Session 7 Design for Assembly- Guidelines

Transcription

1 Session 7 Design for Assembly- Guidelines Lecture delivered by Prof. M. N. Sudhindra Kumar Professor MSRSAS-Bangalore 1

2 Session Objectives At the end of this session the delegate would have understood What are the desirable outcomes of the DFMA process Part count reduction criteria Alternative simple designs through the examples. Thorough knowledge on DFMA guidelines. 2

3 Session Topics Introduction Desirable outcomes of DFMA Principles of reducing parts Assembly system Part- count reduction criteria Simple design alternatives Guide lines for product assembly evaluation 3

4 Introduction Assembly is a key activity in manufacturing. Most consumer products consist of several parts, which must be brought together and secured for the product to function. While assembly is extremely common, it should perhaps be regarded as a necessary evil! It probably has to be done, but should be avoided wherever possible. Much effort has therefore been put in over the years into establishing methods of avoiding the need for assembly. 4

5 Introduction The decisions affecting the assembly content of a product originate in only one place - the design process. Although later decisions can be made as to sequence, method and degree of automation, designers make the fundamental commitments on assembly. "Design for assembly" is a general term for a set of processes, which guide designers in making assemblyrelated decisions 5

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7 Concept Embodiment design Detail design Manufacturing development 0 I II III IV Concept Specification Test and Development Product release evaluation and planning evaluation PEMP Simplified Quality Functional Deployment Voice of Marketplace High level QFD Design for assembly Design for manufacturability Design for Variety/Mass Customisation User oriented Design Design for international Design for serviceability Design for green Design for testability Design technique positioning within the structured development process Concurrent Engineering 7 From Cradle to Graveyard

8 Desirable outcomes of this process are Minimizing the amount of assembly required by a product Easing the handling and manipulation requirements of the product s components Minimizing the number and complexity of securing operations 8

9 Why bother? 1. Assembly is often both expensive and complicated - so why not design in order to avoid assembly? 2. The first and usually most significant principle for ease of assembly is to reduce the number of parts by merging individual components into integrated parts. The need for assembly is removed by combining previously discrete components. 3. This is often possible with "simple" assemblies but in the case of more complex products one must accept the necessity of assembly. 9

10 PRINCIPLE : Reduce assembly by integrating parts PEMP Why assemble? 1. DEGREES OF FREEDOM (MOVEMENT): Various elements must enjoy a degree of mobility in order to achieve the function. 2. MATERIAL DIFFERENTIATION: Successful operation depends on particular material characteristics (e.g. assembly of rubber and gasket, electrical and thermal isolation). 10

11 WHY ASSEMBLE? PEMP 3. PRODUCTION CONSIDERATIONS: Some parts will be easier to produce by division into sub-parts (e.g. the pipe and (flange)stationary sealing surface's division into two, to be assembled by welding/fasteners). 4. ESTABLISHMENT CONSIDERATIONS (REPLACEABILITY): The product may be used in a fixed installation and have to be assembled in a separate process 11

12 WHY ASSEMBLE? PEMP 5. DIFFERENTIATION OF FUNCTIONS: A single agent or a combination of such in the form of more elements can carry out a function 6. PARTICULAR FUNCTIONAL CONDITIONS: In the sense of increased requirements of accessibility, demounting, cleansing, inspection, etc. these can necessitate a division into elements. 7. DESIGN CONSIDERATIONS: Aesthetic requirements can cause a division of the form, which will consequently require assembly. 12

13 Assembly systems Design for assembly is an optimization task which pre-requires knowledge of both design and assembly principles. It is therefore useful to have an overview of some types of assembly systems 13

14 Assembly Systems Manual assembly: Assembly is carried out by the assembler who has simple and mostly passive auxiliary equipment at his disposal, such as tables, fixtures, component boxes, conveyor belt and hand tools. Semi-automatic assembly: Automatic (programmed) machine system, where some operations are manual and adapted to the machine's program. Automatic assembly: A machine system, which follows a program. The system takes decisions on the basis of the program. Such decisions occur as a result of condition of the system and input, and realize the required output. Flexible assembly: This is a system, which allows for a variation of certain product characteristics 14

15 Design for ease of assembly requires expert knowledge of the following: Joining methods and processes; The connection between product design and assembly process; and The connection between product design and the type and quality of assembly systems. 15

