Flexible Manufacturing Systems
Transfer lines and Special Purpose Machines (SPMs) based on ‘hard’ automation components, like mechanical cams and limit switches, have long been a part of manufacturing. The idea behind such automation-also called Detroit-type automation after its origin-is that a component with a large and steady demand over a long period of time is best produced on dedicated lines.
The cost of modifying these lines to make them suitable for other components is prohibitive, but this is not required because of the longevity of the component. Also, the one-time cost of designing (or modifying) and debugging the line is easily recovered because of the increase in throughput and the large volumes. The problem that global industry has faced since the 70s is that the number of products suitable for hard automation has gradually withered because of increased competition and drastically shortened product life cycles.
Continuous product innovation necessitated the development of manufacturing systems, which can accommodate manufacturing flexibility in production volumes, product mixes and product design.
Flexibility of product design is achieved by shortening the ‘time to market’, from conceiving the product to launching it in the market. CAE and rapid prototyping are key enabling technologies. FMSs are technological solutions for flexibility of volume and mix.
The principle of an FMS is that automation is embedded into the software that controls the system and its components. Changes in component design are accommodated by changing the software-these changes are achieved much faster than they would be in a hard automated environment.
Processes can be simulated graphically before actually using them in production, making debugging easier and cheaper. Manufacturing tasks can be run on the system components without loading the part to be manufactured.
This gives the manufacturing engineer feedback on whether things are going the way they should-without risk of damage to the part or to the equipment. FMSs – and their components-are justified financially on the grounds that the investment made is largely an up front investment.
Though there are maintenance costs (and these costs are higher than they are for hard automation systems because of the sophisticated equipment used), the incremental cost of changing the product mix and volume is low. Figure 4.6 shows a typical FMS layout. The building blocks of the FMS are:
- CNC Machines in which the operations performed by mechanical tools on the component being manufactured are dictated by reprogrammable control units. Accuracy is very high and a single machine can perform a massive range of operations.
The key advantage, however, is that the time taken to start production from scratch (that is from the time the design has been frozen) is very short. Using CAE and CAPP, the required machining programmes (called part programmes) can often be generated automatically. The programmes can be rapidly tested through simulation and dry runs.
Figure 4.6: Block Diagram of an FMS
- Industrial robots as understood in industrial engineering, very different from the fictional robots of Star Wars. These robots are jointed structures, which can be manipulated through control software to perform simple functions like pick and place or spot welding, depending on the kind of gripper that is attached to the end of the robot arm. In an FMS, they are typically used for loading and unloading components.
- Automated Guided Vehicles are battery-powered and programmable vehicles used to carry components between machines and storage points. If the system is not very complex, conveyors can be used instead of AGV s. The advantage of AGV s is that they provide for flexible routing.
- Coordinate Measuring Machines (CMMs) use a probe operating within a threedimensional coordinate system to measure the geometrical accuracy of components and thus automate the inspection process.
- Automated Storage and Retrieval Systems (AS/RS) are basically flexibly automated warehouses. An AS/RS is defined by the Material Handling Institute (MHI) as ‘a combination of equipment and controls that handles stores and retrieves materials with precision, accuracy and speed under a defined degree of automation.’ This automation is aided by the use of pallets and standardized containers.
The problem with FMS’ is that there are very few examples of successful implementations worldwide, though the usage of their component technologies (mainly CNC machines and robots) has grown at an explosive rate. A successful example of the use of flexible automation is
Motorola’s pager manufacturing line described by Pine (1993), which achieved a ‘zero set-up time, hands off, true lot size of one, asynchronous pull, build-to order manufacturing system.’
The pagers could be produced in up to 29 million combinations and the order fulfillment time was crashed to one and-a-half-hours. Interestingly, Pine notes that some production tasks had to be left manual even though the goal was to automate everything, ‘whether it made sense or not. ‘ The 80s were marked by a tremendous hype over ‘factories of the future’ and ‘lights out factories’.
