Technical Information

Fast, Accurate Contour Milling withHigh Surface Definition
Manufacturing processes in mold and die making, as well as in aerospace technology, can be optimized through modern milling
technologies like High Speed Cutting (HSC). Economic benefits, however, can only be achieved if both the machine tools and the control
can handle higher traverse speeds than those used in conventional machining.
Machine motions that are very fast and yet true to the contour require very precise control of the acceleration and braking processes
along a programmed contour. In the conflict of interests between machining time, surface quality and geometric accuracy, moderncontrols must be capable of making an approach optimized for the milling machine and the manufacturing process. But end users alsoneed to be able to influence the result of milling with simple changes to the parameters. The path control of the CNC has a decisive
influence on the optimization of machining times under given requirements for accuracy and surface quality.

HSC in mold and die making—requirements on machine tool controls
HSC technology offers many newmachining possibilities for hardened andalloyed tool steel. Next to the classic
die-sinking operations, direct HSC millingof molds in hardened materials is therefore
gaining in economic significance. In
comparison with conventional milling, theessential advantage of HSC machining lies
in its distribution and removal of heatgenerated during the cutting process. High
cutting speeds and feed rates, together
with small depths of cut enable the chips
to carry most of the heat away from theworkpiece.

Figure 1: Free-form machining (doubly curved surface)

September 2007

HSC Machining– Prerequisites and Influences

The high feed rates characteristic of HSC
machining increase requirements for higher
contouring acceleration rates on curved
workpiece contours. This places themechatronic characteristics of a machine
tool squarely in the limelight. If theacceleration rates of the feed drives
increase, greater acceleration forces areintroduced into the structure of themachine tool. This consequently increases
the risk of exciting troublesome machine
oscillations that can result in loss of surface
quality. The control therefore requires astrategy of motion control that minimizes
machining time and yet achieves optimum
surface quality under compliance withaccuracy specifications. The control must
therefore offer both the manufacturers ofmachine tools and their users ways tooptimize the path control.
Machine tool builders require optimal
interfacing of the control to thecharacteristics of the machine. The control
must provide a clearly structured
arrangement of parameters for motion
control and the control loops of the feed
motors. Machine tools are frequently
evaluated by their range of possible
finished parts. Each machining task mustbe performed so that highly dynamic
movements do not produce disturbancefrom machine vibration. The control must
therefore be integrated in the machine toensure high resilience in any machining
task.
Machine tool users require the control toenable them to take into account job
requirements for machining time andworkpiece accuracy. The specified accuracy
requirements must be met with the very
first part without time-consuming tests.
The requirements must be defined in theNC program to ensure a permanent
assignment to the work order. Moreover,to keep the production times for molds anddies in an acceptable range, free form
surfaces are frequently milled with pathdirections reversed between passes.
Meanwhile, the control must generate
reproducible tool paths when approaching
contour elements from opposed directions.
Otherwise one must expect a loss insurface quality.

Influence of data processing on
workpiece accuracy
The manufacture of components withmetal-cutting operations requires
numerous intermediate steps in which thedata of a CAD model geometry istransformed into tool paths:• CAD (Computer Aided Design):
The workpiece contour is normallymodeled with NURBS (non-uniform
rational B-splines). NURBS makes it
possible to mathematically describe
free-form surfaces.
• CAM (Computer Aided Manufacturing):
The tool path elements are calculated
point by point under consideration of themilling strategy and tool compensation
values from the CAD geometry, whereby
the predefined chordal deviation (modelaccuracy) determines the distance
between points.
• CNC (Computerized Numerical Control):
The NC program is converted point forpoint into axis movements and velocity
profiles.Here preset path tolerances aretaken into account. To attain high surface
definition, the deviations betweenadjacent milling paths must remain
significantly smaller than the defined
path tolerances.
• Mechatronics:
The axis movements are available in a
fixed time grid as nominal and actual
movements and are converted over themachine geometry into tool andworkpiece movements. The following
error of the feed axes, deviations fromthe nominal geometry of the machine,
thermal influences and vibrations in themachine frame and motors can reduce
the workpiece accuracy.

