A Simulation Study of FMS ToolAllocation Procedures

INTRODUCTION A flexible manufacturing system (FMS) is a group of

numerically controlled machine tools with an automated material handling system and a central supervisory computer system.

The advent of FMSs has greatly increased the productive potential of manufacturers; however managing the increased number of tools required for these systems and their application has hindered increases in productivity.

Proper tool allocation is important because of the large number of tools that might have to be allocated to the tool magazine and the fact that only a limited number of tools can fit on the tool magazines.

INTRODUCTION lack of attention to tool management has resulted in poor

performance of many FMS.

The question to be addressed then is: Given the limited tool magazine capacity at the machines, what is the impact of using different tool allocation procedures on FMS performance.

PROBLEM IDENTIFICATION The problem being addressed in this paper is that, although

machine capacity might be sufficient, it might not be possible to process all orders required for a particular planning period due to the limited number of tool slots at the tool magazines attached to the machines.

a subset of orders has to be selected and the required tools allocated to the machines before the orders can be processed.

it is important that the time required for tool changes between subsets of orders be minimized. Tool changes affect the efficiency of the FMS.

PROBLEM IDENTIFICATION The first aspect of tool allocation is the loading and

placement of a set of tools in the magazine.

Any tool allocation policy must also take into account the tool replacement strategy to be used.

Daoud and Purcheck argued that more focus should be placed on developing tool management strategies that aim to minimize the frequency of tool changes due to tool wear, rather than tool changes imposed by part type variety.

the need for tool changes due to part type mix might override the need for tool changes due to tool wear. The focus in this study is therefore on tool changes due to part type variety.

Heuristic Tool Allocation Procedures

The complexity of solving the part type selection and tool allocation problems simultaneously for a reasonably realistic FMS has been recognized by several researchers and heuristic procedures have been proposed.

The Heuristic tool allocation procedures studied in this paper are:

1.Tool and part batching

2.Tool sharing

3.Flexible tooling

TOOL AND PART BATCHING Assuming there is enough machine capacity to process all the

parts during a planning period.

the need to divide the parts into batches arises mainly because of limited tool magazine capacity at the machines.

The main criterion of interest in this case is to minimize the number of batches required to process all the parts, thereby minimizing the idle time associated with batch changeovers.

procedure for assigning parts to batches:

1. selecting part types that require the largest number of too slots.

2. first selecting part types that requires the smallest number of tools.

TOOL AND PART BATCHING Tool allocation is incorporated into the batching decision as

follows:

For each part type in the batch, a copy of each tool type needed for that part type on a particular machine is allocated to the machine's magazine.

For example, if two part types use the same tool type, two copies of the tool will be allocated, one for each part type.

In addition, if a particular part type requires multiple copies of the same tool, then depending on the tool life and processing time of the part, the appropriate number of copies of that tool are allocated to the tool magazine.

TOOL AND PART BATCHING This approach ignores tool sharing among part types. Also,

when using this approach, no tool changing occurs until the entire batch of part types is processed.

this approach has the potential to reduce the frequency of tool changes but provides no savings in tool magazine capacity.

Tool tracking is also reduced.

An implication of this strategy is that, because the tool magazine capacity on each machine is fixed, a part will not be assigned to a particular machine for the production window if there is not enough space on the tool magazine to hold the required tools.

This approach can also lead to excessive tool inventory and greater tool handling time.

TOOL SHARING Failure to recognize tool commonality can lead to unnecessary tool

duplication and further underutilization of tool magazine capacity. A suggested approach for overcoming these limitations is the tool sharing approach.

It is assumed that only one copy of each tool type is needed for each part type requiring that tool. Also assume each tool occupies only one slot.

the common tool has to have sufficient tool life to meet the processing requirements of all part orders needing that tool.

more sophisticated tool-monitoring logic will be needed to keep track of the expended tool lives of the various common tools and to keep track of how the remaining tool lives will be allocated among other part orders.

FLEXIBLE TOOLING These flexible approaches are aimed at minimizing the

bottleneck effects of the tool magazine capacity at each machine.

When part types are selected for production, their required tools are also allocated to the machines, and the tool slot consumption at each machine is updated just as in the tool-part batching procedure. Following the completion of the part types requiring those tools, any tools not fully consumed are removed from the tool magazine while another part is being machined. This frees up space on the tool magazine to permit the selection of another part type to be processed and the allocation of the needed tools to the machine.

FLEXIBLE TOOLING The flexible tooling procedure thus has the potential of

reducing cutting tool inventory because it permits tool sharing during the production window. It also leads to higher utilization of the tool magazine capacity because tool slots are not "tied down" by idle tools.

this approach requires frequent tool changes, and thus another constraint is imposed on the system by the mechanism of tool changes.

the use of this approach means that no prerouting of part orders is necessary during prerelease planning.

COMPARISON

PERFORMANCE MEASURE the tardiness of orders, percentage of orders tardy, and

mean flow time of orders processed on the FMS will be determined.

Tardiness refers to the average lateness of all orders completed after their due dates.

The percentage of orders tardy measure represents the percentage of total orders completed after their due dates

Flow time is a measure of throughput and can also provide an indication of the level of work-in-process inventory.

EXPERIMENTAL ASSUMPTION

A static daily demand for parts is assumed rather than a dynamic demand, meaning that a fixed quantity of part orders is introduced into the system in each production period.

the arrival rate was generated by assuming a machine load capacity of 75%.

due date is not a performance measure in this study; rather, due dates are used to preferentially determine the part input sequence at the FMS.

The total processing time per part type is assumed to be based on a 2-Erlang distribution with a mean of 60 minutes.

The cutting life of tools is assumed to be exponentially distributed with a typical mean of 30 minutes.

RESULT

CONCLUSION Tool allocation approach for FMS production planning is particularly

important when the FMS is being used to process a few part types. Although the use of a flexible tooling approach produces results superior to the use of either a tool batching or tool sharing approach at both low and high part mix.

The frequency of tool changes within a production period is still high for the flexible tooling, as reflected in the higher utilization of the robotic material handling unit. Any waiting times at the machines due to tool changes are overridden by the greater opportunity provided by the savings in tool magazine capacity as a result of the frequent removal of tools from the tool magazines. Thus, it is concluded that a tool allocation approach focused on greater utilization of the tool magazine capacity is better than a tool allocation approach that seeks purely to minimize the frequency of tool changes within a given production period.

CONCLUSION Sharing approach allows the benefits of greater tool

magazine utilization to be achieved while at the same time requiring a lesser frequency of tool changes. Thus, a tool sharing approach might be better if the FMS is operated in an environment where the system for tool flow and tool changes is not efficient and imposes a lot of delays on the machines, especially if the FMS is the bottleneck unit in the shop's operations.

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