For years I taught the following regarding Kanban as a method for material delivery. First, Kanban is a pull system, and pull is good, push is bad. Pull tightly controls the amount of inventory in the system.
Second, material handling is an overhead and non-value-adding function, so you should choose the most efficient way to get materials to the various Points of Use, and that is Kanban. Third, most materials are “C” items, and larger Kanban quantities will not drive down inventory turns significantly (down is bad), and allow them to be delivered less frequently. “A” ,”B”, and unique items will probably need to be delivered using other methods.
Finally, having most of the required material stocked permanently in front of the operator increases his/her flexibility to switch from one product to another in a Mixed Model line. All good reasons, right? There are more, and you can list them in the comments section below!
I have noticed, however, a sea change in material delivery philosophy and methods. I’ve seen this in plant tours, in discussions with students in our Material Management workshops, and in articles posted on the web. That’s the only evidence that I have to back up this statement, but here goes: Lean companies are moving away from Kanban (not abandoning it) and towards a higher percentage of sequencing and kitting methods. What is driving this are a number of factors.
TAKE A CLOSE LOOK AT OPERATOR PRODUCTIVITY
Make a video recording of your current production lines and watch the video closely. [Side comment: a great way to document what you see in a video, and to create Standard Work documentation, is a software tool called Timer Pro. You can easily capture work steps, and document VA and NVA work.] Chances are high that you’ll see a lot of part retrieval time, identifying the right parts, reaching and sometimes walking to get them, and positioning them. The more complex the Kanban presentation, the more part retrieval time required. Also, the shorter the Takt Time, the higher the percentage of the total cycle is dedicated to this clearly non-value-adding work.
This form of waste, which falls in the Toyota 7-Wastes classification as the waste of motion, is amplified by the total number of operators on the line. Which are certainly more than the number of material handlers.
Could presenting just what the operator needs, for the specific unit in front of them, reduce some of this waste? Sure, of course. We’ll talk about how to do that further below in this post. What kind of productivity boost can you expect? That depends on your Current State, but I have seen production lines where 20-40% of each cycle was consumed by parts retrieval. Ouch!
THE RISE OF MIXED MODEL COMPLEXITY
Another factor driving companies away from Kanban and towards other delivery methods is the continuing rise of Mixed Model complexity. Over time companies naturally add new products and options, and less frequently get rid of the older ones. The level of complexity therefore grows relentlessly, putting a strain on the material delivery system and particularly on the Kanban system. Remember that a Kanban part is stored permanently at a workstation, whether it is needed there today or not. The amount of physical space required at a workstation for material storage therefore tends to grow over time, and can easily overwhelm the space available. All of this contributes to the point we made previously regarding part selection time.
MISTAKE-PROOFING MATERIAL DELIVERY
The third major motivator for moving away from Kanban material delivery methods is a growing concern with part selection quality. We already discussed the part retrieval time waste, but what happens when the wrong part is retrieved? This is of special concern when the parts are actually interchangeable, like shims of different thicknesses, and where the difference between the parts is not distinguishable visually. Yes, there are various methods that can be used to reduce this risk: Pick To Light systems, color coding, Check-Do-Check verification. All of these add additional complexity and cost to the material presentation effort.
WHAT DO THE BIG BOYS DO?
I recently had a chance to tour the Toyota Georgetown plant in Kentucky, where they make the Camry, and what I saw surprised me. This is a huge plant, and I didn’t see everything, but the Points of Use that I did see were virtually Kanban-free. This was true even on lines where the variability was relatively low, like the engine prep line. So how were materials being delivered?
Integrated into and adjacent to the lines were a series of kitting stations, where parts were selected and placed on a kitting tray, one unit at a time. The trays were then delivered to the beginning of the assembly line via AGVs, circling around in a continuous loop. Yes, Kanban was used in the Kitting Cells, but not in front of the operators. Use of the Kanban method had been pushed upstream, to Kitting Cells and to Kanban Supermarkets, but eliminated from the assembly lines themselves.
I had a chance to discuss this with some Toyota MEs, and they told me that this change had been made within the last few years, that it had resulted in a 5% productivity jump overall (including additional people in the Kitting Cells), and that it eliminated part selection errors. The immediate driver for this change, however, was in floor space savings. Removing all line-side material other than a small kit tray enabled the lines increase capacity and operators without increasing the space needed. This was huge for them.
This method was not unique to Toyota Georgetown. I have seen similar strategies in place at other Lean companies, on recently designed lines. Integrating Kitting Cells with the production lines, and running them at the same Takt Time, is only one of several kitting and sequencing options. Apply the Scientific Method, and pick the system that works best for you.
I should mention also that other Toyota plants operate differently. Toyota Material Handling uses a hybrid material delivery strategy, with commonly-used and smaller items stocked at the workstations using the Kanban replenishment method, while sequencing/kitting the unique parts. This fits much more closely with the material delivery model that we’ve been teaching over the years. They have been growing, and it would not surprise me to see them moving to the Georgetown model over time, in order to conserve factory floor space.See this in the flesh at our Lean Material Management workshop!
So is Kanban dead? Absolutely not, but there are some very interesting changes afoot in the world of Lean Material Delivery, which can have a very positive impact on quality, space utilization, and overall productivity!