A flexible manufacturing system (FMS) is a highly automated Group Technology (GT) machine cell, consisting of a group of processing stations (usually CNC machine tools), interconnected by an automated material handling and storage system, and controlled by an integrated computer system. An FMS is capable of processing a variety of different part styles simultaneously under NC program control at the different workstations.
An FMS relies on the principles of group technology. No manufacturing system can be completely flexible. It cannot produce an infinite range of parts or products. There are limits to how much flexibility can be incorporated into an FMS. Accordingly, a flexible manufacturing system is designed to produce parts (or products) within a range of styles. sizes, and processes. In other words, an FMS is capable of producing a single part family or a limited range of part families.
Flexibility and Automated Manufacturing Systems
Flexible manufacturing systems vary in terms of number of machine tools and level of flexibility. When the system has only a few machines, the term Flexible manufacturing cell (FMC) is sometimes used. Both cell and system are highly automated and computer controlled. The difference between FMS and FMC is not always clear, but it is sometimes based on number of machines (workstations) included. The flexible manufacturing system consist of four or more machines, while a flexible manufacturing cell consists of three or fewer machines. However, this distinction is not universally accepted.
To qualify as being flexible, a manufacturing system should satisfy several criteria. The tests of flexibility in an automated production system are the capability to;
- Process different part styles in a nonbatch mode
- Accept changes in production schedule
- Respond gracefully to equipment malfunctions and breakdowns in the system
- Accommodate the introduction of new part designs.
These capabilities are made possible by the use of a central computer that controls and coordinates the components of the system. First two criteria are most important while last two are soft criteria and can be implemented at various levels of sophistication. If the automated system does not meet these four tests mentioned above, it should not be classified as a flexible manufacturing system or cell.
Integrating the FMS components
An FMS consists of hardware and software that must be integrated into an efficient and reliable unit. It also includes human personnel. In below section, we will learn more about these components and how they are integrated.
FMS hardware includes workstations, material handling system, and central control computer. The workstations are CNC machines in a machining type system, plus inspection stations, parts cleaning and other systems as needed. A central chip conveyor system is often installed below floor level.
The material handling system is the means by which parts are moved between stations. The material handling system usually includes a limited capability to store parts. Handling systems suitable for automated manufacturing include roller conveyors, in-floor towline carts, automated guided vehicles, and industrial robots. The most appropriate type depends on part size and geometry, as well as factors relating to economics and compatibility with other FMS components. Non-rotational parts are often moved in a FMS on pallet fixtures, so the pallets are designed for the particular handling system, and the fixtures are designed to accommodate the various part geometries in the family. Rotational parts are often handled by robots, if weight is not a limiting factor.[AdtoAppearHere]
The handling system establishes the basic layout of the FMS. Five layout types can be distinguished:
- open field
- robot-centered cell
All these basic layouts are shown below in same order.
The in-line layout uses a linear transfer system to move parts between processing stations and load/unload station(s). The in-line transfer system is usually capable of two-directional movement; if not, then the FMS operates much like a transfer line, and the different part styles made on the system must follow the same basic processing sequence due to the one-direction flow. The loop Layout consists of a conveyor loop with workstations located around its periphery. This configuration permits any processing sequence, because any station is accessible from any other station. This is also true for the ladder layout, in which workstations are located on the rungs of the ladder. The open field layout is the most complex FMS configuration, and consists of several loops tied together. Finally, the robot-centered cell consists of a robot whose work volume includes the load/unload positions of the machines in the cell.
The FMS also includes a central computer that is interfaced to the other hardware components. In addition to the central computer, the individual machines and other components generally have microcomputers as their individual control units. The function of the central computer is to coordinate the activities of the components so as to achieve a smooth overall operation of the system. It accomplishes this function by means of software.
2. FMS Software and Control Functions
FMS software consists of modules associated with the various functions performed by the manufacturing system. For example, one function involves downloading NC part programs to the individual machine tools: another function is concerned with controlling the material handling system; another is concerned with tool management; and so on. Table 40.4 lists the functions included in the operation of a typical FMS. Associated with each function is one or more software modules. Terms other than those in our table may be used in a given installation. The functions and given modules are largely application specific.
3. Human Labor
An additional component in the operation of a flexible manufacturing system is human labor. Duties performed by human workers include;
- loading and unloading parts from the system
- changing and setting cutting tools
- maintenance and repair of equipment
- NC part programming
- programming and operating the computer system
- overall management of the system
Applications of Flexible Manufacturing Systems
Flexible manufacturing systems are typically used for mid-volume, mid-variety production. If the part or product is made in high quantities with no style variations, then a transfer line or similar dedicated production system is most appropriate. If the parts are low volume with high variety, then numerical control, or even manual methods would be more appropriate. These application characteristics are summarized in Figure below.
Flexible machining systems comprise the most common application of FMS technology. Owing to the inherent flexibilities and capabilities of computer numerical control, it is possible to connect several CNC machine tools to a small central computer, and to devise automated methods for transferring workparts between the machines. Figure below shows a flexible machining system consisting of five CNC machining centers and an in-line transfer system to pick up parts from a central load/unload station and move them to the appropriate machining stations.
In addition to machining systems, other types of flexible manufacturing systems have also been developed, although the state of technology in these other processes has not permitted the rapid implementation that has occurred in machining. The other types of systems include assembly, inspection, sheet-metal processing (punching, shearing, bending, and forming), and forging.
Most of the experience in flexible manufacturing systems has been gained in machining applications. For flexible machining systems, the benefits usually are;
- higher machine utilization than a conventional machine shop-relative utilizations are 40-50% for conventional batch type operations and about 75% for a FMS due to better work handling, off-line setups, and improved scheduling:
- reduced work-in-process due to continuous production rather than batch production
- lower manufacturing lead times
- greater flexibility in production scheduling