Technical Information

Aspects of Energy Efficiency in Machine Tools
Discussions of the efficient use of energy have become more frequent in many sectors of industry. Machine tools comprise numerous
motors and auxiliary components whose energy consumption can vary strongly during machining. The main spindle drive, for example,
and the coolant system work near their rated power during roughing with a high stock-removal rate, while the power consumption during
finishing is significantly lower. There is a very close interdependence between the individual components and subassemblies of a machine
tool and aspects of productivity and quality. From a detailed examination of manufacturing processes to the power consumption ofindividual components, potential for savings can be evaluated and measures can be defined for the efficient use of energy.

In many branches of the investment goods
industry, energy efficiency has developed
into an important product characteristic.
Geared motors for materials handlingtechnology have been divided into efficiency
classes for some time now. A multitude ofideas has been tried for increasing energy
efficiency in manufacturing with machine
tools. Potential savings result with regard
to the base load of machine tools that
require energy consumption even in
nonproductive phases. The base load isdetermined substantially by the auxiliarycomponents of a machine. Besides the useof energy-efficient motors in the auxiliarycomponents, many possibilities for reducing
the base load can be found in proper energy
management. With energy management,
consumers are specifically switched off by
the machine control in nonproductive
phases.
Measures to support the operator during
setup also increase energy efficiency,
because they shorten nonproductive
phases and reduce the influence of thebase load. Scrap inevitably increases
energy consumption per good part.
Manufacturing with accuracy starting from

November 2010

the very first part can therefore become adecisive factor for the energy efficiency ofa machine tool.

Machine designs with balanced thermal
behavior and precise position measuring
technology have a distinct advantage here.

Energy demand during milling
The power requirements of a milling
process are divided into the following
consumer groups:• Cooling lubricant processing• Compressed air generation• Electrically powered auxiliarycomponents of the milling machine• CNC control package with main spindle
and feed axis motors
The proportionally calculated energy forlighting, ventilation and air conditioning is
added to these groups. The energy demandof a milling process strongly depends on
the size of the milling machine and themachining task.
In this example, an aluminum housing withdimensions of 150 mm x 50 mm x 25 mm
is to be milled on a machining center witha work envelope of 850 mm x 700 mm
x 500 mm. The mean total power
consumption of all the above-mentioned
consumer groups is 13 kW for roughing
and 7.4 kW for finishing. The power balance
during roughing and finishing provide moredetailed information on the distribution ofenergy consumption among the individual
consumer groups.

Housing for the observation of
the power requirements of a milling process
External processing
Cooling lubricant: 5.1 kW
Compressed air: 1.3 kW

Auxiliary components ofthe machine: 3.1 kW
Feed drives: 0.25 kW

13 kW

The cooling lubricant is prepared centrally
away from the milling machine (pumping,temperature stabilization). For roughing, thishas a mean power requirement of 5.1 kW.
For finishing, the mean power requirement
decreases to 1.5 kW. Production readinessconsumes almost no power. Dry machining
offers great potential here for increasingenergy and resource efficiency. In many
milling applications, however, doing without
the cooling lubricant can significantly
increase the scrap rate and therefore
increase mean energy consumption.
The mean compressed air power changes
only slightly in the phases of production
readiness, roughing, and finishing. It
averages approx. 1.3 kW. Compressed air
is required for minimum lubrication of thespindle, tool changing and cleaning theworkpiece. In small quantities it is required
as sealing air (spindle, tool measurement,
linear encoders).
The electrical consumers of the machine
include the CNC control with main spindle
and feed axis motors as well as numerous
auxiliary components (pallet changer,
cooling, hydraulics, automation). The power
consumption of the auxiliary components
varies in the production conditions ofreadiness, roughing and finishing by only
600 W. With a power consumption of 2.5
kW, the auxiliary components largely
determine energy consumption in the2

Electrical
power of themachineSpindle:3.25 kW

CNC control

External processing
Cooling lubricant: 1.5 kW
Compressed air: 1.3 kW
Auxiliary components ofthe machine: 2.8 kW
7.4 kW

Feed drives: 0.25 kW

Electrical
power of themachine
CNC control

Spindle:1.55 kW

Mean power requirement for manufacturing a housing part
Above: roughing, below: finishingproduction readiness condition. A
requirement-oriented deactivation of
auxiliary components therefore offers
substantial potential energy savings.
The CNC control package with feed axis
motors and main spindle require in thisexample just 27 % of the total power
requirement. In both cases, the mean
power consumption of the feed motors is
250 W and is largely determined by theholding power of the vertical axis. Shortpeak values occur only in the accelerating
and braking processes.