CAD

DesignPath generation

CAM
Tool compensation
NC program interpreter
Path control
CNC
Tolerance monitoringVelocity profiles
Mechatronics

Feed rate control
Machine and drives

The need to optimize machining times,
surface definition and workpiece accuracy
results in the following basic requirementson the CNC:
• Effective monitoring of contour
tolerances• Exact reproduction of adjacent paths
after direction reversal• Avoidance of vibration from highly
dynamic movements
For two-dimensional tool movements, theinfluences of the data processing chain on
the workpiece accuracy can be inspectedwith a KGM 182 grid encoder fromHEIDENHAIN. The characteristics of motion
control by a HEIDENHAIN iTNC 530 can beillustrated with a demonstration unit on agantry machine. The KGM serves as thebasis for evaluation of attainable contouraccuracy.

Fast, Accurate, True to Contour– High Speed Milling with the iTNC 530

Effective control of contour tolerances
NC programs for free-form surfaces areusually created with a CAM system andconsist of simple line segments.
HEIDENHAIN controls automatically smooththe block transitions while the tool moves
continuously on the workpiece surface.
This automatic smoothing is controlled byan internal function that monitors thecontour deviations. This function (Cycle 32)
enables the user to define the permissible
contour deviation as desired. The default
value is defined by the machine tool builder
in a machine parameter of the control(typically 0.01 to 0.02 mm). In addition, thetolerance also affects the traverse paths on
programmed circular arcs.

Figure 2: TCP tool path for a ball-nose cutter

On free-form surfaces, the deviation fromthe CAD model geometry can, in the worst
case, consist of the sum of the defined
contour tolerance and the chordal deviation
defined in the CAM system. The result on
the workpiece depends in the end on thetotal characteristics of the machine and thevalues adjusted for the jerk and accelerationof the feed axes.

3a: Tolerance 0.01 mm

The workpiece corner shown in theillustration requires a circular arc path by
the center of a ball-nose cutter (TCP = Tool
Center Point, see Figure 2). Withoutsmoothing of the nominal path data, theY axis of the machine would have toaccelerate abruptly at the transition point.
The resulting jerk causes significant
oscillations of the machine. Moreover, thephysical limits of normal drives prevent
them from generating such virtually infinite
jerk. Without expanded measures in pathcontrol, the unavoidable result is contour
deviations that can assume significant
proportions, depending on the change ofcurvature and contouring speed.
The path control of the iTNC 530 smoothes
the jerk and complies with the given
contour tolerance even during strongchanges in contouring speed (Figure 3).
If a larger tolerance can be defined, theproduction times can be shortened
significantly. In the example shown here,
the machining time is reduced by about
12 % by increasing the contour tolerance
from 0.01 mm to 0.02 mm.

DetailSpherical
cutterWorkpiece
Tool path(with radius compensation)[µm]
20100–10–20

3b: Tolerance 0.02 mm[µm]
20100–10–20

Figure 3: Enlarged detail showing the contour-monitored nominal path of the TCPHigh reproducibility of adjacent paths inalternating directions
Figure 4 shows a detail of a workpiece andthe corresponding line blocks of the TCP
path. Neighboring paths were milled
efficiently with reciprocating movements(multipass milling with direction reversal).
The individual passes consist of only fewstraight-line blocks with very different
lengths. The chordal deviation set in theCAM system is 3 µm.

ForwardBackward

Figure 4: Workpiece contour and associated line blocks with reciprocating traverse.
The points represent the data points in the program.

Figure 5 gives an enlarged representation of
the tool path deviation from the programmedcontour. The deviation is shown withrespect to the ideal line-to-circle transition,
whereas the part program (Fig. 4) exists ofline blocks with a chordal deviation of 3 µm
from the model. The chordal deviation
affects only the curved areas and issuperimposed on the contour tolerance set
in the CNC. HEIDENHAIN controls attainvery high reproducibility in multipass
movements with direction reversal (Fig. 5).
Contour deviation between forward andbackward paths stays negligible, resulting
in very high workpiece surface definition.

ForwardBackward[µm]
20100–10–20

Figure 5: Deviation of the tool path in the curved area of the workpiece contour(feed rate = 10 m/min, tolerance = 0.01 mm)

The workpiece photographs in Figure 6
show the difference that can be made by
optimized motion control. The free-form
surfaces were machined in reciprocating
passes (programmed feed rate 10 m/min,finishing allowance 0.1 mm). The surface
quality of the workpiece shown in Fig. 6a isunacceptable. The machining result using
an iTNC 530, shown in Fig. 6b, illustrates
the high reproducibility of adjacent paths.

