Lesson 3. Herbie with a die

In lesson 2 Herbie worked like clockwork: exactly 2 seconds per part. Goal 4 was just enough. A real machine does not work like that: now the tool is blunt, now the part is awkward. In this lesson Herbie rolls a die, like the machines in FL-01, and we find out why the drum needs a buffer — a stock of work in front of it.

3.1. Idea: a die at Herbie

Click the die on machine 3 twice — it turns orange (a 1…6 die). Now before every part Herbie rolls the die, and the part takes

• 1 point — 2 × 1 / 3.5 ≈ 0.6 seconds;
• 6 points — 2 × 6 / 3.5 ≈ 3.4 seconds;
• on average 3.5 points — the same 2 seconds.

On average Herbie is the same. But after a six he has fallen behind, and after a one he needs several parts in a row at once. If the buffer is empty at that moment, Herbie waits, and that time is lost for the whole factory: Herbie is the bottleneck, nobody will make up for his idle time.

3.2. Assembly

We add no parts. Herbie's die — click the die on machine 3 until it turns orange. The buffer goal — the switches of lesson 2.

3.3. Check

Three shifts, Herbie with a die, SLOW off. Each shift: power off and on → R → S, S (probe on Herbie) → START, watch for 200 seconds.

«Made» — by second 200, «idle» — display I of the idle meter on Herbie.

Your numbers will be a little different — a die is a die — but the order is the same:

• goal 4 — Herbie sometimes waits, and the factory loses almost ten parts;
• goal 6 — Herbie no longer waits, the factory makes as much as without the rope, and there are three times fewer parts inside.

Two extra parts in the buffer are the drum's insurance: while Herbie quickly deals with ones, he has something to take, and meanwhile the source brings new ones. The more unevenly the bottleneck works, the more stock it needs in front of it. Setting the goal «with a margin, just in case» is pointless too: goals 7 and 8 make exactly as much as 6, they only hold more parts inside.