AUTOMATION AND ROBOTICS:ESSENTIAL TECHNOLOGIES

ASSEMBLY ENGINEERING: ESSENTIAL TECHNOLOGIES

The three essential technologies for assembly engineering explained in this section are product design for assembly, simultaneous engineering, and automation of connecting techniques.

Design for Assembly (DFA) and Assemblability Evaluation

Design for Assembly

When a product is designed, consideration is generally given to the ease of manufacture of its individual parts and the function and the appearance of the final product. One of the first steps in the introduction of automation in the assembly process is to reconsider the design of the product to make it simple enough to be performed by a machine.

The subject of design for automatic assembly can be conveniently divided into three sections: product design, design of subassemblies for ease of assembly, and design of parts for feeding and orienting.

The greatest scope for rationalization is in the measures affecting the whole product assembly system. However, these can only be executed for subsequent products, that is, in the long-term. Individual subassemblies can be made easier to assemble for existing products as long as this does not affect the function and the interfaces to the rest of the product remain the same. The success of these rationalization measures, however, is not as great as that concerning the whole assembly system. Measures affecting individual parts have the least potential for rationalization because only small modifications can be made. However, they can be realized quickly with existing products so that the positive effect of the measures can be perceived very quickly (Figure 13).

Various points concerning parts and product design for assembly are summarized below.

Assembly rules for product design and design of subassemblies:

1. Minimize the number of parts.

2. Ensure that the product has a suitable part on which to build the assembly, with accessibility from all sides and sufficient room for assembly.

3. Ensure that the base part has features that will enable it to be readily located in a stable position in the horizontal plane.

4. If possible, design the product so that it can be built up in layer fashion, each part being assembled from above and positively located so that there is no tendency for it to move under the action of horizontal forces during the machine index period.

5. Form subassemblies that can be assembled and inspected independently.

6. Try to facilitate assembly by providing chamfers or tapers that will help to guide and position.

7. Avoid time-consuming fastening operations (e.g., screwing, soldering) and separate joining elements and safety elements (e.g., rivets, safety panes).

8. Avoid loose, bendable parts (e.g., cable).

Assembly rules for the design of parts:

1. Avoid projections, holes, or slots that will cause tangling with identical parts when placed in bulk in the feeder. This may be achieved by arranging that the holes or slots are smaller than the projections.

2. Attempt to make the parts symmetrical to avoid the need for extra orienting devices and the corresponding loss in feeder efficiency.

3. If symmetry cannot be achieved, exaggerate asymmetrical features to facilitate orienting or, alternatively, provide corresponding asymmetrical features that can be used to orient in the parts.

In addition to the assembly design rules, a variety of methods have been developed to analyze component tolerance for assembly and design for assembly by particular assembly equipment.

Assemblability Evaluation

The suitability of a product design for assembly influences its assembly cost and quality. Generally, product engineers attempt to reduce the assembly process cost based on plans drawn by designers. The recent trend has been toward product assemblability from the early design phase in order to respond to the need for reduction in time and production costs.

The method of assemblability evaluation is applied by product designers for quantitatively esti- mating the degree of difficulty as part of the whole product design process. In 1975, the Hitachi Corporation developed the pioneering method of assemblability evaluation called the Assemblability Evaluation Method (AEM). AEM analyzes the assembly structure using 17 symbols, thus aiming to give designers and production engineers an idea of how easily procedures can be assembled (see Figure 14). It points out weak areas of design from the assemblability viewpoint. The basic ideas of the AEM are:

1. Qualification of difficulty of assembly by means of a 100-point system of evaluation indexes

2. Easy analysis and easy calculation, making it possible for designers to evaluate the assembl- ability of the product in the early stage of design

3. Correlation of assemblability evaluation indexes to assembly cost

AEM focuses on the fundamental design phase. The approach is to limit the number of evaluation items so that designers can apply them in practice. The reason for concentrating on the early design phase was to achieve greater savings by considering assemblability issues as priorities from the very

beginning. As a result, however, the accuracy of this evaluation cannot be very high compared to more detailed analyses.

Boothroyd–Dewhurst DFA Method

The Boothroyd–Dewhurst method is widely used. The objective of this method is to minimize the amount of assembly required for existing products or product constructions. This should be achieved first and foremost by reducing to a minimum the number of individual parts that have to be assembled. The shape of the individual parts for handling and assembly is also optimized. The procedure can be divided into three steps:

1. Selection of an assembly principle

2. Analysis of the assembly task (Figure 15)

3. Improvement of design

In the Boothroyd–Dewhurst method, the assembly times and costs as well as the design efficiency (DE) play a decisive role in determining the assembly suitability. The DE is calculated by multiplying the number of individual parts by the calculated costs and comparing this figure with the theoretical minimum number of individual parts multiplied by the ideal costs for assembly. This means that the factor DE always has a value between 0 (very bad) and 1 (ideal assembly system for this product).

The theoretical minimum number of individual parts is determined by examining whether two assembly parts that are associated with another

• Must move toward each other relatively

• Must be made of different materials

• Must be separated from other assembly parts for assembly or dismantling

If none of these three requirements have to be satisfied, both assembly parts may be combined.

Based on the costs and the DE factor, a decision is made on whether reconstruction is possible.

A reconstructed product is then reanalyzed in the same way. Figure 16 shows the original variants and a reconstructed valve. Originally, the lid had to be kept low against the screw compression spring so that both screws could be assembled. Designing the lid with snap connectors for attachment to the casing and the integration of guides to help the screw compression spring into the piston suggested that the number of parts could be reduced from seven to four and the expensive screwing assembly task could be eliminated.

