Principles

The BookPrinciples

Although manufacturing environments vary greatly, there are some underlying principles that are relevant to virtually all manufacturing systems. The basic concepts described here are cited extensively elsewhere in this site where they are used to help describe the behavior of specific systems.

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General Concepts

Principle (Diminishing Returns): The marginal improvement in a system from improvements of a single factor tends to decrease as that single factor is improved.

Material Flow Principles

Principle (Conservation of Material): In a stable system, over the long run, the rate out of a system will equal the rate in, less any yield loss, plus any parts production in the system.

Principle (Capacity): In steady state, all plants will release work at an average rate that is strictly less than average capacity.

Principle (Little's Law): The fundamental long-term relationship between inventory, throughput and flow time of a production system in steady state is:

Inventory = Throughput X Flow Time

Variability Principles

Principle (Variability Pooling): The variability of combined independent random processes is less than the sum of the variability of the individual processes.

Principle (Variability Buffering): Variability in a production system will be buffered by some combination of:

1 inventory
2 capacity
3 time

Batching Principles

Principle (Move Batching): WIP and cycle time increase proportionally in the batch size used to move parts between processes within a production process.

Principle (Serial Process Batching): In a unit operation with significant setups, capacity grows in the batch size, but at a diminishing rate. This implies that the minimum batch size required for stability may be larger than one.

Principle (Parallel Process Batching): In a batch operation capacity grows linearly in the batch size, up to the maximum number of parts the operation can process. This implies that the minimum batch size required for stability may be larger than one. However, because the process must collect batches before running them, inventory and flow time at the operation also grow proportionally in the batch size.

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