Design for assembly

Design for assembly A H REDFORD OepartmentofAeronauticalandMechanicalEngineering,Universityof Salford,UK

A systematic approach to design forassembly is discussed. Product analysis is suggested to reduce the cost of parts and assembly time. Methods of analysis and worksheet compilation are advocated for greater efficiency in re-design.

Keywords: assembly design, worksheet compilation, automatic handling

170

For the past few years, considerable emphasis has been placed on the benefits of improving productivity, and in general this has been taken to mean labour productivity, ie the output/man hour. Improvements in labour productivity are usually achieved either by making more effective use of the workforce or introducing more productive equipment. While both these aims are worthwhile it must be borne in mind that the effectiveness of a manufacturing company is based on its total productivity (the relationship between total output and total input) and improvements in labour productivity do not necessarily meet this aim, since the introduction of high capital cost equipment without an appropriate reduction in labour content or increase in output, orthe payment of excessive bonuses for improved labour commitment can be counter-productive. Consideration of productivity is important to all industries but manufacturing productivity should be considered to be particularly important, because of the relatively large impact that this sector has on the generation of national wealth. Within manufacturing industry the discrete parts/durable goods industries form a major target for productivity improvements since these are under direct attack from economically priced high quality items. These industries which manufacture engines, farm, metal-working and electrical machinery, communications equipment, motor vehicles, aircraft and shipscontributethe major proportion of all manufacturing exports and yet they are not the highly efficient manufacturing units that could be expected. The bulk of their products are produced in small batches in small inefficient factories, using relatively ancient machines and tools. They are also usually very dependent on labour for the handling, insertion and fastening of parts of individual products and they are often provided with tools no more sophisticated than screwdrivers, spanners and hammers. It is not surprising, therefore, that for a wide variety of manufacturing industries, assembly accounts for more than 50 per cent of the total manufacturing cost and more than 40 percent of the labour force. In most manufacturing industries, when a product is being considered for manufacture, consideration will be given asto howthe product will function, its appearance, and how easily the individual parts which make up the product can be made, but virtually nothought is given asto how it can be put together. The main reason for this attitude to design is that while operators cannot manufacture parts without the use of equipment because of physical limitations, they can perform the most complex of assembly tasks using capabilities which even the most sophisticated equipment cannot hope to duplicate. A further problem is that because people are so good at assembly tasks, even for manual assembly, little, if any, thought is given to making this simpler. If the cost of assembly is to be significantly reduced, therefore, this can best be done by designing parts which can be easily handled and products which can be easily assembled, and this applies whether assembly is to be manual or by machine. In general, because little thought has been given to design for assembly it is likely that improvements in this area will yield the biggest reduction in manufacturing costs. In the following a systematic approach to design for assembly is outlined

0142-694X/83/030170-07 $03.00 ~) 1983 Butterworth & Co (Publishers) Ltd

DESIGN STUDIES

and this is illustrated by the use of two simple examples. It must be emphasized that this approach is, in the main, common sense, but because it is applied systematically all factors relating to ease of assembly will be considered and either re-designed or left depending on the balance of overall costs.

DETERMINATION OF THE MOST APPROPRIATE PROCESS There are three basic types of assembly: manual, dedicated and flexible. Manual assembly, asthe name implies, makes use of human capabilities with or without the aid of sophisticated jigs, fixtures and power tools to perform the assembly task. Manual assembly costs remain relatively constant and independent of the quality of parts being used and the required production volume. Further, manual assembly is versatile, in that it can cope with a wide variety of products manufactured in a wide range of batch sizes. Dedicated assembly equipment (special purpose assembly machines) is expensive and requires considerable engineering development. Its performance is very susceptible to the quality of the parts being assembled and, as the name implies, is not easily adapted to assemble more than a single or at the most a small 'family' of products. It cannot be justified for low annual production volumes and, in general, the cost of assembly reduces as the annual production volume increases. Flexible assembly equipment, someti rues Table 1. First digit for assembly process code Annual production volume per shift greater than 0.7 million assemblies

Seven or more parts in the assembly Lessthan seven parts in the assembly

referred to as either programmable or robotic systems, provides for considerable flexibility in production volume and greater adaptability to design changes and different product styles than dedicated assembly equipment. The major factors which determine the most appropriate assembly process are: • total annual production volume • number of parts in the product • quality ofthe individual parts of the product • amount a company is willing to invest to replace labour • number of shifts worked • number of styles of the product • number of design changes which are likelytotake place during the life of the product The factors can be used in a two digit code system which is identified as shown in Table I (the first digit) and Table 2 (the second digit), and this code can then be applied to the array shown in Table 3 which indicates the most appropriate process. Clearly many elements of cost data have to be entered into the algorithms in order to determine the format of Table 3 and in this case 'typical' values have been used. However, in a computer-aided design for assembly package, the user is able to change the cost database to suit the specific conditions of an individual company, and this will change some of the information shown in Table 3. It is appropriate to note that moving from the bottom right to top left of Table 3 indicates a trend of decreasing assembly cost and in the limit, the cost difference will be a factor of between five andten.

