Technical Information

Influence of Position Measurement on Accuracy
in 5-Axis Machining
Productivity and accuracy are important attributes in the competition for machine tools. Five-axis machining provides considerable potential for increasing productivity. In many cases it permits higher metal removal rates than 3-axis machining. Production times can be significantly shortened thanks to a reduction of time required for resetting, for example, or through multi-operation machining in one setup. In
any case, with increasingly complex workpiece geometry, 5-axis machining is becoming an indispensable part of the machining process.
Since significantly larger traverse ranges of linear axes are usually required for 5-axis machining, machines need to provide high accuracy
over the entire working space. Moreover, the two rotary axes in a 5-axis machine tool can greatly influence attainable workpiece accuracy. Besides the rotary axes, in most cases the linear axes must also be moved in order to change the orientation of the cutter to theworkpiece surface. This can cause visible flaws in the traverse range of up to five axes within a small area of the workpiece surface. In
5-axis machining, precision-limiting effects in the drives, such as screw-pitch and transmission error, reversal error or thermally induced
displacement can therefore much more quickly lead to the production of scrap. The positioning accuracy of linear and rotary axes play adecisive role here in the performance of a 5-axis machine tool.

By now, 5-axis machining has become indispensable in many areas of metal-cutting
machining. Clear economic advantages result from the capability to machine workpieces completely in one setup: the doorto-door time of a part can be dramaticallyreduced. At the same time, part accuracy
can be significantly increased.
Beyond this, the additional rotary axes allow better access to complex workpiece
contours, for example cavities in dies ormolds.Often, they permit shorter tools withless inclination to chattering so that evenhigher metal removal rates are achieved.
With 5-axis simultaneous machining, thecutting speed at the tool tooth can be held
within narrow limits even on complex contours. This brings significant benefits withregard to the attainable surface quality.
What is more, the use of highly productive
tools (e.g. toroid cutters) when milling freeform contours would not be possible at allwithout 5-axis simultaneous machining.

September 2011

Fields of Application for 5-Axis Machining

Parts for Aeronautics and Space
Industry
High strength and low weight are essential
for the aeronautics and space industry. Integral construction has established itself as
the way to minimize the weight of “airborne” parts: components with complex
structure are manufactured completelyfrom a single blank. Metal removal levels
can be as high as 95 %. This high “buy-tofly” rate leads to high costs for the raw
material of the blanks.

5-axis machining in automobile manufacturing
Uncounted molds and dies are needed in
automobile manufacturing for sheet metaland plastic processing. Dies for sheet metal forming can be up to 6 m long and need
to be milled at the very high accuracy of± 0.02 mm so that the upper and bottom
dies can work together with the correct
gap. Moreover, a very high surface qualityof all functional surfaces is necessary in order to ensure that the forming tools have along service life.

In the area of structural components, 5-axis
machining opens new opportunities for reducing weight without loss of component
strength. First, a computer-aided topology
optimization is conducted that adapts thegeometry of the component to the respective loads. The result: the material isbrought specifically to where the mechanical load can be highest. In the other areas,material is specifically reduced. For example, the thickness of walls for stiffening canbe easily adapted to the load distribution in
the component. The wall thickness can decrease with increasing height, for example.
This workpiece geometry can be realized ina simple way through 5-axis pocket milling.

When manufacturing the tool contour, themachine has to maintain a very small distance between cutting paths in order to fulfill the requirements for high surface quality. This automatically lengthens the runtimes of the NC programs. The required accuracy of the forming tools is a formidable
challenge for machine tools: high accuracy
during long program run times on large
components necessitates high thermal stability of the machine structure and the feed
drives.
Five-axis machining has long become thestandard for the machining of jet engines.
High efficiency requirements are driving
continuous improvements in the flow characteristics of all jet engine components. The
resulting component geometries are very
complex and are therefore manufactured
exclusively in 5-axis simultaneous milling
movements.

2

Five-axis machining opens new perspectives for shortening machining times because even deeply curved contours offorming tools become more easily accessible. In addition, special tools such as toroid
or radius cutters can be used that permit
significantly larger path spacing and therefore reduce program run times.