16 When can design for assembly be applied? The answer is not "always!. The designers will normally concentrate first and foremost on getting the product to function within the economic limitations laid down. Time is at a premium; as a result the most important activity in the closing phase of design is to get the product detailed so it can be in production as soon as possible, in other words getting the drawing finished. Assembly deliberations can easily become a minor part of a large hectic process - the result being a non-optimal product from the assembly point of view. 16

17 CONCURRENT ENGINEERING PROCESS PEMP 17

18 APPROACH Simplification of the Product Structure. Reduce the number of parts to be assembled, ensure that the remaining parts are easy to assemble handling, insertion and fastening. This method seeks to minimize cost of assembly within constraints imposed by other design requirements. 18

19 Boothroyd Dewhurst DFA method Theoretical Assembly Time Assembly Efficiency = Actual Assembly Time PEMP Theoretical assembly time is the theoretical minimum number of parts multiplied by basic assembly time for one part (average time for a part which presents no handling, insertion or fastening difficulties). Design Efficiency = Theoretical Assembly Time DFA Assembly Time Assembly cost estimates, for each component can be obtained using the softwares marketed by Boothroyd Dewhurst Inc. Reduction in number of parts and other re-design decisions must also be based on the requirements of Design for variety (requirements for mass customization). 19

20 Part Count Reduction Criteria Examine each part against the three criteria as it is added to the product during assembly: 1. During operation of the product, does the part move relative to all other parts already assembled?. Only gross motions should be considered small motions that can be accommodated by integral elastic elements, for example, are not sufficient for a positive answer. 2. Must the part be of a different material than or be isolated from all parts assembled?.only fundamental reasons concerned with material properties are accepted. 3. Must the part be separate from all the other parts already assembled because otherwise necessary assembly or disassembly of other separate parts would be impossible?. 20

21 Part- Count Reduction Criteria A part is a good candidate for elimination if there is No need for relative motion between the part and all other parts already assembled during the operation of the product. Parts can be combined when they do not move relative to other parts in the assembly. 21

22 Part Count Reduction 22

23 Part- count reduction criteria A part is a good candidate for elimination if there is No need for material to be different or be isolated from all other parts assembled. Parts can be combined with other parts when they do not have to be made from different materials. 23

24 Part Count Reduction 24

25 Part- Count Reduction Criteria A part is a good candidate for elimination if there is No need for service or repairability. Must the part be separate from all other parts already assembled because otherwise necessary assembly or disassembly of other separate parts would be impossible? Parts can be combined with other parts if it would not affect the assembly of other parts. Field service does not require their disassembly. 25

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27 DFA - Part Count Reduction Criteria PEMP Must the parts move relative to one another? Must the parts be electrically isolated? Must the parts be thermally isolated? Must the parts be of different materials? Does combining the parts prevent assembly of other parts? Will servicing be adversely affected? 27

28 28 PEMP DFMA Maximize ease of manufacture by simplifying the design through part count reduction. A part eliminated costs nothing to: make store handle orient assemble purchase clean inspect rework service Fewer parts means less of : engineering time drawings and part numbers production control records inventory purchase orders, vendors bins, containers stock locations, buffers movements accounting details calculations, service parts catalogues

29 PRINCIPLE : Apply criteria for necessary assembly to achieve theoretical minimum part count Simple Design Alternatives An optimal design (from the assembly point of view) can only be achieved by considering the alternative possibilities, which in turn can provide degrees of design freedom. 29

30 Simple Design Alternatives 30

31 Simple Design Alternatives PEMP 31

32 What is an optimal result? One can contend that the purchaser is not prepared to pay anything for assembly. Assembly is only a means of achieving coherence in the product and not of contributing to saleable quality. When viewed from this angle it becomes obvious that criteria necessary for an optimal assembly (i.e. low production and assembly costs) must be tempered by the allowable overhead costs to achieve them. Further these desirable criteria should not adversely affect other criteria such as good design, long life and functionality. 32

33 Guide Lines for Product Assembly Evaluation Guideline 1: Overall Component Count should be minimized. The First Measure of Assembly Efficiency is based on number of components or sub-assemblies used in the product. Three rules Components must be separate if the design is to operate mechanically(relative motion) Components must be separate if they must be made of different materials Components must be separate if the assembly / disassembly is impossible. 33

