PDT 111 Manufacturing Process CHAPTER 6 Concept and

PDT 111 Manufacturing Process CHAPTER 6 : Concept and Methodologies of Joining and Assembly Processes Powerpoint Templates Page 1

Course Outcome 4 Ability to analyze and evaluate the concept & methodologies of joining and assembly technology. Powerpoint Templates Page 2

Joining and Assembly Processes: ü Welding Fundamentals ü Welding Methods and Procedures • Brazing, Soldering and Adhesive Bonding • Mechanical Assembly Technology Powerpoint Templates Page 3

Three Joining Processes 1. Brazing 2. Soldering 3. Adhesive Bonding Powerpoint Templates Page 4

Overview of Brazing and Soldering • Both use filler metals to permanently join metal parts, but there is no melting of base metals. • When to use brazing or soldering instead of fusion welding? – Metals have poor weldability – Dissimilar metals are to be joined – Intense heat of welding may damage components being joined – Geometry of joint not suitable for welding – High strength is not required Powerpoint Templates Page 5

Overview of Adhesive Bonding • Uses forces of attachment between a filler material and two closely‑spaced surfaces to bond the parts. – Filler material in adhesive bonding is not metallic. – Joining process can be carried out at room temperature or only modestly above. Powerpoint Templates Page 6

Brazing • Joining process in which a filler metal is melted and distributed by capillary action between faying surfaces of metal parts being joined. • No melting of base metals occurs. – Only the filler melts. • Filler metal Tm greater than 450 C (840 F) but less than Tm of base metal(s) to be joined. Powerpoint Templates Page 7

Strength of Brazed Joint • If joint is properly designed and brazing operation is properly performed, solidified joint will be stronger than filler metal out of which it was formed. • Why? – Small part clearances used in brazing. – Metallurgical bonding that occurs between base and filler metals. – Geometric constrictions imposed on joint by base parts. Powerpoint Templates Page 8

Brazing Compared to Welding • Any metals can be joined, including dissimilar metals. • Can be performed quickly and consistently, permitting high production rates. • Multiple joints can be brazed simultaneously • Less heat and power required than FW. • Problems with HAZ in base metal are reduced. • Joint areas that are inaccessible by many welding processes can be brazed; capillary action draws molten filler metal into joint. Powerpoint Templates Page 9

Disadvantages and Limitations of Brazing • Joint strength is generally less than a welded joint • Joint strength is likely to be less than the base metals • High service temperatures may weaken a brazed joint • Color of brazing metal may not match color of base metal parts, a possible aesthetic disadvantage Powerpoint Templates Page 10

Brazing Applications • Automotive (e. g. , joining tubes and pipes). • Electrical equipment (e. g. , joining wires and cables). • Cutting tools (e. g. , brazing cemented carbide inserts to shanks). • Jewelry. • Chemical process industry. • Plumbing and heating contractors join metal pipes and tubes by brazing. • Repair and maintenance work. Powerpoint Templates Page 11

Brazed Joints • Butt and lap joints common: – Geometry of butt joints is usually adapted for brazing. – Lap joints are more widely used, since they provide larger interface area between parts. • Filler metal in a brazed lap joint is bonded to base parts throughout entire interface area, rather than only at edges. Powerpoint Templates Page 12

Butt Joints for Brazing Figure 32. 1 (a) Conventional butt joint, and adaptations of the butt joint for brazing: (b) scarf joint, (c) stepped butt joint, (d) increased cross‑section of the part at the joint. Powerpoint Templates Page 13

Lap Joints for Brazing Figure 32. 2 (a) Conventional lap joint, and adaptations of the lap joint for brazing: (b) cylindrical parts, (c) sandwiched parts, and (d) use of sleeve to convert butt joint into lap joint. Powerpoint Templates Page 14

Some Filler Metals for Brazing Base metal(s) Aluminum Nickel-copper alloy Copper Steel, cast iron Stainless steel Filler metal(s) Aluminum and silicon Copper and phosphorous Copper and zinc Gold and silver Powerpoint Templates Page 15

Desirable Brazing Metal Characteristics • Melting temperature of filler metal is compatible with base metal. • Low surface tension in liquid phase for good wettability. • High fluidity for penetration into interface. • Capable of being brazed into a joint of adequate strength for application. • Avoid chemical and physical interactions with base metal (e. g. , galvanic reaction). Powerpoint Templates Page 16

