Nailing a horseshoe

For Want of a Nail (Part III)

(This is the third part in a series; see Part I , Part II and Part IV)

In the previous installment we saw how neither the human operators nor the computerized information system had a clear, timely understanding of what inventory was meant to be in a specific location or reserved for a specific purpose. The material handling processes in place did not conform to the principle of visual control (“mieruka”). Although the system was intended as a form of kanban, it did not conform to the principles of kanban because the role of the spare parts pickers had not been considered, and there was no visual feedback for them. This could also be considered a lack of mistake-proofing (“poka-yoke”), and potentially a lack of “5s”, specifically Sort and Set In Order, again from the perspective of the spare parts picker who had no means to distinguish parts available for shipment and parts reserved for production. It also should be considered a dangerously incomplete IT systems implementation of a process, but a reliance on IT systems to “make things work” is itself not understanding the principle or benefit of visual control.

Today’s episode has to do mainly with leveling (“heijunka”). Leveling is always a sore point in implementing a Toyota Production System based process because it is in direct tension with the principle of responding to customer demand. There isn’t any case where customer demand is level and unwavering; even for oxygen, although we are always breathing, as we need more oxygen when active or excited than when resting.

But leveling is fundamental to quality, or more specifically to being able to observe whether results are in line with expectations. If you expect your production line to produce 100 units every day, all of the workers should be working at the same pace every day, and you should minimized if not eliminate mistakes caused by rushing. Further, without having a standard pace for the entire process, it is all but certain that some parts of the process will be able to operate faster than others, and the faster steps will accumulate work in process behind the slower processes. The extra inventory allows the faster process to “hide” errors by recovering while the slower process catches up, a short-term success that obscures the potential for small problems to grow into big ones.

In this particular factory, leveling production was made difficult by the large number of different end products, some of which had constant demand (although varying in total volume) and others with only occasional demand. If the occasional-demand items would politely take turns the production could be leveled and still tied directly to demand, but of course this serendipity was not reliably present.

So the basic system for moving components from storage to the production line relied upon a crew of material handlers to monitor several assembly lines and restock all of them as needed. Most materials we provided via a “two-bin” system: in concept, you start with two full bins. When the first is empty, it is taken away and refilled before the second is used up. (Sometimes more than just two bins are used.) Because the bin quantities were not all aligned to have enough parts for the same number of finished products, and because some parts were used in common across several end products (but other components were not), the rate at which parts would need to be replenished was not reliable. A change-over from assembling one product to another would mean refilling many bins simultaneously; if several lines needed to change over at around the same time, work might back up considerably.

Further, since production lines might build a number of different products and not all were built equally quickly, and not all lines were always producing, there was drastic variation in the level of support needed from material handlers. At the end of the month, and especially at the end of the quarter, pressure to build as much as possible went up sharply.

To compensate for the changing work load on the material handlers, the more senior members of the assembly team were also authorized to retrieve components from storage. When the urgency to maximize production was greatest, production planners (typically responsible for balancing available component parts with current final product demand, a desk-based job) would assist in moving components to the line so that none of the potential builders would lose time moving parts.

The production people were trained, expected, and performance measured in terms of production of complete products. Precision in counting and handling component parts was not part of how their success was evaluated in any direct way.

When parts arrived at the last minute, material handlers (full time or temporary fill-ins) would retrieve the parts directly from the receiving dock, before they had been counted, quality checked, or received into the system at all. Combined with the other uncertainties afforded by the process, the question of how much of an item was available and where it was stored was always unclear.

Several component parts were springs, or spring-like items, which could easily become entangled with one another. These parts, and all small, lightweight parts, were shipped in bags or boxes. Taking parts out of storage, or out of a bin during the assembly process, is very likely to involve pulling out more parts than desired; disentangling was not a straightforward process, and it could be expected that one or more parts could drop off, roll, or bounce into places unknown. These were always small, cheap parts; when pressure was on to build, build, build, it hardly seemed worthwhile to spend time chasing runaway parts. There’s always tons more in the bin.

Ultimately it was not clear how often dropped parts caused shortages versus parts not ever being shipped. Most small, light parts were shipped loose packed in bags or boxes and the count of pieces in each bag or box could vary. The suppliers would write on the box the count, but there was no routine verification by weight. Checking by weight as parts were removed was complicated by the light weight of the parts; they weighed little enough that plastic or cardboard would count as significant number of parts.

It would be relatively straightforward to establish the weight of the parts, establish a minimum stock by weight, and write off the continual shrinkage due to dropped parts or undetectable short shipment. But the parts were bought and tracked by each, and the each-count was always off, and so production was halted for small, cheap parts as often as it was for large expensive parts.

Better than allowing dropped parts and potentially short shipments would be designing purpose-built containers using rods or grooves to keep parts in a consistent orientation, clearly marked with a weight corresponding to an exact count. While it would require a modest investment to design and produce the specialized containers, and possibly working with the supplier to help establish a reliable process to transition the parts from their production equipment to the containers, the payback in reduced uncertainty would have paid back such costs.

For want of a nail, the shoe was lost;
For want of the shoe, the horse was lost;
For want of the horse, the rider was lost;
For want of the rider, the battle was lost;
For want of the battle, the kingdom was lost;
And all from the want of a horseshoe nail.


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