Machine Tool Automation and Productivity
By Ashok Kumar
Selection of the appropriate level of automation for a particular manufacturing situation is determined largely by batch sizes and variety of parts to be processed.
We can measure the efficiency of a manufacturing facility in three basic ways, that is, manufacturing cost, productivity and profit. Manufacturing cost involves not only the cost of machining, assemblying etc. btut also the cost of moving the components from machine to machine and temporary storage. Involved manufacturing processes affect it and how these processes are integrated or arranged in the factory. Other factors affecting it are the labour costs and the quality of prodcut design.
Productivity refers to the efficiency with which labor is utilized. It generally means the average output per man-hour. Increase in productivity will result in increased profits and will make the end product competetive. Machine tool automation is one of the ways of increasing overall productivity. We can define automation as any means of helping so that staff performs their tasks more efficiently. We will consider here machine tool automation.
A general purpose machine tool is installed with drive motors and controls to operate the spindle and slides automatically and to switch on or off the various auxiliary functions of the machine. Transducers are installed to feed back data on the position and velocity of, for example, the side ways to the controller.
Numerically controlled machine tools range in complexity from simple drilling machines to complex machines that utilize a number of controlled machines. The majority of NC machines are basically similar in configuration with general purpose small NC machine tools. The most versatile general purpose small NC machines are vertical milling machines. The most complex NC machine tools are so-called NC machining centers. These machines incorporate multi-axes control and usually horizontal-or vertical –spindle milling machines, fitted with high capacity tool magazines and automatic tool changers. The high capacity tool magazines increase the flexibility of the machines, by enabling complex parts to be processed without any change of tools. The objective in using these machine tools is to carry out a large amount of machining in one work piece set up, and they are capable of performing a wide varity of operations including milling, driling, tapping and boring. Additional motions, using for example, indexing tables, are usually possible so that several orientations of the work piece can be presented during the program.
Selection of the appropriate level of automation for a particular manufacturing situation is determined largely by batch sizes and variety of parts to be processed. The more automated machines are not economical for very small batches as the production costs for set up and programming must be spread over a small number of components. For larger batches, the reduced cycle times for the automated equipment compensates for greter production costs. This result is typical and shows that manual machines are most suitable when only a few components are required and that NC machines are economical for medim batch quantities, with more dedicated automation restricted to large quantity production.
Batch products can be automated only by using programmable devices that are perfectly controlled by a central computer. One of the greatest problems in realizing the objective of complete computer controlled manufacturing facilities is the development of genral purpose, programmable handling devices. One such device is industrial robot. A robot is capable of motion in many axes as per design and handling heavy items. In adopting a robot to a new task, the grippers must usually be redesigned to accommodate the new items to be handled, and the robot must be re-programmed. A coomon task for robot is loading work pieces into the machine tool and unloading the machined components.
FLEXIBLE MANUFACTURING SYSTEMS
A flexible manufacturing system (FMS) is a group of numerically-controlled machine tools, interconnected by a central control system. The various machining cells are interconnected, via loading and unloading stations, by an automated transport system. Operational flexibility is enhanced by the ability to execute all manufacturing tasks on numerous product designs in small quantities and with faster delivery. It has been described as an automated job shop and as a miniature automated factory. Simply stated, it is an automated production system that produces one or more families of parts in a flexible manner. Today, this prospect of automation and flexibility presents the possibility of producing nonstandard parts to create a competitive advantage.
The concept of flexible manufacturing systems evolved during the 1960s when robots, programmable controllers, and computerized numerical controls brought a controlled environment to the factory floor in the form of numerically-controlled and direct-numerically-controlled machines.
For the most part, FMS is limited to firms involved in batch production or job shop environments. Normally, batch producers have two kinds of equipment from which to choose: dedicated machinery or unautomated, general-purpose tools. Dedicated machinery results in cost savings but lacks flexibility. General purpose machines such as lathes, milling machines, or drill presses are all costly, and may not reach full capacity. Flexible manufacturing systems provide the batch manufacturer with another option one that can make batch manufacturing just as efficient and productive as mass production.
OBJECTIVES OF FMS
Stated formally, the general objectives of an FMS are to approach the efficiencies and economies of scale normally associated with mass production, and to maintain the flexibility required for small- and medium-lot-size production of a variety of parts.
Two kinds of manufacturing systems fall within the FMS spectrum. These are assembly systems, which assemble components into final products and forming systems, which actually form components or final products.
A generic FMS is said to consist of the following components:
- A set of work stations containing machine tools that do not require significant set-up time or change-over between successive jobs. Typically, these machines perform milling, boring, drilling, tapping, reaming, turning, and grooving operations.
- A material-handling system that is automated and flexible in that it permits jobs to move between any pair of machines so that any job routing can be followed.
- A network of supervisory computers and microprocessors that perform some or all of the following tasks: (a) directs the routing of jobs through the system; (b) tracks the status of all jobs in progress so it is known where each job is to go next; (c) passes the instructions for the processing of each operation to each station and ensures that the right tools are available for the job; and (d) provides essential monitoring of the correct performance of operations and signals problems requiring attention.
- Storage, locally at the work stations, and/or centrally at the system level.
- The jobs to be processed by the system. In operating an FMS, the worker enters the job to be run at the supervisory computer, which then downloads the part programs to the cell control or NC controller.
BENEFITS OF FMS
The potential benefits from the implementation and utilization of a flexible manufacturing system have been detailed by numerous researchers on the subject.
A review of the literature reveals many tangible and intangible benefits that FMS users extol. These benefits include:
- Less waste
- Fewer workstations
- Quicker changes of tools, dies, and stamping machinery
- Reduced downtime
- Better control over quality
- Reduced labor
- More efficient use of machinery
- Work-in-process inventory reduced
- Increased capacity
- Increased production flexibility
LIMITATIONS
Despite these benefits, FMS does have certain limitations. In particular, this type of system can only handle a relatively-narrow range of part varieties, so it must be used for similar parts (family of parts) that require similar processing. Due to increased complexity and cost, an FMS also requires a longer planning and development period than traditional manufacturing equipment. Equipment utilization for the FMS sometimes is not as high as one would expect. Japanese firms tend to have a much higher equipment utilization rate than U.S. manufacturers utilizing FMS. This is probably a result of U.S. users’ attempt to utilize FMS for high-volume production of a few parts rather than for a high-variety production of many parts at a low cost per unit. U.S. firms average ten types of parts per machine, compared to ninety-three types of parts per machine in Japan. Other problems can result from a lack of technical literacy, management incompetence, and poor implementation of the FMS process. If the firm misidentifies its objectives and manufacturing mission, and does not maintain a manufacturing strategy that is consistent with the firm’s overall strategy, problems are inevitable. It is crucial that a firm’s technology acquisition decisions be consistent with its manufacturing strategy.
If a firm chooses to compete on the basis of flexibility rather than cost or quality, it may be a candidate for flexible manufacturing, especially if it is suited for low- to mid-volume production. This is particularly true if the firm is in an industry where products change rapidly, and the ability to introduce new products may be more important than minimizing cost. In this scenario,
