Position sensors improve efficiency in Grinding Machines.

The requirements for metal and tool pre-machining are increasing continuously. As a manufacturer of grinding machines, Hoffmann Sondermaschinenbau focuses on precision and high surface quality at maximum machining speed. An absolute, non-contact displacement measuring system for grinding head and workpiece positioning has shown to be equally profitable both for the company and its customers. The efficiency of the grinding machines has gone up by 30 %.

Customer-specific grinding machines
The fully hydraulic cut-off grinding and universal grinding machines make Hoffmann Sondermaschinenbau incorporate considerable know-how and eight years of development work. An utmost variety of blanks are machined on fully or semi-automatic machines - from ultra-fine solid carbide rods to high-speed steel rods. Depending on the machine type, the workpieces are cut to size or trimmed to shape.

(Fig. 1)

Increased Efficiency
Maximizing the economic benefit of the machine - this is the ultimate ambition of managing director Oliver Hoffmann and his team. And to meet the customer requirements for minimum tolerances, short cycle times and high availability. "We are particularly proud of the high precision offered by our grinding machines", is the managing director's explanation for the success of his automatic grinding machines. "The precision we offer has made us a market leader. And now we have been able to achieve a significant reduction of the machining time required for each work piece."

Position measuring systems are used to supervise the motion axes of all fully automatic CNC grinding machines. Formerly, Hoffmann used glass scales and magnetic tapes for this purpose. Very soon, however, it turned out that these devices are unable of leading to the desired measurement results. They were subject to contamination by dust and needed frequent maintenance. The resolution of the magnetic tape was insufficient and unable to satisfy the exacting quality demands.

Because of a business partner, Oliver Hoffmann's attention was drawn to Temposonics® position sensors made by MTS Sensor Technologie in Lüdenscheid: "I was quite enthusiastic about this absolute, non-contact and mechanically uncomplicated measuring principle when I first saw it." After the first tests carried out in the field, all machines were changed successively to incorporate Temposonics® R-Series sensors.

(Fig. 2)

Using the magnetostrictive position transducers brings a bundle of advantages for both Hoffmann and its customers. "After having integrated sensors from MTS, the profitability of our machines has grown by 30 % on average," says Oliver Hoffmann. "The sensor is actually the most economic component of the machine."

Cutting and grinding
All grinding machines manufactured by Hoffmann Sondermaschinenbau are tailored to customer specification and are particularly adapted to the material and work piece to be machined. A large number of components of these machines, such as hydraulic cylinders, come from Hoffmann's own production at the Sternenfels site in Baden-Württemberg

The fully automatic S-V7 CNC cut-off grinding machine was developed for high-precision carbide or HSS rod cutting and surface grinding. Work pieces with a diameter from 3 to 20 mm and a length up to 330 mm can be machined in automatic operation. Before cutting them to size, the rods are fed into a prism and placed into the required position in front of the grinding head by a material pusher. The pusher is driven by a hydraulic cylinder with integrated Temposonics position sensor. The position sensor monitors the material feed and ensures highly accurate positioning of the blank in front of the grinding wheel, whereby the position is programmable freely and individually as required. A second MTS position sensor is integrated into the hydraulic cylinder of the grinding saddle and measures the horizontal displacement of the grinding head precisely.

(Fig. 3)

For various grinding processes, e.g. points and chamfers for technical purposes, contours, transverse grooves, cooling channels or individual special solutions on a carbide rod, Mr. Hoffmann recommends the S-V101: "This universal grinding machine has been developed specifically for rod end machining. Double-end machining is possible by rotating the rod in a reversing station. The various grinding operations can be combined freely as required. Due to the modular construction of the machine, it may be custom-made according to the specific requirements." During the machining operation, workpiece positioning and the movements along the ginding axes are monitored by four MTS sensors in total.

Integrated in the cylinder
As opposed to externally fitted position measurement systems, the magnetostrictive Temposonics sensors are mounting safely inside the hydraulic cylinder. Oliver Hoffmann confirms: "By integration in the cylinder, the sensor availability and thus the availability of our machines has improved significantly."

