Manufacturing Systems
8-1
Answers to Questions at the End of Chapter 8 Answers to Questions at the End of Chapter 8 8.1
8.1 To some extent, the automatic factory is still valid today. There are three components in an automatic factory – manufacturing, material handling, and the information system. In terms of manufacturing, some decisive factors to justify automation are:
! Volume of production. Economics of scale can be achieved by mass production and the financial benefit can compensate the high capital cost of an automatic factory
! Expensive machinery. Some industries, such as semiconductor, require expensive machinery. By automating, these machines can be fully utilized to reduce production cost.
! Variability reduction. Manual machining, while still within tolerance, often produce parts with high variability. This variability can be reduced significantly by automation.
From a material handling perspective, automation is desirable to reduce cost in time due to savings in labor cost. In addition to cost saving, some product may require careful handling; therefore automation is an alternative to prevent product damage. In addition, the declining costs of computing and data storage continue to fuel the desire to invest in automation.
8.2
8.2 The semiconductor industry would be a good target for the automatic factory. Machinery for semiconductor production cost dearly and should be fully utilized. Product value is also very high; material-handling automation is needed to avert damage.
Another sector would be continuous flow manufacturing such as chemical products.
8.3
8.3 Advantages of automatic warehouse:
! High throughput
! Reduced labor cost
! Elimination of human error
! Reduced material damage
! Greater security
Disadvantages of automatic warehouse:
! High initial capital cost
! Downtime or reliability of equipment
! Software related problems
! Backup is needed to cover the risk of complete reliance on automation if a disaster should strike.
! User interface and training
! Obsolescence
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! Lack of flexibility
! Risk of having all eggs in one basket if a disaster should strike the warehouse
8.4
8.4 A cross docking facility can be totally automated. A list that is required for a fully automate cross docking facility with respect to the material handling aspect:
! Software for warehouse management. Software must be provided for a fully automated cross-docking facility since there is no man-labor available to make the arrangement to unload/load.
! Automatic material transport equipment for moving the materials. For example: conveyors, racks that is designed to accommodate cross docking facility, i.e. multi-lane directional conveyor lines from receiving dock to the shipping dock.
! Industrial vehicles for transporting from the dock to storage or storage to dock. For example: automatic crane/hoist to load/unload material to/from trucks/containers. This device will allow automated retrieval of loads from truck.
8.5
8.5 Criteria for comparing transfer line and FMS:
! Volume of production
! Number of products manufactured
! Production of families of work parts
! Reduced flexibility in processing orders because of long production line
! Manufacturing lead time
! Machine utilization
! Direct and indirect labor
! Management control
8.6
8.6 Criteria for comparing FMS and SSMS:
! Part transportation requirement
! Number of setup
! Tool allocation and management
! Flexibility
! Capital cost
8.7
8.7 Tool management problem in:
a. FMS
In a FMS environment, tool sharing among machines is common. This trend is perpetuated by limited tool magazine size and more importantly;
keeping a complete set of tools in a magazine may not be economically feasible since tools are generally expensive. Tools can be categorized into
8-3
two types: resident, which reside in a machine permanently and transient, which is shared among machines and kept in central tool storage.
Determining how many resident and transient tools is the problem. This also translates to how to allocate the transient tools among machines and how many transport devices is needed. Given the machining schedule, usually based on order priority, the required tool sequence can be known.
Simulation or integer programming can be used to find the optimal solution. In practice, keeping active inventory of all tools in the cell with their size, type, number and location, improving tool forecasts and warnings of tools changes, reducing delays in the system, and improving tool information reliability are the core of a tool management system.
b. SSMS
SSMS essentially faced the same tools management problems as in FMS.
Tools are still separated into resident tools and transient tools; however the proportion of resident tools is considerably higher than in a FMS setting.
This can be attributed to the nature of SSMS where part only visit a machine once; thus more resident tools are needed. This arrangement is more costly; in return it offers more versatility, more machine utilization, easier part scheduling and higher throughput.
8.8
8.8 Alternatives for moving part as shown:
! Spurs can be removed so that the system will have an on-track upload/offload. An on-track unload/offload system will allow queueing on tracks. If the system has many AGVs and the process of uploading/offloading takes a significant amount of time, then having spurs is more favorable since it allows an off-track upload/offload system.
! Instead of using AGVs, conveyors can be used for transporting materials. When using conveyors, a spine layout can also be
implemented. When using a spine layout, there is no more loop around the system.
8.9
8.9 This is a research question.
8.10
8.10 This is a research question.
8.11
8.11 JIT and lean manufacturing have essentially the same objectives. Both strive for eliminating or minimize waste, produce only what is demanded, minimize the use of time and space resources, and manufacture in the shortest cycle time possible. Different companies have different name for JIT, at Hewlett-Packard it is called Stockless Production, Material as Needed at Harley-Davidson, Continuous Flow Manufacturing at IBM.
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8.12
8.12 It is not applicable to all types of manufacturing systems. Mass production is still the best process to use for high volume, repetitive products. JIT may also be difficult to implement to very low volume or unique products such as in a job shop environment unless there is flexibility in reordering the machine.
8.13
8.13 This is a research question.
8.14
8.14 This is a research question.
8.15
8.15 In a straight line-balancing problem, the set of assignable tasks is limitedto that task whose predecessors have been assigned. In a U line balancing problem, the set of assignable task is enlarge by those tasks whose successors have been assigned, therefore U line balancing problem is more complex since now task grouping not only move in forward or backward direction as in a straight line, but it can also move in both directions at the same time.
In practice, rebalancing of the line is done quite often following demand changes. Rebalancing involves adding or removing machine from on the line or changing the standard time bases on new layout configuration; it also involves determining the number of operators required and assigning the machines that each operator tends.