Russell Davison

I re-design products so that they cost less to make. Redundant parts are eliminated and design features are integrated. The least number of components are used to meet functional requirements. Parts handling is made easier and assembly operations are simplified.

Here's an article that I originally presented at the 6th Annual British Design Engineering Conference ...

Product design for ease of assembly is a factor that should be considered alongside other parameters from the start of product creation. Considerations, such as minimizing the number of parts and reducing the difficulty levels of handling and assembly, are of major importance for both manual and automatic assembly. Additional areas of investigation are needed when employing assembly automation, due to the use of relatively 'sensor-less' human imitators.

Certain design features must be incorporated, for ease of orientation and feeding, and assembly processes must be kept simple and efficient. I describe, on the following 14 posts, how to design a product for automatic assembly and achieve the required production rate, with minimum rectification work of defective products.

A big part of a product's factory cost is dictated by the product designer. Established design goals, such as minimum material usage and the use of standard components, have always been given top priority by the designer to obtain low direct material costs. Manufacturability has played an important part when considering parts forming techniques and, to a lesser extent, assembly techniques. It is now time to change the emphasis from "how parts are to be made" to "how they are to be put together".

Historically, industry's direct labour costs in the developed nations have been acceptably low in the manufacture of medium to high volume, low technology products. This situation has changed over the last decade . Globalisation exposes the significant difference in labour costs between developed and developing nations.

Labour costs can form a major part of the factory cost of a product. In particular, assembly labour costs per product can account for well over half of the total labour costs.

Assembly is the final labour intensive manufacturing process to be conquered by the automation engineer. The comparatively high factory cost caused by assembly labour points engineers to assembly automation as an economic alternative to 'offshoring'. Assembly automation not only gives tangible economic benefits. It also gives other, less quantitative advantages. The added benefits are a greater control over production, lower floor space requirements and higher finished product quality levels. Exclusion of these less quantifiable benefits can have a critical effect upon the economic justification of an automatic assembly system.

Successful implementation of an automatic assembly system involves many disciplines. Harmonizing of a product design and it's assembly system is an iterative process. It relies upon co-operation between the product designer, production engineer, cost accountant, and equipment supplier. Product design changes requested by the production engineer are created by the product designer and evaluated by the cost accountant, for the effect on factory cost.

The design for assembly process starts with an existing product already in production, a prototype ready for the product to go into production, or a set of product drawings - whilst the design is still being finalized. An optimum assembly system will exist for any product, by considering the following points :

- market life of product
- company policy towards automation
- number of product styles
- number of anticipated design changes
- annual production volumes
- number of parts in the product
- individual component properties
- type of assembly operations

Once the most economical form of assembly system has been established, the product can be re-designed for manual or automatic assembly.

The product is assembled with a note of all handling and assembly operations required to complete the product. For manual handling; weight, shape, symmetry, and bulk properties are noted and recorded.

For manual assembly; the process, access, movement, resistance, and alignment of the parts are considered. Time penalties are given to those parts which have difficulties in excess of handling and assembling a standard part. A design efficiency can be calculated for the existing design. The task of the design team is to increase this efficiency and reduce the factory cost. The efficiency can be increased by similar methods to be described for automatic assembly.

Product design for automatic assembly can be implemented at any stage in the manufacturing process. It is best, however, to design for automatic assembly before the product goes into production. The problems associated with re-design, once the product has gone into production are many. Firstly, the cost of design changes, in terms of re-tooling, may outweigh the savings gained by greater productivity. Secondly, an unacceptable lead time may exist from company approval and customer approval to getting the re-designed product into production. Also, once a product has been re­designed for automatic assembly, good productivity gains can often be realized by incorporating the new product into an existing manual assembly line, without investment in an automatic assembly system. This highlights the fact that a product re-designed for automatic assembly always provides savings in manual assembly, when compared with an existing design.

The object of a design for automatic assembly investigation is to increase the design efficiency of the product.