0

RE-DESIGN FOR MANUAL ASSEMBLY

1

If manual assembly is found to be the most appropriate assembly process, then the classification and coding system developed for handling and insertion are both two digit codes which have been designed to accommodate the majority of handling and insertion problems. Associated with handling and insertion codes are time penalities which have been determined experimentally, and which represent the additional time necessary to handle and/or insert 'non-standard' parts.

Annual production volume per shift greater than 0.5 million assemblies

Twenty five or more parts in the assembly 2 Fifteen or more parts in the assembly 3 Ten or more parts in the assembly 4 Seven or more parts in the assembly 5 Less than seven parts in the assembly 6

Annual production volume per shift greater than 0.1 million assemblies

Ten or more parts in the assembly Less than ten parts in the assembly

8

Factors affecting manual handling are considered to be:

Annual production volume per shift less than 0.1 million assemblies

9

• symmetry of the part in the direction of insertion

7

CLASSIFICATION AND CODING FOR HANDLING AND INSERTION

Table 2. Second digit for assembly process code Single product has a market life of three years or more without significant fluctuations in demand. The manual fitting or adaption of parts is not required and the parts are of sufficiently high quality.

Investment in automation encouraged.

Few product styles Several product styles

Few design changes Several design changes

0 1

Investment in automation discou raged

Few product styles Several product styles

Few design changes Several design changes

2 3

Few design changes Several design changes

4 5

Few design changes Several design changes

6 7

Variety of different but similar products, no manual fitting, high quality parts and investment in automation encouraged, Variety of products, rnanual fitting, low quality parts, fluctuations in demand or automation discouraged.

vol 4 no 3 july 1983

171

Table 3. Assembly process code matrix

0

0

1

2

3

4

5

6

7

8

9

AF

AF

AP

AP

AF

AF

AP

AP

AP

MA

MM A1

AP

MA

1

A1

A1

A1

A1

A1

A1

A1 MM

2

AF

AP

AP

AP

AF AP

AP

AP

AP

AP

MA

3

AF AP

AP AF

AP

AP

AF MM

AP MM

AP MM

AP MM

AP

MA

4

A1

A1

A1

AP A1

A1

A1

MM A1

MM AP

AP

MA

5

A1

A1

A1

A1 AP

A1

MM A1

MM

MM

AP MM

MA

A1

MM A1

MM

MM

MM AR

MA MA

6

A1

A1

A1

A1 MM

7

A1 AP

AP

AP

AP

MM A1

AP MM

AP MM

AP

AP

8

MM

MM

MM

MM

MM

MM

MM

MM MA

MA

9

MM

MM MA

MM MA

MA

MM MA

MA

MA

MA

MA

MM- Manual assembly AI-Automatic indexing AP- Programmable

AR

MA MA

• Consider the re-design of high cost h a n d l i n g and/or insertion parts. • After re-design, d e t e r m i n e the n e w design efficiency.

DETERMINATION OF THE SEQUENCE OF ASSEMBLY The simplest m e t h o d of d e t e r m i n i n g the sequence of a s s e m b l y is to dis-assemble the product in a 'reasonable' manner. This w o u l d not g e n e r a l l y be the o n l y o r d e r of dis-assembly, but the results of analyses carried out to date indicate that the effectiveness of the exercise is not significantly changed if o t h e r dis-assembly sequences w e r e considered p r o v i d e d these w e r e practical.

COMPILATION OF THE WORKSHEET A simple p r o d u c t is s h o w n in Figure 1 and the w o r k s h e e t associated w i t h this product is s h o w n in Figure 2. Before the a s s e m b l y efficiency can be d e t e r m i n e d it is necessary to estimate the theoretical m i n i m u m n u m b e r of parts for the product and this is identified by c o l u m n 15 of the worksheet. Three criteria are applied to d e t e r m i n e if a part needs to be separate. These are: • The part has to be capable of m o v i n g relative to its m a t i n g part(s).