34 Part Count Reduction PEMP 34

35 Part Count Reduction PEMP 35

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41 Guideline 2: Make Minimum use of separate fasteners One way to reduce component count is to minimize use of separate fasteners. Standardize use of fasteners whenever required Ex: Volkswagan Beetle can be fixed with a set of screw drivers and and a standard 13mm wrench! Best way to get rid of fasteners in moulded parts is by use of snap fits. 41

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43 Fastener Examples PEMP 43

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45 FASTENING Permanent: weldments,adhesives, etc. Temporary: screws, circlips, split pins, etc. SNAP IN devices: permanent & temporary fastening Achieve assembly and retain attachment by utilising the elastic properties of the component or device. 45

46 FASTENING 46

47 FASTENING ADHESIVES Advantages: Generally less costly Minimum of component distortion Appearance not affected Reduced weight Greater damping of mechanical vibrations Wide area of stress distribution Galvanic corrosion minimised 47

48 FASTENING MECHANICAL FASTENING Advantages: Generally stronger Some can be reused Useful over a wide range of temperatures Dual function for instance component mounting Well established technology with no special training requirement 48

49 SNAP - IN 49

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55 Guide Line 3: Design the Product with a base component for locating other components: This guideline encourages the use of a single base on which all the other components are assembled The base provides a foundation for a consistent component location, fixturing, transport, orientation and strength. 55

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58 Guideline 4: Do not require the base to be Repositioned during the Assembly If Automatic Assembly equipment such as robots or specially designed component-placement machines are used during the assembly it is important that the base be positioned precisely. Hence it is advisable the product does not require repositioning of the base during assembly. 58

59 Guideline 5: Make the assembly sequence efficient If there are N components then there are potentially N! sequences of assembly. An efficient assembly sequence is one that Affords assembly with the fewest steps Avoids risk of damaging components Avoids awkward, unstable, conditionally stable positions for the product and the assembly personnel and machinery during the assembly. Avoids creating many disconnected subassemblies to be joined later. 59

60 Guidelines for designing for manufacturability / assembly Make the assembly sequence efficient - reduce number of interfaces. -avoid disassembly to test functions of assembled groups and products - develop assembly fish bone diagram 60

61 Process List all the components and processes involved in the assembly process List the connection between the components and generate a connection diagram Select a Base component Recursively add the next component Identify subassemblies. 61

62 Guidelines for designing for manufacturability / assembly Design the components to mate through straight line assembly, all from the same direction. 62

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64 Guideline 6: Avoid component characteristics that complicate the retrieval. Three component characteristics make retrieval difficult Tangling, Nesting, Flexibility and Slippery 64

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68 NESTING 68

69 TANGLING 69

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71 Guideline 7 : Design the components for a specific type of retrieval, handling and insertion method: Consider the assembly method of each component during design Manual, Semi Automatic or Robotic assembly. 71

72 Guidelines for designing for manufacturability / assembly Design the components for a defined way of retrieval, handling and insertion 72

73 Guideline 8 : Design the components for end to end symmetry wherever possible: End to end Symmetry means that a component can be inserted in the assembly either end first. The operator need not waste time! 73

74 End to End Symmetry PEMP 74

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77 Ease of Assembly vs User friendliness conflict 77

78 SYMMETRY PEMP 78

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80 Effect of Symmetry on the time required for part handling: Times are average for two individuals and shaded areas represent nonexistent values of the total angle of symmetry 80

81 Guidelines for designing for manufacturability / assembly When the parts can not be made symmetric design the part to be obviously asymmetric 81

82 Guideline 9 : Design the components for symmetry about their axes of insertion wherever possible: The Designer should strive for rotational symmetry also. 82

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86 Guideline 10: Design the components that are not symmetric about their axes of insertion to be clearly asymmetric: PEMP When the parts can not be made symmetric design the part to be obviously asymmetric The goal of this guideline is to make components that can be inserted only in the way intended! 86

87 Design the components that are not symmetric about their axes of insertion to be clearly asymmetric: PEMP 87

88 Design the components that are not symmetric about their axes of insertion to be clearly asymmetric: PEMP 88

89 Guidelines for designing for manufacturability / assembly When the parts can not be made symmetric design the part to be obviously asymmetric. PEMP 89