Spindle and feed-axis motors are among
the central components of a machine tool.
The energy efficiency of a drive component
depends on the ratio of delivered power toconsumed power and is therefore reflectedin the efficiency. The network of drives on amachine tool converts consumed electricalenergy to delivered mechanical power. The
components of the drive network are a
power supply module, drive modules,
motors and the mechanical components.
Data on efficiency typically refer to therated power. In other rated values, theefficiency of individual components canvary significantly. HEIDENHAIN supply
modules and drive modules attainefficiency values of over 95 %.
Power consumption during milling
Here the power consumption of a main
spindle and the feed drives is itemized.

Grid power input

Energy efficiency of the drive
components

SupplymoduleDrivemodules

Point of measurement: input
power of the supply module

Copper loss

Feed drives

SpindleCopper loss
friction loss
Friction losses

Electrical input
power

Deliveredmechanicalpower

Conclusion
Feed drives contribute only a small share of
the total power of a CNC and can therefore
contribute little to increasing the energy
efficiency. On the other hand, the selectionof the spindle can have a significant effecton energy consumption. If a spindle drive
operates far below its rated power, theintrinsic losses of the drive increase inproportion, with negative effects on theenergy balance. If the spindle limits the
maximum possible metal removal rateof a milling process, the milling processinevitably takes longer. The result: energy
efficiency decreases due to the base load
generated by the auxiliary components.
There is also potential for more efficientdesign of milling processes in theconsideration of the efficiency of spindle
motors, for example by using synchronous
instead of asynchronous motors.

SpindleFeed axes

Mean power consumption of the drives during rough facing
Face-milling cutter: D = 60 mm, speed: 4 100 rpm, infeed depth: 4 mm

Mean power [kW] f

Example 2: Circular pocket
The circular pocket is machined with aroughing and a finishing cycle. The mean
power of the feed drives here is 100 W.
The main spindle needs 1.5 kW of power.

Mean power [kW] f

Example 1: Rough facing
During rough facing with paraxial feed rate
the feed motors have a mean power
consumption of 200 W. The main spindle
reaches it rated power at approx. 19 kW.

SpindleFeed axes

Mean power consumption of the drives during circular pocket milling
Circular pocket: D = 20 mm, end mill: D = 10 mm, lateral allowance: 5 mm, infeed depth 2 x 5 mm,
speed: 16 000 rpm

3Efficiency of regenerative supply
modules

Due to the high power requirement ofmetal cutting, the mean input powersinks the more frequently the milling
process is interrupted by tool changes. The
regenerative supply module works more
efficiently as soon as the time interval
between two tool changes is less than
100 s (equals 0.6 tool changes per minute).
In processes with many tool changes per
minute, a regenerative supply module
often proves to be the better choice.
During contour milling with infrequent
tool changes, the advantages are on theside of the nonregenerative system.

Drivemodules

Feed drives

Input power [kW] f

SpindleMilling

Tool change
Input power [kW] f

The number of tool changes is importantfor energy consumption. In one example, amilling operation with 15 kW is interrupted
cyclically by a tool change. Starting thespindle requires a peak power of approx.
60 kW. While the regenerative supply
module briefly returns 48 kW to grid
power, with the nonregenerative supply
module, the kinetic energy is converted toheat.

SupplymodulePoint of measurement: input
power of the supply module

MillingTime [s] f
Input power of the supply module—above: regenerative, below: nonregenerative
Mean input power f

The supply modules of the CNC controls
from HEIDENHAIN are designed both forregenerative and nonregenerative braking.
In a nonregenerative supply module, thekinetic energy released by the braking
process is converted to heat by the braking
resistors. A regenerative supply module
returns this energy to the power grid.
However, the path required for returning
the energy and the necessary components
for smoothing the grid power generate
losses even when the drives have nopower requirement. The power loss
increases slightly even when power is
not being regenerated. A regenerative
supply module operates more efficiently
than a nonregenerative module when
the regenerated energy more thancompensates the higher power loss.
The decision for a regenerative or anonregenerative supply module therefore
also depends on the expected type ofmachine operations.

