Session 8 Design for Manual Assembly

Transcription

1 Session 8 Design for Manual Assembly 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 The different methodology DFMA Advantages of DFMA Standardization of Parts Effects of parts shape in the assembly Guidelines of DFA Design guidelines of assembly 2

3 Session Topics Advantages of DFMA Guidelines and Advantages Standardization Effect of parts on Handling Time DFA Guidelines Choice of Assembly Method Design Analysis For Manual Assembly Design Guidelines for Insertion And Fastening 3

4 Advantages of DFMA 1. Systematic Approach PEMP DFMA provides a systematic procedure to analyze a proposed design from the point of view of a assembly and manufacture resulting in simpler and more reliable products which are less expensive to manufacture and assemble. 2. Team Work DFMA tool encourage dialogue between designers and manufacturing engineers encouraging teamwork to get benefits of of simultaneous or concurrent engineering. 3. Astounding Savings Saving outweigh cost of effort and result in more reliable products. 4

5 Guide lines/advantages: PEMP 1.2 Develop a Modular Design A module is a self contained component with a standard interface to other components of a system. Allows standardization of diversity. Allows Customization by using different combinations of standard components. Resists obsolescence and shorter redesign cycle. 5

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7 1.3 Use Standard Components Stock item is always less expensive than a custom made item. Standard Items Require less lead time. More reliable because characteristics and weaknesses are well known. Order any quantity at any time. Easier to repair and replacements are easier to find. 7

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9 Guide lines/advantages: 1.4 Design Part to be Multifunctional Combine functions wherever possible. Example: Spring and Structural member. Electrical conductors and Structural member. Chassis as an electrical ground. Heat sink and a structural member. PEMP 1.5 Design parts for multiple use. Many parts can be designed for multi use, Example: A mounting plate can be used to mount a variety of components. Spacer can be used as an axle, lever, standoff etc. 9

10 Guide lines/advantages: PEMP Sort all manufactured or purchased goods into two groups a) Parts which are unique to a particular model (I.e. crank shafts, housing etc) b) Parts which are generally needed in all products and /or models (Shafts, flanges, bushings, spacers, gears etc) 10

11 Standardization PEMP Standardization lays down the definitive solution of a repetitive technical or organizational problem with the best technical means available at the time. It is therefore a form and economic optimization limited by the time factors. Standardization determines the best solution of recurring problems Standardization is the systematic unification by those concerned of material and immaterial things for the benefit of the community. It should not lead to the economic advantage of an individual case. It stimulates rationalization and ensures quality in the business, technology, science and administration. It serves the safety of people and things as well as the improvement of quality in all of life. Furthermore it serves a purposeful ordering of the information in the specific standardization area. Standardization takes place on regional, national and international levels. 11

12 Guide lines/advantages: PEMP Divide each group into categories of similar parts (Part families) Create multiuse parts by standardizing similar parts. To standardize, seek sequentially 1) To minimize the number of part categories. 2) To minimize the number of variation within each categories. 3) To minimize the number of design features within each variation. 1.6 Design parts for ease of fabrication Design using least costly materials that just satisfies functional requirements (Including style and appearance) such that material waste and cycle time are minimized. 12

13 Guide lines/advantages: 1.7 Avoid separate fasteners PEMP In automation applications separate fasteners are difficult to feed, tend to jam, require monitoring for presence and torque and require costly fixturing, parts feeder and extra stations. In manual assembly cost of driving a screw can be 6 to 10 times the cost of the screw. Try to use tabs, snap fits. 1.8 Maximise compliance Non compliance (misalignment, tolerance stack up) can lead to use of excessive assembly force, leading to sporadic automation failures and / or product unreliability. Factors affecting rigid part mating include part geometry (Accuracy, consistency), stiffness of assembly tool, stiffness of jigs / fixtures holding the parts, friction between parts. Designed in compliance features include use of generous tapers / chamfers for easy insertion, use of leads and other guiding features, use of generous radii wherever possible. 13

14 Guide lines/advantages: 1.9 Minimise assembly directions All parts to be assembled in one direction. Extra directions mean wasted time and motion, more transfer stations, inspection stations and fixture nests. 14

15 Guide lines/advantages: 1.10 Minimize handling PEMP Position is the sum of location (x,y,z) and orientation (α,β,γ). Position costs money. Parts to be designed to make position easy to achieve and the production process should maintain the position once it is achieved. Increased number of orientation increase equipment expense, increase quality risk and slow feed rates and slow cycle times. To Assist easy orientation - Parts to be symmetrical - If polarity is important accentuate asymmetry, create obvious asymmetry. - Provide features to help guide and locate in proper position. 15

16 Guide lines/advantages: PEMP 1.10 Minimize handling To facilitate robotic part handling - Provide large smooth top surface for vacuum pick-up. - Provide inner hole for spring. - Provide feature of sufficient length for a gripper pick-up. 16

17 Effect of Symmetry on the time required for part handling (grasp,move,orient and place): Times are average for two individuals and shaded areas represent nonexistent values of the total angle of symmetry 17

