3_dfa

Design for Assembly (DfA) Engr. Wan Sharuzi Hj Wan Harun Industrial Engineering, FKM

Introduction • Design is a complex iterative creative process that begins with the recognition of a need of desire and terminates with a product or process that uses available resources, energy and technology to fulfil the original need within some set of defined constraints. • Assembly is a process of joining components into a complex product.

Introduction (cont….) • Design for “X” – manufacturing – safety and reliability prevention – quality (six sigma) – environment – serviceability – testability – functionality – assembly – others ……..

Introduction (cont….)

• Time-based competition,

– Ability to quickly develop, – Produce and distribute new products, – Processes.

• Progressive development, • Diversification to meet customer needs.

Introduction (cont….) • Rapid changes in markets and technology tend to bring shorter product life cycles. • Developing products quickly to reduce lead times correlated with: – Cost, – Quality

Introduction (cont….) • Product development includes all the activities from market needs interpretation and technical possibilities to finished production designs. – – – –

Drawings, specifications, Tools, fixtures and production programs, Prototype production, Test activities.

Approaches to Product Development • Today’s products are becoming increasingly complex. • In many companies, design and manufacturing exist as separate groups. “Over the wall” approach to design.

Design for Assembly (DfA) • Design for Assembly (DfA) is an approach to reduce the cost and time of assembly by simplifying the product and process through such means as: – – – – – – –

Reducing the number of parts. Combining two or more parts into one. Reducing or eliminating adjustments. Simplifying assembly operations. Designing for parts handling. Selecting fasteners for ease of assembly. Minimizing parts tangling.

Design for Assembly (cont….) 



 

Design a product for easy & economical production. Incorporate product design early in the design phase. Improves quality and reduces costs. Shortens time to design and manufacture.

Design for Assembly (cont….) (a) The original design

Assembly using common fasteners

Design for Assembly (cont….) (a) The original design

(b) Revised design

Assembly using common fasteners

One-piece base & elimination of fasteners

Design for Assembly (cont….) (a) The original design

(b) Revised design

(c) Final design

Assembly using common fasteners

One-piece base & elimination of fasteners

Design for push-and-snap assembly

Design for Assembly (cont….) • Different DfA methodologies have been developed. – Complete • Objectivity (procedures for evaluating assemblability) • Creativity (procedures for improving assemblability)

– Systematic • step-by-step procedure

Design for Assembly (cont….) – Measurable • How to measure assemblability objectively, accurately and completely. • Cost information - more objective measure of product assembly quality. • Accuracy of cost estimation - indicator of the quality of the DfA method.

– User-friendly

Design for Assembly (cont….) • Four basic approaches for implementing DfA have been identified: – Design principles and rules, – Quantitative evaluation procedures, – Expert/Knowledge-based approach, – Computer-Aided DFA methods.

Design for Assembly (cont….) • Design principles and rules – Based on human oriented knowledge. – Collectively design data and convert assembly knowledge to design principles, rules and guidelines.

Design for Assembly (cont….) • Quantitative evaluation procedures – Based on evaluation procedure. – Determine the assembly process operation by operation. – Quantitative measure is calculated.

Design for Assembly (cont….) • Expert/Knowledge-based approach – Based on knowledge-based technology – a knowledge base, inference, communication and knowledge acquisition.

Design for Assembly (cont….) • Computer-Aided DFA methods – DFA systems are integrated with CAD systems. – Key role is the representation of technical objects and procedures for extraction and processing of assemblability attributes from 3D CAD models.

Aspects of Design for Assembly • DfA is applicable to: – Products consisting of 20 - 200 parts. – Mainly for mechanical parts (not electronics).

• Requires 1-2 days to perform for a product. • Average 30% improvement in the assembly cost. • Can be performed in various stages in the design process and iterated.

Design for Assembly Guidelines (General) 1. Aim for simplicity     Minimise part numbers, part variety, assembly surfaces, simplify assembly sequences, component handling and insertion, for faster and more reliable assembly.

DfA Guidelines (cont….) 2. Standardise     Standardise on material usage, components, and aim for as much off-the-shelf component as possible to allow improved inventory management, reduced tooling, and the benefits of mass production even at low volumes.

DfA Guidelines (cont….) 3. Rationalise product design - Standardise on materials, components, and subassemblies throughout product families to increase economies of scale and reduce equipment and tooling costs. - Employ modularity to allow variety to be introduced late in the assembly sequence and simplify JIT production.

