Industrial Assembly

The purpose of Chapter 1 is to provide the reader with an overview of the scope of assembly. The chapter begins by describing the fundamental concepts and characteristics of assembly, followed by a review of its historical development (section 1.2) and economic significance (section 1.3). The introduction continues with engineering and management techniques and approaches for rationalizing and improving assembly operations and systems in industry (sections 1.4, 1.5). Most of these rationalization strategies are explained in detail later in the book. The first part of Chapter 1 is concluded (in section 1.6) with a general taxonomy of assembly operations and systems.

Toddlers all over the world are often given toys to teach them, quite early in life, basic assembly skills. Some toys have pegs of different shapes to be inserted in matching openings in a box. Other toys have rings of different diameter, the purpose being to stack and insert them on a post according to size. Such toys help develop the child’s skills and coordination. Naturally, they involve manual dexterity.

In the ‘old days’ assembly methods were selected by engineers only after the product design had been completed, approved and authorized. As long as all the assembly work was manual, human assemblers could be expected to learn how to assemble even complicated products. There were some guidelines on how to plan the assembly method for effective manual assembly. When parts had to be designed for the non-forgiving automatic assembly machines, and later for robotic assembly (whose cost increases exponentially the more forgiving it is expected to be), designers realized the need to ‘design ahead’.

The success of any assembly system depends on the translation of its design into an implemented working facility. The purpose of this chapter is to describe and explain the system elements and how they are combined into an effective working system.

This chapter deals with key aspects of designing and planning assembly systems. Until recently, assembly has been considered by many to be a low technology process, and its costs and importance may have been underestimated. However, with the advent of automated assembly — especially programmable, automated assembly — fundamental issues are being rethought and new methods are being devised to assist designers and planners.

The engineer designing an assembly system is typically concerned with comparing alternative designs and must be able to evaluate the performance that each might achieve in the long run in order to select the most appropriate design. Decisions such as these are, perhaps, best made using a model of steady-state performance.

Design, planning and performance evaluation — the topics of Chapters 4, 5 and 6 — establish the system in which assembly operations are performed. Decisions based on design, planning and performance evaluation establish system capacity by fixing the number of resources (e.g. machines, tools and workers) and by determining fundamentals of the assembly process. Scheduling — and, more generally, production control — must operate within this constrained framework to ensure the timely flow of materials. Thus scheduling decisions are made in a dynamic, time-dependent environment. Even though scheduling decisions are made in a highly constrained environment, they can exert significant influence on the operation of an assembly system. In particular, scheduling is crucial to controlling in-process inventories and cycle times, and to providing competitive levels of customer (i.e. due date) performance. Thus scheduling is an important component in world-class assembly systems.

One unique aspect of assembly is that it involves the merging of part flows. Assembly logistics thus require thoughtful coordination of material flows and present significant challenges to material flow managers. Poor coordination of material flows leads to excessive in-process inventories as well as inability to achieve assembly schedules.

Assembly is often described as the ‘moment of truth’ for products, when finally complete subassemblies, and then the full assembly, can be tested. But it is also recognized that assembly is where previous defects may be ‘buried and hidden’ not to be discovered until much later. By emphasizing ‘quality at the source’ — essentially at every step of the production in — the potential for hidden defects decreases, and the need for the correction of errors by costly repair and rework at later stages diminishes. But inspection during assembly is becoming more necessary because most tests for functionality are still possible only during and after an assembly is built.

In early studies of assembly by Walker and Guest (1952) after World War II, the focus was on manual assembly work in the context of mass production. An emerging technological trend at that time was the assembly line, and the investigation revealed an increasing worker dissatisfaction with machine-paced work. It was predicted that the approach of mass production would prevail and even be ‘exported’ from the US to the rest of the industrial world. The major conclusion was that it would be necessary in the future to develop human-oriented systems and work methods.