Lesson 4. The experiment: push or pull

Now the panel has three ways to release parts:

• push — a part every second, however many are waiting before Herbie;
• Herbie's rhythm (SLOW from FL-02) — a part every 2 seconds, by a timetable: we know in advance that Herbie works 2 seconds;
• rope — a part only while the buffer is below the goal: Herbie pulls work to himself.

In this lesson we compare them. No new parts.

4.1. How to run it

• Herbie has a die (orange);
• switching the way: the SLOW and ROPE switches; the rope goal is 6 (switches 4 and 2);
• each shift: power off and on → R → START, after 200 seconds look at «made» and the buffer.

4.2. Experiment 1: three ways

Push — the output is good, but the factory is swamped: some fifteen parts wait before Herbie all the time. Herbie's rhythm keeps few parts, but with a die it loses output: by the timetable a part arrives once every 2 seconds, even when Herbie, after some ones, is already free. The rope makes as much as «push», and holds three times fewer parts.

4.3. Experiment 2: Herbie sped up

Let's elevate the constraint, as in FL-02: Herbie → 1 second (Alt/⌘ + click on machine 3 three times: 2 → 4 → 0.5 → 1). Keep the die. Two shifts — Herbie's rhythm and the rope, don't touch the switches.

Herbie's rhythm noticed nothing: the SLOW switch releases a part once every 2 seconds, as it was set. To make the factory go faster, the timetable has to be redone by hand. The rope has nothing to redo: it does not know how many seconds Herbie works — it watches the buffer. Herbie began taking parts faster, the buffer began emptying faster, and the source began releasing more often by itself.

4.4. Why

A timetable sets the rhythm in advance, the rope — as it happens. The factory's output is still the bottleneck's output, but the rope fits the whole factory to Herbie's real pace, whatever it is: uneven because of the die or faster after an improvement. All the factory needs for that is two instruments: the buffer counter (lesson 1) and the comparison with the goal (lesson 2).

Goldratt called this «drum — buffer — rope»: the bottleneck beats the rhythm like a drum, a small stock of work waits in front of it, and the rope ties it to the start of the line.

4.5. Check