Five-axis machining in the field of medical technology
In the field of medical technology, demandis high for devices that are adapted to special examinations or therapies. This canmake treatments considerably more precise and reduce aftereffects on the patients. The devices are often characterized
by very complex geometries that make5-axis machining of single parts on millingmachines attractive.
Increasing life expectancy brings with it anincrease in demand for tooth and joint implants. Today, hip and knee replacements
offer many people the opportunity for a significantly better quality of life. In their external form, tooth and joint implants have tobe perfectly fitted to the specific mating
surfaces of a human body. Implants aremostly manufactured on milling machines,
because milling makes even small batch
sizes possible. Due to complex shaping of
implants, medical technology is one of thelargest fields of application for 5-axis machining. A prerequisite for the economical
manufacturing of such sophisticated components is machines with high-accuracy
position measurement for accurate andprecise feed movements.

Requirements of Position Measurement

In 3 axis milling, the feed axes move within
the dimensions of the workpiece plus thetool diameter. Unlike with 3-axis machining,in 5-axis machining the inclination of thetool can be adjusted with respect to theworkpiece surface. If the position of thetool center point (TCP) remains unchanged,a change in the cutter orientation usuallyrequires additional movement in the linear
axes. These compensating movements
necessarily increase the traverse range required by the linear axes. Because increasing traverse range also means increased
positioning error, feed axes of 5-axis machines need significantly higher accuracy
and reproducibility .
The compensating movements of the linear axes are superimposed on the movements of the tool center commanded by
the NC program in X, Y and Z. Due to thissuperimposition, the axis feed rates for the
tool center can significantly exceed theprogrammed feed rate. The increase in
feed velocity results in increased heat generation in the motors, transmission and recirculating ball screw. Depending on theprinciple of position measurement, thegeneration of heat can cause significant
position error. To prevent faulty workpieces,precise position measurement in the feed
axes directly at the moving machine elements is imperative.

B

A
A

A: Traverse range with 3-axis machining
B: Traverse range with 5-axis machining,incl. compensating movements

3

Position acquisition on linear axes
The position of a feed axis can be measured either through the recirculating ballscrew in combination with a rotary encoder, or through a linear encoder. If the axis
position is determined from the pitch of thefeed screw and a rotary encoder (see top illustration), then the ball screw must perform two tasks: As the drive system, it
must transfer large forces, but as the measuring device it is expected to provide ahighly accurate screw pitch. However, theposition control loop only includes the rotary encoder. Because changes in the driving
mechanics due to wear or temperaturecannot be compensated in this way, this iscalled operating in a Semi-Closed Loop.
Positioning errors of the drives become unavoidable and can have a considerable influence on the quality of workpieces.
If a linear encoder is used for measurement of the slide position (middle illustration), the position control loop includes thecomplete feed mechanics. This is referred
to as Closed Loop operation. Play and inaccuracies in the transfer elements of themachine have no influence on position
measurement. This means that the accuracy of the measurement depends almost
solely on the precision and location of thelinear encoder.

As an alternative to using a recirculatingball screw, linear feed axes can also be
driven by linear motors. In this case, theposition of the machine axis is also measured directly by a linear encoder mounted
on the axis slide. For highly dynamic and at

the same time quiet operation, linear motors already depend on high-resolution, accurate linear encoders. For this type ofdrive, the advantages of Closed Loop operation apply unrestrictedly.

Semi-Closed Loop: Position measurement of a linear axis by a rotary encoder in the feed motor

Various conditions of application together
with growing feed rates and forces lead toconstant changes in the ball screw’s thermal condition. Local temperature zones develop on a recirculating ball screw, whichchange with every change in position anddecisively reduce accuracy in the SemiClosed Loop.
High reproducibility and accuracy over theentire traverse range of linear axes cantherefore be achieved only through operation in a Closed Loop. The results are precise workpieces and a drastically reduced
rejection rate.

Closed Loop: Position measurement of a linear axis by a linear encoder

Thermographic image of a recirculating ball screw with increased temperature4

Position acquisition on rotary axes
The basic principle for linear axes also applies to rotary axes. Here, too, the position
can be measured with a rotary encoder on
the motor, or with a high-accuracy angle
encoder on the machine axis.

The rotary axes that are driven directly by atorque motor play a special role. The
torque motors’ special design permits very
high torque without additional mechanicaltransmission. Rotary axes with torque motors require a high-resolution angle encoder
immediately on the machine axis. They arealways operated in a Closed Loop.

If the axis position is measured using a rotary encoder on the feed motor it, too, isreferred to as a Semi-Closed Loop, because the transmission error of the gear
mechanisms cannot be compensatedthrough a closed position loop.
Error of the mechanical transmission in rotary axes are caused by• eccentricity of the gear wheels,• play, or• friction and elastic deformations in thetooth contacts and the bearings of thetransmission shafts.
Beyond this, most prestressed transmissions are subject to significant friction thatheats the rotary axes and can therefore—
depending on the mechanical design—result in positioning error.

