The definition of kanban is “a signal that gives an instruction to get, move, produce, order, or take some other activity with production materials”. Its literal translation from the original Japanese term, though, is “signboard” or “billboard”. Kanbans tell you when to order, what to order, how much to order, and where to order it from.
The ordering, though, is not necessarily from an external supplier. It may also be from an upstream process or some other department within your own company. It may even be an order from a warehouse. You will often hear the words kanban system, kanban, and kanban card, used somewhat interchangeably.
This sample shows some of the information you will likely see on a kanban card.
A kanban may be a location on a floor, a cart, a pallet, or anything else you can think of that conveys a message about what to do regarding the materials.
A kanban tells…
A kanban system is one of the key ingredients to transitioning to pull and to the implementation of Standard Work. It is worth noting that kanban systems, due to the tight linking to standard processes, work best with high quality, low variation parts and materials.
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You are probably already familiar with the underlying concept of kanban, even if you can’t recite its definition or if you don’t know it by that name. In all likelihood, you have a kanban system in your home. Dig down in your box of blank checks from your bank and find your last checkbook. On the front of it you will see a close approximation to a kanban card. That piece of paper with the reorder information on it is essentially the same as a kanban card used by your company. The main difference between that reorder slip and the kanban card, though, is that the former does not have as tight of a link to your actual usage of checks, nor does it factor in the procurement time it takes to receive the new checks.
So, think about how the check process works. As you get towards the end of your box of checks you tear off the reorder form and drop it in the mail. Granted, many banks are transitioning to an online ordering system, but the principle is the same. The paper (kanban card) signals an action to purchase more.
So what, exactly, are kanbans supposed to do? Kanbans act to control inventory levels and to make replenishment systems more visual. But be careful about one incorrect assumption that people frequently make about kanbans:
Kanbans do not reduce inventory.
Kanbans tell you when to order, what to order, how much to order, and where to order it from.
If the amount that the kanban tells you to order is set too high, you will be sitting on a lot of excess. If it is set too low, you will run out.
Let’s go back to that checkbook example. If the reorder sheet said to order ten thousand checks, you’d have way too many—even though it was ordered with a kanban. If the order form requisitioned only three checks, you’d probably run out very quickly.
So, how do you calculate the right quantity for the kanban? The answer to that can be rather complicated. Some companies use a sophisticated kanban calculator. It might include some advanced statistics, required service levels, and a bunch of other data. Other calculators are a little less complicated. They might use more estimates and less hard data.
More advanced calculators can bring inventory down effectively, but they require accurate data—and lots of it—to work well. Sometimes the cost of this complexity outweighs the benefit. While determining a kanban quantity from a formula is all science, there is a bit of art involved in picking the parameters for the formula.
Sometimes, though, rough estimates are made according to general data, and then the levels are tweaked through trial and error to adjust the necessary safety stock to cover variations in demand, lead time, and other factors.
Regardless of how advanced a calculator is, the goal is the same. When a kanban card is ‘dropped’ (the slang for depositing the card in a kanban post—the collection point) the remaining parts must keep the production process going until the order is filled.
So what might affect the quantity that needs to be on hand?
The most important factor is knowing how much of your product your customers are actually buying.
The time it takes to get the order filled. Obviously, the longer the lead time, the greater the quantity needed.
You might want a smaller amount, but your supplier has a policy that requires you to order in larger quantities. This is generally because of an inefficient ordering process. You may end up with more than you want for this reason. (Side note: Some companies deal with this issue by using uneven kanban quantities, but that’s beyond the scope of this article.)
For example, eggs generally come by the dozen. You might need 33, but have to order 36—a multiple of a dozen.
If a supplier can’t deliver on time, or frequently mixes up orders, the kanban quantity must go up—at least until you find a better supplier or convince your current one to improve.
If parts often don’t work right, you need more in the bins to account for the scrap rate. Again…only until the supplier improves quality or is replaced.
The following information is often contained on kanban cards:
The most basic form of kanban is the two-bin system. Guess how many bins (or cards) are used? You got it—two. One is normally somewhere in the fulfillment process, and the other is sitting with the materials in the production are. There is generally a little bit of overlap—due to the existence of safety stock. This is the little bit extra to cover the variation in the resupply process. Obviously, you want as little safety stock as you can get away with.
