By Gaurav Sharma, Rohit Saini 

INTRODUCTION

Development of Computer Numerically Controlled (CNC) machines is an outstanding contribution to the manufacturing industries. It has made possible the automation of the machining processes with the flexibility to handle small to medium batch quantities in part production. Initially, the CNC technology was applied on basic metal cutting machines like lathe machine, milling machine etc. Later, to increase the flexibility of the machines in handling a variety of components and to finish them in a single setup on same machine, CNC machines capable of performing multiple operations were developed. Though advent of CNC machines offers various advantages like increased productivity, consistent quality, reduced scrap rate, higher accuracy, reduced manpower, reduced lead time, just in time manufacturing etc but system is not much flexible.

Over a period of time and with rapid developments in electronics, the machine tool technology has graduated from the concept of stand alone machine tools to system oriented manufacturing. This resulted in the introduction of Flexible Manufacturing Systems (FMS). Thus the emphasis has shifted from mechanical hardware in case of conventional manufacturing to a combination of mechanical hardware, electronics and software which now accounts for 30-50% of the value of modern manufacturing systems.

In the above diagrams, the percentage by values of electronic software and hardware are shown in three different systems.

1. Conventional machine tool.
2. Stand alone CNC machine.
3. Flexible manufacturing systems.

As it is clear from the figure that percentage by value of electronic software and hardware is maximum in FMS. At this stage , we can say that concept of manufacturing has moved from mass production to batch production in medium to large batches and FMS/FMC production systems meet these needs.

WHAT IS FMS ?

A Flexible Manufacturing System consists of a number of processing stations, predominantly CNC machine tools interconnected by an automatic material handling system, served by common tool and workpiece supply system and controlled by a central computer.

It is a highly automated Group Technology machine cell. An FMS is capable of processing a variety of different part styles simultaneously under NC program control at different workstations. No manufacturing system can be completely flexible . It cannot produce an infinite range of products. There is a certain limit of flexibility. In other words, FMS is capable of producing a limited range of part families.

The type of production system stand alone NC type, FMS or transfer line depends on the variety of products and volume of product.

In the mid volume range, advantages of FMS overstand alone NC is that production of several products can be intermixed. Instead of manufacturing on product one at a time on NC machine, various products can be made simultaneously on the system. Set up time for change over is minimized with FMS and average production increases.

ELEMENTS OF FMS

FMS consists of hardware and software that must be integrated into an efficient and reliable unit. It also includes human personnel. Main elements of FMS are:

1. Tool Handling Block.
2. FMS Workstations.
3. Workpiece Handling.
4. Computer Control.

FMS LAYOUTS

The handling system establishes the basic layout of the FMS. The important types layouts are:

1. Loop FMS Layout.
2. Ladder FMS Layout.
3. Open Field Layout.
4. Robot Centered Cell Layout.

Let us discuss each layout one by one.

1. Loop FMS Layout- Basic loop configuration is shown in figure . Parts usually flow in one direction around the loop with the capability to stop at any station. The load/ unload stations are typically located at the end of the loop. A secondary handling system is shown at each workstation to permit parts to move without obstruction around the loop.
2. Ladder FMS Layout- The ladder configuration is the adoption of loop as shown in figure. It contains rungs on which the workstations are located. The rungs increase the possible ways of getting from one machine to next. This reduces the average travel distance, thereby reducing the transfer time between workstations.

Work flow

Open field layout is also an adaptation of loop configuration. It consists of loops, ladders and slidings organised to achieve the desired processing requirements. This layout type is generally appropriate for the processing of large family of parts.

Finally the robot centered cell is a relatively new form of flexible system in which one or more robots are used as material handling system. Industrial robots can be equipped with grippers that make well suited for the handling of rotating parts. FMS layouts designed around robots as the material handling system are therefore used to process cylindrical or disk shaped parts. Work volume of the robot includes the load/unload positions of the machines in the cell.

Material Handling Equipment

The type of material handling equipment that have been used to transfer parts between stations in an FMS include – Roller Conveyors, Cart on track conveyors and other type of conveyor systems; Automated guided vehicle systems and Industrial Robots. All of these handling systems constitute what is sometimes called the primary material systems in the FMS. The primary handling system establishes the basic form of Layout Configuration.

