Definition
Standard in-process stock (標準手持ち, hyōjun temochi) is the minimum quantity of work-in-process that must be present inside a process for the operator to carry out the standardized work sequence repeatedly and without interruption. Standard in-process stock includes the pieces mounted in machines, not only the pieces held between machines. Toyota defines standard in-process stock as the in-process pieces that are necessary at minimum for the work to proceed, including those attached to machines.
Standard in-process stock is one of the three elements of standardized work, with takt time and work sequence. Standardized work does not hold if any one of the three is missing. Older Toyota materials name the first element cycle time. Later Toyota materials name the first element takt time.
Standard in-process stock is a designed quantity, not a buffer. The number is fixed by the layout and the work sequence, written on the Standardized Work Chart, and held constant. More pieces on the floor than the standard means overproduction has occurred. Fewer means the cycle has been interrupted.
Japanese Origin
標準 (hyōjun) means standard. 手持ち (temochi) means "held in hand" or "on hand," the stock a worker keeps to hand in order to keep working. The term is specific to the stock inside one operator's standardized work. Temochi is not the general word for inventory (在庫, zaiko), and temochi is not the accounting word for work-in-process (仕掛品, shikakarihin), although the pieces counted are work-in-process.
Toyota's use of temochi (手持ち, on hand) rather than zaiko (在庫, inventory) is deliberate. Inventory is stock held against the schedule and its uncertainty, and the quantity is set by planners. Temochi is stock held by the operator because the work sequence cannot be performed without it, and the quantity is set by the sequence itself. Calling the pieces inventory would invite sizing them like a buffer. Calling them on-hand stock keeps the count tied to the work.
How the Quantity Is Determined
The rule is short. The standard quantity is decided by asking at which points, and how many pieces, there must be for the defined work to hold together. Two factors set the answer.
Machine layout and work sequence. With the same layout, an operator who works in the order of the process flow needs only the pieces mounted in the machines and holds nothing between processes. An operator who works against the process flow needs one piece between each pair of processes, or two pieces where a machine takes two at once. The direction of the walk changes the count.
Process requirements. Where a quality check must be done at a fixed point, where a part must cool to a set temperature before the next operation, or where oil must drain from a workpiece, the pieces needed for that wait are added to the standard.
In a multi-machine cell the standard often comes to one piece at each machine plus the piece the operator carries. The Standardized Work Chart marks every location where a standard piece is held, so anyone can compare the actual count on the floor to the standard at a glance.
Role in Restraining Overproduction
"Work running ahead" is a common sight on a production line. Time that should be waiting time gets filled with the next operation, the waiting is hidden, and stock piles up in and after the line. Toyota calls this overproduction, the most fundamental waste because it hides the others. The system that keeps each process's standard in-process stock held constant and runs the processes interlocked, so that no process can run ahead, is what Toyota calls the full-work system.
This is why the third element matters to the whole production system and not only to one operator's routine. A held standard exposes waiting instead of hiding it. Exposed waiting is what makes the redistribution of work and the reduction of headcount possible. The sequence of man-hour reduction follows from this. Fix the standardized work, hold standard in-process stock, takt, and work sequence, let waiting appear, then rebalance the line.
Common Mistakes
Treating standard in-process stock as a buffer. A buffer is sized to absorb variation. Standard in-process stock is sized to make the work sequence possible, and nothing more. Adding pieces "to be safe" converts the standard into inventory and hides the abnormality the standard was meant to reveal.
Counting only the pieces between machines. The definition includes pieces mounted in machines. A three-machine cell worked in process order has a standard of three, one in each machine, plus whatever the operator carries.
Confusing standard in-process stock with standard inventory. Standard inventory (標準在庫, hyōjun zaiko) is the controlled stock held between processes or in a supermarket under kanban control. Standard in-process stock lives inside the standardized work of one operator or one line. Both are visualized and both are held to a number, but they are set by different logic and managed by different people.
Leaving the count off the chart. A Standardized Work Chart without marked standard in-process stock locations cannot be audited. The quantity and its locations are part of the standard, and the chart is where they are recorded.