The minimum amount of material or product that must be in the process at all times to ensure smooth operation.
Standard Work is a little underrated concept in Lean Manufacturing. It is not simply standardization or work standards.
Standard Work is composed of three elements: Takt time, Work sequence and Standard Work in Process (SWIP). Takt Time is a fundamental concept of Lean Manufacturing, and Work Sequence is relatively intuitive. SWIP, however, is a bit more complex.
SWIP refers to the minimum necessary in-process inventory (work in process or WIP) to maintain Standard Work. It is not more or less than what is needed. To calculate the appropriate quantity for SWIP, one must ask a number of questions.
While a rough estimate of SWIP can be obtained by using the equation SWIP = Sum of Cycle Times / Takt Time, it is still necessary to determine where exactly this SWIP should be applied. The following steps provide a guide for determining the appropriate quantity of SWIP:
What’s the team size?
Standard Work is the most efficient combination of manpower, material, and machine, and is based on takt, work sequence, and Standard Work in Process (SWIP). By definition, it should include manual work. If a process is fully automated, it is not considered Standard Work. Instead, it is likely an NC program.
To determine the appropriate team size, the sum of manual cycle time is divided by Takt Time. Therefore, one piece of SWIP per person is required. The equation for manual SWIP would than be:
SWIP(manual) = Team member x (1 piece = person)
When determining the amount of SWIP, there should be no rounding, unless there is less than a full person. In that case, round up to the nearest whole number.
Process steps as automatic one-piece cycle machines
Standard Work assumes the use of multiple processes or machines, and separates human and machine tasks as much as possible.
When using an automatic cycle, the worker will only be responsible for loading and unloading, and will not be present during the actual cycle. The automatic cycle time must also be shorter than the Takt Time, ensuring that there is always at least one piece in the machine during each cycle.
This is known as SWIP (single piece auto), and is calculated as the number of single-piece automatic cycle machines multiplied by one piece per machine. There is no rounding necessary as it is not possible to have less than a full machine. However, this only applies to single-piece automatic cycles, and calculations for batch processes or cycles with longer lead times may differ.
Process steps as a single-piece non-machine automatic cycle
The term “non-machine automatic cycle” refers to process steps such as the drying time for paint, curing time for epoxy, and cooling time for hot parts.
These process steps may not involve machines, but they do require a certain amount of time for completion. The ratio of this time to the Takt Time is known as the Single-Piece Non-Machine Automatic (SWIP) cycle.
It is important to note that this value should always be rounded up to the nearest whole number. In some cases, equipment like turn tables or FIFO racks may be used to manage the curing process, ensuring that a finished product is available for each takt, and a new one is added for curing.
Process steps with a batch automatic cycle
Batch processes refer to situations in which equipment is designed to unload and load multiple pieces at a time, rather than one piece at a time.
A common example is heat treatment processes where a vacuum must be maintained and the door cannot be opened for hours. In such cases, a batch of parts is removed and then another batch is loaded. The cycle time per piece may be less than the Takt Time, but the overall automatic Cycle Time is greater than the Takt Time.
The Single-Piece Non-Machine Automatic (SWIP) cycle in this case is calculated as (Automatic time / Takt Time) x 2. The reason for this is that in batch processes, which do not allow for the addition or removal of individual pieces during the Takt, an extra quantity of complete parts is required. This concept can be compared to the idea of a pulley and bucket system used to retrieve water from a well, where one bucket is at the bottom of the well, full of water and another bucket is at the top, full of water, and during Takt, you empty out the bucket one by one and fill it back up one by one.
It’s worth noting that in formulas 2, 3 and 4, manual cycle time is not included in the calculation because rule #1 takes care of that. This is because every manual Cycle Time must be within Takt by definition of Standard Work and since the unload/load time will involve one piece, there is no need to add manual time back into the calculation (in most of the cases).