Grid power input

Every acceleration process of a drive
requires a braking process in return. Theenergy from the drives’ moving masses islargely reconverted to electrical energy.

Without energy recovery
more efficientWith energy recovery
more efficient

Regenerative
Nonregenerative

Tool change per minute f
Efficiency analysis of a supply module in the version with and without regenerative braking

4

Production readiness condition
Machining

MC 1

MC 2

Production
line

Input power [kW] f

A comparison of the power consumptionof two machining centers (MC) and a
production line with three machining
stations illustrates again the potential:
because the consumption of several
consumer groups in the ready condition is
only slightly reduced, the nonproductive
phases must be kept as brief as possible.
With machining centers for smaller
production batches, energy consumption
can be significantly reduced by theselective deactivation of auxiliarycomponents. Beyond this, potential
savings result from the use of energyefficient pumps in the coolant/lubricant
circuit.

Input power [kW] f

Deactivation of auxiliarycomponents as needed

CNC
Aux. components
Cooling lubricant
Air

MC 1

MC 2

Production
linePower requirement of three manufacturing processes according to operating conditions andconsumer groups

However, consistent switch-off of auxiliarycomponents (hydraulics, spindle cooling) or
the compressed-air supply can also havethe opposite effect. If the sudden removal
of waste heat from auxiliary components
or of the temperature-stabilizing effect ofmedia causes thermal displacement in themachine frame, it can result in scrap parts,
which impair the energy balance of a
production process. The selective switchoff of auxiliary components therefore
functions best on machines with little
inclination to thermal displacement. In any
case, careful planning of the energy saving
effects is a prerequisite.
The CNC can be used as the central control
unit for the energy management of amachine tool and its associated periphery.
The iTNC 530 is provided with special PLC
functions for linking events in the production
process (e.g. NC stop) with outputs forcontrolling auxiliary components. Delaytimes can be assigned to events so that,for example, motors can be locked anddisconnected from current after standstill.
Functions for deactivating various auxiliary
devices, axes, light in the working space,etc. can be created on this basis. These
basic functions can be created as desiredby the machine tool builder and adapted to
the respective machine model. For theuser, a further step is helpful, in which theenergy management can be adapted to his
specific usage habits. In the illustrated
example, the deactivation functions (axislocking, standby, emergency off, switch off)
can be configured for various operating
modes.

Example of a configuration menu for machine tool builders for deactivating auxiliarycomponents, axes, light, etc.

Energy management: Possible configuration mask for the user

5

Reduction of machine idle timethrough optimal support during
setup
Because of the relatively high base energy
load of machine tools, non-cutting times
have a decisive effect on the energy
efficiency of a cutting process. To reduce
the energy requirement per part, noncutting periods such as tool and set-up
times should be kept as small as possible.
In principle, whatever increases themachine’s productivity is efficient.
Non-cutting times during setup
The values in the blocks of NC programs
are given with respect to datums on theworkpiece that must first be measured
on the fixed workpiece. A workpiece
touch probe ensures the fast and safe
measurement of datums. Predefined touchprobe cycles and a delay-free coupling
between the touch probe and control
considerably reduce non-cutting time andat the same time increase machining
accuracy. Datums can be set automaticallyusing the TS touch probes in combination
with the probing functions of the TNC
controls. This makes it possible to avoid
errors during setup that inevitably result in
bad parts.
Touch probes can reduce non-cutting time,
improve production quality, prevent scrap,
and increase productivity. This reduces theenergy requirement per manufactured part.
The possibilities for reducing non-cutting
times are illustrated in these two examples.
Here the set-up times are compared with adial gauge and HEIDENHAIN touch probes
on a machine tool with a base load of 4 kW.
Task 1• Align the workpiece blank parallel to theaxes• Set the datum in the working plane at acorner• Set the tool axis datum at the top
surface of the blank
Task 2• Align a workpiece paraxially using two
holes• Set the datum of the working plane in
the center of the first hole.
• Set the tool axis datum at the top
surface of the blank

Task 1

Task 2Time savings
With a TS touch probe from HEIDENHAIN,
this setup operation is performed with atime saving of about 4 minutes, or approx.
72 %.

