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

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

In the last two years the traditional use of gear transmission for the two rotary

axes is being substituted for the use of the torque motor directly inserted in the

rotary joints. Eliminating gears increases the stiffness and avoids backlash, with

more precise rotation; see Sect. 4.5.

Over the last years, several alternative solutions based on the parallel kinemat-

ics mechanisms have been developed, such as the Hermes head by Fatronik

the Sprint Z3™ head by DS Technologie

, as shown in Fig. 3.6. These solutions

allow fast orientation of the tool up to ±40º in every position with minimal motion

of the headstock.

3.3.3 A Main Spindle with an Auxiliary Spindle

In addition to the spindles described in the previous section, there are other con-

figurations that combine a main spindle with an auxiliary spindle mounted along-

side it in parallel, with different transmission systems to take advantage of the

possibilities of each spindle according to the specifications of the job. For exam-

ple, there are machines which combine the different qualities of belt drive and

direct drive with a pair of spindles mounted together – one is the main spindle,

the other the auxiliary – so it can be used in a combination of high speed/high

power applications.

The main spindle is belt-driven since, as we have explained, this drive provides

high power at low speed. This spindle would be suitable for roughing and semi-

finishing with roughing mill tools of considerable diameters.

A direct drive spindle acting as an auxiliary spindle, completely integrated into

the machine tool, can be coupled to the main spindle. The direct drive spindle will

carry out low power/high speed operations. In this way, the auxiliary spindle will

be ideal for high speed operations providing high quality surface finishing and

dimensional precision, using smaller-diameter ball end tools.

Fig. 3.6 a Detail of the DS Technologie

Sprint Z3 head and spindle. b Hermes head by

Fatronik

3 Machine Tool Spindles 83

3.3.4 Twin Spindles and Multi-spindles

Some machine tools can be equipped with two or more spindles also known as

“twin spindles” or multi-spindles; see Fig. 3.7, and Figs. 1.15b and 1.32b in

Chap. 1. The main advantage here is that two or more identical components can be

machined at the same time in the same machine, thereby increasing productivity

considerably. On the other hand, these machines usually lose dimensional precision

in comparison with conventional machine tools. There are several reasons:

• The distances between the axes of the spindles are not exactly the same, due to

the design and build of the machine.

• The diameter, length and wear of the tools in the twin and multi-spindles used

are also variable.

• The temperature and therefore the effects of thermal expansion at specific

points throughout the machine are not the same.

Fig. 3.7 Two images of the Nicolas Correa

Neptuno, an example of multi-spindle configuration

3.3.5 Automatic Head Exchange

In certain situations, automatic head exchange may be the best option; for exam-

ple, when there are fluctuations in the production system requirements in terms of

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

flexibility or productivity, demand for the machined parts, bottlenecks in the pro-

duction plants or stoppages for maintenance. This system allows the heads of the

machine tool to be changed automatically using hydraulic toolholder systems (i.e.,

hydraulic chucks). Figure 3.8 shows the automatic head exchange system of the

Cyport CP machine by Edel

3.4 Basic Elements of the Spindle

The main components of a modern spindle are described in this section: a) the

motors, which propel the spindle, b) the bearings, which allow the relative motion

Fig. 3.8 The Cyport CP machine automatic head exchange system by Edel

3 Machine Tool Spindles 85

between shaft and machine, c) the shaft, d) the drawbar and e) the tooling system,

f) the sensors, nowadays essential to monitor the process and diagnose spindle

problems, and g) the housing which not only accommodates all the elements but

also contributes to the cooling and refrigeration of the spindle itself.

3.4.1 Motors

The motors provide the mechanical motion to the spindle-tool system. They rotate

the shaft to create relative motion between tool and workpiece and provide power

and torque to the cutting tool in order to perform operations such as drilling, mill-

ing, and grinding.

Machine tool drives use electric motors that may be either direct current (DC),

brushed and brushless motors, or alternating current (AC), and either synchronous

or asynchronous motors. We will now describe the main characteristics of these

different motors and their utility in the field of machine tools.

3.4.1.1 DC Motors

DC motors, whether brushed or brushless, are common in machine tools and al-

though they are more frequently used as feed drives, they have also applications as

main drives. Brushed motors, also known as conventional DC motors, use internal

commutation, while brushless motors use external commutation to create the AC

current from the direct current. Brushless motors have many applications in the

field of machine tools, despite the fact that they are more expensive than conven-

tional DC motors, since their design features provide greater performance and

Fig. 3.9 High frequency spindle units FS45-50/1 Brushless DC and FS33-60/0.15 by SLF

(Spindle-und Lagerungstechnik Fraureuth Gm

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

efficiency. The main characteristic is that the change in polarity of the rotor can be

carried out without brushes. In this way, efficiency is enhanced by reducing fric-

tion. Figure 3.9 shows two high-frequency spindle units used in applications for

small CNC milling and engraving machines, the FS45-50/1 Brushless DC and the

FS33-60/0.15 from SLF

3.4.1.2 AC Motors

AC motors convert alternating current into rotating torque. The AC motor consists

of a stator, which is the fixed part that produces a rotating magnetic field by means

of an alternating current, and the rotor, which rotates due to the rotating torque

generated by the magnetic field. In machine tools, synchronous and asynchronous

AC electric motors are currently used.

