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Question:

I am new to value stream mapping. How do I manage machine centers that are members of several value streams? Our primary business is custom metal fabrication, so our cell layouts can’t necessarily be built around one value stream because different parts might require different processing. How can we approach value stream mapping with flexibility to accommodate different custom products?

Answer:

In the first cut of value stream mapping, you should attempt to optimize around 70 percent of your flow. After that, hand over the process to process owners or value stream managers on the shop floor to further improve it. To achieve this, you need to divide the entire product line into product families and then tackle similar products that make up a family as one unit to come up with the value stream map.

When you have a resource that is used by multiple product families, take into account the percentage of time the resource gets consumed by the product families. This will help you optimize the flow. The shared resource can then be placed in a layout so that it optimizes the flow of the product family that uses the resource the most.

In a custom setting, it is difficult to pinpoint on a day-to-day basis exactly what part of the resource will be used by which part family. Therefore, in custom shops it makes sense to use a generic representative product to help create a value stream map and optimize flow.

For example, in a custom machine shop, the general flow of products is cutting, turning, milling, heat treatment, grinding, and assembly. Some products might not need some of the steps, but you can create a generic representative product to allow you to follow the general sequence and optimize flow. Taking this approach, you can also create generic work cells through which products can flow to optimize the overall flow.

The main goal of value stream mapping is to create flow such that the percentage value-added time ratio (%VAT) is maximized. The %VAT is the total processing time required for a product family divided by the total amount of time it stays in manufacturing, stated as a percentage. This allows you to see for how much time the product was actually being worked on during the period that it was moving through the various operations in the shop. 

Merwan Mehta, Ph.D. 

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