Semi-Closed Loop: Position measurement by rotary encoder on the feed motor.
Errors of the drive mechanism are not recognized.

In a Semi-Closed Loop, the error in thetransmission of rotary axes leads to substantial positioning error and to significantly
reduced repeatability. The errors of the rotary axes are transferred to the geometryof the workpiece, which can greatly increase the number of rejected parts.
The positioning accuracy and repeatability ofrotary axes can be decidedly improved withthe use of precise angle encoders. Since
the axis positions are no longer measured
on the motor, but rather directly on the rotary axes of the machine, this is referred to as
Closed Loop operation. Errors of rotary axis
transmissions have no influence here on positioning accuracy. The accuracy with which
a rotary axis can move to a certain axis position over a long period is also decidedly
increased. Economic manufacturing withminimal scrap is the result.

Closed Loop: Position measurement by an angle encoder on the machine axis.
Errors of the drive mechanism are compensated.

Torque motor: Rotary axes with torque motors are operated in a Closed Loop.

5

Machining Examples
Five-Axis Simultaneous Machining with Very High Precision

Telstar soccer ball
The soccer ball for the FIFA World Championships of 1970 and 1974 were named after the
Telstar, the first non-military communication
satellite, which was launched into space by
the NASA and AT&T in 1963. The Telstar ballbears over 20 white hexagonally and 12 black
pentagonal pieces that were sewn together.
This ball serves as the model for a workpiece that can be milled on a 5-axis machine tool. The resulting workpiece is being
used as a test piece to prove machining accuracy with 5-axis milling.
The Telstar workpiece was manufactured inthree machining steps on the basis of preturned blanks:• Three-axis milling of the pentagons withvertical paths and inclined cutter• Three-axis milling of the hexagons withhorizontal paths and inclined cutter• Five-axis milling of the seams
A perfect optical appearance of the Telstar
workpiece is possible only if the “seams”
between the pentagons and hexagons aremilled with consistently superb precision
despite a machining time of over twohours.

Five-axis manufacturing of the Telstar World Championship ball of1970 and 1974

Machining sequence for 5-axis machining of the Telstar workpiece

Milling the pentagons:
3-axes, inclined cutter
Climb milling
Machining time: 22 minutes
Feed rate:
6 m/min
Cutter:Ø = 16 mm
Line spacing:
1.5 mmInclination angle: 40°

6Milling the hexagons:
3-axes, inclined cutter
Climb milling
Machining time: 2 h 17 min
Feed rate:
6 m/min
Cutter:Ø = 16 mm
Line spacing:
0.2 mmInclination angle: 40°

Milling the seams:
Five axes simultaneously
Machining time:
Feed rate:
Cutter:
Inclination angle:

11 minutes
0.4 m/minØ = 25 mm55°

At the intersections, exactly three seams
meet that are milled with various cutter inclinations. An intersection can therefore bemachined precisely only if the cutter’s tool
center point (TCP) is positioned with threediffering inclinations at exactly the same
point.
However, the three inclination angles of thecutter require large compensation movements in the linear axes. For each seam, amuch different position of the linear and rotary axes results at the TCP position of anintersection.
During milling, positioning errors in one ormore feed axes inevitably prevent the TCP
positions from coinciding when milling thethree seams at their intersection. High accuracy in the positioning of all feed drives is
indispensable for precise machining of theseams, including the intersections.

Semi-Closed Loop: influences of the drive mechanics impair
machining accuracy. The seam width varies. The seam intersections
are visibly inaccurate.

For a simple visual evaluation of the Telstar
workpiece’s accuracy, a 0.15 mm deep
seam is manufactured throughout with a
25 mm diameter cutter. This results in avery flat seam cross section, so that eventhe smallest error in the seam’s depth(±10 µm and less) causes clear fluctuationsof the seam width.
If the Telstar workpiece is manufactured ona machine in a Semi-Closed Loop, the positioning accuracy and repeatability is limited by the transmission error of the recirculating ball-screw in the linear axes and thetransmission in the rotary axes. The result:the seam width fluctuates on the Telstar
workpiece. The intersections cannot coincide exactly with each seam, so that a clear
displacement of the seams’ centers results.
In a Closed Loop, the feed axes attain very
high positioning accuracy and repeatability
over the entire traverse range. This makes
precise machining of neighboring sections
on the workpiece possible, even with largechanges to the cutter orientation and substantial periods between the individual machining steps. The attainable precision isvisible in the intersections of the seampaths. Each intersection is approached in all
three neighboring seams. In addition, theseam width remains constant over the entire circumference of the Telstar workpiece.

