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ktr0713Success Story

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Machine tools are robust in their basic construction and therefore almost unbreakable. A new trend shows that there are a few good reasons to let precision machines with retrofitting shine again in new splendor - at least in terms of dynamics and precision. Luhatec has had its very own experience here: in the comprehensive retrofitting of their precision machine tools and retrofitting with modern servo drives, the machine builders will no longer forget that the use of a backlash-free, torsionally flexible coupling from KTR the performance of the machines increased.

ktr10713Many mechanical engineers and special machine builders can be found in the production halls and in prototyping, in addition to a state-of-the-art machine park, there are also niches where older machine tools do their job. In some companies, they only occasionally "have to do" and are therefore gladly still used for very specific tasks. Of course, this is no coincidence and is not exemplary for the application described here. Nevertheless, the precision machine tools produced in the recent past can achieve a very long service life. The experts especially praise the high rigidity of the basic construction of older machines and therefore often hesitate to invest in new equipment. It would take a while for the residual stresses in the new machines to diminish over time.

For these reasons, the retrofitting of high-quality machining centers is currently in vogue. The drives and controls are usually renewed with the aim of improving and updating the precision and dynamics of the machine. A comparatively inconspicuous component in the drive train plays an important role here: The shaft coupling should also be adapted to the changed drive dynamics.

Flexible with identical machines

Those who heed this recommendation can benefit from a true leap in performance of the machine. The responsible persons of Luhatec Maschinenbau GmbH in Bad Bentheim can report about this from first hand. The company offers mechanical machining, mainly with the machining of prismatic parts. Here are, for example, dimensionally accurate attachments of large engines, steering axles for forklifts and a variety of housing for the drives of mobile machines.

For such challenging tasks, Luhatec has the necessary know-how and the right machine park. On eleven identical Scharmann Solon 3 horizontal machining centers, parts weighing up to 2,5 t can be machined with high precision. All machines are equipped with a double pallet changer, high-pressure internal tool cooling and CNC-controlled precision turntable. For smaller components up to 500 kg weight, there are eight Steinel FFZ 260 horizontal machining centers available.

Luhatec deliberately bought the first Scharmann milling centers. "We started 2001 with five machines that we took over from the engine production of an automobile manufacturer, says Erich Luttermann, one of the two managing partners of the company. "The machines have worked there for five years crankcase of twelve-cylinder engines. They are robust, very reliable and the rigidity of the machine bed is very high. "

For complex parts, Erich Luttermann and his business partner Heiko ten Hagen aim to take as many work steps as possible in one set-up to save processing time and thus costs. Another competitive advantage is the high level of technical competence of the company, which employs 32 employees: not only standard repairs such as the regular replacement of ball screws are realized with our own resources. Even with extensive conversions, the production technicians have experience.

Replacement of drive and control technology

That's why Luhatec also started the retrofit project alone, and the managing director reported on the background: "Two years ago, we decided to renew the drives and controls on all Solon machines because the machine's mechanics are still outstanding State. Nevertheless, we needed more momentum and, in addition, wanted to avoid problems with spare parts procurement. "

However, despite the company's extensive engineering know-how, the retrofit project on the first machine ended with a smooth disappointment. "The spindle motor whistled very loudly and no precise machining was possible. Although we now know the machines inside out, we could not find the cause, "says the Managing Partner.

Digital Servo Drives: Damping is important

ktr20713In this situation, Luhatec received support from the servo drive manufacturer. This recommended the mechanical engineers, the torsionally stiff coupling by a backlash free dreleastic "Rotex GS" coupling from KTR. "The engine manufacturer had the experience that digitally controlled servo drives tend to swing when they are designed to be completely stiff. For this reason, he generally recommends using torsionally flexible Rotex GS couplings in the axle spindle drives. "

On this recommendation, Luhatec turned to KTR Kupplungstechnik GmbH in Rheine. The specialist in the design of drive systems in highly dynamic spindle drives developed the first Rotex GS over twenty years ago. Their engineers have designed the right coupling based on the motor data within a short time and found the necessary compromise between stiffness and vibration damping for this demanding application. Since this coupling is a product from the KTR catalog program, the components could then be quickly delivered to Luhatec.

As a backlash-free jaw coupling with defined torsional elasticity, the Rotex GS has proven itself in the highly dynamic drives of machine tools in both original equipment as well as retrofit. The Shore hardness of the sprocket allows the damping properties to be adapted exactly to the drive parameters. Johannes Deister, Product Manager of KTR Kupplungstechnik GmbH: "Our experience shows that with digitally controlled, highly dynamic drives, a certain degree of damping is required, which has a performance-enhancing effect. If the drive system is one hundred percent stiff, for example, using a bellows or steel disc clutch, one would have to artificially limit the acceleration values, and that would be counterproductive. "

Difference like day and night

ktr30713The fact that you need a certain degree of damping and elasticity in highly dynamic positioning drives may seem surprising at first glance. The people in charge at Luhatec were initially skeptical whether the replacement of a torsionally stiff by a torsionally flexible coupling can solve the problems. But after they had replaced the clutch, the machine immediately achieved the expected dynamics and precision. "The result was really amazing - a difference like day and night", Luttermann is still happy today.

The advantages of a retrofitting in practice could be recognized quickly. The machine not only works faster but also more precisely: "We notice that, for example, when milling large fits. Here, the precision is greater, especially at the transitions, because the cross-axis motion sequences are better regulated, "sums up the managing director. Thanks to the larger main memory of the controller, it is now easier to work off demanding tasks such as die-cutting, and the newly installed direct measuring system also contributes to higher precision of the components.

Retrofit will continue

With these benefits in mind, it's no surprise that Luhatec is continuing the retrofit program, increasing the precision and performance of existing machinery. Now that experience has been gained, the number of strokes will be increased: Luhatec plans to retrofit additional Scharmann machining centers and, of course, re-use KTR rotationally flexible Rotex GS couplings for the spindle drive.

Picture above: Luhatec primarily processes drive components that have to meet high demands on precision and dimensional accuracy.

The author Johannes Deister is Product Manager at KTR Kupplungstechnik GmbH in Rheine.

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