• Drum: the pacing mechanism of the process; that is, the "drum" that soldiers march to.
• Buffer: where work product accumulates--such as holding inputs to a very slow job down the line. The buffer's job is to regularize a process with varying performance.
• Rope: the policy when to turn a job on or off that feeds a buffer.

A process is called a "constraint" when it causes other processes to stall.

Only optimizations to constraints matter. If a change does not speed up the critical path (PERT terminology) then the change doesn't actually speed up the system.

Non-constraints tend to produce more than they consume; excess or "protective" capacity.

The drum is essentially the pace car for the entire system. This is either the slowest constraint in the system or a target set by management.

Buffers accumulate work product and hold it until a process can consume it. The size of buffers needs to be adjusted (how much can you afford to store, how long,

Rope is the policy for turning processes on and off to refill the buffers. (for example a set-reset latch, which turns on when a buffer goes below half and off as it approaches 75%.)

"Simplified" only has a shipping buffer: the market is treated as a constraint, and buffers are there to regularize the due/shipping dates.

Buffer management

Some rough ideas are to tune buffers with red/yellow/green signals, based on fill percentage, and tune so the buffer is only full 5% of the time, and decently full ("protective capacity") 25-30%.

References

• All About Lean
• Velocity Scheduling System