1. Batch Quantities

Making something in large batches has several negative effects. The first thing which Westerners recognised was that stock levels are partly a function of order sizes. We had a formula for economic batch sizing in which the cost of set-up was offset against the cost of holding the stock. Our theoretical average stock level was half the order quantity + whatever element of safety stock we had built into our plans so reducing the order size would reduce our average stock.

There were, however, other considerations. A piece of plant cannot be immediately responsive to all demands upon it if it makes parts in greater quantities than are required at the time. Responsiveness, and hence service to our customers (whether they be external or the subsequent operations within our own plant) requires that we manufacture components in small batches.

We knew that smooth workloads make management of the manufacturing process far easier and had established smooth finished product plans with the adoption of Master Production Scheduling. However, no matter how smooth our final assembly plans, we still had lumpiness elsewhere.

The major contributor to parts being made in large batches is, of course, set-up times. Shigeo Shingo, a quality consultant hired by Toyota, had set about effectively eliminating set-ups. The accounting conventions that led Western businesses to make significant quantities of parts that may not be used were also shown to be ludicrous.

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