Time savings
With a TS touch probe from HEIDENHAIN,
this setup operation is performed with atime saving of about 5 minutes, or approx.
77 %.

Energy savings
With ten setups per day and 220 working
days, 580 kWh are saved per year. That
approximately equals the annual energy
requirement of five refrigerators of theefficiency class A++.

Energy savings
With ten setups per day and 220 working
days, 730 kWh are saved per year. That
equals approximately the annual energy
requirement of six refrigerators of theefficiency class A++.

Touch probe

Touch probe

1 min 25 sec

1 min 30 sec

Dial indicator

Dial indicator
5 min 30 sec

6

6 min 30 sec

Measuring systems for theenergy efficient operation of
servo-controlled motors

Positioncontroller

In the control loops of spindle motors anddirect-drive feed axes, even the smallest
disturbance in the feedback signals canresult in large fluctuations in the motor
current.

An encoder with low signal quality can
result in greater energy loss on the motors.
In addition, the additional energy
requirement of the active cooling has
to be included in the energy balance.
To increase the energy efficiency of the
motor, the encoders should feature highsignal quality.

ddt

Angle encoder

Motor

Motor current [A] f

Control loop with rotational direct drive (torque motors)
6
3
0
3

A
B

6
0.4

0.6

0.8

1.0

1.2

1.4

1.6

2.0

1.8

–1

Time [s] f

Shaft speed approx. 3 minMotor current [A] f

A comparison of the effects of an opticaland a modular magnetic encoder on arotary table with direct drive illustrates themeaning of high-quality position signals.
The RCN 226 angle encoder with 16 384
lines generates only barely noticeable
disturbances in the motor current, anddevelops little heat. Because of themagnetic scanning principle, the encoder
has notably fewer signal periods.
Significant disturbances occur in the motor
current with the same controller settings.
For example, at a shaft speed of approx.
30 min–1 the effective value of the current
is 15 A greater than for an optical encoder.
The consequence is greater heatgeneration in the motor.



40
20
0
20
40
0.4
0.6
0.8
1.0
Shaft speed approx. 30 min–1

A
B
1.2

1.4

2.0
1.8
Time [s] f

1.6

Motor current of a direct drive with position encoder
A: With low interpolation error (optical encoder)
B: With high interpolation error (modular magnetic encoder)
Temperature [°C] f

The interpolation of the signal of a position
encoder includes short-range deviations
within one signal period (interpolation error)
typically of approx. 0.5 % of the signal
period. If the frequency of the interpolation
error increases, the feed drive can nolonger follow the error curve. However,additional current components are
generated by the interpolation error.
Therefore, if the torque remains constant,the energy consumption and efficiency ofthe motor get worse. The additional energy
required is converted to heat. The heatmust typically be dissipated by a motor
cooling system, which itself consumes
energy.

Velocity
controllerTime [h] fTemperature variations of a direct drive with optical (A) and magnetic (B) position encoder

7

Minimizing scrap through closed
loop technology
Bad parts reduce the productivity of amanufacturing process and therefore
greatly contribute to energy costs per
manufactured part. A primary cause of thegeneration of bad parts is the thermal driftof feed axes running on recirculating ballscrews.

Measurementof velocity andposition

The temperature distribution along ballscrews can be changed very rapidly by
feed rates and moving forces. On machine
tools in a semi-closed loop (i.e. without
linear encoders, Figure 1 above) the
resulting changes in length (typically:
100 µm/m within 20 min.) can causesignificant flaws in the workpiece.
If a linear encoder is used for measurementof the slide position (Figure 1, below), an
increase in the ball screw’s temperature(Figure 2) has no influence on accuracy.
This is referred to as operation in a closed
loop because the mechanical errors of thedrive are measured in the position control
loop and are therefore compensated.