Applying Filler Metal Figure 32. 4 Several techniques for applying filler metal in brazing: (a) torch and filler rod. Sequence: (1) before, and (2) after. Powerpoint Templates Page 17

Applying Filler Metal Figure 32. 4 Several techniques for applying filler metal in brazing: (b) ring of filler metal at entrance of gap. Sequence: (1) before, and (2) after. Powerpoint Templates Page 18

Heating Methods in Brazing • Torch Brazing - torch directs flame against work in vicinity of joint. • Furnace Brazing - furnace supplies heat. • Induction Brazing – heating by electrical resistance to high‑frequency current in work. • Resistance Brazing - heating by electrical resistance in parts. • Infrared Brazing - uses high‑intensity infrared lamp. Powerpoint Templates Page 19

Soldering • Joining process in which a filler metal with Tm less than or equal to 450 C (840 F) is melted and distributed by capillary action between faying surfaces of metal parts being joined. • No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond. • Soldering similar to brazing, and many of the same heating methods are used. • Filler metal called solder. • Most closely associated with electrical and electronics assembly (wire soldering). Powerpoint Templates Page 20

Soldering Advantages / Disadvantages Advantages: • Lower energy than brazing or fusion welding. • Variety of heating methods available. • Good electrical and thermal conductivity in joint. • Easy repair and rework. Disadvantages: • Low joint strength unless reinforced by mechanically means. • Possible weakening or melting of joint in elevated temperature service. Powerpoint Templates Page 21

Solders • Usually alloys of tin (Sn) and lead (Pb). Both metals have low Tm. • Lead is poisonous and its percentage is minimized in most solders. • Tin is chemically active at soldering temperatures and promotes wetting action for successful joining. • In soldering copper, copper and tin form intermetallic compounds that strengthen bond. • Silver and antimony also used in soldering alloys. Powerpoint Templates Page 22

Mechanical Means to Secure Joint Figure 32. 8 Techniques for securing the joint by mechanical means prior to soldering in electrical connections: (a) crimped lead wire on PC board; (b) plated through‑hole on PC board to maximize solder contact surface; (c) hooked wire on flat terminal; and (d) twisted. Powerpoint wires. Templates Page 23

Soldering Methods • Many soldering methods same as for brazing, except less heat and lower temperatures are required. • Additional methods: – Hand soldering – manually operated soldering gun. – Wave soldering – soldering of multiple lead wires in printed circuit cards. – Reflow soldering –used for surface mount components on printed circuit cards. Powerpoint Templates Page 24

Wave Soldering Figure 32. 9 Wave soldering, in which molten solder is delivered up through a narrow slot onto the underside of a printed circuit board to connect the component lead wires. Powerpoint Templates Page 25

Adhesive Bonding • Joining process in which a filler material is used to hold two (or more) closely‑spaced parts together by surface attachment. • Used in a wide range of bonding and sealing applications for joining similar and dissimilar materials such as metals, plastics, ceramics, wood, paper, and cardboard. • Considered a growth area because of opportunities for increased applications. Powerpoint Templates Page 26

Terminology in Adhesive Bonding • Adhesive = filler material, nonmetallic, usually a polymer. • Adherends = parts being joined. • Structural adhesives – of greatest interest in engineering, capable of forming strong, permanent joints between strong, rigid adherends. Powerpoint Templates Page 27

Curing in Adhesive Bonding • Process by which physical properties of the adhesive are changed from liquid to solid, usually by chemical reaction, to accomplish surface attachment of parts. • Curing often aided by heat and/or a catalyst. – If heat used, temperatures are relatively low. • Curing takes time ‑ a disadvantage in production. • Pressure sometimes applied between parts to activate bonding process. Powerpoint Templates Page 28

Joint Strength • Depends on strength of: – Adhesive – Attachment between adhesive and adherends • Attachment mechanisms: – Chemical bonding – adhesive and adherend form primary bond on curing – Physical interactions - secondary bonding forces between surface atoms – Mechanical interlocking - roughness of adherend causes adhesive to become entangled in surface asperities Powerpoint Templates Page 29

