A key factor in machining quality

Technical Article

HEIDENHAIN Axis Motors
for Machine Tools

HEIDENHAIN axis motors for machine tools

A key factor in machining quality
For greater accuracy and surface quality, machining firms invest a lot of timeand expense in machine tools, measurement technology, tooling, training, andcontrols with specialized functionality and options. But machining quality is alsostrongly affected by the axis motors.

Resistant to disturbances
A system’s disturbance resistance is important for its ability to handle the disturbances arising from vibrations or
milling forces. The higher the disturbance resistance, the better. Disturbance resistance is heavily influenced by
the inertia ratio between the motor and the moving mass (load) of the feed axis. Thus, the larger a motor isrelative to its load, the more disturbance resistance a system has when subjected to fluctuating milling forces orvibrations. Figure 1 shows how high a sudden change in load must be in order to temporarily cause a given
position deviation in a drive system (as a function of the inertia ratio between the motor and the load).

To achieve exceptional machining results, all of a machine tool’s components must be designed and wellmatched for this demanding requirement. The axis motors are no exception. Along with the motor’s maximum
torque, other key criteria include the torque ripple and the inertia ratio between the motor and its load. These
criteria directly affect the quality of the workpiece being manufactured.
Axis motors specifically designed for machine tools, such as the QSY series of HEIDENHAIN motors, provide abalanced moment of inertia and very low torque ripple for exceptional machining results and dynamic motioncontrol. Their disturbance resistance and acceleration capability are therefore well balanced. For use in machine
tools, their high-accuracy optical rotary encoders and high mechanical rigidity are also very beneficial.

Figure 1
Required sudden change in load for a given position deviation as a function of theinertia ratio between the motor and its load

If the inertia difference between the motor and its load is high, then the loop gain will need to be reduced.
Lowering the loop gain lowers the disturbance resistance, causing the overall system to react more sensitively
to load disturbances, such as those induced by milling forces or vibrations. Therefore, a balanced inertia ratio
between the motor and the load should be selected for machine tools. A balanced inertia ratio ensuressufficiently high disturbance resistance in order to render the overall system insensitive to external load
influences, thereby eliminating their effect on the machining results while still allowing for high loop gains.

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Dynamic

SmoothA motor designed for the highest possible moment of inertia would contradict the goal of having the highest
possible acceleration capability. That’s because the inertia of the motor has a significant effect on the acceleration capability of the overall system. This direct correlation can be seen in the formula for calculating acceleration
capability and in the graph in Figure 2:

In addition to the above-mentioned external influences, the motor itself can introduce disturbances into thesystem that influence machining and workpiece surface quality. A critical factor in this is the amount of torqueripple, which is induced by the motor even despite a sinusoidal current. As a result, the output torque fluctuatesslightly over the course of a complete revolution of the motor shaft.

ώ=Mmax(JLoad + JMotor)

With a given maximum motor torque Mmax and a given load inertia JLoad, the acceleration capability ώ of theoverall system decreases as the motor inertia JMotor increases. Conversely, a motor with low inertia delivers
high acceleration. That’s because the higher a motor’s own inertia is, the more torque it must produce in orderto accelerate the given load as desired.

Figure 2
The acceleration capability of a motor as a function of the inertia ratio
between the motor and the load

In order to investigate the torque ripple of various motor designs, a comparative measurement was performed.
The Z axis of a machine tool was first driven by a HEIDENHAIN axis motor designed for machine tools and thenby an automation-industry axis motor not designed for machine tools but rather merely adapted for this application. Otherwise, the setup and test conditions were identical. A machining operation was performed for anoblique surface on a sample workpiece. During the face milling operation, the Z axis was moved at a feed rateof F = 6250 mm/min for 60 mm in the Y direction and for 17.5 mm in the Z direction.
These tests confirm the anticipated effect of the torque ripple—not only based on the measured contour
deviation (the difference between the contour actually milled and the desired contour, Figure 3) but also
through the visible effects on the sample workpiece’s surface.

Figure 3
Contour deviations measured during the test: a HEIDENHAIN axis motor with low torqueripple (green line) and an adapted motor with significantly higher deviations (red line);
contour deviation magnified by a factor of 1000

When it comes to torque performance, cost-effectiveness becomes a factor as well: the higher the maximum
torque required, the more expensive the motor will be. This is because higher performance requires more or
better magnetic materials for larger or optimized motors. As a result, acceleration behavior and cost considerations call for a ratio between the motor and the load that is both balanced and well-adapted to the intended
application.

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Figure 5
The HEIDENHAIN motor designed specifically for machine tools
produces an evenly angled surface without visible shading.

Test setup: machining an oblique surface using
differing motor designs for the Z axis

The HEIDENHAIN axis motor optimized for machine tools exhibits low torque ripple and produces an evenly
angled surface without visible shading (Figure 5).
The automation-industry axis motor merely adapted to machine tools shows the effects of torque ripple in theform of clearly visible shading on the oblique surface (Figure 6).

Expressed in measured values, the HEIDENHAIN axis motor
achieves an effective contour deviation of just 0.2 µm at thelevel of visually discernible deviations. By contrast, the effective
deviation of the axis motor adapted to the machine tool is
1.2 µm, which is greater by a factor of six.

Figure 6
Workpiece with clearly visible shading on the oblique surface,machined using an automation-industry motor merely adapted
to machine tools

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