Velocity
measurement

Positionmeasurement
Figure 1

Figure 2

8

Example: Machining a coupling lever foran airfoil.
In order to evaluate the accuracy that canbe achieved in the manufacture of small
batches, a coupling lever is manufactured
twice from the same workpiece blank. The
second workpiece is simply machined
10 mm below the first. Between the twomachining operations, twenty machining
cycles for the same part are executed
above the blank. In semi-closed-loop
operation, the contour of the second
workpiece deviates from the contour of thefirst workpiece, as is shown by an edge. In
the semi-closed loop, this offset between
the workpieces is 44 µm. With the use of
linear encoders in a closed loop, no edge
results. A reproducible accuracy from thevery first part is thus guaranteed.

Semi-closed loop
Thermal offsets

Closed loop:
No thermal offsets

Energy savings in a closed loop
The manufacture of 22 coupling levers inthis example takes 2 hours. The bore of
two holes at a distance of 350 mm is to be
manufactured with the tolerance of IT7,
which allows error of ± 28 µm.
To manufacture 22 good parts in a semiclosed loop, the machine first has to run
the NC program cyclically for 25 min inorder to ensure compliance with the IT7
tolerance. During warm-up, the energy
consumption is only approx. 10 % below
the value for milling.
As a result, the energy costs per good partin the semi-closed loop, are 19 % higher
than for manufacturing 22 parts in a close
loop with linear encoders. If 50 parts are
manufactured on a milling machine in thesemi-closed loop with preceding warm-up
phase, a power requirement during millingof 8 kW on 220 working days ensues for an
additional energy requirement of 660 kWh.

9

Conclusion
Analyses of metal-cutting processes showthat the power consumption of a CNC
control with feed-axis and spindle motors
frequently comprise only 25 to 30 % of thetotal required power. On the other hand,the auxiliary components in the machine orits environment play a dominant role in theenergy balance.
There are numerous possibilities for amore efficient use of energy. Significant
savings can be realized through the proper
selection of components for a machine tool
alone. Whether a CNC control shouldfeature energy recovery to line power
depends on the frequency of tool changing
during milling operations or workpiecechanges during turning. The regenerated
energy is then available for otherconsumers in the machine’s environment.
Further energy savings are possible
depending on whether spindles can beclosely adapted in their speed and torqueto the machine’s range of operations. If a

10

universal design of the spindle is required,
the spindle motor may have to be run at
low efficiency rates—with the expectedconsequences for energy consumption.
The selection of the position encoder canhave a decisive effect on the efficiency ofspindle motors and direct drives. Encoders
with insufficient resolution and accuracy
generate high current values in the position
control loop. Position encoders with highline counts are essential for the highefficiency of servo-controlled drives.
It has been proven that linear encoders
increase accuracy and therefore contributeto higher precision and reproducibility ofmachining results. This makes it possible toreduce waste in production and, as an
immediate result, the energy requirement
per good part.
Components for processing coolinglubricants, for supplying compressed air
and hydraulics and for cooling occupy adominant share of the total power. If amachine has particularly efficient pump
motors, this alone can save significant
amounts of energy. Another opportunity isoffered by energy management on the
CNC control with selective switch-off ofcomponents. This makes it possible, forexample, to delay the switch-off of pumpdrives after a machining operation has
ended.
Even in the production process itself,
potential energy savings can be found.
With the relatively high base load ofmachine tools, a reduction of
nonproductive phases is a high priority.
Here the CNC controls from HEIDENHAIN
can help in many ways. Beginning withideas for user-friendly program editing tofunctions for workpiece setup with
HEIDENHAIN touch probe systems, all theway to more efficient and exact motion
control in three and five-axis machining,users profit from the know-how collected
by HEIDENHAIN in decades of experiencein metal-cutting technology.

For more information
For more information on these topics andon HEIDENHAIN products, please e-mailus at info@heidenhain.de or call
+49 8669 31-0. The brochures andTechnical Information documents shown
below can also be downloaded fromwww.heidenhain.de.

