Incorporating cycle-time into models

This section presents a process for incorporating cycle-time into your models. At its essence, the process is quite simple. The process involves four basic steps, as outlined below:

1. Determine the "starting line" in a conserved flow chain.
2. Time-stamp the flow(s) which represent the starting line.
3. Determine the "finish line" in the conserved flow chain.
4. Tap into the flow associated with the finish line using a connector and converter. Define the converter using one of the cycle-time builtin functions.

Let's take a closer look at each step in the process.

1. Determine the starting line: In this step, you're deciding what flow(s) in the process should be considered as the starting line, for the purpose of generating cycle-time metrics. Any flows in a conserved flow chain can be used as the starting line. In the following figure, for example, one could choose from among many potential starting lines. Any flows downstream from the starting line you choose can be used for the generation of cycle-time metrics.

Thus, if you identified the flows enter queue 1 and enter queue 2 as the starting lines, you then could collect cycle-time data such as the wait time for process 1, the total time for process 1, and the cycle-time for the overall process. If, on the other hand, you decided to use enter process 3 as your starting line, the associated cycle-time calculation would apply to Process 3 only.

Notes:

• When you have multiple entry points to a main chain, you will want to consider making each entry point a "starting line." In Figure 10-2, if enter queue 1 is designated as the starting line but enter queue 2 is not, only the material which moves through the main chain via enter queue 1 will be considered in the subsequent cycle-time calculation.
• It's possible to designate successive stages in a main chain as starting lines. In Figure 10-2, for example, you might want to designate both enter process 1 and enter process 3 as starting lines. However, if both flows are time-stamped, material that moves through enter process 1 will be time-stamped again, when it moves through enter process 3. This material will lose its earlier time-stamping.

2. Time-stamp the flows which represent the starting line: To time-stamp a flow, begin by entering the flow's define dialog. As indicated in the following figure, you'll find a check box next to a small stopwatch icon at the top-right of the dialog. This check box is the time-stamping check box. Check the box. When you return to the diagram, the flow's appearance will change to indicate that it has been time-stamped. An illustrative time-stamped flow is also shown below.

3. Determine the "finish line": The finish line is the point in the conserved-flow chain where you want to collect the cycle-time information. As shown below, the finish line can be any flow which is downstream of the starting line in the chain. There must be a time-stamped flow upstream in the conserved flow chain, if cycle-time calculations are to have meaning.

4. Tap into the flow associated with the finish line: To wrap up your work, you'll use a connector and a converter to tap into the finish line flow.You can define the converter using one of the cycle-time builtin functions. As illustrated below, this step is straightforward. When you finish, the converter will sport a partially-filled clock face. This is your visual indication that the converter is generating cycle-time data. When you subsequently run your model, as the time-stamped flow volume crosses the finish line its time-stamp will be used to generate the desired cycle-time data.

Notes:

• It's possible to tap into a conserved flow chain in multiple places, so as to collect cycle-time information from multiple-stages in a conserved-flow chain. Each metric will be generated based upon the closest upstream time-stamp.
• By using multiple converters tapped into the same downstream flow, you can calculate a number of different cycle-time metrics at the same point in a process.
• Cycle-time builtins are available only in converters. They will be gray unless the converter is tapped into a flow in a conserved flow chain, and you have time-stamped a flow somewhere upstream in the same conserved flow chain.