MA- Manual assistedassembly AF- Automatic free transfer AR- Robotic

• s y m m e t r y of the part p e r p e n d i c u l a r to the direction of insertion • r e q u i r e m e n t f o r g r a s p i n g tools • interaction of r a n d o m parts (do t h e y t a n g l e and/or nest?) • requirementfortwo-handed operation • h a n d l i n g difficulty due to factors o t h e r than g e o m e t r y (fragile, s l i p p e r y etc) • part thickness • part size Factors w h i c h affect m a n u a l insertion are considered to be: • w h e t h e r or n o t t h e part is a fastener • the difficulty associated w i t h restricted v i e w or obstructed access • the stability o f t h e part after insertion (does the part need h o l d i n g d o w n for s u b s e q u e n t operations?) • the ease w i t h w h i c h a part can be a l i g n e d and positioned • degree of resistence to insertion • t y p e o f f a s t e n i n g process

sOc2ew( 2 ) ( s t e e l ) ~

(~

?

03

Cover(steel)

04 Sprinq (steel)

05 Piston stop (nylon)

06 Piston (aluminium)

07 With the a p p r o p r i a t e classification and coding data, it is possible to analyse a p r o d u c t design systematically for ease of assembly.

Main block (plastic)

SEQUENCE OF DESIGN ANALYSIS The various stages of the analysis are: • • • •

D e t e r m i n e the sequence of assembly. C o m p l e t e t h e design for m a n u a l a s s e m b l y w o r k s h e e t . D e t e r m i n e the a s s e m b l y efficiency. Consider the e l i m i n a t i o n of p o t e n t i a l l y r e d u n d a n t parts.

172

Figure I. Original design for piston assembly

DESIGN STUDIES

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DESIGN EFFICIENCY E

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Figure 2. Piston assembly: design for manual assemb/y worksheet

• The part hasto be manufactured from a different material to its mating part(s). • Either assembly or dis-assembly, if required, would not be possible if the part were not separate.

02 Snop on c o v e r ond stop ( p l o s t i c )

DETERMINATION OF ASSEMBLY EFFICIENCY The assembly efficiency is defined as the standard time to assemble the theoretical minimum number of parts divided by the actual assembly time. Tests have indicated that the time taken to handle and insert a 'simple' part is two seconds, and this figure has been used to determine the assembly efficiency for the product analysed in this paper. However, the standard assembly time is subject to the specific problems of individual industries, and this and the other basic data can be changed proportionately to suit the requirements. It should be noted that while improving assembly efficiency absolutely, it is more important to relate the assemble efficiency of the new design to that of the old design. A 10-20 percent improvement in assembly efficiency represents a halving of assembly cost even though the final efficiency, due to conflicting factors which prevent re-design for assembly, is not particularly high.

03 S p r i n g (steel)

04 Piston ( o l u m i n i u m )

-

-

ELIMINATION OF POTENTIALLY REDUNDANT PARTS In this particular example three parts have been considered to be potentially redundant: the cover, because it can be combined with th'e piston stop since it does not move relative to the piston stop and can be made of the same material, and the cover screws, since it should be possible, under appropriate circumstances, to eliminate all separate fasteners. In this particular example the combined cover and piston stop and the main block

vol 4 no 3july 1983

Figure 3. Redesigned piston assembly

173

D E S I G N FOR M A N U A L A S S E M B L Y W O R K S H E E T 1

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DESIGN EFFICIENCY

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could be designed such that the cover is a snap-fit on the main block. It was accepted that functional, manufacturing and aesthetic requirements would still be met by the new design. Clearly, if any other essential design requirement were to be impaired by a re-design for assembly exercise, this would not be acceptable.

RE-DESIGN OFHIGHCOSTHANDUNG OR INSERTIONPARTS From the worksheet shown in Figu re 2 it can be seen that of the parts that remain after the elimination of redundant parts, the spring is a high cost handling part (5s to handle) whilst the piston is a high cost insertion part (4sto insert). It w a s t h o u g h t that nothing could be doneto improve the design of the spring to improve its handling properties, but that the piston could be re-designed to improve its insertion properties. The problem associated with the piston is that, on insertion, the piston has to be released before the piston spindle enters the main block and this sometimes results in the piston tilting and the spindle not aligning with the hole in the main block. Two reasonable re-design alternatives exist: first, the spindle could be extended such that it entered the hole in the main block before the piston had to be released. This was discounted on the grounds that it would impair the function of the product. Second, a small tub spindle could be added to the top of the piston which could be gripped bythe operator and which would then facilitate easy insertion. This was considered acceptable, and if the volume above the piston was considered to be critical this could be accommodated by slight re-design of the piston stop. The re-designed product is shown in Figure 3 and its worksheet is shown in Figure 4 where it can be seen that the assembly efficiency has increased from 21 per cent to

