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

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30 L. Norberto López de Lacalle and A. Lamikiz

advantage; at the same time small ones are more highly recommended to ensure

precision and controllability of the drive train.

The calculation of screws is based on preventing two mechanical problems.

First, buckling due to shaft working under compression when the nut displaces

along the screw. Second, critical whirling speeds must be avoided, only in the case

when the shaft rotates and the nut moves along the screw. The other case is when

the screw is fixed and the motor rotates the nut; evidently the motor must be placed

onto the carriage. See Chap. 4 for further details about ball screw design and use.

The thermal growth of the screw is a key aspect to be considered and reduced.

During the normal function of ball screws, heat coming from the friction of balls

movement on the thread causes a significant longitudinal dilation of the screw.

Approximately 7–10% of the power is converted into heat. If the shaft has been

placed with pre-stress, some dilation is absorbed by this good practice. However

some microns always appear. To reduce dilation to a minimum, one solution in

machines for high precision is the forced fluid cooling of the screw, internal to the

hollowed out shaft, or even the forced cooling of the nut. This increases the price,

but maintains thermal growth within machine requirements.

Nevertheless, if the axis position is measured by a direct linear scale, thermal

growth becomes considerably less important, because CNC works to reach the

desired axis position until it matches with that measured by the linear scale. There-

fore, the thermal insulation of external measurement scales from machine heat

sources is essential.

1.8 The CNC Implementation

Usually the CNC and drive supplier can be selected by the user, who chooses the

brand and model of the CNC for a machine. This is the case of manufacturers of

small and medium machines, who try to be flexible regarding user wishes. This is

an important competitive factor for their businesses. However in the case of large

machine tool companies, the use of their own controls and the need to simplify the

production chain means this option is not suitable. Big machine manufacturers

usually sign an agreement with a CNC provider to develop specific CNCs for their

machines, being commercialised with the machine tool trademark.

Basic CNC functions are shown in Fig. 1.20, particularised for a three-axis ma-

chine. In more detail:

• The basic operating system: All current controls are based in microprocessor

architecture similar to a PC; therefore they need a basic operating system.

• The program translator: The part is programmed in ISO code (the so-called

G-code) or a special NC language. The former is useful to learn how to use ma-

chines; unfortunately it is too simple for the new machine capabilities. That is the

way CNC manufacturers have developed their own languages. The translator

always reads some blocks ahead to give time for performing all trajectory calcula-

1 Machine Tools for Removal Processes: A General View 31

Fig. 1.20 The CNC integration

32 L. Norberto López de Lacalle and A. Lamikiz

tions before the tool reaches the next positions. The higher the machine feed is, the

further ahead the blocks must be processed.

• The interpolator. This is the most intelligent CNC function, when 3D, 5D (usu-

ally three translations and two tool orientations) or even multi-axis trajectories

must be converted into axis positions. The high precision required leads inter-

polator to make calculations at every few microns along toolpaths. Calculation

methods are spatial transformations using homogeneous matrices, tool radius

compensation and other geometrical corrections (see Chap. 5).

• Axes control. When the position to be reached is obtained by the interpolator the

speed of each axis motor is also obtained. The speed value is the input for the

motor drive. Through control techniques, PDI being the simplest, motion in each

axis is kept up to the real position reaches the target position. Therefore there

must be a position measurement device, an encoder or a linear scale shown in

Fig. 1.19, to know the real axis position at any given time.

• The user interface. This is today a decisive selling factor due to the high com-

plexity of machines and operations to be done. Users need on-line assistance

utilities to make programs free of errors for the new developed machines. Ma-

chines for special applications, like tool grinders or gear shapers or the new

multi-task machines (see Figs. 1.3 and 1.4), require programming utilities

which enable users to generate machining programs for complex parts.

• The Programmable Logic Controller (PLC) which is included in the same hard-

ware architecture today with the rest of the control cards, for enabling

a high speed communication with the basic NC modules. PLC controls the auxil-

iary machine functions (generally coded as M functions in ISO), running

a program implemented by the machine builder and taking into account the value

of digital inputs collected from simple sensors placed in the machine. The PLC

digital outputs are connected to switches or electrovalves to command the tool

magazines, the coolant circuit, the pallets transfer system and other auxiliary de-

vices. Another important surveillance function is related to the end-of-stroke

switches, placed at the ends of all guides to prevent moveable components from

leaving the slideways: when a microswitch detects a sliding component is on an

end-of-stroke the PLC immediately stops the axis feed motor.

Nowadays, control manufacturers produce two or three basic CNC platforms

which can include several options for different kinds of machines. Therefore the

machine tool manufacturer must adapt them to each specific machine model, fol-

lowing several steps:

1. The definition of machine axes in relation to CNC ones, giving also the positive

and negative senses and strokes of each axis.

2. Control parameters of each axis must be adjusted to the machine dynamics.

3. PLC programming for each machine model with its user options.

4. Customization of the user interface. General CNC control models are prepared

for the milling machines, lathes or universal grinders most commonly sold.

However, for specific applications special help utilities to prepare NC programs

are absolutely necessary.

