Drum-Buffer-Rope for Lower Inventory.

Author:Umble, M. Michael

Executive Summary

The drum-buffer-rope logistics system can help facilities operate with a minimum of inventory and expenses. By applying a DBR system and using stock and time buffers appropriately, you may be able to boost your manufacturing plant's efficiency. The first step is to decide whether your company's facility is a V-, A-, or T-plant.

"We're different!" manufacturing managers often claim, asserting that their plant is unlike any other. Every manufacturing organization does have unique characteristics and problems that are a function of company culture, management style, and established policies. But each plant also shares significant common ground with plants from a variety of industries.

A system known as V/A/T plant classification provides a useful structure for classifying and analyzing manufacturing operations, providing managers with valuable perspectives about how to manage their process effectively Let's look at how various stock and time buffers that are required to support the drum-buffer-rope (DBR) logistical system are used in each type of plant environment.

What are V-, A-, and T-plants?

V/A/T plant classification was developed primarily by Eli Goldratt. Around 1980, while running a consulting organization called Creative Output, Goldratt noticed that manufacturing plants in very different industries seemed to have similar characteristics and problems. From this experience he developed the V/A/T classification system.

Product flow diagrams are used to determine the structure of a plant. Three specific categories of points are of special interest in product flow diagrams: divergence points, convergent assembly points, and divergent assembly points. Divergence points are steps in the product flow at which material may be transformed into two or more distinctly different materials. Convergent assembly points are points at which two or more component parts are assembled to form a single parent item. Divergent assembly points occur when a number of common component parts may be combined or assembled in a variety of ways to form a large number of possible parent items. These points are illustrated in Figure 1.

The product flow diagram of a specific manufacturing environment may include divergence points, convergent assembly points, and divergent assembly points. However, one of these three categories will usually dominate. This observation led to the development of three basic plant classification categories: V-plants, A-plants, and T-plants.

Characteristics of V-plants

Product flow diagrams for V-plants are characterized by divergence points throughout the production process. In such plants, a single piece of material can be increasingly transformed at each divergence point into a very large number of distinctly different end items. The general shape of the product flow diagram resembles the letter "V," hence the designation V-plant. The typical structure of the product flow diagram for a V-plant is shown in figure 2. The Figure shows a single piece of material that can be transformed by four different resources (R1 through R4) into eight different materials (M8 through M15).

An example of a V-plant is a steel rolling mill that performs annealing, rolling, heat treating, and slitting operations. Sheets of steel are first sent through annealing, which heats and softens the steel in preparation for the rolling operation. At rolling, the sheets are crushed into a variety of thicknesses. Heat-treat furnaces then temper the steel to numerous combinations of strength and hardness. Finally, the steel is cut into strips of desired width at the slitting operation. The rolling, heat treating, and slitting operations represent significant divergence points in the product flow that can transform a single sheet of steel into several thousand different end items.

Most basic producer or converter plants -- such as textile, paper, chemical, and basic metal processes -- are V-plants. Many fabrication plants that produce a wide variety of products from basic materials, such as metal, plastic, wood, and cotton fibers, are also V-plants. For example, the basic structure of product flow diagrams in a textile plant would be very similar to the product flow diagrams for a steel rolling mill. In a textile plant, operations include dyeing, weaving, finishing, and sewing -- divergence points at which a given fabric is transformed into an increasingly larger variety of products. Thus, despite their obvious differences, rolling mills and textile plants typically share many critical characteristics. The same is true of all plants that are dominated by divergence points and have the general V-plant configuration. Three characteristics are typical of V-plants:

* The number of end items is large compared to the number of raw materials.

* All end items sold by the plant are processed in essentially the same way.

* The equipment is generally capital-intensive, highly specialized, and typically requires lengthy set-ups.

Since V-plants are characterized by common materials processed through a series of divergence points, poorly designed operating procedures may cause excess inventory long lead times, and constant expediting efforts to meet shipping schedules. For example, minimum batch sizing policies or a focus on efficiencies may cause material to be processed in batches larger than necessary to satisfy current customer demand. If this occurs, those materials needed to fill current orders wait needlessly in queues, leading to larger work-in-process and finished-goods inventories and inflated production lead times. If the material is allocated to produce the wrong products, the resulting material shortage for current orders will require additional materials to be expedited and production priorities to be reshuffled.

Characteristics of A-plants

A-plants are characterized by convergent assembly points throughout the process. In such plants, a large number of purchased or fabricated component parts and materials are combined to form subassemblies that...

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