Tales from the Quality World- Heijunka
Tales from the Quality World- Episode 6: Heijunka
Very early in my career in quality engineering, my first position was primarily focused on continuous improvement.
Following the principles of the Toyota Production System and Shigeo Shingo, I performed plant layout revisions for better flow of product (the waste of movement) and reduction of inventory (the waste of excess production).
Most people understand kanban quite readily as a visual indicator to trigger a round of production of a given product. This can be implemented on production lines that require change-over from product to product. The inventory managed by the kanban, of course, has to be calculated to create enough buffer to ship to the customer while incorporating into the timing the necessary change-over time.
Properly used, kanban systems can remove the need of management and supervisors to manage inventory to focus on other aspects of the business.
But many kanban systems do not utilize heijunka. I borrowed a very good illustration of a simple heijunka from a Wikipedia article. It is the heijunka which really links a production operation to the needs of its customers!
For the given part(s) to be produced, the production line(s) must know the customer demand. Using the illustration, let’s consider these products bumpers. Five different bumpers are being produced at this manufacturing site. Each card in the box represents one rack or container of a given mumber of bumpers. Clearly, bumpers A and B are in high demand, whereas C and D are in demand about half that of the first two. Bumper E even less so.
Imagine bumpers A and B go onto vehicles of high volume and demand in the marketplace such as a Ford 150. Bumper E, though, is used on the Ford electric Mustang.
The cards are loaded to the heijunka to reflect actual use a the customer’s facility. Across the top of the box over a two shift operation, times are assigned to each column. The forktruck operator swings by the box every fifteen minutes or thirty minutes as designated and pulls cards from that column and loads the Semi-trailer with racks of each bumper in that column.
The cards pulled then go into a lot-making board. The board would consist, in our example, of five columns. Each column would have a trigger point determined by capacity plus change-over time consistent with customer demand. As cards are pulled from the heijunka and moved to the lot-making board, at some point on one of our five bumpers A through E, we would reach a trigger point for one of the bumpers, requiring the production line to change over and produce parts to meet the number of cards of that letter.
Production managers would cease to manage when to change over as well as how many to produce. The system would manage this process.
At my company for which I was doing this work, I would pull the cards that accumulated after production and arrange them in order of their placement in the heijunka box and give them to a supervisor to put in the box. However, after a couple weeks I was getting complaints from the supervisor that they were running out of a given bumper and the kanban system wasn’t working.
The following morning I again pulled all the cards for inventory that was produced on the line and organized them for the heijunka box. The order might be something like this: A-B-A-B-C-D-A-B-E-A-B-A-B-C-D-A-B-A-B-E You get the idea. Because of higher demand, bumpers A and B will be pulled more often.
Remember, change-over on the production line doesn’t happen until a trigger point is reached. This indeed might result in the production line doing more frequent change-overs from A to B to C to A to B to D to E, etc.
But recall, the supervisor ran out of a given bumper to load the trailer and complained the kanban failed. Did it? And recall I just gave him the new cards sequenced for the heijunka box. I happened to walk past the supervisor’s office and spied through the window that he was manipulating the cards! Instead of the demand based sequence I handed him, he was sorting the cards into A-A-A-A-A-B-B-B-B-B-C-C-D-D-E.
Notice how his ‘re-arrangement’ results in fewer change-overs. But it also ignores customer demand cycle and manipulated the kanban system resulting in inventory imbalance.
I watched him walk the cards over to the heijunka box and place them in his manipulated order. I approached him and asked why he changed the order of the cards from that which I had given him. He said he wanted to reduce the number of change-overs (naturally).
I told him that by doing so HE had created the shortage of one of the bumpers he had complained about. I said he didn’t need to worry about inventory using the kanban system.
He said- if he wasn’t managing the parts what was he supposed to do?
“Manage your people,” I responded. Train them, improve their skills, cross-train.
Back then, many manufacturing supervisors were promoted from the shop floor and only knew parts. And upper management didn’t develop the supervisor’s skills, hence supervisors didn’t develop staff skills. A lot has changed in the last twenty years and I see far more supervisors working on developing their staff. But