Joint Design • Adhesive joints are not as strong as welded, brazed, or soldered joints. • Joint contact area should be maximized. • Adhesive joints are strongest in shear and tension. – Joints should be designed so applied stresses are of these types. • Adhesive bonded joints are weakest in cleavage or peeling. – Joints should be designed to avoid these types of stresses. Powerpoint Templates Page 30

Types of Stresses in Adhesive Bonding Figure 32. 10 Types of stresses that must be considered in adhesive bonded joints: (a) tension, (b) shear, (c) cleavage, and (d) peeling. Powerpoint Templates Page 31

Joint Designs in Adhesive Bonding Figure 32. 11 Some joint designs for adhesive bonding: (a) through (d) butt joints; (e) through (f) T‑joints; (b) and (g) through (j) corner joints. Powerpoint Templates Page 32

Adhesive Types • Natural adhesives - derived from natural sources, including gums, starch, dextrin, soya flour, collagen. – Low‑stress applications: cardboard cartons, furniture, bookbinding, plywood. • Inorganic - based principally on sodium silicate and magnesium oxychloride. – Low cost, low strength. • Synthetic adhesives - various thermoplastic and thermosetting polymers. Powerpoint Templates Page 33

Synthetic Adhesives • Most important category in manufacturing. • Synthetic adhesives cured by various mechanisms: – Mixing catalyst or reactive ingredient with polymer prior to applying. – Heating to initiate chemical reaction. – Radiation curing, such as UV light. – Curing by evaporation of water. – Application as films or pressure‑sensitive coatings on surface of adherend. Powerpoint Templates Page 34

Applications of Adhesives • Automotive, aircraft, building products, shipbuilding • Packaging industries • Footwear • Furniture • Bookbinding • Electrical and electronics Powerpoint Templates Page 35

Surface Preparation • For adhesive bonding to succeed, part surfaces must be extremely clean. • Bond strength depends on degree of adhesion between adhesive and adherend, and this depends on cleanliness of surface. – For metals, solvent wiping often used for cleaning, and abrading surface by sandblasting improves adhesion. – For nonmetallic parts, surfaces are sometimes mechanically abraded or chemically etched to increase roughness. Powerpoint Templates Page 36

Application Methods • Manual brushing and rolling • Silk screening • Flowing, using manually operated dispensers • Spraying • Automatic applicators • Roll coating Powerpoint Templates Page 37

Adhesive is dispensed by a manually controlled dispenser to bond parts during assembly (photo courtesy of EFD Inc. ). Powerpoint Templates Page 38

Advantages of Adhesive Bonding • Applicable to a wide variety of materials. • Bonding occurs over entire surface area of joint. • Low temperature curing avoids damage to parts being joined. • Sealing as well as bonding. • Joint design is often simplified, e. g. , two flat surfaces can be joined without providing special part features such as screw holes. Powerpoint Templates Page 39

Limitations of Adhesive Bonding • Joints generally not as strong as other joining methods. • Adhesive must be compatible with materials being joined. • Service temperatures are limited. • Cleanliness and surface preparation prior to application of adhesive are important. • Curing times can limit production rates. • Inspection of bonded joint is difficult. Powerpoint Templates Page 40

Mechanical Assembly Technology 1. Threaded Fasteners 2. Rivets and Eyelets 3. Assembly Methods Based on Interference Fits 4. Other Mechanical Fastening Methods 5. Molding Inserts and Integral Fasteners 6. Design for Assembly Powerpoint Templates Page 41

Mechanical Assembly Defined • Use of various fastening methods to mechanically attach two or more parts together. • In most cases, discrete hardware components, called fasteners, are added to the parts during assembly. • In other cases, fastening involves shaping or reshaping of a component, and no separate fasteners are required. Powerpoint Templates Page 42

Products of Mechanical Assembly • Many consumer products are assembled largely by mechanical fastening methods. – Examples: automobiles, large and small appliances, telephones. • Many capital goods products are assembled using mechanical fastening methods. – Examples: commercial airplanes, trucks, railway locomotives and cars, machine tools. Powerpoint Templates Page 43

Two Major Types of Mechanical Assembly 1. Methods that allow for disassembly – Example: threaded fasteners 2. Methods that create a permanent joint – Example: rivets Powerpoint Templates Page 44

