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Use Little’s Law and Finish What You Start!

By Bernard Milian

When I was a kid, my parents used to tell me: Finish what you start before you move on! It’s a simple piece of advice that is still relevant today and yet is largely underrated in the industry.

The translation into a production environment is simple: When you start working on a production order, bring it to completion as quickly as possible, to deliver a saleable product that generates revenue and satisfies the customer.

In financial terms, this translates into reducing the work in progress. The term describes what it is: Something that is “in progress” means that it is started but not finished, and so when we were kids, we would have been slapped on the wrist for having lots of things in progress and not much finished… tidy your room and finish your homework please!

The calculation formula that applies to this is called “Little’s Law.” When I tell you it’s for the little ones… and the adult ones we’ve become.

What is Little’s Law?

The law is simple: the WIP is equal to the throughput multiplied by the lead time. If you produce 1000 per day and have 5 days lead time, your WIP is 5000, so if you reduce the WIP to 2000 your lead time will drop to 2 days. You “just” need to reduce the WIP to satisfy the customers with a shorter lead time.

With the advent of Lean in the 90s, we often put production in line, i.e. we linked the different operations that make it possible to obtain a product as quickly as possible. This is the very essence of WIP reduction: we manufacture products in flow, from A to Z, with a sequence that allows for a minimal lead time. This is what Henri Ford did with the assembly line of the Ford T.

However, this line approach has its limitations and does not apply to all contexts.

The more diversity you have, the more complex it is to set a line.

The more complex your routings are and the more specialized and expensive your production resources are, the less likely you are to put them in line.

You are therefore faced with a dilemma: Use specialized and expensive production means efficiently, or finish what you start as soon as possible.

Some examples of this type of situation: Machining sequences of complex parts, chemical or pharmaceutical processes that require campaigns, integration of complex systems that include many elements.

In these contexts, it is easy to lose sight of the real purpose: To finish quickly what you start. We are mainly focused on the efficient use of the means of production, especially since, unlike in-line manufacturing, we do not see the flow. On a line, the previous part must be evacuated because the next one is coming. When you don’t have a line, you can’t see the flow, and very quickly you have things started everywhere that are waiting for the next operation.

We also often have products that have been started with missing components, and that are waiting for the complete kit. We also have non-conformities that are waiting for an analysis and a decision.

Work-in-progress is increasing, delays are worsening, and priority conflicts are accumulating.

Yes, when we didn’t finish our homework, we would pile up the backlog, and it was hard to keep track.

We need to change our focus. In the short term, in the operational horizon in which we must make decisions and deal with priorities, our means are defined and fixed. Our machines and their opening times are defined. Our staff and their skills are determined. All we can do on a daily basis is to get the work to flow through as quickly as possible, to satisfy the customer’s needs, and therefore to finish as quickly as possible the production orders we start.

That’s the role of production teams: To get the flow going as fast as possible – it’s not to maximize the OEE of all resources.

It must be admitted that this is complicated in certain production contexts, for the very reason that it is difficult to see the flow. This is where a digital solution like Intuiflow brings its added value on the shop floor. Displaying priorities unambiguously, advancing orders quickly, and releasing production orders with complete component kits at the right time – to allow everyone to finish their homework in a tidy room…

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