Maximum Precision in Couplings for CNC Machines

They respond to the need of flexible production management, where intelligent systems are no longer centralized, but integrated into the machines; it is thus possible to create production based on innovation, and not only aimed at reducing costs. Couplings are a fundamental part of their engineering, as they can offer high productivity and reliability.

by Stefano Vinto

Precision, reliability, programmability and high productivity are the advantages of CNC machines, which represent the heart of the manufacturing industry of the present and future. The widespread use and continuous improvements of CNC machine tools have a significant impact on the productivity of the manufacturing industry. The advent of Industry 4.0 today requires further progress of these machines.

Industry 4.0 and cyberphysical production systems

Information Technology, or IT, has led to significant progress and to a radical transformation of the manufacturing industry on a global scale. The use of computers in smart devices has led to an increasing requirement for services and infrastructure in the IT industry, delivered through networks such as the cloud. These smart devices are increasingly interconnected via both local wireless networks and the Internet. In the so-called Fourth Industrial Revolution, the advent of IoT and Ios enabled the creation of networked resources and services for the entire production process, giving rise to Smart Factories and Factories of the Future. In the production sector, cyber-physical production systems (CPPS) play a key role. A CPPS system generally includes smart machines, storage systems and other production facilities. With CPPS systems, production may be planned more flexibly and efficiently.

The evolution of machine tools and the introduction of CNC languages

Machine tools came into being when the tool path was guided by the machine tool itself, replacing that imposed by the human hand. The first numerical control (NC) machines were developed at the Massachusetts Institute of Technology (MIT) in the second half of the 1940s, following some research activities linked to the creation of the components of a new aeroplane designed by Lockheed. These activities led to the adaptation of conventional machines to numerical control. In the decades following the development of the first NC machines at the MIT, the development of the first numerical control programming languages began. Among these, the most commonly used was the G – code, which was standardised and approved as the EIA RS-274-D standard; this standard was then almost entirely transcribed into the ISO 6983 standard. This language is currently used even in the most advanced CNC machines, with few differences and updates by control system manufacturers. The advent of the language itself has thus determined the introduction of CNC machine tools, thanks in part to progress in computers which began in the 1970s. In a modern CNC machine, a microcomputer stores the machining programs previously prepared by the operator and controls the functioning of the machine itself. The further development of CNC machines has also enabled the creation of sets of interconnected machines controlled by a single central computer in order to create coordinated machining programs.
This solution not only provided one of the prerequisites for the introduction of 4.0 Machine Tools, but it also gave rise to Direct Numerical Control (DNC) systems. Modern CNC machines are now essential components of Flexible Manufacturing Systems (FMS) or Flexible Manufacturing Cells (FMC), solutions which allow high levels of flexibility in both large series and small batch production. Modern CNC machines have also radically changed the scope of design tools, giving rise to the CAPP (Computer Aided Process Planning) and CAM (Computer Aided Manufacturing) systems.

Cyberphysical machine tools: vertical and horizontal integration

4.0 Machine Tools define a new class of machine tools, just like the CPPS systems seen above. These systems are characterized by being smart, well connected, widely accessible, with greater adaptability and autonomy. With 4.0 Machine Tools it is possible to achieve both vertical and horizontal integration. Cyberphysical production systems include smart machinery, storage systems and production facilities with end-to-end integration based on ICT technologies. Cyber-physical machine tools (CPMT) are an essential part of CPPS systems. They consist in the integration of a machine tool, machining processes, calculation and networking systems; integrated computers and networking enable the monitoring and control of machining processes, with feedback circuits through which machining processes may influence calculations and vice versa.
Machine tools characterized by vertical integration support end-to-end digital integration throughout the entire engineering process, including the design, process programming, manufacturing and assembly phases. In order for this to be achieved, an internal production environment is required, based on models which: are an integral part of the technical references; evolve through all stages of production; are integrated across all disciplines involved in product development (for instance, design, process programming, manufacturing and inspection). Machine tools characterized by horizontal integration are those which have a high degree of interconnection both with each other and with other plants and production resources (for instance, industrial robots, conveyor belts, on-site measuring systems and computerized systems for controlling production and relevant business processes of the company, respectively called MES and ERP). The main requirements to achieve the appropriate levels of horizontal integration are two: the type of machine-to-machine communication needs to evolve from a syntax-based model to one based on semantics; machine tools must be able to acquire precise and reliable data from machines and/or their components. Such data may be measured directly by sensors or acquired by control systems.
4.0 Machine Tools will therefore be smart machine tools with self-diagnosis, self-maintenance and self-optimization features.

The role of the coupling supplier

The requirements that a component supplier must meet in order to best meet the requirements of machining centre and machine tool manufacturers are manifold: accuracy is particularly important, because the results achieved with linear motors are difficult to surpass. The accuracy of processes and the reduction of allowance margins are therefore crucial. Specifically, couplings must be versatile, with allowances and kinematics adaptable to the physical properties of the machines. Besides, in order to keep abreast with the increasing efficiency required by the growing competitiveness, they need to be resistant, in order to withstand the ever higher cutting and movement speeds of the axes; finally, they must be remarkably resistant to wear and temperature variations throughout the entire life cycle of the plant. These properties are all present in the stainless steel bellows couplings of the BK series manufactured by R+W Italia.
These are very sturdy couplings capable of working at high revolutions per minute, that is, at high speed. They are also precise, resistant to stress, wear and temperature variations: qualities that make them suitable to adapt to the particularly high dynamics of CNC machines.
The precise, backlash-free BK metal bellows couplings have a low moment of inertia, no need for maintenance, virtually unlimited service life and above all high reliability. In addition to metal bellows couplings, R+W provides a complete range of solutions for all transmission and torque limiting requirements, such as EK series elastomer couplings, SK series torque limiters and ZA and EZ series line shafts.
The elastomer couplings in the EK series combine high flexibility with good strength. They dampen vibrations and impacts by compensating for shaft misalignment. Many elements influence the design of elastomeric couplings: factors such as load, actuation and temperature affect the insert’s life span. The elastomeric element is available in different shore hardnesses, to always find a suitable compromise between the damping properties, torsional rigidity and misalignment correction for most applications. SK torque limiters, absolutely backlash-free, allow to protect the motor system in case of overload, disconnecting it from the driven part within milliseconds. Extremely precise, they transmit torque with great accuracy and intervene only when necessary. They also allow quick and easy reassembly as soon as the cause of the overload is removed.
Finally, the line shafts in the ZA-EZ series are ideal for connections with large axial distances, often found in packaging machines. They are easy to assemble and disassemble without moving or aligning the elements to be connected. R+W offers a range of line shafts up to 6 metres long, which do not require intermediate support. Available in special versions in terms of materials, allowances, dimensions and performance, R+W line shafts, if well dimensioned and correctly mounted, do not require any maintenance and last practically indefinitely.