Längenmessgeräte
für gesteuerte Werkzeugmaschinen

Brochure
iTNC 530
Contouring Control

Brochure
Modular Magnetic
Encoders

BrochureLinear Encoders
for NumericallyControlled
Machine Tools

Produktinformation

iTNC 530

Baureihe ERM 200

Die vielseitige Bahnsteuerung
für Fräsmaschinen, Bohrwerke
und Bearbeitungszentren

Magnetische
Einbau-Messgeräte

Oktober 2007

Juni 2007

Brochure
Exposed Linear
Encoders
Absolute
Winkelmessgeräte

OffeneLängenmessgeräte

Juni 2007

Brochure
Absolute Angle
Encoders
with Optimized
Scanning

TNC 320
Die kompakte Bahnsteuerung
für Fräs- und Bohrmaschinen

mit optimierter Abtastung

Oktober 2010

Januar 2009

DrehgeberJuli 2010

MANUALplus 620

Juni 2006

September 2009

Brochure, CD-ROM
Touch Probes

Messgeräte fürelektrische Antriebe

Winkelmessgeräteohne Eigenlagerung

Tastsysteme

November 2007

September 2007

Oktober 2006

Technical Information
Accuracy of Feed Axes
Technische Information

Genauigkeit von Vorschubachsen

Bearbeitungsgenauigkeit von Werkzeugmaschinen

Jahr für Jahr werden bei neuen Werkzeugmaschinen Efﬁzienz- und Leistungssteigerungen nachgewiesen. Durch immer höhere Vorschubgeschwindigkeiten und Beschleunigungen werden Bearbeitungszeiten fortlaufend reduziert. Gleichzeitig ermöglichen ständig steigende Arbeitsgenauigkeiten immer engere Tolerierungen der Werkstücke. Dadurch lassen sich einerseits zunehmend kritische Teile fertigen, andererseits wird der Zusammenbau komplexer Baugruppen vereinfacht. Selektive Handmontagen und Nachbearbeitungsschritte
können oft entfallen. Eine höhere Genauigkeit der Teile hat zudem meist eine Erhöhung der Funktionsfähigkeit von Baugruppen zur Folge.
So steigt zum Beispiel die Lebenserwartung von Getrieben bei gleichzeitig geringerer Geräuschentwicklung.

Produktivität und Genauigkeit sind entscheidende Wettbewerbsaspekte von Werkzeugmaschinen. Schnelle Wechsel der Einsatzbedingungen von Werkzeugmaschinen erschweren jedoch Produktivitäts- und Genauigkeitssteigerungen erheblich. So müssen in der Teilefertigung immer kleinere Losgrößen wirtschaftlich und genau gefertigt werden. In der Luft- und Raumfahrt ist höchste Zerspanleistung beim
Schruppen gefordert, während im anschließenden Schlichtprozess mit sehr hoher Genauigkeit gefräst werden muss. Beim Fräsen hochwertiger Formen sind große Zerspanraten beim Schruppen und eine makellose Oberﬂächenqualität als Schlichtergebnis erforderlich.
Gleichzeitig werden aber höchste Bahnvorschübe benötigt, um die notwendigen feinen Bahnabstände mit vertretbaren Bearbeitungszeiten fertigen zu können.

Im Gesamtfehlerbudget einer Werkzeugmaschine spielen die Positionierfehler der Vorschubachsen eine entscheidende Rolle. Sie werden
im Folgenden näher betrachtet und anderen Fehlern gegenübergestellt. Dabei stellt sich die thermische Dehnung der Kugelgewindespindel als das Hauptproblem bei der Positionserfassung mit Spindel und Drehgeber heraus. Der dadurch verursachte Positionierfehler überwiegt oft die thermisch bedingten Strukturdeformationen und geometrischen Fehler von Bearbeitungszentren. In mehreren Versuchen
wird der Einﬂuss der Erwärmung des Kugelgewindetriebs – auch in Abhängigkeit von seiner Lagerung – auf das Bearbeitungsergebnis
dargestellt. Wird ein Längenmessgerät zur Positionserfassung verwendet, spielt jedoch die thermische Dehnung der Spindel keine Rolle– die Positionsdrift ist vernachlässigbar. Bei steigenden Anforderungen an Genauigkeit und Geschwindigkeit von Werkzeugmaschinen gewinnt deshalb die Positionserfassung mit einem Längenmessgerät zunehmend an Bedeutung.

Das Reduzieren thermischer bzw. systematischer Fehler einer Werkzeugmaschine
durch langwieriges Optimieren einzelner
Fertigungsschritte ist bei ständig wechselnden Aufträgen und der damit verbundenen
sinkenden Größe der Fertigungslose kaum
noch möglich. Die ´Genauigkeit des ersten
Teils´ gewinnt immer größere Bedeutung.
Insbesondere die thermischen Fehler von
Werkzeugmaschinen ﬁnden dadurch zunehmend Beachtung.