In one of the most infamous cases in manufacturing, General Motors invested billions of dollars in FMS-type technologies without any substantial return on investment. The problem with such efforts was that they were driven chiefly by a desire for technological nirvana, without a sound understanding of manufacturing basics. A common goal was to eliminate human intervention in the process of working. After the diffusion of lean production methods, executives have realized that there is no substitute for a motivated shop floor worker, willing to learn and improve work processes. As Duimering et al. (1993) have argued, it is important to differentiate between flexible technology and Organizational flexibility. The manufacturing executive’s goal must surely be the latter and not the former. Flexible automation technologies are widely used today-
CNC machines are common in low-volume or high-accuracy environments and robots are used for spot welding-but they are used to augment sound manufacturing processes.
Rapid prototyping is an emerging technology that promises to radically change product development. It takes CAD data to actually generate a three dimensional prototype of the component being designed. This can be achieved in minutes.
In the most commonly used rapid prototyping method, a CAD database is used as an input to generate wax or nylon slices of the component, layer by layer. A laser works on a fine layer of powder, first tracing the boundary of the slice and then sintering the powder particles within the boundary. This process is repeated until a life-size, three-dimensional prototype is complete.
Though prototypes are currently made from a narrow range of materials, such as wax or nylon, the availability of a ‘real’ prototype offers huge advantages for the product development team. To mention a few, the ‘manufacturability’ of the component can be studied more easily by manufacturing engineers, the component can be checked for proper fit with other parts and feedback can be generated from the market on aesthetic aspects. The goal of some companies, as reported in a survey by The Economist (1994), is to move towards free form fabrication (FFF}-a kind of virtual manufacturing. ‘They (free form fabricators) need no factory, no supply chain; just the raw material, the design and, perhaps, an operator. Some types of product-personalized porcelain, perhaps-may well goes all the way from design to department store in the world of information.
Lean Production Tools
Two key lean production shop floor tools are Poka Yoke and SMED. Both of these are attributed to Shigeo Shingo, the famous Japanese industrial engineering consultant who was instrumental in setting up the Toyota Production System. We shall briefly discuss these ideas without going into techniques, which have been more than adequately dealt with by others, including Shingo himself
Poka Yoke devices are ‘mistake-proofing’ devices aimed at making it possible to achieve zerodefects in production. Shingo had originally termed these devices Baka Yoke, which translates into ‘fool proofing’. He changed the name after a worker was offended by the use of the term
Baka Yoke because it seemed to imply that she was a fool. The philosophy of Poka Yoke is very simple. Shingo realized that while human errors are unavoidable, the key to zero defects is to prevent errors from turning into defects. This can be done if processes are made mistake prooftools are used such that the worker either cannot make a mistake at all, or is immediately informed if a mistake has been made. Of course, all this pre-supposes an Organizational climate in which workers are not alienated from management objectives.
Single Minute Exchange of Dies (SMED)
SMED is generally viewed as the key technical breakthrough that enabled the move towards of pull systems and one-piece flow. Again, we refer the reader to more authoritative sources for a detailed technical description of SMED (see Chapter 2 for an introduction). What was revolutionary about SMED, though it may seem commonsensical in retrospect, was Shingo’s outof- the-box approach to the much-studied and little-applied theoretical problem of Economic
Order Quantity (EOQ). In the classical EOQ model, the objective is to find a production (or ordering) lot size at which the total cost of operation in minimized. The two incremental costs that are involved are set-up costs and inventory carrying costs. The conventional wisdom was that set-up costs increase as the lot size decreases, because more time is wasted. (and capacity lost) with each set up. Against this, larger lot sizes imply a larger average inventory and hence a larger inventory carrying cost. The optimal lot size is one which achieves the best trade-off by minimizing While researchers in, the West focused on increasingly sophisticated EOQ models,
Shingo decided to attack the concept of set-up costs. This was in keeping with the lean production philosophy of eliminating waste. Shingo gradually realized that what lead to set-up costs was simply a lack of focus on eliminating all waste of time in set-up activities. When a setup is necessary on a machine, only a handful of activities actually cause the machine to be stopped. Shingo referred to these activities as internal setup activities. Most set-up activities (for example, bringing a jig or a fixture to the machine) are external-they can be performed even when the machine is running. Shingo prescribed a simple set of procedures for minimizing set-up time–list set-up activities, segregate them into internal and external, perform external activities off-line, and most importantly, convert internal activities to external to the extent possible.
Simple but effective methods, like the use of clamps instead of threaded fasteners, were adapted to crash internal set-up activities. The objective was to complete set-ups in single digit minute values-less than 10 minutes. He called this set of procedures SMED after this objective.