The Boothroyd–Dewhurst method aims primarily at reducing assembly costs by means of an integrated construction system. Other measures, such as product structuring, sorting, the construction of subassemblies and the differential construction technique are not considered. The evaluation of the suitability of the assembly system requires that the product be a constructed one. This means that assembly suitability is taken into consideration only very late in the construction process. The Boothroyd–Dewhurst DFA method is also available for PCs.

Simultaneous Engineering

In the planning and realization of assembly systems, it is not only the selection of the correct operating materials which is important for the project success but also the optimal composition of the project

groups. Teamwork, also called simultaneous engineering, is certainly the most decisive precondition for an optimal and mutually coordinated design of the product and assembly system. The team is made up of product developers, assembly system planners, quality management, purchasing, and sales and can vary depending on the project tasks and decisions to take. However, steps must be taken to ensure that all members of the team are aware of the rules of simultaneous engineering.

Another opportunity for interdisciplinary work arises when there is pressure to shorten product development times. By involving the assembly system planner in the construction process, it is possible to begin much earlier with the assembly system design. Some of the development and planning work may then be executed parallel to one another. The development and realization times are shortened and the costs for the whole project are usually reduced. Additionally, the products can be brought onto the market much earlier due to the shorter development times.

Figure 17 shows a few examples of how simultaneous engineering can help shorten the product development times for new products in comparison with preceding models.

Experience from numerous projects has also shown that, by means of simultaneous engineering, the number of product changes during the development phase can be significantly reduced.

Simultaneous engineering is not really a new way of thinking. However, in recent times it has been applied more consistently and systematically than ever. This is particularly true in Europe. Many Japanese companies have been practising simultaneous engineering successfully since the mid-1970s.

Connecting Technologies

The establishment of connections is one of the most common assembly tasks. The automation of this task is thus one of the central problems confronting assembly rationalization.

Screws

Among the most important assembly procedures is the making of screw connections. The connecting process ‘‘screwing’’ has therefore been examined in detail, and the use of flexible handling appliances such as screw assembly systems has further been tested in typical industrial tasks. The problems of positioning errors, controlled correction of the screwing tool, and diameter flexibility for handling different screw sizes without changing tools have been solved through the development of special hardware modules. The system offers the possibility, in assembling metric screws, of increasing the joining speed, handling screws with different heads (including Phillips), self-tapping in wood and / or plastic, and mounting hexagon head cap screws of different diameters without changing tools.

Rivets

Like screwing, riveting is one of the classic connecting techniques. In recent times, rivets have in fact been gaining in popularity again through the development of new types and manipulation tools and the increasing replacement of steel and iron by other materials.

The greatest market potential among the different types of rivets is forecast for blind rivets. Access to the workpieces being joined is required from only one side, thereby offering the ideal conditions for automation. A blind riveter for industrial robots has been developed based on a standard tool that, apart from its size and weight, is also notable for its enhanced flexibility. With the aid of a changeover device, for example, different rivet diameters and types of blind rivet can be handled.

Self-Pierce Riveting

In contrast to traditional riveting, self-pierce riveting does not require pilot drilling of the steel plate at the joint. The most frequent technique is self-pierce riveting with semitubular rivets, characterized by the rivet punching out only the punch side of the sheet. Joining simply results from noncutting shaping of the die side and widening of the rivet base on the sheet die side. The necessary dies are to be matched with the rivet length and diameter, workpiece thickness, as well as number of interlayer connections, thickness variants, and materials. The workpiece punch side is flat when countersunk punching head rivets are used, whereas the die side is normally uneven after die sinking.

Punching rivet connections with semitubular rivets are gastight and waterproof. Punching rivets are mainly cold extruded pieces of tempering steel, available in different degrees of hardness and provided with different surfaces. Ideal materials for application are therefore light metals and plastics or combinations of both. Punching rivet connections with semitubular rivets show a far better tensile property under strain than comparable spot welding connections.

Press-fitting

Press-fitting as a joining process does not require any additional elements, making it particularly desirable for automation. At the same time, however, high seam joining and bearing pressures occur that may not be transferred to the handling appliance.

A press-fitting tool was therefore developed with allowance for this problem. It confines the bearing pressures within the tool with very little transfer to the handling appliance. Various process parameters, such as joining force, joining path, and joining speed, can be freely programmed so as to increase the flexibility of the tool. A tolerance compensation system is also integrated in the tool that compensates any positioning errors (axis and angle shift). For supply of the connecting elements the tool also has interfaces to interchangeable part-specific magazines (revolving and cartridge mag- azines) and the possibility of supply via formed hoses.

Clinching

Clinching is a way of joining sheet metal parts without connecting elements. A specially designed press-driven set of dies deforms the parts at the joint to provide a friction-locked and keyed connec- tion.

Because no connecting elements are required, the problem of organizing, preparing, and feeding these parts does not apply. For this reason, this relatively recent connecting technique is being used for an increasing number of applications.

A robot-compatible, freely programmable tool has been developed for flexibly automated clinch- ing. This tool is notable for its compact outer dimensions and low handling weight. With this robust but sensitive robot, tool joining forces up to 50 kN can be generated. Different sets of dies for the specific connecting task in question can also be used by means of the automated changeover system. The sensor system integrated in the tool allows precise and individual process control. Rapid working strokes and joining force can all be freely programmed. The quality and success of the join at each point are checked and documented.