1 Machine Tools for Removal Processes: A General View 33

5. Finally, tuning tests using idle movements and some machining tests are per-

formed. These tests allow the fine adjustment of CNC parameters to machine to

achieve the maximum precision and speed, as is explained in the next section.

To improve machining process reliability and facilitate the maintenance of ma-

chines, several sensors and monitoring techniques can be managed by the CNC.

Spindle consumption, collision detection, thermal growth of spindle and the bal-

ance of the main spindle are monitored by the CNC in high end machines.

1.9 Machine Verification

After machine construction, a thorough verification of machine precision and per-

formance must be made. For this purpose some tests have been defined. An important

result is the determination of the CNC compensation parameters for machine axes.

Nonetheless, tests can also be used for several purposes, thus as acceptance tests by

machine buyers, for comparisons between similar machines, for periodical machine

checking throughout machine life, etc. Three kinds of tests can be mentioned.

First, geometric tests for measuring straightness, parallelism or concentricity er-

rors. ISO 230:2006, ISO 10791:1998, ISO 13041:2004 and other standards defined

by the ISO Technical Committee 39/SC 2 (“Test conditions for metal cutting ma-

chine tools”) specify methods for assessing the accuracy and positioning repeat-

ability of numerically controlled machine tool axes by the direct measurement of

individual axis. In the standards, methods for linear and rotary axes are suggested.

The second group includes procedures for studying the interpolation control,

based on laser interferometers, the “Ball-Bar” device for the relative movement of

two Cartesian axes, or position sensors for measurement of machine repeatability.

When several rotation axes are simultaneously considered there are no standard

tests, excepting those for bi-rotary heads in machines with a horizontal or vertical

Z axis. However, standards do not include tests to check five-axis machines with

tilting tables (RRLLL configuration in Fig. 1.7) or other structures.

Another important datum to achieve the best machine-CNC adaptation is the

lag-error measurement using the internal utilities implemented in modern CNCs

for monitoring the behaviour of the axis drives with respect to the axis positions.

The last group includes “testparts” (see Chap. 6) to check the behaviour of

lathes and machining centres during process. Currently, milling tests are only

focused on three-axis milling centres. The most well-known is the NAS work-

piece (defined in ISO 10791-7:1998). But this test does not include complex

surfaces, which is why different testparts have been designed in the last years,

such as the so-called Mercedes or NCG parts (available at NC-Gesellschaft asso-

ciation [18]). However, none of them are yet included in the ISO standard nor-

mative. Furthermore, there is no specific part for testing five axes machining

centres, so customers have to define their own tests to check the machine they

are interested in buying, which is highly time consuming.

34 L. Norberto López de Lacalle and A. Lamikiz

1.10 Typical Machines for Several Applications and Sectors

At the design stage, a machine is considered for a particular industrial application,

but using rather non-specific requirements. As was explained in Sect. 1.2.1, the

workpiece type and size, operations to be performed, productivity and the preci-

sion to achieve will define the machine specifications. In the following section

some machines typical of important sectors are described; likewise other examples

will be described in depth over the next chapters.

1.10.1 A Machine for Big Structural Turbine Parts

The vertical lathe is a typical machine for this application. The workpiece comes

from forging, being disks or cases. Machining operations are turning, boring and

milling. The maximum turning diameter can be up to 3

m and part weights of

tons are not uncommon. The plate rotation speed is in the range of 200

rpm

with a power of 50–100

kW. The ram stroke is long, more than 1,500

mm. In the

ram a turning tool or a milling headstock can be placed, in the latter case with

a power ranging from 20 to 50

kW. Two machines of this type by P. Carnaghi

are

shown in Fig. 1.21.

Another example of a big machine with two rams mounted on the same screw

is shown in Fig. 1.22, by Ibarmia

. This is a drilling and milling machine focused

on the drilling of circular rings for wind turbine support towers. Both rams are

independent although they can work in parallel.

Fig. 1.21 The Pietro Carnaghi

vertical lathes, models ATF14 and AP100

1 Machine Tools for Removal Processes: A General View 35

Fig. 1.22 The Ibarmia

drilling machine

1.10.2 A Horizontal Milling Centre for Automotive Components

In the highly competitive automotive sector, companies are encouraged to reduce

times and prices while maintaining sufficient quality. Therefore, machines must

provide high metal removal rates, fast feedrates, greater accuracy and stability,

increased overall productivity and finally, the flexibility to meet changing produc-

tion volumes.

Commonly produced parts are blocks and cylinder heads in iron or high silicon

casting, component cases, gears, crankshafts, rods, or small pieces for the trans-

mission and suspension systems. In Chap. 13 more examples are described.

Automotive components are usually machined in horizontal three-axis machin-

ing centres, provided with a B axis for part rotation or in some cases with a simple

rotary indexing table (Fig. 1.23). Horizontal centres bring benefits to an applica-

tion which vertical ones are usually unable to, including faster power spindles and

more power spindles, larger tool magazines, a smaller footprint and chip disposal.

Parts are held and fixed on universal pallets, which can be transferred by auto-

mated systems. In some cases the inclusion of a swivel head or rotation table en-

ables the production of more complex shapes.

