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

Подождите немного. Документ загружается.

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

Table 3.1 Comparison of properties of bearing steel balls and ceramic balls

Properties (units)

Conventional

steel bearing

Hybrid ceramic

(Si

) bearing

Young’s modulus (GPa) 208.00 315.00

Hardness (VickersRc) 060.00 078.00

Density (g/cm

) 007.80 003.20

Max. usage temperature (ºC) 120.00 800.00

Coefficient of expansion (10

–6

/K) 011.50 003.20

Poisson’s ratio 000.30 000.26

Thermal conductivity (W/mK) 045.00 035.00

Chemically Inert 000No 00.Yes

Electrically conductive 00.Yes 000No

Magnetic 00.Yes 000No

Typically, steel raceways and silicon nitride Si

balls are used. The ceramic

balls may weigh up to 40% less than the metallic ones; therefore they need less

energy to keep moving and there is a reduction in centrifugal loading and skid-

ding. What is more, ceramic balls have a higher stiffness so, at high speeds, they

suffer less deformation due to centrifugal forces. As a consequence, ceramic balls

can roll up to 30% faster than steel balls with the same service life.

The ceramic balls have a lower affinity with the steel raceways so there is very

little wear from adhesion. In steel balls the microscopic cold welds between ball

and raceways break during the rotation creating surface roughness that increases

friction forces and the risk of excessive heat.

They also have a lower friction coefficient, which means that they operate at

lower temperatures. This is an interesting feature, as it increases not only the bear-

ings life but also the lubricant life.

Experimental tests have demonstrated that hybrid bearings operate at lower vi-

bration levels. As they have a higher stiffness and less mass, they also have higher

natural frequencies and dynamic stiffness.

All these factors mean that with ceramic balls precision is improved and the

bearing reaches higher speeds and has a longer service life, often twice that of the

conventional metallic bearings. However, using them requires special attention

from the machinist to avoid collisions because their main drawback is the high risk

of brittle fracture when there is a collision.

3.4.2.3 Lubrication

Lubrication of the bearings plays a key role in how they perform and function. The

lubricant prevents direct contact in bearings forming a fluid film between ball,

race and cage. In steel bearings, direct metal-to-metal contact is avoided so wear

and corrosion are reduced. Lubricating systems may use either grease or oil/air

lubricants.

3 Machine Tool Spindles 93

Grease lubrication is the most commonly used for conventional machining and

for some HSM cases where the required spindle rotational speeds are not too high.

This kind of greasing is cheap and permanent, so it needs minimal maintenance.

However, they cannot run continuously at a d

⋅

N factor above 850,000, since high

temperatures degrade the grease and the risk of bearing damage rises.

If higher speeds are required in the bearings, oil-air lubrication is more suitable,

as it lubricates, cleans and cools the bearings. However, this is a more expensive

method because it needs monitoring, controlling and maintenance, as well as

a continuous supply of clean dry air; see Fig. 3.15. It is sensitive to variations in

Fig. 3.15 The oil-air lubrication method, above, requires adding a lubrication system to pump,

control and mix the lubricant. In comparison, grease lubrication, below, is simpler. Here the

circuit of air that creates an overpressure in the spindle nose is presented. Schema

ics by GMN

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

the composition of the air-oil mixture; an excessive amount of oil can heat the

bearing while insufficient oil leads to premature wear. There are several methods

for mixing the oil and air and transporting it to the bearing: oil-mist, oil-air, oil-jet

and pulsed oil-air. The oil mist lubrication is difficult to control, measure and

transport, since the mist can degrade into large drops of oil and the uniform distri-

bution of the lubricant becomes difficult. The oil-air method is more controlled

and requires a high pressure pump and mixing valves. The advantage is that the

mixture is produced close to the bearings and an accurate quantity is provided.

The oil-jet method is based on a high pressure pump that provides lubricant di-

rectly to the bearing races. It is used in high performance spindles with high loads

and speeds. However, it requires a complex pump, an oil tank and a temperature

control system. Finally, the pulsed oil-air method injects oil in very small quanti-

ties with a frequency of injection that can be periodic or dependent on the spindle

working conditions.

Performance evaluation of the lubricant and of the lubrication method must

take into account factors such as the length of operation times between mainte-

nance operations, the maximum rotational speed, the heat generated and loss

through friction, as well as certain environmental aspects such as recycling, dis-

posal, etc.

