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Method - Datum

When we design the process to have a “Build-in Quality” condition, I learned that the necessary tools are not just Andon & Poka-yoke. There is something far more important.


Datum. Datum is defined as the following in ISO5459:2011.

“One or more situation features of one or more features associated with one or more real integral features selected to define the location or orientation, or both, of a tolerance zone or an ideal feature representing for instance, a virtual condition”

It is the basis of geometric dimensioning and tolerancing and the subject will cover datum line, surface, etc. It is something that every engineer knows and used.


On top of this GD&T, Toyota asked an interesting question.

“What will happen to the product if different processes used different datum?”

Once I got a project to reduce the setup time of a line. I had pretty good confidence about this project but after observing the setup for two hours, I was completely confused. Nothing made sense. In the end, I asked the engineer how many types of products are produced on this line and the answer was one. Then I asked, why are we doing “Setup”?

In the end, we found out that all supplying processes were using different “Datum”, causing variations in the product.

The lesson that I learned was that datum is not an independent decision of each process designer. Rather a product & every process engineering should work together to accomplish build-in quality.

“Areas” or the standardized work.

Datum is not just an engineering topic. It is an important topic for the operators. Their standardized work should start from the datum. “Set the part” should mean “align the datum of the product and the fixture”. If they can not set the part to the datum, they should be allowed to call for support and immediately fixed. In volume centric environment, a broken datum is usually ignored. Broken pallets and/or conveyors are also a similar problem of not respecting datum. Unless there is culture or management that respects datum is on the shop floor, building quality into the process is hard.

“Solid” or the organizational approach.

One important thing for the organization is, as I already wrote above, that all stages & functions need to use the same datum. Starting from product design, everybody should use the same datum. If not, that stage or function should be considered as a high-risk area of quality.

Since I was introduced to this datum, I start wondering where is the datum of a vehicle. I learned that it is at the “Front of the vehicle”. If so, what does “Front of the vehicle” means for TPS? The front of the vehicle is the trigger of standardized work. It is the trigger of Andon, Kanban, etc. The entire system moves based on the datum. So instead of saying standardized work starts when the “Front of vehicle” moves, we might have to say when the datum moves into the area, standardized work starts. And by intentionally designing the datum to the direction of the flow, it becomes easier for everybody to respect the datum.

Datum is the critical point of process design.

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