18 Effect of part thickness on handling time: 18

19 Effect of part size on handling time: PEMP 19

20 Effect of symmetry on handling time when parts nest or tangle severely(disentangling time is not included) PEMP 20

21 Effect of clearance on insertion time: PEMP 21

22 Effects of restricted access and and restricted vision on initial engagement of screws: 22

23 Manual Assembly Small parts all within easy arm reach-workers sitting Large parts require major body motions for acquisition mechanical assistance may be required 1.very low volumeclean room or worker at beginning of learning curve 4.modular assembly center largest part shorter then 35 in. 5. Custom assembly layoutproducts assembled one-at-a-time 2.bench assembly repetitive work 6.flexible assembly layoutproducts may be assembled in batches 3.multi station assembly Manual assembly methods 7.installation-assembly on site 8.multi-station assembly 23

24 Acquisition time for items not stored within easy reach of the assembly worker: 24

25 DFA Guidelines PEMP 1. Minimize part count by incorporating multiple functions into single parts. (Iredale 1964) 2. Modularize multiple parts into single subassemblies (Crow 1988) 3. Assemble in open space, not in confined spaces. Never bury important components (Tipping 1965) 4. Make parts to identify how to orient them for insertion. (tipping 1965) 5. Standardize to reduce part variety (Tipping 1965) 6. Maximize part symmetry. (Iredale 1964, Paterson 1965) 7. Design in geometric or weight polar properties if non-symmetric (Tipping 1965) 8. Eliminate tangly parts. (Iredale 1964, Tipping 1965) 9. Color code parts that are different but similar in shape. 25

26 DFA Guidelines 10. Prevent nesting of parts (Iredale 1964, Tipping 1965) 11. Provide orienting features on non symmetries (Iredale 1964, Tipping 1965) 12. Design the mating features for easy insertion ( Iredale 1964, Tipping 1965, Balswin 1966) 13. Provide alignment features (Baldwin 1966) 14. Insert new parts into an assembly from above (Tipping 1965) PEMP 15. Insert from the same direction or very few. Never require the assembly to be turned over. (Tipping 1965) 16. Eliminate fasteners (Iredale 1964) 17. Place fasteners away from obstructions 18. Deep channels should be sufficiently wide to provide access to fastening tools. No channel is best 19. Providing flats for uniform fastening and fastening ease. 20. Proper spacing ensures allowance for a fastening tool. 26

27 Choice of Assembly Method PEMP Make a choice between manual, special purpose automatic or programmable automatic assembly based on good estimate of the most economical assembly method. Information required: Production volume per shift Number of parts in assembly Single product or a variety of products Number of parts required for different styles of the product. Number of major design changes excepted during the product life. Investment policy regarding labor saving machinery. 27

28 Choice of Assembly Method These questions, combined with several tables, are used to generate recommendations for the general type of process to be used for assembly, from the following categories: Purely manual; Manual with Mechanical assistance; Dedicated automation on indexing machines; Dedicated automation on station linked by free transfer device; Flexible automation using programmable or selectable work heads (pick & place units); Flexible automation using programmable manipulators (Robots) The outcome of this stage also includes an indication of the relative cost of assembly, which can be used for comparisons between designs or benchmarking across competitors. 28

29 Design analysis for manual assembly The key element here is the identification of ESSENTIAL parts. Essential parts are those which; Must have gross relative movement to all previously assembled parts during product operation, OR Must be made of a different material from all previously assembled parts, OR Must be separate for reasons of assembly or necessary disassembly. Agenda for redesign the number of non essential parts must be minimized. 29

30 Coding system and design data for handling and assembly Parts, which survive the initial review, are now assessed for difficulty of handling and presentation. Assessment areas are include: Parts symmetry (About two orthogonal axes) Parts feeding / presentation difficulty (weight, size, tangling, nesting, shingling ) Access to the site of assembly; Direction of assembly; Fitting and manipulation difficulty (skill levels, need for tools etc.; time estimates are generated for standard operations); Security of assembly (need for / presence of fasteners and inter stage transfer). This process produces cost estimates for the assembly process alone. 30

31 Design for Manual Assembly Comparing an IDEAL assembly time with an estimated actual assembly time required for a particular product design. To calculate the IDEAL assembly time the theoretical minimum number of parts is first determined by asking the following questions of each part in the assembly: 1. Does the part move relative to all other parts already assembled? 2. Must the part be of a different material than or isolated from all other parts already assembled? 3. Must the part be separate from all other parts already assembled because otherwise necessary assembly and disassembly of other parts would impossible? If the answer to the part under consideration YES the number of parts is entered into the calculation otherwise a zero is assigned. 31

32 Design for Manual Assembly Theoretically minimum number of parts is the sum of the numbers assigned to each parts in the assembly IDEAL assembly time is calculated based on theoretical number of parts each of which can be assembled in ideal time of 3 sec. This ideal time assumed that each part is easy to handle and insert and that about one third of the parts are secured immediately upon insertion with well designed snap - fit elements. To estimate ACTUAL assembly times, penalties in seconds are assigned for handling difficulties, insertion difficulties associated with each part in the assembly (standardized data based on time study are used) Actual assembly time is the sum of the handling and insertion times. DESIGN EFFICIENCY = Ideal Assembly time Actual assembly time 32