As important as the lubrication is a correct sealing of the spindle to avoid im-

purities from entering the bearings. The most common method is based on creat-

ing an over-pressure inside the spindle in such a way that flow comes out in the

rear and the spindle nose so that particles cannot enter; see Fig. 3.15. Another

method usually combined with the over-pressure is the use of labyrinth seals in

the spindle nose.

3.4.2.4 Magnetic Bearings

Active magnetic bearings (AMBs) work on totally different principles than con-

ventional rolling bearing systems, as they support loads thanks to magnetic levi-

tation. AMBs consist of electromagnets which produce a magnetic field using a

constant electric power flow. At least front and rear radial magnetic bearings and

an axial magnetic bearing are needed. A set of power amplifiers supply current

to the electromagnets, and a controller and a set of gap sensors work together in

a closed loop, monitoring and controlling the position of the rotor within the

levitation gap. Since physical contact is avoided, the effects derived from con-

tact, such as friction, temperature increases and wear are also reduced. Those

characteristics make them a good solution for high speed spindles. What is more,

the electromagnets can be used as actuators and sensors of the spindle running

conditions. That is the reason why the use of hybrid spindles with rolling bear-

ings and AMBs is being explored, using the AMB to absorb the process dynamic

loads and the rolling bearings to position and support the shaft [1]. However, the

main drawback is that they are expensive and complex.

3 Machine Tool Spindles 95

3.4.2.5 Hydrostatic Bearings

In hydrostatic spindles, a fine layer of oil is injected at high pressure to produce what

is known as a hydrostatic equilibrium in the interior of the bearing. These bearings

have ceramic pads which contain the oil injected at high pressure to support the shaft.

This method avoids some disadvantages of ball bearing systems, such as low

damping, a limited service life, and the problems derived from heating and vibration.

In comparison, hydrostatic bearings are practically wear-free, operate with little

noise, and provide excellent stiffness and damping; hence, they perform better under

dynamic loads. The oil is injected at low temperatures so it reduces the thermal ex-

pansion of the shaft. They cannot reach the same d

⋅

N as roller bearings though. What

is more, nowadays the cost of the oil supply and sealing system makes then more

affordable. Also, at low speeds they perform better than hydrodynamic bearings, due

to a high load carrying capacity and friction conditions. Hence, these bearings are

nowadays used in applications as grinding, milling and turning machine tools.

3.4.2.6 Hydrodynamic Bearings

The operation mechanism of hydrodynamic bearings is based on the fluid-wedge

principle. For machine tool applications, the shaft is supported by tilting-pad ele-

ments that can be spring loaded or can move due to the flexibility of the material

they are embedded into. The working speed of the spindle must be in the fluid

friction range since, at lower speeds or when the spindle is started or stopped, the

wear rate increases as it runs under boundary lubrication conditions. These bear-

ings were traditionally used in grinding machines where rotational speeds are high

enough and remain approximately constant. However, nowadays, they have gen-

erally been substituted by hydrostatic bearings.

3.4.2.7 Aerostatic Bearings

The operation principle is similar to that of hydrostatic bearings. The advantages of

these bearings are low friction, low heat generation, accurate running, long life, and

simple construction because there is no need for seals. However, they have a small

load-carrying capacity, low damping and a tendency to vibrate. Hence, they are

suitable for very high speed applications with low loads. In the machine tool indus-

try, they are used for precision grinding machines and ultraprecision applications.

3.4.3 The Toolholder

In milling, the toolholder is the interface between the cutting tool and the spindle

and it ensures the transmission of the rotational movement of the spindle to the

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

cutting tool. The selection of the toolholder is defined by the performance and

possibilities of a machine tool. It requires a simple system to clamp and unclamp

the tool, allows automatic tool changing within the machine tool, ensures a precise

and rigid adjustment with the spindle and it must guarantee a proper eccentricity

and balancing. A defective toolholder can have negative effects on the dimen-

sional precision and surface quality of the piece, and it can reduce the service life

of the cutting tool and the spindle. Nowadays, hollow toolholders are made in such

a way that the cutting fluid can flow through the spindle to the tool tip.

A toolholder for milling has three main parts: the taper, the flange and the col-

let pocket. The taper is the conical part that is introduced into the spindle. The

flange is the part that serves to hold the toolholder when it is in the tool changer

storage magazine of the machine tool. The collet pocket is the part of the tool-

holder that actually holds the cutting tool.