90 Guideline 11: Design the components to mate through straightline assembly, all from the same direction. This guideline intended to minimize the motions of assembly has two aspects: Motions should mate through straight line motions and this motion should always be in the same direction. Thus assembly process will never require re-orientation. (Down is the preferred single direction) 90

91 Single Axis Pyramid assembly PEMP 91

92 Guideline 12: Make use of chamfers, leads and compliance to facilitate insertion and alignment and overcome handling difficulties To make the actual insertion or mating of a component as easy as possible each component should guide itself into place. 92

93 Make use of chamfers, leads and compliance to facilitate insertion and alignment and overcome handling difficulties PEMP 93

94 Make use of chamfers, leads and compliance to facilitate insertion and alignment and overcome handling difficulties PEMP 94

95 Make use of chamfers, leads and compliance to facilitate insertion and alignment and overcome handling difficulties 95

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100 DESIGN FOR EASY INSERTION 100

101 DESIGN FOR EASY INSERTION 101

102 DESIGN FOR EASY INSERTION 102

103 DESIGN FOR EASY INSERTION PEMP 103

104 DESIGN FOR EASY INSERTION PEMP 104

105 DESIGN FOR EASY INSERTION PEMP 105

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109 Guidelines for designing for manufacturability / assembly Avoid Visual obstruction. Simultaneous fitting operations. PEMP Mistake proof the assembly design (poka yoke) so that the assembly process is unambiguous. 109

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111 Overconstrained design leads to unnecessary complexity and redundant parts. 111

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114 Possible To Assemble? PEMP 114

115 Possible To Assemble? PEMP 115

116 Possible To Assemble? PEMP 116

117 Possible To Assemble? PEMP 117

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122 Guideline 13: Maximize Component Accessibility: Assembly can be difficult if components have no clearance for grasping. Assembly efficiency is also low if a component must be inserted in a awkward spot. PEMP 122

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124 Guidelines for designing for manufacturability / assembly PEMP Maximize component accessibility, provide easy visibility of mating locations. Minimize the number of: parts and fixings design variants assembly movements assembly directions 124

125 Facilitate easy identification and differentiation PEMP 125

126 AVOID ADJUSTMENTS PEMP 126

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129 Minimise part count by incorporating multiple functions into single parts 129

130 Modularise multiple parts into single sub - assemblies PEMP 130

131 Design open enclosures to permit assembly in open space, not in confined spaces. Never bury important components PEMP 131

132 Parts should easily indicate orientation for insertion PEMP 132

133 Standardize to reduce part variety PEMP 133

134 MAXIMISE PART SYMMETRY PEMP 134

135 For automated assembly, design in weight polar properties across non - symmetries 135

136 Eliminate tangy parts PEMP 136

137 Facilitate identification of similarly shaped but different parts PEMP 137

138 Provide orienting features on non - symmetries 138

139 Provide orienting features on non - symmetries 139

140 Design the mating features for easy insertion PEMP 140

141 Provide alignment features PEMP 141

142 Insert new parts into assembly from above PEMP 142

143 Insert from the same direction, or very few. Never require the assembly to be turned over PEMP 143

144 Eliminate fasteners PEMP 144

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146 Place fasteners away from obstructions Tool accessibility 146

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148 Part access for insertion difficult Tool access difficult 148

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150 Provide flats for uniform fastening and fastening ease PEMP 150

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152 Provide space for fastening tool PEMP 152

153 CHECK LIST - DESIGN FOR ASSEMBLY MINIMISE THE NUMBER OF: Parts & Fixing Design Variants Assembly Movements Assembly Directions PROVIDE: Suitable Lead In Chamfers Automatic Alignment Easy Access For Locating Surfaces Symmetrical Parts or Exaggerate Asymmetry For Simple Handling and Transportation 153 PEMP

154 CHECK LIST - DESIGN FOR ASSEMBLY PEMP AVOID: Visual Obstruction Simultaneous Fitting Operations Parts which will Tangle or Nest Adjustments which affect prior adjustments The possibility of Assembly Errors 154

155 Summary The designer should understand The desirable outcomes of the DFMA process Part count criteria Alternative simple designs through examples DFMA guidelines 155