Increasing Six Axis Machine Tools Accuracy with Laser Tracer

Präwest Präzisionswerkstätten, a high-tech machining company based in Bremen (Germany), is a leader in the field of 5-axis milling of complex parts. An innovative measuring method helps the company improve the accuracy of its four biggest machine tools. Präwest captures the machine tools’ volumetric deviations with Etalon’s LaserTracer-NG and compensates them in Heidenhain’s machine tool control through the software option KinematicsComp. The result: Dimensionally accurate workpieces without any readjustments — even in case of demanding freeform parts with diameters of up to 2.5 m.

Präwest’s product portfolio starts where other turning and milling workshops have to give in. “We solely manufacture parts with complex characteristics,” explicates Gaylord Klammt, Head of Business Area Industry at Präwest. “Everything the market considers as exceptional would be a standard issue for us.”  

A true Präwest masterpiece is, among others, a one-of-a-kind shrouded impeller made of stainless steel, which has an external diameter of 1.780m, thus breaking the mold of common dimensioning. Producing such complex parts with highest quality and simultaneously having a lock on the costs would be impossible without a sophisticated manufacturing strategy.

High tech machinery with self-designed 6-axis machines

The large-scale acquisition of top-notch machine tools contributes to Präwest’s high manufacturing quality. More than 40% of the machines are newer than three years. Last year alone, Präwest invested in ten new milling machines. But that was only the beginning. Despite extensive research, Präwest couldn’t track down a perfect machine tool for the manufacturing of large parts. Hence, the precision manufacturer decided to develop its own 6-axis milling machines with parallel axes. The machine tool control comes from Heidenhain.

The iTNC 530 impresses all with efficient tool path planning

We use Heidenhain’s iTNC 530 contouring control,” says Gaylord Klammt. From Präwest’s point of view, the contouring control stands out due to its practically oriented configuration with optimized motion control and jerk limitation, rounding of series of straight-line segments, and contour pre-calculation. “We really like its logic for path planning — or better, for path compensation,” states Gaylord Klammt. “Heidenhain thinks in tolerances, not in functions. The setting options have been designed in a straightforward way, helping us simultaneously push the envelope of contour accuracy and traverse speed in programming. This includes the certainty that adjustments of one particular setting will not have side effects on other elements. This approach corresponds to our very own understanding of efficient process optimization.”

Milling in different sectors led to mismatch

Although Präwest’s 6-axis milling machines were constructed in the finest possible way, their geometrical accuracy was still not yet meeting Präwest’s high demands. The milling experts detected systematic geometry errors of the linear axes and rotary axes. “Consequently, the question was: How do I manage to produce a dimensionally accurate part in six axes of movement when the part is more than two meters long and the accuracy tolerance is only ± 0.05 mm ?”, clarifies Gaylord Klammt.

Measurement system LaserTracer-NG calculates compensation values along all movement axes and transfers them to the control software

ISO verification with Etalon LaserTracer

The measurement equipment manufacturer Etalon provided the answer: The company has developed a self-tracking laser interferometer specially dedicated to increasing the accuracy of machine tools by means of volumetric compensation. With Etalon’s technology, it is possible to calculate exact compensation values for spatial errors in all degrees of freedom. With micrometer accuracy, the system analyses position errors, straightness errors, rotation errors (pitch, yaw and roll), and squareness between the axes in the whole working volume of the machine tool. Angular positioning deviations, axial and radial movements, and wobbling of the rotational axes are comprehensively recorded as well. Thus, the LaserTracer-NG fundamentally differs from conventional laser interferometers that involve elaborate aligning and generate a large amount of work for determining pitch, yaw, and roll.

After measurement data acquisition, Etalon’s software calculates the volumetric compensation data. As it is interfaced with the machine controller, this data gets automatically transferred to the machine.

KinematicsComp corrects the kinematic description of the machine tool

Rotation axis calibration with Etalon LaserTracer

To assure the smooth processing of the compensation data, Etalon works closely with the controller manufacturers. “The software option KinematicsComp, which is available for the Heidenhain TNC 640 and iTNC 530 controls, makes it possible to store a detailed description of the machine errors in the control. Subsequently, KinematicsComp automatically compensates the position errors that result from static errors of the physical axes. It considers the positions of all rotary and linear axes as well as the current tool length,” explains Gero Günther from the control manufacturer Heidenhain. “This procedure increases the precision of the tool center point in the whole working space.”     

Measurement is a “true breakthrough“

To experience the performance of the LaserTracer-NG in connection with KinematicsComp first hand, Gaylord Klammt invited both Etalon and Heidenhain to conduct test measurements.

In the end, the Präwest team was more than impressed by the results. “Suddenly, this milling machine started to produce with geometrical perfection. Furthermore, we were able to understand the weak points of the machine. A true breakthrough with respect to manufacturing quality, contour accuracy, and process reliability! After the test measurement, we quickly decided to acquire a LaserTracer-NG system, plus KinematicsComp.”

Machine accuracy greatly increased

When performing volumetric compensation on a milling machine, Präwest experienced that the geometric accuracy of the workpiece increases significantly, particularly when machining complex parts. “We are speaking about dimensions that would never have been possible with other methods”, states Gaylord Klammt. The improved machining accuracy comes with perfect timing as Präwest is planning to enter a new business area: the prototype manufacturing for the aerospace industry. “Our customers long for the highest precision. Meanwhile, we want to keep up our profitability. Thanks to volumetric compensation, we are in the position to realize maximum quality in production with manageable effort, even in the case of extremely tight geometric tolerances. The technology helps us reliably uphold our reputation as a precision manufacturer.”

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