3.4.3.1 Tapers

Several technological solutions have been developed from the need to standardise

how the cutting tool is held in machine tools. These standards are BT, CAT (also

known as V-Flange), ISO (also known as SK) and HSK; see Fig. 3.16. A recent alter-

native is the Big-Plus

dual contact. We should also mention the R8 collet toolholder

technologies and the Morse taper collet which are used in manual machines.

The conical forms such as BT, CAT and ISO, are usually used in conventional

milling machines. In contrast, the HSK toolholders are more suitable for high speed

milling. The conical toolholders as BT, CAT and ISO are coupled to the spindle via

“two cones in contact”. The centrifugal force that increases with the rotational

speed of the spindle, together with the thermal effects, tends to push the cone into

the spindle, thus increasing the retention force. The displacement that the cone

experiences within the spindle may provoke imprecision in the machining.

Fig. 3.16 Standard toolholders by Maritool

, from left to right: BT50, CAT50, ISO30, HSK50A

3 Machine Tool Spindles 97

ISO toolholders are more widespread in Europe for traditional machining. They

are clamped to the machine by means of hydraulic or pneumatic actuators. The

dimensions of the ISO cones are standardised and classed as ISO 30, ISO 40 and

ISO 50, according to the external diameter of the cone. In the US, however, CAT

toolholders are more widely used in traditional machining.

The CAT toolholder was created by the Caterpillar Corporation in order to

standardise the tool holding systems in their machine tools. These holders are

designed according to the size of the cones and are classified as CAT-30, CAT-40

and CAT-50. The BT toolholder is similar to the CAT, with slight differences,

such as the fact that it is symmetrical with respect to the rotary axis of the spindle,

which improves performance.

HSK, now in the ISO 12164-1:2001, stands for hollow taper shank in German.

This toolholder has certain advantages over the others in terms of HSM where

rotational speed is greater than in conventional milling. As rotational speed in-

creases, the centrifugal force increases the retention force on the cutting tool by

means of milled drive keys that are located within the shank of the mechanism.

The keys tend to expand in the interior of the toolholder because of the centrifugal

forces in such a way that the metal-on-metal contact is reinforced and pressured

within the spindle axis. Thus, the contact stiffness is increased and more aggres-

sive cutting operations can be undertaken; see Fig. 3.17. The retention force of an

HSK toolholder can be up to twice that of a conventional toolholder, such as, for

example, a BT. In the BT interface, the clamp pincers are on the outside of the

shank, so they tend to lose contact and retention force as centrifugal force in-

creases. The HSK toolholders outperform the ISO toolholders by up to 5 times in

terms of radial stiffness. In addition, HSK toolholders offer greater repeatability.

There are various designations for HSK toolholders. There is a number to indicate

the external diameter of the flange (25,

32,

40,

50,

63,

80 up to 100

mm) followed by

a letter ranging from A to F. The letters most suitable for high speed machining are

Clamped position

Tool-changing

position

Fig. 3.17 Clamped and tool-changing positions of the HSK toolholder

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

A, B, E and F. The letter A is used for applications requiring moderate torque and high

rotational speed with automatic tool change. Letter B is for high torque, high speeds

and automatic tool change. Letter C is for moderate torque and high rotational speed,

but manual tool change. The letter D is for high torque, high speed and manual tool

change. The letters E and F are also for HSM applications, but with low torque and

very high speeds. E and F designate HSK toolholders that are designed for very high

speed operations that either have no framework or for cutting operations that may

negatively affect the equilibrium of the machining process, resulting in the appear-

ance of chatter. The diameter of the toolholder determines the stiffness. In Fig. 3.18,

the bending moment of several HSK tapers with different diameters and clamped

with the force recommended by the standards are compared.

The recently developed Big-plus™ system consists in simultaneous dual con-

tact to ensure that the toolholder is located accurately in the spindle. The taper and

face of the machine tool spindle fit together due to the elastic deformation of the

spindle, so that the taper and flange face fit simultaneously. This system permits

interchangeability with the standard toolholders and machines and is being offered

by many machine-tool manufacturers.

An alternative is the ICTM standard (Interface Committee for Turning-Mills)

which is the standard developed by 17 companies in Japan as the interface of turn-

ing centres (turning

milling) based on the two-face restraint type ISO 12164-

1:2001. It is compatible with HSK-A type.

