Time Metrics in Lean November 2, 2008
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Lead Time = the average time it takes for one unit to go through the entire process – from start to finish – including time waiting between sub-processes.
Calculation = Cycle Time x units of WIP x number of operations + time delays between processes (planned and unplanned)
Example 1: Cycle Time of 120 seconds x 1 unit of WIP x 1 operation + 0 time delays = Lead Time of 2 minutes
Example 2: Cycle Time of 120 seconds x 100 units of WIP x 2 operations + 0 time delays = Lead Time of 400 minutes
Notice how the number of units of Work In Process (WIP) radically increases lead time. This is one of several reasons that Lean is so obsessed with small batch sizes. “Time delays between processes” is often by far the greatest contributor to Lead Time – and often presents the greatest opportunities for quickly eliminating waste. (“Low hanging fruit”).
Processing Time = The time a product is being worked on by an Operator.
Processing time is observed with a stopwatch or video camera – following one unit being processed by one operator – all the way through the process (or sub-process).
Processing Time = Manual Work + Walking + Waiting
Notice that Machine Time is NOT included in Processing Time. If the Operator has nothing better to do than stand around to wait for the machine to finish doing its thing, then that is called “Wait time”, and Wait Time is included within Processing Time. Processing Time is all about the Operator (not the machine).
Machine Time = The time that a machine is working on a product.
Machine time is the total time that the machine is working on the product. Whether or not the Operator has something better to do than to stand around waiting for the machine to finish has no influence on Machine Time.
For example – If an automatic machine is running for 60 seconds, and the Operator has something valuable to do for 20 seconds, and then has 40 seconds of “Wait time”, the Machine Time is still 60 seconds.
Process Lead Time = The time that a product is “being worked on”. (with or without an Operator)
Process Lead Time is measured in the same unit of measure as Lead Time – (usually days) – while Processing Time is measured in the same unit of measure as Takt Time and Cycle Time (usually seconds).
If the Operator is involved in every moment of the process, then Process Lead Time is simply Processing Time calculated into a tiny decimal fraction of a day.
Cycle Time = The average time between completed units “coming out the end of the pipe”
Example: the cycle time of motors assembled at the rate of 120 per hour would be 30 seconds per unit
Machine Cycle Time = The average time between completed units coming out of a machine
Example: A machine might have Machine Time of 60 seconds, but if the machine makes batches of 6, then Machine Cycle Time is 10 seconds.
Effective Cycle Time = Cycle Time adjusted for all the factors that reduce Working Time Available. (Also known as Output Pace)
Takt Time = Planning drumbeat. How often completed units NEED to come out the end of the pipe – as established by customer demand (also known as “Rate of customer demand”)
Calculation = Working Time Available / Target Units to Produce (usually calculated per week or per shift)
Example: 420 working minutes per shift / 210 Target Units to Produce during that shift = Takt Time of 2 minutes per unit. But Takt Time and Cycle Time are always measured in seconds – so Takt Time would be 120 seconds.
Value Add Time = Time of those work elements that actually transform the product in a way that the customer is willing to pay for. (also known as Value Creating Time)
Value Add Time < Cycle Time < Lead Time
Non Value Add Time = Cycle Time minus Value Add Time
Adjusting Cycle Time to Takt Time
Takt Time and Target Cycle Time are determined by customer demand, for which we usually have little control. So we adjust Cycle Time to meet Takt Time, by adjusting:
1) Available production time (the number and length of shifts)
2) The number of work cells making the same item (thereby increasing the Takt Time for each)
3) The number of end items produced in a cell (thereby increasing or decreasing demand for that cell)
4) The number of operators in the work cell
Target Cycle Time = Operational Takt Time adjusted for other factors
Target Cycle Time must be less than or equal to (and is usually equal to) Operational Takt Time.
(In many environments, Takt Time, Operational Takt Time, and Target Cycle Time are all the same, and the single term “Takt Time” can be used. In other environments, the differences can become important.)
