Defining cycle times…and other manufacturing terms
Operations are described using common terminology and mapping tools. Here are some definitions and simple examples.
The word ‘operation’ evokes the image of a large factory, but it’s any process involving a set of inputs of one or more steps that result in a more valuable set of outputs. A car plant is certainly an operation, but so is a hospital, the claim-processing department of an insurance company, or a person doing the washing. Each of these operations can be described using specific terms. Using laundry as a metaphor, here’s a breakdown of what the terms mean.
Cycle time. It’s the average time it takes to complete a step or set of steps within an operation. The cycle time for the washer is 30 minutes while the cycle time for the dryer might be 45 minutes to an hour. In a 10-washer operation, the cycle time for a single load would be three minutes (30 minutes divided by 10 washers). If the dryer is large enough to run two loads the cycle time per washer-load will be half of the cycle time per dryer-load.
Dependent steps. They can only be done when a previous step has been completed. The interaction of dependent steps creates much of the need for operations management.
Bottleneck. Within a set of dependent steps, this one defines the speed at which the entire operation runs. Generally, the step with the longest cycle time will be the bottleneck. Imagine there’s lots of laundry. As soon as the first load is in the dryer, the second load starts in the washer, and so on. Once the line is full, the dryer will determine the speed of the operation because it will still be drying the first load when the washer finishes its cycle on the second load.
The bottleneck is often an important area of focus for improving the capacity of an operation. Increasing the bottleneck’s capacity will increase overall capacity, but increasing the output of a non-bottleneck step may have no effect.
Manufacturing lead time. This is the average amount of time it will take a new set of inputs to move all the way through the operation, assuming no unusual measures are taken. A load of laundry would spend one cycle (45 minutes) in the washer, including idle time, another cycle in the dryer (90 minutes), and then two-thirds of a cycle being folded (120 minutes). From laundry bag to clean and folded will average two hours. Because folding takes place after the bottleneck, the load doesn’t have to stay there for a full cycle.
Idle time. Sometimes you only need to do one load of laundry, but because the steps in the process are dependent, two machines (including the folder) will be idle part of the time.
Since many operations are capable of completing their tasks faster than the bottleneck operation, it’s not sensible to run them at full capacity. If you run the washer and dryer non-stop all day, you would accumulate extra loads of wet laundry waiting to be dried. Eventually the washer would have to stop running to let the dryer catch up.
Work in process (WIP). This refers to laundry still in the washer, the dryer or being folded. WIP is sometimes discussed in dollar terms, or whatever units are moving through the operation. Once the line is full, there would always be a load either in the washer or waiting to be put into the dryer and another load in the dryer. We would also have a load being folded, but since it doesn’t have to wait for anything, that step will be empty some of the time. Ignoring the possibility that folding is delayed by loading, expect to have a load of laundry in-process at the folding step for 30 minutes out of every 45 minutes of cycle time for two 2/3 loads of WIP.
Buffer. Sometimes an operation will have storage space where WIP from one step accumulates before being worked at the next step. There are many reasons for having a buffer. Suppose we want to run the washer non-stop in the morning to get as many loads as possible finished before the afternoon, but we need space for them while the dryer catches up. In larger operations, a buffer ensures the bottleneck is never starved for inputs. Since the bottleneck sets the pace, loss of production there implies lost production for the entire operation.
These terms put together and properly executed ensure that won’t happen; and if applied at home, they map out an efficient way to get the laundry done.
Richard Kunst is president and CEO of Cambridge, Ont.-based Kunst Solutions Corp., which publishes the “Lean Thoughts” e-newsletter and helps companies become more agile, develop evolutionary management and implement lean solutions.
This article appears in the March 2015 issue of PLANT.