I originally presented this article, "A Software Aid to Product Design for Robot Assembly", as a guest speaker at an I.Mech.E. Congress on Automotive Technology ...

INTRODUCTION

Original work by industrial researchers into classifying and coding parts for automatic parts handling, more than 30 years ago, led to considerations of good design features for automatic handling.  Further research work resulted in a classification and coding system for manual handling, manual insertion and automatic insertion.  This work culminated in the production of assembly system designer guidelines and these were later converted to computer software package to help product designers in the Design for Assembly process. With the increasing interest in the use of industrial robots for assembly, an obvious extension to the work on product design was the development of appropriate classification and coding systems for assembly robots and the translation of this into a user friendly computer based system.

PRODUCT DESIGN FEATURES FOR VARIOUS FORMS OF ASSEMBLY

There are inherent design rules for all forms of assembly and these are independent of the assembly process being used.  There are other rules which are process dependent.  The more important considerations are:

Number of Parts:- For all forms of assembly, reducing the part count, through considering the potential redundancy of every part, leads to a reduction in assembly and component manufacturing costs.
 
Parts Handling:- A further example of a common design requirement is for parts handling where, although manual handling is completely different to automatic handling, both benefit from an increase in the symmetry of a part.   Similarly, both methods cannot easily accommodate minor asymmetrical features, nesting and tangling parts, very small or large parts, etc. As a result of this commonality, features which allow a part to be automatically handled easily are invariably useful for manual assembly and, in general terms, unless there is a significant manufacturing cost penalty, parts can always be designed for automatic handling.

Parts Insertion:- The requirements for the various types of assembly vary significantly for some insertion operations. For manual handling, the emphasis is on access and sighting.  For automatic assembly, the main features are alignment, ease of insertion and stability after insertion.  An additional potential problem is the direction of insertion for robotic assembly.  For fastening operations, regardless of assembly method, the most economic operations use integral fasteners and the most expensive require threaded fasteners.

Parts Gripping:- In manual and automatic assembly (ignoring the features already mentioned related to size), gripping doesn’t create technological or economic problems. In robotic assembly, however, gripping features can be very significant and parts should be designed so that the least number of different grippers are required. This reduces costs and often reduces non-productive assembly time.

Assembly sequence:- The optimum sequence of assembly is very much dependent upon the type of assembly. For single worker assembly, the sequence of assembly is not important and it’s often determined by operator preference.  In manual line assembly, sequence is controlled by line balancing considerations.  In automatic assembly, sequence is related to the basic logic of the equipment and is controlled by the quality and, under some circumstances, the cost of the parts to be assembled.  The sequence of assembly is determined by gripper requirements in single station robotic assembly, where the emphasis is on reducing significant non-productive time, such as either gripper changing or turret indexing.

ASSEMBLY ALTERNATIVES

In manual assembly, the two categories are single worker and line, with many variations incorporating features of both methods. It is generally not too difficult to identify the most appropriate form of assembly.

For automatic assembly, the choice of equipment is limited  and selection is based on the number of parts, cost and component quality. Again, it is not difficult to identify the most appropriate equipment.

In single-station robotic assembly however, the selection of the most appropriate equipment is more difficult.  There are many assembly robot types with different characteristics.  Additionally, there are many parts handling possibilities and various gripper options.  Although basic design for robotic assembly is essentially independent of the particular assembly cell configuration, both the product designer and system designer need some help to evaluate the performance and economics of alternative systems. Software applications have been developed for robotic assembly to serve both these functions.

Firstly, the product design is analysed by investigating its operation sequence relationship, handling features, gripping features and its insertion features. The user is asked to configure a system by specifying the robot to be used.  The cost and performance specification for three popular assembly robots is built into a robot data file and these can be increased by the user at any time. The software application determines the most appropriate assembly sequence, based on interdependencies and the type of robot to be used.  It then evaluates various parts feeding options. These options are based on feeder characteristics built into the system and they can be increased to include new types of automatic feeders.

The software application offers re-design possibilities to reduce the handling cost, where only expensive feeding methods can be used or when only manual handling is possible.  If the robot type is unsuitable, due to lack of capability, then this is reported and the application user can either modify the robot data file, enhancing the robot specification, or select another robot.  If the number of grippers required is excessive then various re-designs for easier gripping are proposed.  The application also takes into account existing equipment utilisation.  This is important because greater utilisation reduces the assembly cost.

CONCLUSIONS

The product and system design software application is a useful tool for evaluating robotic assembly.  It can be modified and extended to reflect advancements in robots, feeders and grippers.  It gives a quick evaluation of the suitability of a product’s design and determines the effect of changing assembly system parameters. These tasks could be done manually, using data sheets, but it is time consuming because of the large number of permutations of the various equipment types.