors provides rated torques of up to approxi-

mately 5,000

Nm and a range of speeds from 200

rpm to 1,000

rpm. The peak

torque tends to be in the vicinity of 80% higher than the rated.

Fig. 4.15 a Complete torque motors. b Built-in torque motors (Siemens

)

4 New Developments in Drives and Tables 147

Fig. 4.16 Direct rotary drives in profile grinding machines [5]. a Gear grinding. b Ball screw

grinding (Doimak

)

In Fig. 4.16 two examples of rotary drives with torque motors are shown, in

grinding machines.

In Sect. 3.3.2, the application to two-rotary milling headstocks is commented

upon, where torque motors are substituting to hypoid gear transmissions. Other

example is the machine by Ibarmia in Fig. 1.7 in Chap. 1, where the tilting move-

ment of headstock is provided by a torque motor.

4.6 Guidance Systems

The guidance systems, together with the drive, determine the operation of the

machine tool slides. The technologies used for machine tool applications are de-

scribed below.

Historically, the type of guidance most used has been friction. Normally, the

machine manufacturer also fabricates the guidance system. Presently, the roller-

based guidance has gained much popularity thanks to the high quality and per-

formance that commercial components provide and the fact that the manufacturer

can integrate these into his machine. On the other hand, for very demanding appli-

cations in regard to accuracy and stiffness, the hydrostatic guidance has been and

continues to be the best option from the technical viewpoint.

4.6.1 Friction Guides

The friction guide (Fig. 4.17) is the most widely used type of guidance in machine

tool applications.

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

4.6.1.1 Materials

Different pairs of materials have been used to manufacture friction guides. Cast-

ing, steel, bronze and certain polymers has been used as sliding materials. A key

factor to ensure the controllability and smooth operation of the guide is to avoid

the stick-slip phenomenon which appears when the static friction coefficient is

higher than the dynamic friction coefficient (see Fig. 4.18).

Fig. 4.17 Different configurations of friction guides (by Busak Shamban

)

Fig. 4.18 Comparative diagram of frictional behaviour, by SKC

4 New Developments in Drives and Tables 149

Fig. 4.19 Coated slideways and worm rack in a table of a gantry milling machine (by SKC

)

The development of polymeric materials with additives which favour lubrica-

tion, for example, Turcite™ Slideway or SKC

(see Fig. 4.19) or Moglice

, has

allowed to a great extent to reduce the stick-slip phenomenon.

4.6.1.2 The Load and Damping Capacity

Among the favourable characteristics of the friction guides are the high load ca-

pacity they provide, up to 140

MPa, its strength against impact and the capacity to

provide considerable damping. On the other hand, its use will be limited to slides

with relatively low speeds, under 0.5

m/s.

4.6.1.3 The Manufacturing

In order to obtain a good quality friction guide it is necessary to work the slide-

ways to ensure a uniform contact of the bedplate and slide. This finishing opera-

tion, the scraping, requires highly experienced labour. In addition to guaranteeing

a uniform contact, it is of interest to carry out a series of markings on the surface

in order that contact is produced on a multitude of small projections. In this man-

ner, adhesion problems are avoided when specular surfaces are matched, with

these being more important when the sliding is at low speeds.

It is appropriate that lubrication be used to favour a smooth movement. For this

purpose, slots in the vicinity of one millimetre in depth on one of the surfaces are

prepared. In order to force the movement and distribution of the lubricant, it is

appropriate that the slots be on the moving part.

The application of polymeric-based material can be carried out in several ways.

The most classic consists in bonding a polymer sheet a few millimetres thick over

one of the parts, machining this once the bond has set in order to correct any

flaws. The surface which slides on the polymer should be slightly warmed, for

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

example, by induction, to minimize wear and tear. Presently, it is more common to

inject the polymer in a liquid state in order that once it is cured it adapts to the

shape of the guide being manufactured. For this, the surface which will receive the

polymer is prepared with a very rough surface finish to favour the adherence and

is also provided with injection holes. The bedplate and slide are matched, main-

taining the space that the polymeric material will occupy between them. When the

injected material has cured, in general after 24 hours, the bedplate and slide are

separated and the surfaces are finished.