Closed Loop: Thanks to the precise linear and angle encoders
from HEIDENHAIN, errors in the drive mechanics have noinfluence on the result of machining. The seams are milled tothe correct width, and the intersections all match up exactly.

7

Five-Axis Simultaneous Plain Milling

Polygonal workpiece
Five-axis machining provides considerable
potential for reducing machining times. The
lateral surface of a polygonal workpiece isfinished simultaneously with a plain milling
cutter in five axes in one revolution of theworkpiece. Compared with 3-axis machining of the lateral surface with a ball-nose
cutter, 5-axis machining reduces machining
time to 30 %.
Precise machining with five simultaneously moving feed axes place stringent requirements on the accuracy of the feed
drives. Depending on the type of position
measurement, the errors of the additionally required rotary and tilting axes cancause significant limitations on workpiece
quality. It is therefore essential to pay special attention to the correct encoder and its
integration into the control loop.

To examine the effect of position measurement on the result of machining, workpieces were machined with the B axis separately in a Closed Loop and in a
Semi-Closed Loop. All other axes were in apermanent Closed Loop.

The illustrations below show the effect of
eccentricity in a gear stage. The transmission with precisely assembled gears transfers the motion of the drive motor without
error to the tilting axis. If a transmission
gear wheel is eccentric, the movement of
the tilting axis has a sinusoidal error.

In a Semi-Closed Loop, mechanical errorsof rotary axes can cause position errors
and therefore result in inaccuracy. Tilting
axes often have drives with multiple-stage
gearing. For uniform motion in a tilting axis,
all gear components must have high manufacturing accuracy and be assembled precisely. Even small eccentricity errors of thegear can cause distinct fluctuations in the
speed of the tilting axis.

The periphery of the polygonal workpieceis machined in one revolution of the C axis
with simultaneously interpolated movement of the B axis. During machining of
the lateral surface, the B axis is tilted inand out five times in a tilting range between 1° and 19°.

The illustrated polygonal workpiece was
manufactured on a milling machine whose
tilting axis has a transmission reduction of
i = 120. The motor pinion has a diameter of
40 mm. The radial runout of the pinion is± 0.058 mm.

i=1
Movement of thetilting axis

DrivemotionActualmovement

Error-free transmission
Transmission with an
eccentricity error

Ideal movement

360°

Position errors on rotary axes due to eccentricity in thetransmission, gear reduction i = 1

i = 120
Movement of
the tilting axis

DrivemotionActualmovement

10”
Error-free transmission
Transmission with an
eccentricity error
3°
Ideal movement

Position errors on rotary axes due to eccentricity in thetransmission, gear reduction i = 120

8

If the tilting axis is controlled in a SemiClosed Loop, the sinusoidal position error
generated by the transmission cannot be
detected by the drive controller. One revolution of the drive motor with eccentric pinion causes motion errors of the tilting axis
in the range of ± 10 angular seconds. The
error repeats itself every 3° with respect tothe tilting angle.

In a second experiment, the tilting axis
was controlled in a Closed Loop. Here a
HEIDENHAIN angle encoder with opticalscanning was employed. The angle encoder measures the axis position directly at
the tilting axis. In this way the effect of thetransmission error from the gears is immediately detected. The drive controller reacts
immediately to very small deviations andgenerates the equivalent countermovement of the feed motor. The transmission
errors therefore have no effect on the result of machining.

In a first experiment, the polygonal workpiece is machined in the Semi-Closed
Loop. The position errors generated by thetransmission are clearly visible as surface
ripple on the lateral surface of the polygonal workpiece. The resulting form deviationis ± 0.015 mm. The transmission error isvisible only on the sections of the workpiece at which the tilting axis moves.

Thanks to the HEIDENHAIN angle encoders on the rotary axes, the lateral surface
can be milled in five axes simultaneouslywith short machining time and, at the sametime, high surface quality and accuracy.

Semi-Closed Loop: Polygonal workpiece with surface error.

Closed Loop: Polygonal workpiece with very high surface definition.