DfA Guidelines (cont….) 4. Use the widest possible tolerances Reduce the tolerance on non-critical components and thus reduce operations, and processing times.

DfA Guidelines (cont….) 5. Choose materials to suit function and production process Avoid choosing materials purely for functional characteristics, material choice must also favour the production process to ensure product reliability.

DfA Guidelines (cont….) 6. Minimise non-value-adding operations The minimisation of handling, excessive finishing and inspection will reduce costs and lead time.

DfA Guidelines (cont….) 7. Design for process - Take advantage of process capability to reduce unnecessary components or additional processing, such as the porous nature of sintered components for lubricant retention. - Design in features and functions to overcome process limitations, such as features to aid mechanical feeding. - Avoid unnecessary restriction of processes to allow mfg. flexibility in process planning.

DfA Guidelines (cont….) 8. Teamwork - Promote concurrent engineering. - Establish a product or project based development organisation involving a formalised multi disciplinary/departmental teamwork structure. - Success is dependent on senior management buy-in, an open door culture, staff training and development, and an ongoing continuous improvement programme.

DfA Guidelines (cont….) All approaches to achieving an easy to assemble product can be summarized by the following two product statements. 1. Minimize the number of parts in an assembly. 2. Ensure that the remaining parts are easy to assemble.

DfA Guidelines (cont….) • Guideline description – The major advantage of practicing DfA. – Fewer parts reduce everything else needed to manufacture a product.

• Implementation techniques – Consider the part for elimination. – Use common parts.

DfA Guidelines (cont….) – Eliminate features or functions not of value. – Eliminate fasteners. – Design multifunctional parts.

• Outcomes – Improves quality, reduces opportunities for defects. – Shortens product design time.

DfA Guidelines (cont….) – Reduces the total material cost. – Simplifies vendor selection. – Reduces labor content. – Reduces assembly time. – Simplifies the assembly process, factory layout. – Reduces need for unique tools. – Reduces warehouse/stockroom space.

Design for Assembly Methods • Several systems being used in industry: – The DfA method exploited by Boothroyd Dewhurst Inc., USA. – The Hitachi Assemblability Evaluation Method (AEM) by Hitachi Ltd, Japan. – The Lucas Design for Assembly Methodology by Lucas-Hull, UK.

Where DfA fits in the design process? • The design process is an iterative, complex, decision-making engineering activity that leads to detailed drawings by which manufacturing can economically produce a quantity of identical products that can be sold. • The design process usually starts with the identification of a need, proceeds through a sequence of activities to seek an optimal solution to the problem, and ends with a detailed description of the product.

Where DfA fits in the design process? (cont…) •

• • •

Design process involves 3 main phases: i. Feasibility study, ii. Preliminary design, iii. Detail design. During the feasibility study, plausible solutions are explored, and rough checks are made on performance and product costs. Performance specifications for the most promising ideas are quantified during the preliminary phase. The detail design results in the piece-part and assembly drawing.

Where DfA fits in the design process? (cont…) • Design for assembly (DfA) – considered at all stages of the design process. • Concepts – analyzed against cost and performance. A systematic analysis of product assemblability – performed. • If cost and performance analyses require a concept to be redefined, the efficiency of assembly – analyzed before final approval.

Boothroyd-Dewhurst DfMA Methodology Analyse for Assembly OR

OR

Analyse for Special-purpose assembly transfer machines

Analyse for Manual Assembly Analyse for Robot Assembly

General Design Guidelines for Manual Assembly • The process of manual assembly is divided into 2 separate areas: – Handling (acquiring, orienting and moving), – Insertion and fastening (mating a part to another part or group of parts).

Design Guidelines for Part Handling 1. Having end-to-end symmetry and rotational symmetry about the axis of insertion (Fig 1a). 2. Asymmetrical if part cannot be made symmetrical (Fig 1b). 3. Provide feature that will prevent jamming (Fig 1c). 4. Avoid features that will allow tangling (Fig 1d). 5. Avoid parts that stick together, slippery, very small or hazardous to the handler (Fig 2).

