CIM: MÜGE BAŞARAN 030090704 WEEK 11 (2)

PRODUCT DESIGN FOR ROBOT ASSEMBLY

Group: DFX synonyms | DFRA

There is no old definition

New definition:

As with product design for high-speed automatic assembly, one objective with robot assem-bly is to provide the designer with a means of estimating the cost of assembling the product—but in this case using robots. However, several important design aspects are affected by the choice of robot assembly system; a choice which, in turn, is affected by various pro-duction parameters such as production volume and the number of parts in the assembly. Three representative types of robot assembly systems can be considered, namely:

1. Single-station with one robot arm

2. Single-station with two robot arms

3. Multistation with robots, special-purpose workheads, and manual assembly stations as appropriate

For a single-station system, parts that required manual handling and assembly, and that must be inserted during the assembly cycle, present special problems. For reasons of safety it would usually be necessary to transfer the assembly to a location or fixture outside the working environment of the robot. This can be accomplished by having the robot place the assembly on a transfer device that carries the assembly to the manual station. After the manual operation has been completed, the assembly can be returned in a similar manner to within reach of the robot. The use of special-purpose workheads for insertion or securing operations presents similar problems to those for manual assembly operations. Two different situations can be encountered. The first involves the insertion or placement of the part by the robot without it being secured immediately. This operation is then followed by a transfer of the assembly to an external workstation to carry out the securing operation; a heavy press fit would be an example. The second situation is where a special-purpose workhead is engineered to interact directly at the robot workfixture. This might take the form of equipment activated from the sides of or underneath the workfixture to carry out soldering, tab bending, or twisting operations, spin riveting, and so on, while the robot has to place and, if necessary, manipulate the part. These major problems with single-station systems do not occur with a mult ista t ion sys-tem, where manual operations or special-purpose workheads can be assigned to individ-ual stations as necessary. This illustrates why it is important to know the type of assembly system likely to be employed when the product is designed. In order to determine assembly costs, it is necessary to obtain estimates of the following:

1. The total cost of all the general-purpose equipment used in the system, including the cost of robots and any transfer devices and versatile grippers—all of which can be employed in the assembly of other products if necessary.

2. The total cost of all the special-purpose equipment and tooling, including special-purpose workheads, special fixtures, special robot tools or grippers, special-purpose feeders, and special magazines, pallets, or part trays.

3. The average assembly cycle time—that is the average time to produce a complete product or assembly.

4. The cost per assembly of the manual labor involved in machine supervision, load-ing feeders, magazines, pallets, or part trays and performing any manual assembly tasks.

Classification systems and databases have been developed for the purpose of cost esti-mating [2]. The information presented allows these estimates to be made and includes one classification and data chart for each of the three basic robot assembly systems. In these charts, insertion or other required operations are classified according to difficulty. For each classification, and depending on the difficulty of the operation, relative cost and t ime factors are given that could be used to estimate equipment costs and assembly times. These costs and time estimates are obtained by entering data from the appropriate chart onto a worksheet for each part insertion and separate operation.

(Geoffrey Boothroyd,Peter Dewhurst,Winston A. Knight, Product Design for Manufacture and Assembly, Third Edition, PG.206-208)

ASSEMBLABILITY EVALUATION METHOD (AEM)

Group: increasing producibility

There is no old definition.

New definition:

The product design review used an analytic design improvement procedure called the assemblability evaluation method (AEM). This was developed at Hitachi (Awane et al., 1981; Hashizume et al., 1980). AEM analyses assembly structures using approximately 20 symbols which give designers and production engineers a quantitative measure of how easily products can be assembled. This analysis highlights weaknesses in product design (location and reason) in terms of as-semblability (assembly producibility). Figure 11.2 shows the three basic features of AEM.

Firstly, evaluation indices are quantified, based on a hundred point maximum. This allows an easy determination of the difficulty of assembly operations and easily identifies design features in need of improvement. The evaluation indices can be used as a common language for designers, production engineers, and managers. The indices also constitute an effective management tool.

Secondly, AEM is easy for designers to learn and use. This makes it possible to evaluate and improve the assemblability of a product in the early design stages.

Finally, assemblability evaluation indices are correlated to assembly cost. This allows the deduction of the standard time for assembly and assembly costs. Designers can then directly evaluate the effect of assemblability improvement in terms of cost.

Two indices are used in AEM: the assemblability evaluation score, which is an index of design quality; and the assemblability cost index. The assemblability evaluation score assigns a value of 100 points to the most easily assembled case. Points are deducted for elements which reduce assemblability.

Figure 11.3 shows that the estimated assembly operation costs, using normalized values, for various product types are very close to the actual production costs. This proves that this method is sufficiently accurate for practical purposes and that assembly operation costs can be easily and accurately estimated at an early design stage. Designers can effectively and efficiently improve product design during the early sta es of development b means of the evaluation and improvement iterations shown in Figure 11.4. This method provides a number of advantages:

(a) facilitation of factory automation;

(b) reduction in assembly labour;

(d) improved reliability of products and automated equipment.

Table 11.1 gives examples of design improvements in a VCR mech-anism. The results of these improvements are summarized in Table 11.2.