Elements of standard work: takt time, sequence and WIP

Why it matters

Parts for the same product on a bench do not show that the method there is defined. If K01 places the seal first while K02 prepares the cover first, the two sequences may not provide the same quality. If one checks the part before tightening it and the other leaves the check until the end, comparing cycle times alone is incomplete. Standard work explains how to repeatedly produce an acceptable product safely. Writing takt time, work sequence and required work in progress together establishes a traceable link between the target and practice. The document describes more than the worker's hand movements; it also shows which conditions must be ready for work to start. The standard therefore depends on how material is presented at the bench.

Takt time expresses customer demand as an interval within the production period. Net available time is divided by demand for the same period; minutes and seconds must not be confused. Planned breaks are subtracted from available time. Time lost to breakdowns or rework does not reduce demand; it remains separately visible as a loss. The measured cycle of the work sequence may be shorter than takt time. This difference is no reason to make the worker work continuously at maximum speed. Safe repetition of the same method, sustainable replenishment and the product meeting its acceptance criteria are examined separately. Takt is a measure of demand; it is not a performance target derived from the fastest observed cycle. Recalculate when the period and demand change.

Required work in progress is the minimum number of parts that must be in specified locations to sustain the defined work sequence. The part being processed is included. Counting every raw part in the box confuses replenishment inventory with the requirement within the cycle. Conversely, counting only waiting parts makes the part in the fixture invisible. Explain the instant and boundaries used for the count. Input storage, the working fixture and the accepted output area are different scopes. To say that a part is required, show which step becomes impossible when it is removed. Habit, a large table or a full box alone is not proof of necessity.

Reducing inventory can help; however, unconditionally reducing all work in progress to zero is not a reliable rule. If a part is in the fixture during processing, removing it to achieve zero inventory eliminates the operation. Excess parts must be distinguished from essential parts. At the same time, the number specified in standard work is not an unchanging natural constant. A new fixture, a different replenishment frequency or a changed inspection sequence may change the requirement. Define the new safe method first, then test inventory and cycle time again. A reduction on paper is unacceptable if it causes starvation or nonconforming products. Improvement means explaining what is necessary and removing what is unnecessary with evidence; it means more than writing a smaller number.

Formula