Fig 1 : Geometrical features affecting part handling

Fig 2 : Some other features affecting part handling

Design Guidelines for Insertion and Fastening 1. Design with little or no resistance to insertion. – Provide chamfers, – Provide clearance. (Fig 3 – 6)

2. Standardize – common parts, processes and methods (Fig 7). 3. Use pyramid assembly (Fig 8). 4. Avoid holding parts down (Fig 9). 5. Design a part which can be located before it is released (Fig 10).

Fig 3 : Incorrect geometry can allow part to jam during insertion

Fig 4 : Provision of air-relief passages to improve insertion into blind holes

Fig 5 : Design for ease of insertion: assembly of long stepped bushing into counter-bored hole

Fig 6 : Provision of chamfers to allow easy insertion

Fig 7 : Standardize parts

Fig 8 : Single-axis pyramid assembly

Fig 9 : Provision of self-locating features to avoid holding down and alignment

Fig 10 : Design to aid insertion

Design Guidelines for Insertion and Fastening 6. Listed in order of increasing manual assembly cost: – – – –

Snap fitting Plastic bending Riveting Screwing

(Fig 11). 7. Avoid the need for repositioning (Fig 12).

Fig 11 : Common fastening methods

Fig 12 : Insertion from opposite directions requires repositioning of assembly

Design Guidelines for Insertion and Fastening • Guidelines are insufficient for a number of reasons: – Does not provide any means to evaluate a design quantitatively for its ease of assembly. – No relative ranking of all the guidelines that can be used to indicate which guidelines result in the greatest improvements in handling and assembly. – These guidelines are simply a set of rules, provide the designer with suitable background information to be used to develop a design that will be more easily assembled than a design developed without such a background.

Development of the DFA Methodology (Cont.) • Analytical method (1977) for determining the most economical assembly process and analyzing ease of manual, automatic and robot assembly. • Experimental studies – to measure the effects of symmetry, size, weight, thickness and flexibility on manual handling time. • Additional experiments – to determine the effects of part thickness of the grasping and manipulation, the effect of weight on handling time for parts requiring two hands for grasping and manipulation.

Development of the DFA Methodology (Cont.) • Experimental and theoretical analyses – to determine the effects of : – chamfer design on manual insertion time, – part geometry on insertion time, – obstructed access and restricted vision on assembly operations.

• Classification and coding system for manual handling, insertion and fastening processes – developed and presented. – Time standard system in estimating manual assembly times.

Development of the DFA Methodology (Cont.) • To evaluate the effectiveness of DfA method, two-speed power saw – analyzed for initial and new design. Initial design No. of parts : 41 parts Estimated assembly time : 6.37 minutes New design No. of parts : 29 parts Estimated assembly time : 2.58 minutes

• 29% reduction in part count. • 59% reduction in assembly time.

Boothroyd-Dewhurst DfA Method • Addresses the problems of: – determining the appropriate assembly method, – reducing the number of individual parts that must be assembled, – ensuring that the remaining parts are easy to assemble.

Boothroyd-Dewhurst DfA Method (Cont.) • The methods of assembly are classified into three basic categories: – Manual assembly, – Special-purpose transfer machine assembly, – Robot assembly.

Boothroyd-Dewhurst DfA Method (Cont.) • Based on the parameters (no. of parts and production volumes) that influence assembly efficiency and hence cost. • Using chart for assessing the available assembly methods. • A set of rules and charts for assembly efficiency.

Boothroyd-Dewhurst DfA Method (Cont.) • Manual assembly process can be divided into two separate scopes of tasks: – Handling – Insertion and fastening

Boothroyd-Dewhurst DfA Method (Cont.) • Assembly efficiency for manual assembly 3 x NM EM = TM NM = theoretical minimum number of parts TM = total manual assembly time

Pneumatic piston sub-assembly

Worksheet for pneumatic piston subassembly

Redesign of pneumatic piston sub-assembly

Worksheet for redesign of pneumatic piston subassembly

Theoretical Minimum Parts Assessment • The theoretical number of parts is determined  by asking three questions:

– Does this part move relative to another? – Do the mating parts have to be made of different  materials? – Do the parts have to be separate to allow servicing  before or after assembly?

• If the answer to any of these questions is ‘yes’,  then that part cannot be eliminated.

Assembly Efficiency (AE) • 2 main factors influence the assembly cost of a product:

– The total number of parts in a product, – The ease of handling, insertion and fastening of the parts.