Im Hinblick auf stark wechselnde Einsatzbedingungen in der Fertigung gewinnt die thermische Genauigkeit von Werkzeugmaschinen immer
mehr an Bedeutung. Besonders bei kleinen Fertigungslosen und damit ständig wechselnden Maschinenaufträgen kann kein thermischstabiler Zustand erreicht werden. Gleichzeitig gewinnt aber die Genauigkeit des ersten Werkstücks eine große Bedeutung für die Wirtschaftlichkeit von Fertigungsaufträgen. Ständige Wechsel zwischen Bohren, Schruppen und Schlichten verstärken die Schwankungen im
thermischen Zustand einer Werkzeugmaschine. Während der Schruppbearbeitung steigt die Fräsleistung auf Werte oberhalb von 80 % an,
beim Schlichten werden Werte unterhalb von 10 % erreicht. Die zunehmenden Beschleunigungen und Vorschubgeschwindigkeiten sindverantwortlich für die Erwärmung der Kugelumlaufspindeln in den linearen Vorschubantrieben. Daher spielt die Positionserfassung in den
Vorschubantrieben eine zentrale Rolle bei der Stabilisierung des thermischen Verhaltens von Werkzeugmaschinen.

Die folgende Betrachtung zeigt, dass thermische Fehler gerade bei den Vorschubachsen sehr ausgeprägt sein können. Im Gegensatz zu den Strukturdeformationen
können Fehler der Vorschubachsen durch
die Wahl einer geeigneten Messtechnik mit
einfachen Mitteln deutlich reduziert werden.

Eine wesentliche Quelle thermischer Verlagerungen sind die Vorschubachsen auf
Basis von Kugelgewindespindeln. Abhängig von den Vorschubgeschwindigkeiten
und -kräften können sich die Temperaturverteilungen auf den Kugelgewindespindeln sehr schnell ändern. Die dabei entstehenden Längenänderungen (typisch:
100 μm/m innerhalb von 20 min) können
auf Werkzeugmaschinen ohne Längenmessgeräte zu signiﬁkanten Fehlern am
Werkstück führen.
September 2007

für Werkzeugmaschinen

Technical Information
Machining Accuracy of
Machine Tools

Technische Information

Messgeräte für Vorschubachsen mit Direktantrieben
Direktantriebe haben sich in vielen Bereichen der Halbleiter- und Elektronikherstellung einen festen Marktanteil erobern können. Auch im
Werkzeugmaschinenbereich ﬁnden sie zunehmend Verwendung. Für die Direktantriebstechnik sprechen geringer Verschleiß, niedriger
Wartungsaufwand und eine höhere Produktivität. Dazu müssen jedoch Steuerung, Motor, Mechanik der Vorschubachse und Positionsmessgerät optimal aufeinander abgestimmt sein.
Die Leistungsfähigkeit von Direktantrieben wird entscheidend durch die Wahl des Positionsmessgeräts beeinﬂusst, den Direktantrieben
stellen hohe Ansprüche an die Güte der Positionssignale.
Optimale Messsignale• erhöhen die Qualität der produzierten Werkstückoberﬂäche,• reduzieren Vibrationen im Maschinengestell,• verhindern Lärmbelastungen durch geschwindigkeitsabhängige Motorgeräusche und• vermeiden zusätzliche Wärmeentwicklung im Motor.

Abbildung 1

Erwärmung eines Kugelgewindetriebs beim Abzeilen mit einem mittleren
Vorschub von 10 m/min. Die Thermograﬁe-Aufnahme zeigt Temperaturen
von 25 °C
bis 40 °C.

Technical Information
Encoders for
Feed Axes with
Direct Drives

Die Leistungsfähigkeit eines Direktantriebs wird also entscheidend durch die Wahl des Positionsmessgeräts beeinﬂusst. Messgeräte mit
optischem Abtastverfahren zeigen Vorteile bei Genauigkeit, Laufruhe und Erwärmungsverhalten eines Direktantriebs. HEIDENHAIN liefert
ein breites Programm von Längen- und Winkelmessgeräten, die sich aufgrund ihrer technischen Eigenschaften speziell für Direktantriebe
eignen.