Why Use Mechanical Assembly? • Ease of assembly – can be accomplished with relative ease by unskilled workers. – Minimum of special tooling required. – In a relatively short time. • Ease of disassembly – at least for the methods that permit disassembly. – Some disassembly is required for most products to perform maintenance and repair. Powerpoint Templates Page 45

Threaded Fasteners • Discrete hardware components that have external or internal threads for assembly of parts. • Most important category of mechanical assembly. • In nearly all cases, threaded fasteners permit disassembly. • Common threaded fastener types are screws, bolts, and nuts. Powerpoint Templates Page 46

Screws, Bolts, and Nuts • Screw - externally threaded fastener generally assembled into a blind threaded hole. • Bolt - externally threaded fastener inserted into through holes and "screwed" into a nut on the opposite side. • Nut - internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form. Powerpoint Templates Page 47

Screws, Bolts, and Nuts Figure 33. 1 Typical assemblies when screws and bolts are used. Powerpoint Templates Page 48

Some Facts About Screws and Bolts • Screws and bolts come in a variety of sizes, threads, and shapes. • Much standardization in threaded fasteners, which promotes interchangeability. • U. S. is converting to metric, further reducing variations. • Differences between threaded fasteners affect tooling. – Example: different screw head styles and sizes require different screwdriver designs. Powerpoint Templates Page 49

Head Styles on Screws and Bolts Figure 33. 2 Various head styles available on screws and bolts. Powerpoint Templates Page 50

Types of Screws • Greater variety than bolts, since functions vary more. • Examples: – Machine screws - generic type, generally designed for assembly into tapped holes. – Capscrews - same geometry as machine screws but made of higher strength metals and to closer tolerances. Powerpoint Templates Page 51

Setscrews Hardened and designed for assembly functions such as fastening collars, gears, and pulleys to shafts. Figure 33. 3 (a) Assembly of collar to shaft using a setscrew; (b) various setscrew geometries (head types and points). Powerpoint Templates Page 52

Self-Tapping Screws • Designed to form or cut threads in a pre‑existing hole into which it is being turned. • Also called a tapping screw. Figure 33. 4 Self‑tapping screws: thread‑forming, and thread‑cutting. Powerpoint Templates Page 53

Screw Thread Inserts • Internally threaded plugs or wire coils designed to be inserted into an unthreaded hole and accept an externally threaded fastener. • Assembled into weaker materials to provide strong threads. • Upon assembly of screw into insert, insert barrel expands into hole to secure the assembly. Powerpoint Templates Page 54

Screw Thread Inserts Figure 33. 6 Screw thread inserts: (a) before insertion, and (b) after insertion into hole and screw is turned into insert. Powerpoint Templates Page 55

Washer • Hardware component often used with threaded fasteners to ensure tightness of the mechanical joint. • Simplest form = flat thin ring of sheet metal. • Functions: – Distribute stresses. – Provide support for large clearance holes. – Protect part surfaces and seal the joint. – Increase spring tension. – Resist inadvertent unfastening. Powerpoint Templates Page 56

Washer Types Figure 33. 8 Types of washers: (a) plain (flat) washers; (b) spring washers, used to dampen vibration or compensate for wear; and (c) lockwasher designed to resist loosening of the bolt or screw. Powerpoint Templates Page 57

Bolt Strength Two measures: • Tensile strength, which has the traditional definition. • Proof strength - roughly equivalent to yield strength. – Maximum tensile stress without permanent deformation. Powerpoint Templates Page 58

Stresses in a Bolted Joint Figure 33. 9 Typical stresses acting on a bolted joint. Powerpoint Templates Page 59

Over-tightening in Bolted Joints • • • Potential problem in assembly, causing stresses that exceed strength of fastener or nut. Failure can occur in one of the following ways: 1. Stripping of external threads. 2. Stripping of internal threads. 3. Bolt fails due to excessive tensile. stresses on cross‑sectional area. Tensile failure of cross section is most common problem. Powerpoint Templates Page 60

Basic Functions of Threaded Fasteners • To provide relative rotation between external and internal threads during fastening. • To apply sufficient torque to secure the assembly. – Product designer often specifies required preload to secure assembly. – Assembly operator must apply the right torque to achieve the specified preload. Powerpoint Templates Page 61