Finally, there is also a wide range of other toolholders that have been specially de-

signed by companies, like the Sandvik Coromant

Capto™, Kennametal

KM™ or

Komet

ABS™. These toolholders feature a polygonal-shaped coupling, and are

18kN

21kN

13kN

13 kN

390Nm

350Nm

270Nm

220 Nm

100

200

300

400

500

600

700

HSK 50 DIN Standard HSK 63 DIN Standard HSK 63/80 DIN

Standard

HSK 80

kN Nm

Fig. 3.18 Bending moment on several HSK tapers with the corresponding clamping force

3 Machine Tool Spindles 99

used in lathes and in milling centres, for a quick tool change. Figure 3.19 shows

a collet chuck.

When manipulating these toolholders it is important to pay particular attention

to cleaning and maintaining the cones (especially high-precision ones, such as

those for HSK), since swarf and lubricant can negatively affect their performance.

If the contact surfaces are insufficiently clean, the quality of the contact decreases,

which will impede the cone settling within the spindle. It is advisable to blow the

collet clean before proceeding to clamp it.

3.4.3.2 Clamps

There are various methods for securing the tool in the toolholder: mechanical

chucks, hydraulic chucks and shrink-fit chucks. The clamping requirements of the

tool in the toolholder are similar to those of the toolholder in the spindle: a) the

maximum stiffness of the union; b) the precise alignment of the tool axis with the

spindle shaft axis and c) avoiding unbalances.

Mechanical clamping by means of chucks (collet chucks) is the most wide-

spread method. In essence, the tool is loaded into the chuck which holds the tool

when tightened. Segmentation of the chuck enables it to deform to a certain extent,

allowing the pressure to be distributed uniformly over the tool shank. It is a low-

cost system, even for HSM, because the jaws in the chucks are interchangeable, or

adjustable, so tools of different diameters can be held.

An improved version of mechanical clamps is the “great power chuck” provid-

ing a 10% higher torque and axial rigidity. It is based on needles inside two

slightly angular cones, as shown in Fig. 3.20.

Fig. 3.19 Section of a “great power” collet chuck, by Laip

, showing the internal needles to

make the radial force

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

Fig. 3.20 Schematic representations of a collet chuck, hydraulic chuck and shrink-fitted tool shank

Figure 3.20 shows a schematic diagram of the clamp mechanism of a collet

chuck, a hydraulic chuck and a shrink-fitted chuck.

Hydraulic chucks employ a metal membrane to envelop the tool shank. Next to

the membrane is a perfectly sealed hydraulic reservoir, which is connected to an

actuating screw which can increase the pressure of the fluid. The increase in fluid

pressure is transmitted to the membrane which secures the cutting tool. Hydraulic

chucks are expensive systems. Moreover, it is not possible to secure tools of dif-

ferent diameters in the same chuck, although some manufacturers offer additional

3 Machine Tool Spindles 101

membranes that can be introduced between the tool and the chuck so that the same

chuck can accept different diameters of tools. However, this has the disadvantage

of an inevitable amount of misalignment.

Shrink-fit chucks are solid blocks with a high-precision opening in which the

cutting tool is loaded. At room temperature, the opening is smaller than the tool.

The chuck heats up by means of an induction heater, and the thermal expansion

allows the tool to be introduced. Then the chuck cools down again (in some

systems a cooling mechanism is provided), and the opening shrinks to its original

size gripping the tool in place. The stiffness of this holding system is very high.

Unlike the hydraulic chuck, which needs an internal structure and other elements,

such as actuating screws, the shrink-fit chuck can be much more symmetrical

and free from the imperfections that may reduce equilibrium when rotating. Al-

though shrink-fit chucks are simpler than hydraulic chucks, they are also more

expensive.

Furthermore, a heating system needs to be acquired and, in some cases, a cool-

ing system as well.

The choice of the toolholder definitely affects the machining dynamic perform-

ance and it is conditioned by the spindle characteristics. Comparing the frequency

response function (FRF) measured at the tip of a tool of 16

mm diameter and

mm overhang in a power collet chuck, a collet chuck, an hydraulic chuck and

500 1000 1500 2000 2500 3000 3500

0.2

0.4

0.6

0.8

1.2

1.4

x 10

-6

Frecuencia (Hz)

Flexibilidad dinámica en el eje Y

pinza conica

hidraulico

pinza cilindrica

termico

Collet

chuck

Power

collet

chuck

Shrink fit

chuck

Hydraulic

chuck

collet chuck

power collet chuck

hydraulic chuck

shrink fit chuck

Dynamic compliance in radial direction (m/N)

Frequency (Hz)

Fig. 3.21 Dynamic flexibility measured at the tool tip for four different toolholders with the

same tool in the same spindle