Method - Datum

Toyota is a mechanical engineering company. If so, what is the connection between mechanical engineering and the Toyota production system? To understand the link, this post is about datum reference.

Datum reference or 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 the datum line, surface, etc. It refers to the geometry of an object and measures it against a standard or desired value. Every engineer knows and uses it.

On top of this, GD&T and there is an interesting question.

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

A product came from three different suppliers and was followed by one machining process, but multiple machines with varying clamping methods were used. Each of these processes was designed by various engineers, setting different datum references in the process. The final assembly line had precise operations to do. Yet, depending on the flow of the product, the distortion of the product was different. Assembly operators knew some of this weirdness. So, depending on the supplier, they tried to adjust the process even if it was the same part number. But this was guessing based on where the part came from—the process of adjusting was lengthy and unpredictable. This adjustment disappeared when we learned the variations in the datum references and solved the issue cheaply.

The same datum reference should be used throughout the material's flow. If not, processes that use a different datum reference should be highlighted as high-quality risk locations. At the very least, the impact of using different datum references in the material flow should be understood with caution.

Datum is not just an engineering topic. It is an essential topic for the operators. Their standardized work should start from the datum reference. “Set the part” should mean “align the datum reference of the product and the fixture.” Such detail should be described inside the work element sheet and trained by following the job instructions. If they can not set the part to the datum reference, they should call for support and immediately get the problem fixed. In a volume-centric environment, a broken datum reference is usually ignored. For example, broken pallets and conveyors do not respect datum reference similarly. Building quality in the process is complicated without a system or management that respects the datum reference on the shop floor.

An organization must use the exact datum reference for all stages and functions. Starting from product design, everybody should use the same datum reference. If not, that stage or function should be considered a high-risk quality area.

Since I was introduced to this datum reference, I wondered where a vehicle's reference is. I learned that it is at the “Front of the vehicle.” If so, what does “Front of the vehicle” mean for TPS? The front of the vehicle is the trigger of standardized work. When the “front of product” enters an area, we should start the standardized work. If the work is not completed when the “front of product” leaves an area, we should pull the Andon. As the “front of product” passes the area, we use parts and eventually trigger the Kanban. It is the trigger of Andon, Kanban, etc. The entire system moves based on the datum reference crossing the line. So, instead of saying standardized work starts when the “Front of the product” moves, we might have to say when the datum reference moves into the area, standardized work starts. This makes sense since there are products with decorations at the front that have little meaning in terms of work. By intentionally designing the datum in the direction of the flow, it becomes easier for everybody to respect the datum reference.

This discussion about where the reference point is will become an important topic when applying TPS to a non-manufacturing environment. For example, for aviation travel, where is the datum reference? If we say it is on the body of the customer, then all the time, the customer searching for where to go will be ignored. Waiting in line will be neglected since the customer hasn’t reached the work area. The worst will be that each process defines the reference point based on what is convenient for them. When that happens, the customer will be left in the blind spot of service.

Datum reference is a mechanical engineering concept. However, we should consider it when implementing the Toyota Production System.