Performing processing of both small and large batches. Families of similar workpieces, but also morphologically different. Same materials and much more. Here are the duties to be performed by the transfer machines. With flexibility, accuracy, output and reliability.
Transfer machines fall within
the more general category of automatic machines, interpreting a more advanced concept of machine tools, in accordance with the changing needs of the market. Over recent years, companies had to deal with a highly competitive often nervous market, and had to “invent” new strategies to keep pace with the changing needs. Beyond processes and staff reorganization, innovation and diversification have had a leading role. Flexibility and operational efficiency have become must-haves, a sort of “password” to stay in business in a competitive manner, through continuous technological innovation, offering increasingly performing solutions, which were probably unimaginable only a few years ago. Transfer machines are suitable to this scenario, thanks to their ability to perform different processes on different workpieces, differing as for geometries, sizes and materials. It is not easy. It is a continuous challenge.
Automated machines and transfer
When talking of automatic machines, the first definition regards their operating architecture, i.e. the internal organization of the means to “transfer” workpieces and the means for working (or, more generically, operate) on them.
The architecture is defined in light of the transfer mode of the workpiece and the type of transfer means, the number and type of present Specific Machine Tools (i.e. tools which implement a given function provided by the production cycle), the number of Productive Units simultaneously machined by a Specific Machine Tool, the type of operational coordination among the various Specific Machine Tools.
In automatic machines two macro groups of architectures may be identified, according to the motion of the product, which can be either continuous or intermittent. In the case of intermittent motion, the various pieces of Specific Machine Tools operate on workpieces at rest, while in the case of continuous motion, the different Specific Machine Tools work on moving products. Machining centers and transfer machines are related to the architecture with intermittent motion; in particular, the transfers fall within the machines with synchronous intermittent transfer.
The limit of transfer machines has functional nature and is related to the fact that the transfer takes place only after all the Specific Machine Tools have performed their task. Obviously, in case of jams or failures of the tool, the transfer of all Production Units does not occur and the machine blocks. As for reliability, the machine can be considered as a serial system of elements, each with its own degree of reliability, for which the stop or malfunctioning of a subset is followed by the stop of the entire system. In order to improve total reliability, besides acting on the reliability of individual subsets, which must be high and as homogeneous as possible, it is generally recommended to have a restricted number.
To be remembered: the total reliability of a serial system is always lower than that of the subset with lower reliability, hence the need for high and homogeneous reliability.
Operational saturation is fundamental to make the machine efficient. In the design of transfer machines, the most critical operation in terms of duration is detected and, on the basis of this assessment, the necessary dwell time is set. The dwell time will then be added to the transfer time which is a function of the step, the mass to be transferred, from the stability characteristics and the motorization system. If in the cycle there are very short operations that allow for the execution in one single station, of course, the execution may be performed provided that the sum of the execution time is less than (or equal to) the dwell time assessed. It should be noted that the advantage which can be obtained is not so much in terms of increase of productivity, but rather in the reduction of machine size.
General construction aspects
There are two basic configurations for transfer machines: with in-line arrangement and with circular arrangement. In the case of in-line provision, stations are arranged to operate on product units which translate according to a rectilinear direction, advancing with constant pace thanks to a system capable of generating a linear intermitted motion.
The circular arrangement provides that the product units are located on the worktable in angularly equidistant points. Even in this case the table is equipped with intermittent motion. During processing, the workpiece moves in successive positions in a circular trajectory. The table where the trajectory is located can be either vertical or horizontal: the vertical solution is generally preferred when the moveable masses are small, as well as the trajectory radii. In the designing phase, much importance is given to the choice of the motion law and to the motion generating systems.
Transfer machines with line transfer have different strengths, among which the absence of centrifugal forces (if there are no curved sections), the good ratio moving masses – total mass of the transport organs, the ability to manage even big-sized pieces, the accessibility to the various stations and the fact that, as the number of stations increases, the growth of dimensions and inertias is still acceptable since the increase in size occurs only in the longitudinal direction. By contrast, the longitudinal dimensions are important, the structure is complex and, in certain configurations, it uses only a portion of the available stations, the positioning accuracy may be compromised by play and/or wear. In the case of machines with circular path transfer, the transfer element is constituted by a rotary table which rotates around the central axis.