6a6b

Figure 6: Multipass process with direction reversal, reproducibility of adjacent cutter paths
6a: Deviations between adjacent paths result in a poor surface
6b: Milling result with an iTNC 530: Consistent surface with forward and backward
traverseEffective avoidance of vibrations duringhighly dynamic motion
The feed velocities required for HSC milling
technology place machine controls before
a great challenge. Short machining times
can be achieved only at higher meancontouring feed rates. However, wherever
the milling path includes small radii, thevelocity has to be drastically reduced inorder to keep path deviations within thepermissible tolerance band. In addition,
the accelerating and braking motions cancause frame vibrations that impair
workpiece surface definition.
Jerk and acceleration are smoothed in theextraordinary motion control offered by
HEIDENHAIN. This can suppress machine
vibrations very effectively. If necessary, thecontrol automatically reduces theprogrammed feed rate to reduce theexcitation of vibration to a minimum.
Effective prevention of excessive machine
vibration enables a part program to run atvery high velocity and thereby decisively
reduces machining times.
Figure 7 shows the actual tool path ofa machine tool on a programmed
two-dimensional contour. If the jerk is notsmoothed, the machine's acceleration
phases excite vibrations (Figure 7a). Motioncontrol by the iTNC 530 from HEIDENHAIN
effectively prevents excessive vibration(Figure 7b) Once again, the workpiecesurface shown in Figure 8 clearly illustrates
the benefits of motion control byHEIDENHAIN controls. Movement along
the illustrated circle segments requires achange of axis acceleration at every point,
which usually causes machine vibration(Figure 8a). By smoothing the jerk, theiTNC 530 makes it possible to realize highsurface definition without disturbing effects
from vibration (Figure 8b).

7a7bNominalActualNominalActualFigure 7: Measured actual positions, recorded with a grid encoder at a rounded cornerwithout and with position nominal value filtering (7a and 7b, respectively) of the NC data.

8a8bFigure 8: Effects of machine vibration on a workpiece:
8a: Without jerk smoothing, vibration in the Z axis produces notches in the surface
8b: Motion control by the iTNC 530 systematically prevents surface problems caused byvibration

Summary
HSC milling technology has a decisive
influence on manufacturing processes inmold and die making as well as inaerospace technology. The required feedvelocities place machine controls before agreat challenge. In the conflict of interests
between machining time, contour accuracy
and surface definition, the HEIDENHAIN
iTNC 530 control ensures that machining
complies with selected preferences. Thismeans that the tool paths are planned sothat• machine vibrations are prevented,• accuracy requirements are fulfilled, and• machining time is minimized.

Moreover, the iTNC 530 achieves highreproducibility of neighboring milling paths
to enable users to meet very highrequirements on surface quality andreduce machining time for reciprocating
multipass milling.
The iTNC 530 has set new standards in theharmonization of the control, the drives andthe machine frame. This makes it possible
to manufacture a broad range of componentsof required quality starting with the firstpart.

Consistently Upward Compatible– A Promising Future with HEIDENHAIN Contouring Controls

For over 25 years, HEIDENHAIN has been
providing customers with contouring controls
for milling, drilling and boring. The controls
have undergone continuous development
during this period: many new features havebeen added—also for more complex machineswith many axes. The basic operational
technique, however, has remained the same.
The machinist who has been working at amachine with TNC does not have to relearn.
On the iTNC 530 he immediately uses all of
his previous experience with TNCs, programming and working as before.
2004: iTNC 530
with smarT.NC
2003: iTNC 530 withWindows 2000
2001: iTNC 530

1997: TNC 426 M
TNC 430
1993: TNC 426 C/P

These contouring keys from the TNC 145
are also on the iTNC 530

1988: TNC 407
TNC 415

1987: TNC 355

1984: TNC 155

1983: TNC 150

1981: TNC 145, thefirst contouring control
from HEIDENHAIN

For more information• Catalog: iTNC 530 Contouring Control
DR. JOHANNES HEIDENHAIN GmbH
Dr.-Johannes-Heidenhain-Straße 583301 Traunreut, Germany{ +49 (8669) 31-0
| +49 (8669) 5061
E-Mail: info@heidenhain.dewww.heidenhain.de636 225-21 · 10 · 4/2008 · F&W · Printed in Germany · Subject to change without notice