Synchronous AC motors are distinguished by having a rotor that spins syn-

chronously with the oscillating field of the current that drives it. They have a con-

stant rotation speed determined by the frequency of the grid to which they are

connected and by the number of pole pairs of the rotor. The rotational speed N,

also known as the synchronism speed, is easily calculated in rpm using Eq. 3.1:

=⋅ (3.1)

where f is the grid frequency (e.g., 50

Hz in Europe) and p is the number of poles.

The main advantages of synchronous AC motors are that their position and speed

can be accurately controlled with open loop controls and that speed is independent

of the load, so it can be accurately maintained.

Asynchronous AC electric motors, also known as induction motors, differ from

synchronous motors in that the latter have current supplied to the rotor which

Fig. 3.10 Section of an AC motor of a machine tool spindle. The cavities for cooling can also

be seen around the motor

3 Machine Tool Spindles 87

creates a magnetic field. The type of rotor winding, slip ring motor or squirrel

cage motor, conditions the motor behaviour. Figure 3.10 shows an AC asynchro-

nous motor.

The rotating speed of these motors is managed by frequency converters, which

are the electronic power stages that control the voltage and frequency provided to

the motor. The “vector control” techniques have improved the performance of

these motors, due to the improved speed control and reduced acceleration and

deceleration times.

3.4.1.3 Torque Motors

Torque motors are direct drives based on permanently excited multi-pole brushless

synchronous AC motors with a hollow-shaft rotor. As eddy-current losses increase

with the number of pole pairs, the maximum number of pole pairs is limited, al-

though they still have a relatively large number of magnetic pole-pairs. Conse-

quently, torque motors are mainly designed for low speed applications, usually

below 1,000

rpm.

They have relatively large diameter-to-length ratios and large outer and inner

diameters, so it can be said that they are large hollow shafts with very low inertia.

In terms of performance, they combine high torque levels, high dynamic responses

due to small electrical time constants, high efficiency due to permanent magnets

and high angular and dynamic stiffness. Finally, the large mechanical air gaps

(0.5 to 1.5

mm) make mounting and aligning them easy. More information on

torque motors is given in Sect. 4.5.2.

3.4.2 Bearings

Bearings are mechanical elements designed to reduce friction between an axle and

its support. For this reason, the efficiency of the bearings is especially important

for the spindle performance. Thus, although it is not a new mechanical element,

bearings technology has been and is being studied comprehensively [13,

21].

The bearings used in a machine tool have to meet the demands of the spindle in

terms of rotational speed, load capacity and life. That is the reason why rolling

bearings are the most common solution. Using taper, roller, deep groove bearings

or angular contact ball bearings in machine tool spindles depends on the applica-

tion. Angular contact ball bearings combine precision, load carrying capacity and

high rotational speed. On the other hand, taper bearings provide higher load carry-

ing capacity and stiffness, but they do not allow high rotational speeds. The race-

ways of deep groove bearings are of a similar size to the balls that run in them.

They can take heavy loads but may present misalignment problems. Roller bear-

ings allow a certain amount of axial movement of the axis with respect to the sup-

port. They are suitable for large radial loads.

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

The manufacturing tolerances for bearings dimensions and characteristics are

defined by the American Bearing Engineers Committee (ABEC). High speed

spindles bearings meet ABEC 9 to have rotational precision and minimise runout.

The maximum speed at which bearings can operate is the d

⋅

N factor, also known as

speedability, where d is the mean bearing diameter and N is the speed in rpm.

A high speed spindle requires a d

⋅

N up to 1,500,000 and the only rolling bearings that

can reach those values are the angular contact ball bearings. These bearings can oper-

ate under axial and radial loads. They have a nominal contact angle, typically 15º to

25º defined by the ball-raceways contact line and are perpendicular to the axis. In

drilling, the spindle has higher loading in the axial direction, so higher contact angles

should be selected. In milling, radial loading is higher so lower angles are required.

In order to operate correctly, bearings must be preloaded by applying a constant

thrust load. The preload eliminates axial and radial play, increases the bearing

stiffness, reduces runout, prevents ball skidding under high accelerations and

reduces contact angle variation at high speeds. However, too much preload leads to

excessive heat generation in running conditions and shortens the bearing life. The

preload can be applied a) using springs, b) through axial adjustment or c) using

duplex bearings. The first method uses a spring that loads the non-rotating raceway.

