Basics of CONWIP Systems (Constant Work in Progress)

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CONWIP -go-round with different parts – like roller coaster seats with people

There is broad agreement in industry that a pull system is in almost all cases better than a push system. The most famous way to establish a pull system is to use a kanban system. The idea of kanban is so much associated with pull production that the two terms are sometimes even used synonymously. However, there are other ways to implement pull. Another useful approach is CONWIP, standing for Constant Work In Progress and developed by Mark Spearman and Wallace Hopp in 1990.  In this small series of posts, I would like to go into the details of CONWIP and its similarities to and differences from kanban. This first post will explain the basics, the next two posts will go into more details by answering some frequently asked questions, and the fourth post will discuss advantages and disadvantages of CONWIP.

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A Critical Look at Goldratt’s Drum-Buffer-Rope Method

Drum Buffer RopeEliyahu Goldratt developed different methods on how to manage production systems. These methods are nowadays known as the Theory of Constraints, or TOC for short. One key method described is called Drum-Buffer-Rope, or DBM for short. Similar to Kanban or CONWIP, it aims to constrain the work in progress (WIP) in the system. There is much discussion on which method is better than the other, although the result often depends heavily on with which method the respective author earns its living.  In this post I will present how Drum-Buffer-Rope works, and discuss its advantages and shortcomings.

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Theory and Practice on FiFo Lanes – How Does FiFo Work in Lean Manufacturing?

FiFo LaneFiFo lanes are an important part of any lean material flow. They are a very simple way to define both the material flow and the information flow. In this post I want to tell you why to use FiFo, how to use FiFo, and the advantages of FiFo, as well as show you a few examples of FiFo lanes.

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How Many Kanbans? – Estimation Approach and Maintenance

Number of Kanban
How many Kanban?

In my previous two posts, I described how to calculate the number of kanbans (Post 1 and Post 2). However, this calculation is complex, and the result is nothing more than a very rough estimate. Hence my preferred method for determining the number of kanbans is, broadly speaking,  “just take enough, and then see if you can reduce them.” In this post, I would like to explain this approach and also discuss how and when to update the number of kanbans.

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How Many Kanbans? – The Kanban Formula, Part 2

Number of Kanban
How many Kanbans?

This is the second post on kanban calculation (if possible, please read the first post on kanban calculation first). There are two possible approaches. First, you can calculate the number of kanbans using a kanban formula (due to its length, split into a first post and this second post). Alternatively, you can estimate the number of kanbans and adjust the system as it is running (as shown in a third post).

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How Many Kanbans? – The Kanban Formula, Part 1

Number of Kanban
How many Kanbans?

One frequent and tricky question when designing a pull system is to determine how many kanbans to use in the system. There are two possible approaches. First, you can calculate the number of kanbans using a kanban formula. Due to the length of the process, I have broken this into two posts (For the second part click here). Alternatively, you can estimate the number of kanbans and adjust the system as it is running (as shown in a third post).

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The Problem of Losing Kanban – Different Kanban Types

Kanban cardPull production using Kanban is one of the major achievements of the Toyota Production System and hence lean manufacturing. The work in progress is limited by the number of Kanban. Overproduction is avoided by producing only if a part is taken out of the supermarket and the Kanban card is returned to the start of production. However, this Kanban system works only if the Kanban  returns to the start of production. Losing Kanban means not reproducing goods sold. In this post I would like to talk about different methods to prevent the loss of Kanban, including different Kanban types.

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Simple Triangle Kanban System for Office Supplies

One Process Kanban LoopKanban systems are a huge help in industry, ensuring a steady availability of parts and products without excess stock. However, Kanbans are not necessarily limited to industrial use. In this post I will describe a simple Kanban system for office supplies. This system is also sometimes known as triangle kanban. One benefit of this system is that it will improve the availability of pens, paper, and other supplies. However, a second major benefit is that this provides a risk-free opportunity to  train your people in creating and using Kanban systems.

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