• AE is obtained by dividing the theoretical minimum assembly time by the actual assembly time. Ema = Nminta / tma Nmin = theoritical minimum no. of parts ta = basic assembly time = average time for a part tma = estimated time to complete the assembly of the actual product

Classification System for Manual Handling • Classification no. consists of 2 digits: each digits is assigned a value of 0 to 9. • First digit is divided into 4 main groups: – – – –

First First First First

digit digit digit digit

of of of of

0–3 4–7 8 9

• Second digit of handling code is based on flexibility, slipperiness, stickiness and fragility of a part.

Class’n Sys. for Manual Insertion and Fastening • Concerned with the interaction between mating parts. • Basic assembly tasks: screw, weld, rivet, force fit, etc. • Design features that significantly affect manual insertion and fastening times: – – – – –

Accessibility of assembly location, Ease of operation of assembly tool, Visibility of assembly location, Ease of alignment and positioning during assembly, Depth of insertion.

Class’n Sys. for Manual Insertion and Fastening • The two-digit code numbers range from 00 to 99. • First digit is divided into 3 main groups: – Group I : First digit of 0 – 2 : Part is not secured immediately after insertion. – Group II : First digit of 3 – 5 : Part secures itself or another part immediately after insertion. – Group III : First digit of 9 : Process involves parts that are already in place.

• Groups I and II are further subdivided into classes that consider the effect of obstructed access and restricted vision on assembly time.

Class’n Sys. for Manual Insertion and Fastening • The second digit is based on the following group divisions in the first digit: – Group I : For a first digit of 0 – 2 : Classifies whether holding down is required to maintain orientation or location. – Group II : For a first digit of 3 – 5 : Classifies whether the fastening operation involves a simple snap fit, screwing operation or plastic deformation. – Group III : For a first digit of 9 : Classifies mechanical, metallurgical and chemical processes.

• For each two-digit code number, an average handling or insertion and fastening time is given, as shown in Fig 15 and 16.

Fig 15 : Manual handling - estimated times (seconds)

Fig 16 : Manual insertion - estimated times (seconds)

Effect of Part Symmetry on Handling Time • Symmetry – one of the principal geometrical design features that affects the times required to grasp and orient a part. • Assembly operations involve 2 component parts: – the part to be inserted, – the part (assembly) into which the part is inserted.

• Orientation involves the proper alignment of the part to be inserted. Divided into 2 distinct operations: – Alignment of the axis of the part that corresponds to the axis of insertion, – Rotation of the part about the axis.

Effect of Part Symmetry on Handling Time (Cont.)

Fig 17 : Alpha and beta rotational symmetries for various parts

Effect of Part Symmetry on Handling Time (Cont.) α -symmetry: rotational symmetry about an axis perpendicular to the axis of insertion.

α = 1800

α = 3600

α = ?

Effect of Part Symmetry on Handling Time (Cont.) β -symmetry: rotational symmetry about its axis of insertion.

β =0

0

β = 1800

β =?

Effect of Part Symmetry on Handling Time (Cont.) Thickness: The length of the shortest side of the smallest rectangular prism which encloses the part. Size: The length of the longest side of the smallest rectangular prism that can enclose the part. Thickness Size Thickness = radius General Rule

Exception when cylindrical and Diameter < Length

Effect of Part Symmetry on Handling Time (Cont.) • Several different approaches have been employed to determine relationships between the amount of rotation required to orient a part and the time required to perform that rotation. • Two most commonly used systems are: – The methods time measurement (MTM) – Work factor (WP)

Effect of Part Symmetry on Handling Time (Cont.) • In the MTM system, the “maximum possible orientation” is employed, which is one-half the beta rotational symmetry of a part. • The effect of alpha symmetry is not considered. • This system classifies the maximum possible orientation into 3 groups: – Symmetric – Semi-symmetric – Non-symmetric

• These terms refer only to the beta symmetry of the part.

Effect of Part Symmetry on Handling Time (Cont.) • In the WF system, the symmetry of a part is classified by the ratio of the number of ways the part can be inserted to the number of ways the part can be grasped preparatory to insertion.

Effect of Part Symmetry on Handling Time (Cont.) • The relation between the symmetry of a part and the time required for orientation is determined by the summation of the alpha and beta symmetries, given by: Total angle of symmetry = α + β

• The effect of the total angle of symmetry on the time required to handle a part is shown in Fig 18. • The shaded areas indicate the values of the total angle of symmetry that cannot exist.

