Every shop has variability: late material, a dull tool, a rushed drawing, a new operator, a crane tied up. Your layout and workflow either absorb that variability gracefully—or they amplify it.
Find the constraint
The constraint is the step that limits overall output most days. It might be obvious (a slow CNC), or sneaky (programming, material handling, edge finishing, QA).
A practical test:
- If you doubled the speed of one step, which one would increase shipped output?
That’s your constraint.
The point of a buffer
A buffer exists to keep the constraint working even when upstream hiccups happen. But buffers have a cost:
- More WIP to track
- More space consumed
- More hiding places for defects
- Longer lead time
So the trick is small buffers in the right places, not “more space everywhere.”
Use “protected staging”
For your constraint, define two zones:
- Input buffer: enough to keep it running through normal variability (often 30–90 minutes of work, not a full day).
- Output buffer: enough to prevent downstream blocking (pack/assembly congestion).
Make them physical, labeled, and measurable. If they’re not explicit, the shop will create “invisible buffers” that are harder to control.
Prevent WIP from becoming inventory
When WIP gets too large, lead time grows and problems hide longer. A simple approach:
- Limit WIP per product family or per lane
- Use visual signals (cards, marked squares, rack slots)
- Track rework separately so it doesn’t pollute “clean flow”
What to do when the buffer is always empty (or always full)
- Always empty: upstream is unstable or the constraint is being starved by material handling/programming. Fix the feeding process.
- Always full: downstream is blocking or the constraint is producing the wrong mix. Fix downstream capacity or scheduling discipline.
Buffers are not a moral failure—they’re a tool. Used well, they protect throughput. Used poorly, they bury your problems under pallets.