174

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05-WasherS(two)

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D i a p h r o 9 m a s s e m b l y - OI Fixture - 91

Figure 5. Original design f o r d i a p h r a g m p l a t e a s s e m b l y

DESIGN STUDIES

DESIGN FOR AUTOMATIC ASSEMBLY WORKSHEET 1

2

3

4

5

6

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Cat Figure 6. Diaphragm plate assembly: design for automatic assembly worksheet

62 per cent, an increase in assembly efficiency of 41 per cent. RE-DESIGN

FOR AUTOMATIC

ASSEMBLY

The techniques used for re-design for automatic assembly are similar to those used for manual assembly, the only basic difference being that the coding system for automatic handling consists of a three digit code. This code deals sequentially with basic shape, basic symmetry and properties and orientations of asymmetric features. The two digit code for insertion deals with the type of insertion (fastener or non-fastener) and the direction of insertion for the first digit and the quality of insertion and/or the type of fastener for the second digit. Figure 5 shows a simple product which can be analysed as shown in the worksheet of Figure 6 which shows that for this particular product only one part is actually necessary, as determined by the criteria for a separate part. In this example, when consideration was given to manufacturing the diaphragm plate and the bearing housing asan integral part it wasthought that this would be uneconomic. As an alternative to this, either the diaphragm plate orthe bearing housing could be manufactured with an integral rivet, but, again, this was considered to be impractical. Yet another alternative would be to glue the diaphragm plate to the bearing housing, but it was thought that for functional reasons this would not be satisfactory. It was finally decided that the diaphragm plate would be joined to the bearing housing by a single central separate rivet and Figure 7 shows the proposed redesign. Figure 8 shows the worksheet for the new design and it can be seen that the feeding and orienting code for the diaphragm plate has changed from 008 to 001 whilst the insertion code has changed from 08to 00. Further, although the feeding and orienting code of the bearing housing has remained unchanged, its insertion code has changed from 08 to 00. The total assembly cost per part for the assembly

vol 4 no 3 july 1983

of the re-designed product is 0.5 p with an assembly efficiency of 9 per cent, compared with an assembly cost of 2.23 p and an efficiency of 1.8 per cent for the original design. This neglects the reduced cost of the transfer system which would be required for the re-designed product, the reduced cost of parts and also the cost of administering the larger number of parts of the original product. With an annual production volu me of 2.5 x 106 this would give a minimum saving in assembly costs of £43 000.

OZ- Diaphraqm plate

Bearing housinq

O4 - Rivet

Diaphragm assembly - OI Fixture -91

Figure 7. Redesigned diaphragm plate assembly

175

DESIGN FOR AUTOMATIC 1

2

3

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5

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ASSEMBLY

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12

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DESIGN EFFICIENCY

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iD Nos: 01-89 Parts of the final assembly 90 Unsecured group of parts 91-99 Tools, fixtures, etc.

Figure 8. Redesigned diaphragm plate." design for automatic assembly worksheet

It is important to note that the saving in manufacturing costs is significant. Without exception, in re-design exercises carried out to date, the savings in manufacturing costs have always been greater than the savings in assembly costs.

the data for automatic insertion has been obtained from the manufacturers of automatic insertion equipment. The database for the handbook and the design techniques have been incorporated into a computer-aided design facility and either the handbook or the software can be purchased.

H A N D B O O K OF DESIGN FOR A S S E M B L Y CONCLUSIONS The h a n d b o o k o f design for a s s e m b l y developed jointly at

the University of Massachusetts and the University of Salford embodies most of the elements discussed in this paper. The handbook consists of six basic sections which are: • • • • • •

choice of assembly process analysis of design for manual assembly coding systems for manual handling and insertion design data for manual handling and insertion analysis of design for automatic assembly coding systems for automatic handling and insertion

The data for manual handling and insertion has been obtained from experiments; the data for automatic handling has also been obtained from experiments while

176

The benefits which result from applying a systematic approach to design for assembly may be summarized as follows: • There is typically a 20-50 per cent reduction in the number of piece parts. • The number of parts which are suitable for automatic handling can be increased considerably. • There is invariably a decrease in piece part manufacturing costs at least as large as the decrease in assembly costs. • When the user of the system becomes proficient the exercise does not take very long to perform per part analysed and is thus a very low cost activity whilst the potential benefits are substantial.

DESIGN STUDIES