4.6.2 Rolling Guides

Presently, the roller-based guides are the most widely used in machine tool appli-

cations. There are a large variety of models and sizes which adapt to most machine

tool slide requirements.

Two large families of rolling guides can be distinguished. On the one hand,

there are those without recirculation, and on the other, there are guides with recir-

culation.

4.6.2.1 Linear Bearings without Fecirculation

The rolling guides without recirculation (Figs. 4.20 and 4.21) tend to be used

when the stroke of the slide is relatively short. It basically consists of rolling ele-

ments, balls, rollers or needles, placed between moving parts. In general, a cage is

used to prevent contact between different rolling elements and to handle the as-

sembly with greater facility.

The strong points of this type of guide are the very high stiffness and load ca-

pacity, due to the high number of rolling elements which can be used, and high

accuracy in the movement.

To guide a slide, it is necessary to use at least two guides, in order to restrict all

degrees of freedom except that of movement. The preload is attained by making

one guide work against the other; for example, by means of screws supported

Fig. 4.20 Linear rolling bearings without recirculation, by THK

4 New Developments in Drives and Tables 151

directly on the rail or on an intermediate jib which distributes the force on a more

uniform basis causing minor deformations of the rail and improving accuracy.

Normally, the supply of this type of guide includes the guide rails. Sometimes,

due to lack of space availability, for example, it is necessary to use an element of

the machine as a roller track. In this case, it is necessary to temper and harden this

area to prevent accelerated wear and tear.

4.6.2.2 Recirculating Rolling Guides

On the other hand, there are the recirculating rolling systems (Fig. 4.22). These

consist of a shoe, within which the rolling elements, balls or rollers recirculate, as

they move along the length of a guide rail.

The guide rail has at least four roller tracks, such that each shoe restricts all the

degrees of freedom except that of movement. In this case, the preload is attained

using rolling elements of a slightly oversized diameter. Generally, the manufac-

turer offers the same product with different degrees of preload.

Presently, the recirculating rolling guide manufacturers have obtained designs

with high-load capacities vs forces transverse to the movement as well as vs

moments. To refer to the guide size, the tendency is to use the rail width in mm,

with there being rails available from size 15 up to size 65 or even larger. The

load capacities of a guide family from the manufacturer THK are indicated in

Table 4.4.

Fig. 4.21 Linear rolling bearings without recirculation, by INA

Fig. 4.22 Recirculating rolling guide model, by THK

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

Table 4.4 Load capacity of THK

guiding systems

Basic load rating

Outer dimensions

Radial, reverse radial and side

Height Width Length Dynamic Static

M W L rating rating

C C0

Model No.

mm mm mm kN kN

SHS 15

V 24.0 034.0 064.4 014.2 024.2

SHS 20

V 30.0 044.0 079.0 022.3 038.4

SHS 25

V 36.0 048.0 092.0 031.7 052.4

SHS 30

V 42.0 060.0 106.0 044.8 066.6

SHS 35

V 48.0 070.0 122.0 062.3 096.6

SHS 45

V 60.0 086.0 140.0 082.8 126.0

SHS 55

V 70.0 100.0 171.0 128.0 197.0

SHS 65

V 90.0 126.0 221.0 205.0 320.0

The preload is necessary to eliminate gaps. In addition, the preload increases

the stiffness of the skids. An average preload can increase the shoe stiffness by

a factor of three in respect to a shoe which is not preloaded, and makes it less

dependent on the load applied.

On the other hand, the drawbacks of a high preload are the increase of friction

and reduction of the useful life of the guidance system.

4.6.3 Hydrostatic Guides

In case of hydrostatic bearings (Fig. 4.23 and Fig. 1.16 in Chap. 1), the surfaces of

the components which slide are separated by a thin oil lamina. The pressure main-

Fig. 4.23 a Hydrostatic V-flat guide. b Hydrostatic guide with wrap-around (Hyprostatik

)