Aufbau von Direktantrieben
Entscheidender Vorteil der Direktantriebstechnik ist die sehr steife Ankopplung an
die Vorschubmechanik ohne weitere mechanische Übertragungselemente. Dadurch
erlaubt der Regelkreis wesentlich höhere
kV-Faktoren als ein konventioneller Antrieb.

Thermische Stabilität von Werkzeugmaschinen
Lösungen zur Vermeidung thermisch bedingter Maßabweichungen an Werkstücken
rücken stärker denn je in den Fokus desWerkzeugmaschinenbaus. Aktive Kühlungen, symmetrisch aufgebaute Maschinenstrukturen und Temperaturmessungensind heute bereits gängige Maßnahmen.

Vorschubeinrichtung eines Fräs-Bearbeitungszentrums

März 2006

Die Bahnsteuerung für Zyklenund CNC-Drehmaschinen

Brochure
Angle Encoders
without Integral Bearing

Technische Information

Die Genauigkeit heutiger Werkzeugmaschinen wird mit einer zunehmenden Zahl
neuer und überarbeiteten Prüf- und Abnahmetests ermittelt. Waren früher rein geometrische Abnahmetests an der Tagesordnung, so werden heutzutage verstärkt
dynamische Tests wie Kreis- und Freiformtests, thermische Untersuchungen z.B.
nach ISO/DIS 230-3 und – im Fall von Produktionsmaschinen – Fähigkeitsuntersuchungen bei der Abnahme bzw. routinemäßigen Überprüfung eingesetzt. Die
unterschiedlichen Einﬂüsse des Schneidprozesses, der geometrischen und thermischen Genauigkeit, der statischen und
dynamischen Steiﬁgkeit sowie des Positionierverhaltens der Vorschubachsen auf dieerzielbare Genauigkeit des Werkstücks lassen sich immer differenzierter analysieren.
Für den Anwender der Maschine werden
dadurch deren Fehler immer transparenter.

Brochure
MANUALplus 620
Contouring Control

Winkelmessgeräte
mit Eigenlagerung

Brochure
Encoders forServo Drives

Brochures
TNC 320
Contouring Control
TNC 620
Contouring Control

September 2007

Brochure
Angle Encoders
with Integral Bearing

Brochure
Rotary Encoders

CD-ROM
iTNC Presentation

Geschwindigkeitserfassung an
Direktantrieben
Bei Direktantrieben ist kein zusätzlicher Drehgeber zur Geschwindigkeitsbestimmung
vorhanden. Lage und Geschwindigkeit werden über das Positionsmessgerät – Längenmessgerät bei Linearmotor, Winkelmessgerät bei rotatorischem Antrieb – bestimmt.
Da die mechanische Übersetzung zwischen
Drehzahlmessgerät und Vorschubeinheit
fehlt, muss das Positionsmessgerät übereine entsprechend hohe Auﬂösung verfügen, um auch bei langsamen Verfahrgeschwindigkeiten eine exakte Geschwindigkeitsregelung zu ermöglichen.

Geschwindigkeitsregler

Lageregler




d
dt

Winkelmessgerät

Motor

Regelkreis bei rotatorischem Direktantrieb (Torque-Motor)
Durch die höheren kV-Faktoren, die bei Direktantrieben möglich sind, steigt auch der
Einﬂuss der Signalqualität der Positionsmessgeräte auf das Positionierverhalten.

Direktantriebe erfordern deshalb Positionsmessgeräte mit kleinen Signalperioden undhoher Signalgüte.

September 2005

11

DR. JOHANNES HEIDENHAIN GmbHdevelops and manufactures linear andangle encoders, rotary encoders, digital
readouts, and numerical controls.
HEIDENHAIN supplies its products to
manufacturers of machine tools, and of
automated machines and systems, inparticular for semiconductor andelectronics manufacturing.

DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany{ +49 8669 31-0
| +49 8669 5061
E-mail: info@heidenhain.dewww.heidenhain.de751 261 · 00 · A · 02 · 11/2010 · PDF

HEIDENHAIN is represented in over
50 countries—mainly through its own
subsidiaries. Sales engineers and service
technicians support the user on-site withtechnical information and servicing.