In these machines the tool change time “chip to chip” must be as low as possi-

ble; currently times of 2–4

seconds are usual. Very rapid idle movements between

operations or tool positioning are also required.

Spindles are in the range of 6–12,000

rpm powered with 60–90

kW motors to

be able to move large diameter multi-insert rotary tools. Versions with a geared

drive or a direct connection of main motor to spindle are possible, the torque being

higher in the geared versions (in the range of 600–1,100

Nm).

36 L. Norberto López de Lacalle and A. Lamikiz

Tool magazines are the largest in machine tools, with an optional storage of up

to more than 400 tools.

Some machines include as an option two twin spindles and tables or drums to

fix several parts in one setup; see Fig. 1.24. When this is done, the borderline

between individual machines and transfer machines (see Sect. 1.2.9) becomes

diffuse.

On the other hand, vertical centres are applied to wheel, steering, brakes or sus-

pension components, for small and medium production.

Fig. 1.24 a Twin spindle and drum part change system. b Double spindle for engine head ma-

chining, by Heller Maschinenfabrik

Fig. 1.23 Horizontal machining centre, model Makino

a51. a Detail of the spindle and column

of Makino’s A77. b Automated system for production of engine blocks

1 Machine Tools for Removal Processes: A General View 37

1.10.3 A Milling Centre for Moulds

In this case, a CNC milling machine with a 1

metres working zone is com-

mon, with three axes and a vertical spindle as the main solution, and five axes as

a widely used option. For small moulds the C-frame is typical, such as the model

shown in Fig. 1.25. For medium size moulds the gantry frame becomes more inter-

esting due to its improved stiffness, with a sliding table where the mould is placed

with bolts or fixtures.

For big moulds the gantry type with a travelling beam and with a two rotary

head allows machining complex shapes. However, for medium and small moulds

the two-axis rotary bed is stiffer and therefore more widely used

As to the main motion, an electro spindle with 20,000–25,000

rpm and 17–25

is a typical picture but a direct drive by a flexible coupling is also very widely used.

This latter solution is cheaper, and more robust against collision and allows

12,000

rpm and even more. Currently, electrospindles are able to give enough torque

even at a low rotational speed allowing roughing to be performed in the high speed

milling centre starting from the hardened steel material.

Ball screws are the economical and high performance solution for drive trains,

allowing an acceleration of 0.8–1

g. In high end machines linear motors are used.

Machines are not usually provided with pallet transferring, because machining

on moulds is a long operation not requiring too many tools and, therefore, the

setup time is not critical.

Fig. 1.25 The five-axis milling machine Seaska, by Kondia

1.10.4 A Milling Machine for Big Dies and Moulds

As mentioned above, these machines are gantry-type, with a transversal travelling

beam and a long ram for the Z movement. The heavy die is fixed on the table,

38 L. Norberto López de Lacalle and A. Lamikiz

whereas all movements are done by the tool. Usually a two rotary head is used for

sculptured surface milling, as shown in Fig. 1.26. There, to obtain the maximum

possible accuracy, the B and C axes are both driven by brushless motors and the

transducers are mounted directly on the axes. In addition, the axis kinematics

chains have a crown gear-worm screw system with a variable pitch for the com-

pensation of the backlash.

New models of rotary heads based on torque motors for the orientation axes

have been launched in the last four years. This is going to be the main design solu-

tion in the near future, at least for medium and high level machines. Torque mo-

tors are ring-shaped and can be placed directly on the rotary joints, eliminating

gear transmissions.

Some machines for large dies can change the spindle head automatically, se-

lecting a head with a geared spindle for roughing and another with two axes and

a high speed spindle for finishing. Rapid-change connectors must be mounted at

both the ram end and base of the head for the electricity, oil and coolant supplies.

1.10.5 Conventional Machines for Auxiliary Operations

Today, manual machines are mainly used as auxiliary machines, for special opera-

tions. This group includes the universal milling machine, the press drill and the

engine lathe which are always present in workshops, with only slight modifica-

tions in relation to the inventions of the second half of the 19th century. An easy

and inexpensive option is the inclusion of measurement rules in each axis for the

digital display of axis positions, as in the case shown in Fig. 1.27.

Fig. 1.26 Rotary axis head in a milling machine, by Fidia

1 Machine Tools for Removal Processes: A General View 39

Fig. 1.27 Milling machine FU152 of Lagun

Basically all metal technicians have learnt their basic skills and the “art of

working metals” on these machines, since machining operations are more similar

in these machines than in the recently launched CNC machines. In Fig. 1.27

a current universal milling machine is shown, with automatic feedrates on 3 axes

(X, Y, Z) and continuous variation speeds for main spindle and feedrates. Options

for this model are a universal milling head, a universal milling head with quill

feed, a universal dividing head, a precision circular table and attachments for slot-

ting, rack milling and gear hobbing. In Fig. 1.28 an engine lathe and its feed gear-

box and turret is also shown. This model reaches 2,200

rpm with a 5.5

kW head-

stock motor.

Fig. 1.28 Engine lathe, model SP250 by Pinacho

. a Detail of the feed gearbox.

b Universal turret