It is the simplest and provides a constant preload. For high speed applications

a hydraulic or a pneumatic system can be used to optimise the preload according to

the rotating speed. Axial adjustment provides a fixed preload by using spacers or

precision lapped shims. This method requires great precision to avoid excessive

preloading due to the setup or the thermal growth in running conditions. Duplex

bearings combine several angular contact bearings and the preload is given by the

mounting configuration. With this method the bearings share the loads and an

overall increase of stiffness is achieved. The bearings can be stacked in several

ways; see Fig. 3.11, Fig. 3.12 and Fig. 3.13:

UNLOADED PRELOADED

Fig. 3.11 Back-to-back bearing mounting; “o” is the total preload offset

3 Machine Tool Spindles 89

UNLOADED PRELOADED

Fig. 3.12 Tandem bearing mountings; “o” is the total preload offset

• “Back-to-back” or DB mounting is the most common technique. The inner

races are first relieved and then clamped together eliminating relief clearance.

This is also called “O” mounting due to the shape of the contact lines. It is used

when there is a good alignment between housing and shaft. With the correct

preload, this method provides good accuracy and high rigidity.

• “Face-to-face” or DF mounting: Outer races are relieved and then clamped

together. Also called “X” mounting due to the shape of the contact lines. It pro-

vides a higher radial and axial stiffness and good accommodation of misalign-

ment. However, the speedability is lower than back-to-back mounting.

PRELOADED

Thrust

UNLOADED

Fig. 3.13 Face-to-face and tandem bearing mounting; “o” is the total preload offset

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

• “Tandem” or DT mounting: the inner and outer races have proportional offsets,

so the contact lines are parallel. This provides higher load-carrying capacity

due to load sharing. Heavy trust loads are countered in one direction but revers-

ing loads cannot be taken like DB or DF mountings.

If the spindle is going to operate under high loads, einstead of a set of two bearings,

three or more bearings can be assembled following the mentioned arrangements.

Regarding the arrangement of the bearings in the spindle, the spindle housing

and the shaft locate the bearings. Generally they are arranged as follows: at the

nose of the spindle there are one or more pairs of angular contact bearings and,

near the rear of the spindle shaft, there is another pair that may be angular contact

bearings, deep groove bearings or roller bearings. In fact, a usual and simple con-

figuration consists on locating a tandem pair at the spindle nose and another at the

rear, set to a global DB setup, as shown in Fig. 3.14. However, the selection and

Fig. 3.14 The spindle industry provides different spindle configurations (Courtesy EDEL

Top, the configuration of angular bearings in the spindle: global DB configuration with DT

arrangement in the front bearing, and DT in the rear bearing

3 Machine Tool Spindles 91

location of bearings depends on the requirements of the spindle design and there

are a huge variety of arrangements.

3.4.2.1 The Service Life of the Bearings

The common cause of bearing failure is the fatigue. The bearings’ life is affected

by the radial and axial loads, vibration levels, working speeds, temperature and

lubrication. Generally, a spindle bearing reaches 5,000 to 7,000 working hours of

good use. Now we will briefly describe the methods used in the DIN/ISO 76

(static load rating) and DIN/ISO 281 (Dynamic load rating, life rating), which are

usually provided by the bearings manufacturers. Equation 3.2 allows us to calcu-

late the dynamic load rating C in Newton’s for two or more bearings. Generally,

C is a factor specified by the manufacturer.

0.7 bearing

Ci C=× (3.2)

where C

bearing

is the load rating of single bearing in Newton’s and i is the number

of bearings in the bearing set. The equivalent dynamic load P is calculated as

follows in Newton’s.

FYFXP ×+×= (3.3)

where X and Y are the radial factor and the axial factor, respectively. F

, F

are the

radial load and the axial load, respectively and are also given in Newton’s. The

nominal life rating L

10h

can be calculated in hours using the following equation

which is based upon a 10% probability of failure:

⎛⎞

=⋅

⎜⎟

⋅

⎝⎠

(3.4)

where N is the spindle speed in rpm, C is the dynamic load rating and P is the

equivalent dynamic, both previously calculated. The adjusted life rating L

nah

hours is calculated as:

123 10nah t h

LaafL=⋅ ⋅⋅ (3.5)

where f

is the factor for operating temperature and the factors a

and a

, are pro-

vided by the supplier.

3.4.2.2 Metallic and Hybrid-ceramic Bearings

The bearings used by machine tool manufacturers have traditionally been metallic,

but in recent years, hybrid-ceramic bearings have been gaining popularity due to their

improved performance and suitability for high-speed machining; see Table 3.1.