Page 140 - How To Implement Lean Manufacturing

How to Do Lean—The Four Strategies to Becoming Lean 119

The equation for takt, or takt time, is the available work time divided by the customer
demand for that work time interval.

An Example Calculation
For example, let’s say we produce two ten-hour shifts and each shift includes a 30-minute
lunch and two ten-minute breaks. So the available work time is 20 hours – (2 × 50
minutes) = 18.33 hours. Our normal work schedule is five days per week and we have
nine holidays, so our work year is 365 – (2 × 52) – 9 = 252 days per year. The customer
has a contractual agreement to purchase 500,000 units per year.
To calculate takt, let’s use the work week, since that is a common planning interval
(If you use the day or the month, the answer will be the same—try it).

• Available time is 18.33 h/day × 5 d/wk = 91.67 hours, which equals 330,000
seconds/wk
• Customer demand is 500,000 units/yr ÷ 52 wks/yr = 9615 units per week
• Takt = 330,000 seconds/wk ÷ 9615 units/wk = 34.3 second per unit
In simple terms, we need to produce one good unit every 34.3 seconds to stay in
step with our customer’s demand. This is the synchronization time, to synchronize sup-
ply externally. If it is met, the first strategy has been executed successfully.

How to Handle Model Mix Leveling
It is common on many cells to produce several models of the same basic production
unit. These models, taken as a whole, are often referred to as a family of products
because they use many of the same parts and many of the same processing steps. In
that case, the takt equation remains unchanged. It is still available work time divided
by customer demand but must be calculated for each model. The complication is not
in the takt equation; rather, it is setting up the cell so the units can be produced simul-
taneously.
Frequently, the concept of model mix leveling is avoided. The typical logic used to
avoid doing the leveling goes like this, “Since my customer comes for his pickup on
Friday, and as long as I have the entire shipment made by then, it makes no difference
whether I make the models at a uniform rate during the week or in a batch, just as long
as I have all the models completed by pickup day.”
However, let’s look at a specific example. For example, you produce five models,
A,B,C,D, and E and it takes exactly one day to produce the contractual volume of each
model. Your work schedule is five days per week and his pickup is first thing Monday
morning. So you make A on Monday, B on Tuesday, and so on, finishing with E on Friday,
and all five models are ready for pick up the following Monday. This causes spike
demands of raw materials as the various models—that is, batches go through the sys-
tem, but under normal circumstances it does not sound too compelling to force model-
mix leveling. But what if something abnormal happens? For example, you get a call on
Wednesday and your customer says, “By the way, we want to change our pickup for
next Monday. We will not need Model A, B, or C but still need the normal weekly vol-
ume but with a mix of 50 percent D and 50 percent E. Well, you now have produced only
A, B, and C and the customer only wants D and E. In this case, if you work Saturday and