The UMass method of representing the efficiency of a product design is :

Design efficiency, E = N (H + A) / T

Where :
E = automatic assembly design efficiency
N = theoretical minimum number of parts
H = automatic handling cost per part
A = automatic assembly cost per part
T = total operation cost

A 100% efficient product design has the following qualities :

- It uses the theoretical minimum number of parts
- The number of insertion operations equals to the theoretical minimum number of parts
- The difficulty level in feeding is similar to feeding a 2.5 cm cube at 1 per second
- The difficulty level of insertion is similar to that of inserting a standard part at 1 per second

Well designed products have UMass design efficiencies between 20% and 30%. Poorly designed products have efficiencies less than 5%. Alternative product designs can be compared, in terms of design efficiency, to obtain the most economic design.

Any detailed investigation into the design of a product results in a more productive method of assembly. The parts and assembly operations used to put together the parts can be viewed with either manual or automatic assembly in mind. When designing for automatic assembly, remember that the intricate feedback loop that co-ordinates human motors is not present in economically justifiable automatic assembly systems. For example, parts that are manually picked up the wrong way round can have their orientation corrected. The human assembly worker detects this error through sight or touch and quickly corrects the orientation. Similarly, defective parts can be detected and discarded by a human assembly worker.

Automatic workheads do not detect rejects without the aid of complex sensor systems. A defective part arriving at the workhead causes a jam and the workstation is down for a period of time. These events are minimized by having high component quality levels and re­structuring the inspection routines. Most manual assembly systems have three inspection stages - goods inward, during assembly, and upon final assembly of the product. Parts or assemblies which do not fall within quality bands, at each of these stages, are rejected. Automatic assembly equipment requires higher quality components and, therefore, greater quality control is required at the goods inward stage than for manual assembly. Automatic assembly equipment, fed with high quality parts, gives a higher quality finished product than manual assembly. The consistency of an automatic system, aided by high quality parts creates a high quality product.

Dedicated assembly systems, compared with flexible assembly systems, are generally intolerable of defective parts. It has proved effective to install a 100% inspection station prior to the workhead to maintain up-time in some installations.

The ease with which a part can be presented to a workhead automatically at the required feed rate, and assembled within the specified cycle time, depends upon the; design of the part, design of the equipment, and the method of assembly. Each element of assembly automation has it's own maximum performance characteristic. For example, the cycle time of a pick and place unit depends upon the degrees of freedom, the actuator stroke and the actuating medium, i.e. compressed air, hydraulic oil, DC servo motor. A feeder is limited by its maximum conveying velocity and the insertion process is affected by the positional accuracy of the rotary table, robot, or platen location.

The product designer must analyse the product and consider if it can be economically assembled in it's present state, or decide if design changes are necessary for automatic assembly. In practice, the majority of products assembled manually require design changes to make automatic assembly viable. The product designer must consider what further benefits can be gained from more re­designing of design features, assembly operations, or even the elimination of parts. The process of design for automatic assembly is best effected by a systematic approach. A structured method for evaluating designs to identify inefficient features has been developed by UMass and other organisations.

An automatic feeding device and, at least, one workhead is required for each component to be automatically assembled into a part-built product. A significant reduction in cost is achieved by eliminating a part from a product. Designers should strive for the irreducible number of separate components per assembly, consistent with its performance and fitness for purpose. An investigation into the function of the product exposes redundant parts and these should be eliminated.

Fasteners, which are separate from the component being secured, should be avoided. Fastening technologies of the future are based on adhesives, ultrasonic welding, soldering, resistance welding, clip fastening, and twisted tab joining. Fasteners can be classed as being permanent or semi-permanent. Permanent fasteners do not permit removal, e.g. adhesives. Semi-permanent fasteners do permit removal, e.g. screws.

The range of plastic snap-in fasteners is classed as permanent or semi-permanent, as they can be removed with the aid of special tools. It is not feasible to repair products with these permanent joints. This leads to 'throw away' products, upon a fault occurring, unless self-contained sub-assemblies are used in standard modules.

Parts integration dictates that groups of components should, where possible, be manufactured as a single part by chip-less forming, e.g. precision die-casting, precision plastic moulding, powder metallurgy, investment casting, fine blanking, and high energy rate forming. These methods produce, in one single operation, the features of a number of parts without the use of fasteners. Also, features for identification by the automatic bowl feeder tooling can be cast into the part.

Each part in an assembly serves a purpose with the aid of functional features. The designer should group a number of parts together to form a single part with multi-functional features.

Create a precedence diagram for the assembly operations. Identify redundant areas of the operation and incorporate the functional parts of these operations into other operations.