In the case of machines with transfer according to circular path, the transfer element is constituted by a rotary table which rotates around the central axis. The number of possible stations is estimated between 4 and 32, but typically we remain between 6 and 12 this because, as the number of stations increases, with unaltered pitch, the radius of the table increases linearly, but the mass increases according to cubic law and inertia parameters have increased with fifth order law. The movement with circular path has several strengths, such as a marked positioning accuracy due to the absence of backlash and wear of the rotary table which has no joints. The structure is simple and rigid, and has reasonably affordable costs. On the other hand, there is the presence of centrifugal forces on the Production Units. And the high inertia of the table as the number of stations increases, as well as the dimensions which considerably increase as the number of stations increases, due to the poor use of space in the central table. Accessibility is generally inferior compared to in-line transfer. This system is not suitable to the transfer of big-sized workpieces due to the high dimensions which both table and inertias should have.
Productivity, efficiency… and much more
Each application has unique needs, which in any case must be carefully considered in order to provide tailor-made solutions. Whatever the specific need, productivity and efficiency are a common background which is accompanied by flexibility. Hence, the need to provide high quality standards with innovative solutions capable to allow for continuous work cycles, in three shifts, seven days a week, with increasingly compressed cycle times and which maybe allow for unattended work. One should therefore focus on reliability, which can only be achieved with careful planning. But this is not enough. The environment and energy efficiency are hot topics today, to be pursued with tenacity, by optimizing consumption and studying increasingly efficient technologies. And aesthetics. Often considered as the last of today’s needs, as it has been highlighted by research carried out in several countries, machines’ aesthetics is undergoing re-evaluation. As a matter of fact, besides the technological level, a satisfying look helps convey a sense of reliability and accuracy, creating a man-machine synergy.
Direct Drive Technology
The principle at the basis of the Direct Drive technology, or Direct Transmission, for table rotation expects the energy required for movement to be generated by an electric servomotor, without mechanical systems, gearboxes, gears, belts, etc., for motion transformation. Compared to conventional solutions, the Direct Drive system turns out to have a simpler mechanical structure, resulting in greater reliability and repeatability of the performance. Since there is no gearbox, all the connected problems do not exist, such as wear, friction, etc., with resulting benefits from both transmission and energy efficiency. The reduced mechanical complexity leads to a reduction of the components, and consequently to a reduction of the direct costs of purchasing and installing, besides the economic benefits as a relapse of the enhanced performance.
Transfer machines in short
It is a particular combined machine, where the workpiece, which moves according to a pre-set rate, reaches the various production units, where the various operations are performed.
The elements characterizing a transfer are the systems which ensure the correct positioning of the workpiece, the operating units responsible for handling and advancement, possibly grouped in a station to allow simultaneously very different processing, and organs which transfer the workpiece to various stations, ensuring its stability and in synchrony with the processing times. Even in the most advanced transfer machines, tool change is automatic.
Linear and circular: different trajectories, different “characters”
Transfer machines with line transfer have different strengths, among which the absence of centrifugal forces (if there are no curved sections), good ratio moving masses – total mass of the transport organs, the ability to manage even big-sized pieces, accessibility to the various stations and the fact that, as the number of stations increases, the growth of dimensions and inertias is still acceptable since the increase in size occurs only in the longitudinal direction. By contrast, the longitudinal dimensions are important, the structure is complex and, in certain configurations, it uses only a portion of the available stations, the positioning accuracy may be compromised by play and/or wear. Movement with circular path has several strengths, such as a significant positioning accuracy due to the absence of backlash and no wear of the rotary table which has no joints. The structure is simple and rigid, and has reasonably affordable costs. On the other hand, there is the presence of centrifugal forces on the Production Units. And high inertia of the table as the number of stations increases, as well as the dimensions which considerably increase as the number of stations increases, due to the poor use of space in the central table. This system is not suitable to the transfer of big-sized workpieces due to high dimensions which both table and inertias should have.