33 Design Guidelines for Part Handling In general, for ease of part handling, a designer should attempt to: 1. Design a part that have end to end symmetry and rotational symmetry about the axis of insertion. If this cannot be achieved try to design parts having the maximum possible symmetry. 2. Design part that, in those instances where the part cannot be made symmetric, are obviously asymmetric. 3. Provide features that will prevent jamming of parts that tend to nest or stack when stored in bulk. 4. Avoid features that will allow tangling of parts when stored in bulk. 5. Avoid parts that stick together or are slippery, delicate, flexible, very small or very large or that are hazardous to the handler (I.e. parts that are sharp, splinter easily, etc) 33

34 . We must consider the basic shape of the parts being assembled. Two basic categories are prismatic and rotational.. Rotational parts tend to roll when placed on a surface,suggestion that they will need some sort of holding fixture.this also means that during assembly,they must be supported by hand if not in a stable position when working.. Prismatic parts tend to have at least one stable orientation that allows them to be rested on surfaces.unlike rotational parts, if the prismatic parts are made to be stable when put in their final position, then they are much easier to fasten.. 34

35 The size of an object is generally the size of its largest major dimension,and thickness is the smallest major diameter.. There are a number of criteria that can be used to determine how easily a part can be handled. -a high size/thickness can be a measure of fragility. -large size values can indicate large weights. -small size values can indicate the need for special tools. 35

36 Design Guidelines for Insertion and Fastening For ease of insertion a designer should attempt to: 1. Design so that there is little or no resistance to insertion & provide chamfers to guide insertion of two mating parts. Generous clearance should be provided, but care must be taken to avoid clearance that will result in a tendency for parts to jam or hang up during insertion. 2. Standardize by using common parts, processes, and methods across all models and even across product lines to permit the use of higher volume processes that normally result in lower product cost. 3. Use pyramid assembly provide a progressive assembly about one axis of reference. In general, it is best to assemble from above. 4. Avoid, where possible, the necessity for holding parts down to maintain their orientation during manipulation of the subassembly or during the placement of another part. If holding down is required, then try to design so that the part is secured as soon as possible after it has been inserted. 36

37 Design Guidelines for Insertion and Fastening For ease of insertion a designer should attempt to: 5. Design so that a part is located before it is released. A potential source of problem arises from a part being placed where, due to design constraints, it must be released before it is positively located in the assembly. Under these circumstances, reliance is placed on the trajectory of the part being sufficiently repeatable to locate it consistently. 6. When common mechanical fasteners are used the following sequence indicates the relative cost of different fastening processes, listed in order to increasing manual assembly cost. (a)snap fitting, (b)plastic bending, (c)riveting, (d)screw fastening. 7. Avoid the need to reposition the partially completed assembly in fixture. 37

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41 Selected manual part handling time standards, seconds (Parts are within easy reach, are not smaller than 6mm, do not stick together, and are not fragile or sharp). For Parts that can be grasped and manipulated with one hand without the aid of grasping tools. No Handling difficiencies Part nest or tangles Thickness >2mm < 2mm Thickness >2mm < 2mm Sym (deg) = Size 6mm < Size Size 6mm < Size (Alpha + Beta) >15 mm size > 6mm >15 mm size > 6mm < 15mm < 15mm Sym < < = Sym < < = Sym < 270 Sym =

42 For parts that can be lifted with one hand but require wo hands because they severely nest or tangle, are flexible or require forming etc Alpha < = 180 Size >15 mm Alpha = 360 Alpha = 360 6mm < Size size > 6mm < 15mm

43 Parts inseted but not secured immediately or secured by snap fit Secured by separate operation or part No holding down Holding down Required Required Secured on insertion by snap fit Easy to not easy Easy to Not easy Easy to Not easy align to align to align to align align align no access or vision difficulties Obstructed access or ristricted vision Obstructed access and ristricted vision

44 Parts inserted and secured immediately by screw fastening with power tool Easy to align Not Easy to Align 0 1 No access or Vision difficulty Ristricted vision only Obstructed access only

45 Selected separate operation time, seconds Screw tighten with Manipulation reorientation Addition of power tool or adjustment non solids

46 Coffee mill assembly tree PEMP 46

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49 DFA Analysis for the KRUPS COFFEE MILL PEMP 49

50 Parts cost estimation and manufacturing process optimization PEMP As well as estimating the cost of assembly, the M of DFMA estimates the basic cost of manufacturing the components, identifies suitable basic manufacturing processes and then optimizes parts for the relevant process. The DFMA method is available on paper or in software. The software is now available as part of a package of Design for --- products marketed by Boothroyd Dewhurst Inc. 50

51 Summary Guidelines for assembly have been discussed like - modular assemblies - use of standard parts and assemblies - multifunctional parts - multiple use parts - avoid separate fasteners - maximise compliance - minimise assembly directions - minimise handling Effect of part size, part geometry, assembly clearances and part acquisition times on manual assembly time have been tabulated 51