Available time for changeovers per period (Ta∆), also called available time for (internal) setups, represents the time per a given period (e.g., day, shift, week) during which a machine, equipment, or resource can be changed over (i.e., from one product to another, prepared for a different medical procedure, cleaned for another customer). Ta∆, is foundational to every part, every interval (EPEI), changeover distribution, and kanban-sizing calculations.
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One principal lean objective is to continuously shrink changeover times using setup reduction strategies. Here, we are primarily concerned with internal setup (i.e., the time during the setup when a given resource is not available to produce or service the customer). This is also synonymous with “changeover time” (the time elapsed between the last part from the prior run and the first good part immediately following the changeover), unless external setups (the activity that is conducted before and after the internal) from one changeover “crashes” into another, or there are insufficient human resources to execute the external such that it then becomes internal. The smaller the internal changeover, the higher the potential changeover frequency, and the smaller the batch size, the shorter the lead time.
Figure 1:here
Although Ta∆ can be exploited with shorter changeover times, Ta∆ can also be expanded often by attacking the elements that consume the balance of available time within a day or shift. For example, the targeted resource can be used or run during breaks and lunches, or those planned unavailable times can be modified. Also, for example, unplanned time losses can be countered through the application of total productive maintenance, and yields can be improved through mistake proofing. Additionally, cycle times can be reduced through machine kaizen.
The following conceptual discussion about the “purist view” vs. the “rationalist view” is useful.
Purist view
The lean purist’s perspective is that the time incurred for “unplanned” losses (e.g., machine breakdowns, adjustments, and idling) should not be excluded from Ta∆. In other words, the calculated takt time (Tt) should presume these losses are less than minimal. It is also presumed that scrap is negligible.
If and when a process can’t repeatedly and regularly satisfy customer requirements, then the losses, if they are indeed the root causes of missed Tt, will be appropriately highlighted and aggressively addressed through kaizen. As previously mentioned, losses are often reduced through the application of setup reduction, total productive maintenance, variation reduction kaizen, etc.
Losses that go beyond very minor unplanned interruptions, such as reduced speeds, changeovers and adjustments, tool changes, IT system crashes, phone outages, rework, and equipment breakdowns, can tend to be “hidden” when Ta∆ is reduced for such items. The same notion applies when Ta∆ is reduced to accommodate increased production quantities to offset yield losses.
A reduced Ta∆, let’s call it “rationalized Ta∆,” accommodates less frequent changeovers, making the waste of stoppage and yield losses recognizable, and yet it allows the buildup of inventory or queues. In the end, the process may satisfy its quota, but it will be done with hidden waste and unevenness, and sometimes very public and painful overburden. We know that “hidden” is the antithesis of lean, essentially accommodating the waste and obscuring it—and allowing it to live on.
Figure 2:here
Figure 3:here
Rationalist’s view
Lean is all about taking care of the customer. Ultimately, the lean practitioner must defer to common sense in the pursuit of taking care of the customer. Accordingly, it may be prudent to temporarily rationalize Ta∆ to reflect the impact of unplanned losses, including yield losses (and facilitate the satisfaction of the customer requirements). Purists for the most part can grudgingly accept this accommodation if the situation is plainly acknowledged (i.e., not hidden) and leaders actively and effectively pursue the root causes of the losses.
Figure 4:here
Figure 5:here
Some considerations:
• It is pragmatic to calculate Ta∆ using weekly, monthly, or even quarterly data to “smooth out” variation in demand, unplanned downtime, adjustment losses, and other situations. However, the lean practitioner is obligated to understand the variation and address it appropriately, as well as understand the implications to EPEI, replenishment intervals, and kanban-sizing calculations.
• If demand is dynamic, the available-time calculation should be refreshed periodically.
• A rule of thumb: A Ta∆ target is approximately 10 percent of available time, although this will vary based on the value stream characteristics.
© Mark Hamel. First appeared March 27, 2014, on the Lean Math blog.
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