Optoelectronic safety devices cut machine tool cycle time

The use of industrial safety technology for ensuring machine and system safety and occupational safety in the spirit of the Machinery Directive requires a high sense of responsibility and sound expertise. Regardless of whether for new machines or for modifications to existing systems, the important thing for the designer is to integrate the safety technology into the machine in such a way that optimum productivity, ergonomics and profitability are achieved while incorporating and taking into account the relevant standards and regulations.

Taking the relevant standards into account can, however, become a challenge. The standards are constantly changing; some are replaced and others are added. In the end, the number of harmonised standards is rising. It can therefore be difficult and expensive for a company's engineers to acquire their own safety know-how, particularly when the state of safety technology is also continuously developing.

Leuze electronic is one of the technological leaders in the area of optoelectronic safety sensors, so for this reason the company offers safety consulting within the scope of its Machine Safety Services. Alexander Hiller, who responsible for the development of the new MAKA Systems GmbH DC 7 traveling column CNC machining centre (pictured right), chose to benefit from this safety consultancy service during the implementation of an optional optoelectronic safety device.

Short cycle times

Alexander Hiller explains: "The DC 7 CNC machining centre is very compact, precise and designed for short cycle times. A typical area of application is aluminium machining in the automotive industry. The high precision of the DC 7 is due to the stable design of frame and units carriers, as well as a special clamping system for heavy machining equipment. The impressive productivity results from, among other things, the possibility to process vertically arranged workpieces in parallel. For this purpose, the machine can be equipped with two independently operating five-axis units.

Another reason behind the high performance of the DC 7 is a robust and fast turntable (see right), which enables parts to be loaded while others are being machined. Only the turning time of just two seconds is included in the cycle times. Hiller adds: "Depending on the primary processing time, the opening and closing times may, however, also be relevant for the standard fast-action roller door of the protective device on the turntable. Both opening and closing take three seconds, for a total of six seconds."

With the aim of eliminating these times, Hiller designed an alternative optoelectronic protective system. This is based on vertically arranged safety light curtains, which are mounted on three sides instead of the mechanical enclosure with the fast-action roller door. To protect the area enclosed by the safety light curtain ('stepping-behind' protection), either a horizontal ankle-height safety light curtain or area laser scanner can be used.

Suitable safety components

For the DC 7 turntable, the Leuze electronic MLC 500 safety light curtains were found to be suitable. They conform to type 4 in accordance with IEC/EN 61496 and satisfy the requirement of category 4 and Performance Level PL e according to EN ISO 13849- 1. With robust design in a very slim housing, they form a solid foundation for stable, reliable system operation. Even in the basic version, the MLC 500 safety light curtains offer the necessary functions for the DC 7 point-of-operation guarding, as well as for danger-zone and access guarding with hand and finger detection. For additional, practical function extensions such as automatic start/restart interlock (RES), as well as contactor monitoring (EDM) or, moreover, for various operating modes such as muting, device linking or various scan modes, further device versions (Standard and Extended) are also available.

For 'stepping behind' protection, which can be realised with multiple MLC 500 sensors arranged horizontally, the use of an RSL 400 safety laser scanner is also recommended (see right). With these compact devices, Leuze electronic says it sets a new standard worldwide. With an operating range of up to 8.25m, two independently adjustable configurations and two safety-related switching output pairs (OSSDs) enable the RSL 400 to perform two different protection tasks simultaneously with just one device, while satisfying fully the relevant requirements in EN ISO 13849-1 and the EC Machinery Directive.

Safety distance calculations

Following a meticulous risk assessment and a detailed design process, the main question that remained was the determination of the minimum safety distances and minimum heights of the protective device with respect to approach speeds of body parts and with regard to possibly reaching over. EN ISO 13855 provided the corresponding basis for the calculation. Unlike the more commonly used EN ISO 13857 for physical guards, this standard explicitly addresses electro-sensitive protective equipment. Described in this standard is the procedure for determining the required safety distances between protective field and danger zone as a function of the respective stop times.

Hiller comments: "With Leuze electronic's help, we optimally dimensioned the optoelectronic protective device for the turntable of our DC 7 so that maximum safety is ensured and, at the same time, minimal space is required in the access area." The main goal - elimination of the opening and closing times for the standard roller door - has therefore been achieved. At the same time, the safety light curtains offer considerably more freedom of movement for the operator than the fixed enclosure. Hiller concludes: "A welcome side effect is that, unlike the fully enclosed safety enclosure, the optoelectronic protective device is open above. The heavy machining equipment can thereby be much more easily changed on the turntable by means of a crane."

Follow the links for more information about the MLC 500 safety light curtains and RSL 400 safety laser scanners from Leuze electronic.