(Fig. 4)

For direct stroke measurement in the cylinder, the pressure-resistant sensor rod containing the sensing element immerges into the open piston rod. The sensor head, a rugged aluminum housing, accommodates the complete electronics for active signal conditioning. Dual encapsulation ensures high safety of operation and optimum protection against electromagnetic interference. A ring-shaped permanent magnet mounted on the bottom of the piston travels non-contact along the sensor pipe and transmits the measuring point to the sensing element using magnetomechanical effects.

Maintenance-free without sealing air
The magnetostrictive sensor operating principle ensures a long life cycle without wear of the sensing elements. Since the position is detected through the sealed sensor housing without contact, the ingress of dust is prevented. Due to protection type IP68, the position sensors operate safely also in a difficult environment. Cost-intensive maintenance and regular cleaning of the position measuring system are not necessary. Managing director Oliver Hoffmann declares: "MTS sensors just don't get soiled. In the past, customers used to call us for cleaning or repairing the position sensors. And, of course, for tedious and time-consuming re-calibration."

Not only the maintenance expenditure, but also the downtimes of the grinding machines have decreased: "Apart from the costs, machine downtimes are bad for our image. No matter which machine component is defective. MTS sensors are devices we can rely on. Ever since we use them, not a single customer has called us because of a sensor failure."

As Temposonics sensors don't use optical effects for position measurement and have an encapsulated sensor housing, the need for expensive sealing air to prevent penetration of dust is omitted. This is beneficial both for Oliver Hoffmann and his customers: "Without sealing air, we can save machine components and enjoy a price advantage in the market. Our customers can make enormous savings as well. They don't need separate sealing air facilities and can save a lot of energy."

Easy installation
Also during installation, Temposonics sensors help Oliver Hoffmann save a considerable amount of money and time: Easy-to-mount mechanics facilitate the cabling and handling expenditure. "It is a relief that no cables have to be taken into account - no need to worry about the cable run. Thus we have become more economic also from the machine constructor's point of view."

Another plus: Before delivery, the sensors are factory-set according to ordering specification. On-site adjustment is not necessary. Each position sensor is checked using a Laser interferometer during production. Moreover, a reference run when starting the machine or after emergency switch-off is omitted due to absolute position measurement. The absolute measured value is always available without delay. According to Oliver Hoffmann's experience, the sensor position measurement is "immediate and precise also under vibration."

Increased displacement speed
Under the aspect of economy, the most important argument is the dynamics of the MTS sensors. According to Oliver Hoffmann, a significant increase of the machining speed has been realized: "With up to 10 m/min, the displacement speed of our machines has improved by 30 %. For our customers, reduced cycle times mean that they can produce larger daily quantities. The sensor alone would be even faster." Indeed, the displacement speed of Temposonics R series sensors without inhibiting factors is unlimited.

Minimum production tolerances
In addition to the machine speed and availability, the dimensional accuracy of machined work pieces has also increased clearly. The tolerance is as low as ± 25 µm. "This is an accuracy we have never reached when using a glass scale or a magnetic tape system," says Oliver Hoffmann. "We are highly satisfied of the precision offered by Temposonics. Now we can realize considerably enhanced control accuracy." The MTS sensors measure with a resolution of 1 µm and a linearity of ± 0.01 % F.S. Their reproducibility of 4 µm is particularly important for series production, because the result of the position measurement is always exactly the same. When changing models, continuous adjustment of the new machining position is quick and easy. The operator determines the carbide quality, the diameter and the required length. The machine calculates many position parameters independently.

The advantages of MTS sensors for grinding machines manufactured by Hoffmann Sondermaschinenbau are already well known to insiders. Oliver Hoffmann confirms: "Customers who use earlier machines with different position measuring systems already ask for replacement by Temposonics sensors." This is not surprising, because increased availability, better machine speed and precise positioning of blanks and tools ensure optimum economy and grinding results with minimum tolerances.

At-a-glance sensor data
Model: Temposonics RH
(suitable for direct installation into hydraulic cylinders)
Interface: SSI, synchronous mode
Protection: IP68 (cable connection)
Resolution: 1 µm
Reproducibility: 4 µm
Data transmission rate: up to 10 kHz
Special features: Internal linearization


For more information, please contact :-

MTS Sensors Technologie GmbH & Co. KG
Tel: +49 2351 95 87-0
Fax: +49 2351 56 491

August 2009

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