Lesson 1: The busy plant

Walk onto almost any shop floor and ask how it is managed. The answer is usually some version of: we make sure every work centre is productive. Machines are measured on utilisation, supervisors on output per shift, and an idle machine is seen as money burning.

It sounds like common sense. The rest of this course is about why it so often produces the opposite of what it promises: piles of inventory, late orders and a constant feeling of firefighting.

Pause and think: a plant has spare capacity on every machine but one. Orders are still late. What is the most likely reason?

The workers are not working hard enough
The plant needs a better scheduling software
Work is being done on the wrong things at the wrong time
The plant needs more machines

In plant after plant, the capacity is there. What goes wrong is the sequence: non-constraints run ahead on work that isn’t needed yet, and the order that is needed waits behind it. That is a management problem, not an effort or a technology problem.

Why a plant gets complex

A plant with one product on one dedicated line is easy to run. Real plants have some or all of these:

Many end products. Dozens or thousands of variants, each with its own routing.
Convergent points. Assemblies that need several parts to arrive together.
Divergent points. A common material or part that can go into several different products.
Common resources. One machine working for many products, with set-ups when it switches.
A bottleneck. One resource with less capacity than the work asked of it.

Each one adds a way for “keeping everyone busy” to go wrong. Let’s look at five plants, from simple to complex, and predict the problems. These are the cases I use in my operations sessions; try to answer before you open the explanation.

Case 1: one product, one line, one bottleneck

NB1 is faster than the bottleneck. Its supervisor is measured on output and keeps it running all day. What happens?

The plant ships more, because NB1 produces more
A pile of work builds up in front of the bottleneck, and the plant ships no more than before
NB2 becomes the bottleneck
Nothing changes at all

The bottleneck sets the pace of the whole line. Everything NB1 makes beyond that pace just waits: money tied up in work in progress, a longer lead time, and not one extra sale. An hour saved at NB1 is a mirage.

Case 2: one non-bottleneck feeds two products

NB1 makes part A for the bottleneck and part B for NB2. What can go wrong when NB1 chases efficiency?

Nothing: NB1 has spare capacity
Only product B can be late
Only the bottleneck can be starved
Either way, something suffers: over-producing A piles work in front of the bottleneck and delays B; over-producing B starves the bottleneck

If NB1 runs long batches of A, a pile builds in front of the bottleneck while B is delayed, and the late start can turn NB1 and NB2 into temporary bottlenecks. If it runs long batches of B instead, B piles up as finished goods while the bottleneck starves, and every minute the bottleneck stands idle is throughput lost for good.

Case 3: the same part goes two ways

NB1 makes one part that can become product B (through the bottleneck) or product C (through NB2). Both NB1 and NB2 are kept busy. What happens?

Parts get “stolen” for C, piling up C stock while the bottleneck starves for B
Both products are delivered on time
The bottleneck moves to NB2 for good
Only inventory goes up; delivery is unaffected

This is material stealing. A busy NB2 grabs parts to make C that nobody needs yet, the bottleneck starves, and NB1 has to make the same part again to catch up, which can turn it into a temporary bottleneck too.

Case 4: two assemblies, some shared machines

NB1 works for both assemblies. Switching it from part A to part B takes a short set-up; switching from A to D takes a long one. What does an efficiency-minded supervisor do, and what follows?

Runs parts in due-date order; nothing goes wrong
Runs only product 2’s parts
Avoids the long set-up, so parts for one assembly pile up while the other assembly starves
Asks for a second NB1

Prioritising to save set-ups puts parts out of step. Assembly 1 has plenty of one part and none of the other; assembly 2 waits for D. That is de-synchronisation: lots of parts in front of assembly, and nothing that can be assembled.

Case 5: two products share an assembly and a material

Components for E are plentiful, but component C for F hasn’t arrived. The assembly team doesn’t want to stand idle. What do they do, and what follows?

They wait for C; nothing goes wrong
They assemble extra E: E piles up as finished goods, F is delayed, and the common material is used up on the wrong product
They build F without C
They ship E early, which is always good

Keeping the assembly busy produces E that isn’t needed yet, eats the common material X, and leaves F late when C finally arrives. Excess stock, a late order and misallocated material, all from one idle hour avoided.

The pattern

In all five plants the same thing happens: when every work centre tries to be productive, work gets done that isn’t needed yet, and work that is needed waits. The busier the floor looks, the worse the flow.

The next lesson gives these problems names, so you can spot them on a real shop floor.

Check your understanding

Answer all five. Get four right to complete the lesson.

1. What is the most common way shop floors are managed?

By the constraint’s schedule
By customer due dates
By keeping every work centre productive and busy
By minimising inventory

Utilisation and output per work centre are still the default measures in most plants.

2. A non-bottleneck feeding a bottleneck produces more than the bottleneck can take. The result is…

More inventory and a longer lead time, with no extra sales
More sales
A faster bottleneck
Lower operating expense

The bottleneck sets the pace; the extra just waits.

3. A common part is used for a product nobody needs yet, while the bottleneck starves. This is called…

De-synchronisation
Over-activation
Resource stealing
Material stealing

The part was taken by the wrong product. It typically happens at divergent points.

4. Which feature of a plant creates a risk of parts arriving out of step at assembly?

A single dedicated line
Convergent points, where several parts must come together
Having only one product
Having spare capacity

Assemblies need every part at the same time; anything that batches parts separately puts them out of step.

5. A plant has idle capacity on most machines but delivers late. The most likely cause is…

Lazy workers
Old machines
Work done on the wrong things at the wrong time
Too few products

The capacity is usually there. The sequence is wrong.