Fig 18 : Effect of symmetry on the time required to handle a part

Effect of Part Thickness and Size on Handling Time • 2 major factors that affect the time required for handling during manual assembly are: – the thickness – the size of the part.

• The thickness of a “cylindrical” is its diameter whereas, for non-cylindrical parts, the thickness is defined as the maximum height of the part with its smallest dimension extending from the flat surface (Fig 19). • Cylindrical parts are defined as parts having cylindrical or other regular cross sections with 5 or more sides.

Fig 19 : Effect of part thickness on handling time

Effect of Part Thickness and Size on Handling Time (Cont.) • When the diameter is greater than or equal to its length, the part is treated as non-cylindrical. • Fig 19 shows the parts with a “thickness” greater than 2 mm present no grasping or handling problems. • For long cylindrical parts,

Fig 20 : Effect of part size on handling time

Fig 21 : Examples of parts that may require tweezers for handling

Fig 22 : Effect of symmetry on handling time when parts nest or tangle sevely

Fig 23 : Geometries of peg-and-hole

Fig 24 : Effect of clearance on insertion time

Fig 25 : Points of contact on chamfer and hole

Fig 26 : Chamfer of constant width

Fig 27 : Geometry of part and peg

Fig 28 : Geometry of disk and hole

Fig 29 : Effects of restricted access and restricted vision on initial engagement of screws

Fig 30 : Effect of number of threads on time to pick up the tool, engage the screw, tighten the screw and replace the tool

Fig 31 : Effect of obstructed access on time to tighten a nut

Fig 32 : Effects of obstructed access and restricted vision on the time to insert a pop rivet

Fig 33 : Effects of holding down on insertion time

Fig 34 : Effects of holding down and realignment on insertion time for difficult-to-align part

Fig 35 : Controller assembly

Fig 36 : Completed worksheet analysis for the controller assembly

Fig 37 : Conceptual redesign of the controller assembly

Fig 38 : Completed analysis for the controller assembly redesign

Fig 39 : Rearrangement of connected items to improve assembly efficiency and reduce costs

Fig 40 : Design concept to provide easier access during assembly

Fig 41 : Design to avoid adjustment during assembly

Fig 42 : Overstraint leads to unnecessary complexity in product design : (a) overstrained design; (b) sound kinematic design

Fig 43 : Overstrained leads to redundancy of parts : (a) overstrained; (b) kinematically sound

Hitachi Assemblability Evaluation Method • Effective tool to improve design quality for better assemblability. • To facilitate design improvements by identifying “weakness” in the early design process, by the use of two indicators:

Hitachi Assemblability Evaluation Method 1. An assemblability evaluation score ratio, E, used to assess design quality by determining the difficulty of operations. 2. An assembly cost ratio, K, used to project elements of assembly cost.

Hitachi Assemblability Evaluation Method

Assemblability evaluation and design improvement flow diagram

Assemblability Evaluation Procedure

Theory of Evaluation • Assembly operations are categorized into approx. 20 elemental assembly tasks. • Each task is assigned a symbol, indicates the content of the task. • Each of the elemental tasks is subject to a penalty score which reflects the degree of difficulty of the task.

Theory of Evaluation (Cont.) • The sum of the various penalty scores for a part are then modified by the attaching coefficients and subtracted from 100 points to give the assemblability evaluation score for the part. • Total assemblability evaluation score is then calculated. Score of 80 is acceptable.

Theory of Evaluation (Cont.)

Examples of the AEM symbols and penalty scores

Assemblability evaluation & Improvement examples

Case Study • Electric Thermo-Pot • Before improvement – Number of parts : 92 parts – Estimated assembly time : 24.58 minutes – Estimated assembly cost : Yen2949.35 – AEM score : 72.7

Case Study (Cont.) • After improvement – Number of parts : 59 parts – Estimated assembly time : 11.77 minutes – Estimated assembly cost : Yen1412.80 – AEM score : 80.4

Case Study (Cont.) • Percentage of reduction/increment – Number of parts : 36% – Estimated assembly time : 52% – Estimated assembly cost : 52% – AEM score : 7.7%

Summary

• Most appropriate solutions to any assembly problem depends on the production conditions such as: – – – –

production volume, life expectancy, available equipment, product market life.

• Good methodology must have considerable merit.

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