This is where the biggest difference in the calculators is seen. For simple calculators, it is like leaving for work 5 minutes early every day. You just pop in a little extra with each bin. For advanced calculators, the math takes into account standard deviations and statistics and your expected service level—a measure of how often you run out of parts. You might be able to get that five minutes early to work down to 3:47. Doesn’t sound like much of a gain regarding coming to work on time, but when you multiply it out by all the different part numbers in a factory, the savings can really add up.
When parts get expensive or have very long lead times, you might see more than two bins. Instead of being in a supplier’s production queue at one spot for thirty parts, you might have three orders for ten parts each in various stages of the ordering process. One might be somewhere in the internal ordering process, one could be at the supplier’s production area, and the last one might be on the UPS truck. Plus, you’d have a fourth card in your own production area. Obviously, the card is not really in those places—just the parts that the card matches up to.
Think of it like a merry-go-round. If you stand in one spot, the horses file past you. If it had only two horses, each one would have to carry a big load. Add in two more horses, and their loads are cut in half. Multi-card kanban systems are more complicated, and far more prone to errors and lost cards than two-bin systems are, but they do help reduce inventory in some cases.
A common variation of the multi-bin kanban system example is when an empty rack is a signals to move a component through a fabrication process. I frequently use Go-Karts as an example. In my ficticious company, we have a rack for each Go-Kart chassis. The empty rack is a kanban that assembly sends back to fabrication to signal them to produce another one. The racks wind their way through the whole production process. In a two bin system, there might be ten Go-Karts per rack. In my system, we have twenty racks that each hold a single vehicle.
So why kanban and not a computer system? Because it is visual. It is easy to miss something on a computer report. It is hard to walk into a production cell and not notice that a bin is empty, or that a card is missing. Problems become immediately apparent. Of course, many kanban system link into a computer system—a purchaser may just scan a barcode to place an order. But the visual management at the point of use for a kanban is hard to beat.
Plus, operators can be trained to review parts levels in their area. If they notice that both bins are nearly always full, they can point out an opportunity to reduce inventory. If the safety stock is nearly used up with each kanban cycle, they can see that the inventory is set too low.
In addition to the mulit-kanban system, there is also a specialized type of kanban called a ‘supermarket’. This is generally used on mixed model lines. There are a set number of items stored in designated spots. When the downstream process pulls an item, a kanban card or the empty space signals to the upstream process what to build next. The quantities of each type of item are dictated by the model mix. It can be confusing to manage productivity for upstream processes using this type of system when the items come from different areas.
As effective as kanban systems are for helping structure your inventory management efforts, there are some common pitfalls to avoid.
Kanban systems only work when teams follow the rules consistently. If rules are followed sporadically, there will almost certainly be parts shortages.
Material with no kanban card attached, cards lying around, and past due cards without any action are all abnormal conditions and indicators of problems.
People like to hedge. If there’s a secret stash, the kanban system won’t work to highlight problems.
Kanban systems work like beacons. When suppliers are unreliable, parts quality is poor, or lead times are too long, kanban systems make the problems very noticeable. Don’t discard the kanban system—fix the problems.
When demand changes, the kanban quantities will have to change as well. For demand spikes a single special order, often called a ‘white card’ can be placed. For permanent changes, the quantity should be adjusted. Microsoft Excel and Word work together nicely to print cards quickly (merge functions) when demand changes.
Different style cards can signal different things. Triangle cards may be internal. Suppliers may be color-coded. Parts racks can themselves be kanbans. Use kaizen to improve the kanban process.
In kanban systems, your life tends to get easier. Switching to kanban can be a chore (lots of cutting and laminating and Velcro), and it can be unnerving to see your safety net of inventory go away, but most people soon see the benefit.
Once the system is in place, dropping cards for operators is a breeze. Many companies even have material handlers who come past all the work areas collecting cards and empty bins and replenishing materials. When this is done with a standard process, the person is known as a water strider or mizusumashi in Japanese.
The biggest hang-up people have is in maintaining the discipline to keep the system running effectively. It is easy to stick a card in your pocket and bring it home on accident. Or during crunch time, it is easy to grab from the second bin before you drop a card. Be careful—a kanban system, as robust as they can be, will fail if the process is ignored.
Lead by example. Walk through the work areas every day looking for problems with materials. Stress following the process. Make sure you don’t take a shortcut when time gets tight.
Focus on making the system visual so problems are immediately visible. When you transition to kanban, start with the highest value parts first (use an ABC inventory analysis). You’ll set yourself up for the biggest inventory reduction that way.
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