In addition to the primary handling system, many FMS installations makes use of a secondary handling system. Secondary handling system is located at each workstation and in use to transfer work from primary system to the machine tool or other processing station. Its function is to position and to locate the parts with sufficient accuracy and repeatability at the workstation for processing. Buffer storage of parts may also be provided at each workstation by the secondary system.

AGV implies Vehicle guidance and routing.

It includes

1. The vehicle guidance and routing
2. Traffic control and safety.
3. System management.

The term guidance system refers to the method by which the AGVs pathways are defined and the vehicle control system that follow the pathways. There are two principal methods currently in use to define the pathways along the floor embedded guide wires and paint strips. Out of these two, guide wire system is more common in use in warehouses and factory applications.

Guide Wire Methods

In guide wire method, the wires are usually embedded in a small channel cut into the surface of the floor. Channel is typically about 1/8^th inch wide and ˝ inch deep. After guide wires are installed, the channel slot is filled so as to eliminate the discontinuity of the floor surface. A frequency generator provides the guidance signal carried in the wire. The signal is of low voltage(< 40 V), low current(< 400 ľA) and has frequency in the range of 1 to 15 KHz. This signal level creates a magnetic field along the pathway that is followed by a sensor on board each vehicle operation of typical illustrated in the fig. 4.

Two sensor coils are mounted on the vehicle on either side of the guide wire. When the vehicle is moving along a course such that guide wire is directly between the two coils, the intensity of the magnetic field measured by each coil will be equal. If the vehicle strays to one side or another, or if guide wire path curves, the magnetic field intensity at two sensors will be different. This difference id used to control the steering motor, which makes the required change in vehicle direction to equalize the two sensor signals, thereby tracing the different pathways.

Computer Control System

FMS 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 co-ordinate the activities of the components so as to achieve a smooth overall operation of the system. It accomplishes the function by means of application software.

FMS software consists of modules associated with the various functions performed by the manufacturing system. For example one function involves down loading NC part programs to individual machine tools, another function is concerned with controlling the material handling system; another is concerned with tool management and so on. Data and commands are sent from central computer to the individual machines and other hardware components. Data on execution and performance are transmitted form the components back to the central computer.

Human Labour:

An additional component in the addition of an FMS is human labour. Duties provided by human workers include:

• Loading and unloading parts from system.
• Changing and setting cutting tools.
• Maintenance and repair of equipment.
• NC part programming.
• Programming and operating the computer system.
• Overall management of system

Aantages of FMS

With the introduction of FMS, flexibility is introduced in system to change part variety.

1. Higher productivity.
2. Higher machine utilization is achieved.
3. Number of rejections in goods obtained is the minimum.
4. Quality of the product is improved.
5. Better control over the whole Production process.
6. Just in Time manufacturing is achieved.
7. Minimally manned operations.

A study conducted on utilization of stand alone machining centre in Europe indicated a utilization of 2554 hrs out of 8760 hrs available, which works out to be 29 %. Idle time includes Sundays, holidays etc. With the application of FMSs, the utilization of CNC machining centres increased to 53 % from 29 %.

Applications

FMS has found its applications in fields where a fairly large volume and a wider variety of parts are required. It is for this reason that it is used in :

1. Manufacturing of Tractors, Earthmoving and Agricultural machinery.
2. Production of Defence related parts and Aerospace equipment.
3. Manufacturing of Machine tools and Automobiles.

Recent Developments

FMS is not only adopted in advanced countries but also in the Indian industry. In the last five years, some of the leading Automobile, Machine Tools and Defence sector industries have installed FMSs.

HMT has already developed an FMS on pilot basis and has undertaken the supply of FMS to the Defence Sector under a joint working agreement with an overseas manufacturer. Though the FMS requires higher initial investment, its benefits are substantial in the long run. Therefore it can be predicted that the adoption of FMSs in the Indian Industry is likely to grow in the coming future as the demand for flexibility and productivity is ever increasing.