B-axis tilts

B axis is
stoppedMachining a polygonal workpiece with plain milling and rotary andtilting axes

9Machining in Opposite Orientations

Requirements for the accuracy ofrotary axes
The sides of prismatic components are often machined in opposite orientations. In
this case, one side is machined first—perhaps by using a tilting axis to incline theworkpiece with respect to the tool. The
workpiece is then turned 180° by a rotarytable to machine the opposite side.
Machining in opposite orientations places
stringent demands on the positioning accuracy of the rotary table. Even small angularerrors in the turnabout movement result inerrors in the parallelism of opposite sidesof the workpiece. On a workpiece with a
500 mm edge length clamped at the center of a rotary table, a positioning error of
only two millidegrees results in an error of
0.01 mm perpendicular to the surface.

Positioning errors can occur on rotary axes
when the machining position is measured
at the shaft of the drive motor (SemiClosed Loop), because the errors in the rotary axis transmission (play, elasticity, radial
runout of the gear shaft) cannot be included in the position control and are not compensated. Depending on the design of thedrive mechanics, Semi-Closed Loop control
can result in positioning error in the turnabout movement of ± 10 millidegrees andmore.
The accuracy of a rotary table can be dramatically improved by using precise angle
encoders that measure worktable movements directly. The transmission error in
the gears of rotary tables are ascertainedby the angle encoder and therefore compensated in the position control (ClosedLoop). In Closed Loop operation, the precision of the angle encoder largely decides
the accuracy of the turnabout movement.
Angle encoders with optical scanning canmake values of less than 0.3 millidegree
possible.

180°

To illustrate the influence of positioning accuracy in rotary tables on machining in opposite orientations, a text was carved outof the sides of a cuboid component. Firstthe component is probed on a 5-axis milling machine on one of the workpiece sides
in the tilted plane. Then a text is cut out
with a ball-nose cutter and 3-axis interpolated climb milling and up-cut milling. The
characters have a height of 0.025 mm.
Then the component is turned around on
the rotary table by 180° in order to carveout a second text with the same characterheight on the opposite side.

180°

Cutter not
engagedEngraving
depth

Engraving
depth

constant

variable

Closed Loop
Precise positioning, contour
is machined precisely

10

Semi-Closed Loop
Positioning error leads to contour damage on the workpieceFirst a workpiece is milled in a Closed
Loop (see 9 on the workpiece). Then, asecond workpiece is machined on thesame machine in a Semi-Closed Loop(see on the workpiece). The difference
becomes apparent very soon. While thetexts milled in Closed Loop control with
precise linear and angle encoders weremachined without error on both sides, in
the Semi-Closed Loop, machining errors

are recognizable: on the back of the component milled in a Semi-Closed Loop the engraving is deeper at left while at right the tool
is simply cutting air.
Position-dependent and direction-dependent
positioning errors in the Semi-Closed Loopcause a slant in the machine table and workpiece. This results in a plainly flawed engraving and the rejection of the workpiece.

Semi-Closed Loop: Influences on the drive mechanism(e.g. transmission errors) can impair the machine’s accuracy andtherefore the machining accuracy and surface quality.

Closed Loop: Accuracy-limiting influences from the drive mechanics have no influence on the results of machining. Thanks to theprecise angle encoders from HEIDENHAIN, a high contour accuracy and excellent surface definition of the workpiece are achieved.

11EncodersFive-axis machining places particularly demanding requirements on the accuracy of
feed drives because the traverse ranges
and axis feed rate grow in comparison with
3-axis machining. The generation of heatand mechanical transmission error in thefeed drives turn position measurement of
the feed drives into a decisive factor formachining precision. Scrap and costs areminimized with the correct position acquisition.
Therefore, linear encoders for linear axes
as well as angle encoders for rotary andtilting axes are indispensable for machine
tools on which high positioning accuracy
and a high machining speed are essential.
Because linear and angle encoders from
HEIDENHAIN ascertain the movement of
the axis directly and immediately. Mechanical transfer elements therefore have no influence on position measurement—kinematics errors, thermal errors andinfluences of forces are measured by theencoder and taken into account in the position control loop. This makes it possible toeliminate a number of potential error sources:
For linear axes:• Positioning error due to thermal behaviorof the recirculating ball screw• Reversal error• Errors due to deformation of the drive
mechanics by machining forces• Kinematics errors through pitch error in
the recirculating ball screw
For swivel, tilting and rotary axes:• Mechanical transmission errors• Reversal error• Errors due to deformation of the drive
mechanics by machining forces

DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany{ +49 8669 31-0
| +49 8669 5061
E-mail: info@heidenhain.dewww.heidenhain.de895 283 · 00 · A · 02 · 9/2011 · PDF

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