Methods to Apply Required Torque 1. Operator feel ‑ not very accurate, but adequate for most assemblies. 2. Torque wrench – indicates amount of torque during tightening. 3. Stall‑motor ‑ motorized wrench is set to stall when required torque is reached. 4. Torque‑turn tightening - fastener is initially tightened to a low torque level and then rotated a specified additional amount. Powerpoint Templates Page 62

Rivets • Unthreaded, headed pin used to join two or more parts by passing pin through holes in parts and forming a second head in the pin on the opposite side. • Widely used fasteners for achieving a permanent mechanically fastened joint • Clearance hole into which rivet is inserted must be close to the diameter of the rivet. Powerpoint Templates Page 63

Types of Rivets Figure 33. 10 Five basic rivet types, also shown in assembled configuration: (a) solid, (b) tubular, (c) semitubular, (d) bifurcated, and (e) compression. Powerpoint Templates Page 64

Applications and Advantages of Rivets • • • Used primarily for lap joints A primary fastening method in aircraft and aerospace industries Advantages: – High production rates – Simplicity – Dependability – Low cost Powerpoint Templates Page 65

Tooling and Methods for Rivets 1. Impact - pneumatic hammer delivers a succession of blows to upset rivet. 2. Steady compression - riveting tool applies a continuous squeezing pressure to upset rivet. 3. Combination of impact and compression. Powerpoint Templates Page 66

Interference Fits • Assembly methods based on mechanical interference between two mating parts being joined. • The interference, either during assembly or after joining, holds the parts together. • Interference fit methods include: – Press fitting. – Shrink and expansion fits. – Snap fits. – Retaining rings. Powerpoint Templates Page 67

Press Fitting • Typical case is where a pin (e. g. , a straight cylindrical pin) of a certain diameter is pressed into a hole of a slightly smaller diameter. • Possible functions: – Locating and locking components ‑ to augment threaded fasteners by holding parts in fixed alignment with each other. – Pivot points - to permit rotation of one component about the other. – Shear pins. Powerpoint Templates Page 68

Shrink and Expansion Fits • Assembly of two parts (e. g. , shaft in collar) that have an interference fit at room temperature – Shrink fitting - external part is enlarged by heating, and internal part either stays at room temperature or is contracted by cooling – Expansion fitting - internal part is contracted by cooling and inserted into mating component - when at room temperature, expansion creates interference • Used to fit gears, pulleys, sleeves, and other components onto solid and hollow shafts Powerpoint Templates Page 69

Snap Fits • Joining of two parts in which mating elements possess a temporary interference during assembly, but once assembled they interlock. – During assembly, one or both parts elastically deform to accommodate temporary interference. – Usually designed for slight interference after assembly. • Originally conceived as a method ideally suited for industrial robots. Powerpoint Templates Page 70

Snap Fit Assembly Figure 33. 13 Snap fit assembly, showing cross‑sections of two mating parts: (1) before assembly, and (2) parts snapped together. Powerpoint Templates Page 71

Retaining Ring • Fastener that snaps into a circumferential groove on a shaft or tube to form a shoulder. • Used to locate or restrict movement of parts on a shaft. Figure 33. 14 Retaining ring assembled into a groove Powerpoint Templates Page 72 on a shaft.

Stitching • U‑shaped stitches are formed one‑at‑a‑time from steel wire and immediately driven through parts to be joined. • Applications: sheetmetal assembly, metal hinges, magazine binding, corrugated boxes. Figure 33. 15 Common types of wire stitches: (a) unclinched, (b) standard loop, (c) bypass loop, and (d) flat clinch. Powerpoint Templates Page 73

Stapling • Preformed U‑shaped staples are punched through the two parts to be attached. • Supplied in convenient strips. • Usually applied by portable pneumatic guns. • Applications: furniture and upholstery, car seats, various light‑gage sheetmetal and plastic assembly jobs. Powerpoint Templates Page 74

Molding Inserts and Integral Fasteners • Permanent joining methods that involve shaping or reshaping one of the components by a manufacturing process such as: – Casting – Molding – Sheet-metal forming Powerpoint Templates Page 75

Molding Inserts • Placement of a component into mold prior to plastic molding or metal casting, so that it becomes a permanent and integral part of the molding or casting. Figure 33. 17 Examples of molded‑in inserts: (a) threaded bushing, and (b) threaded stud. Powerpoint Templates Page 76

Why Use Molding Inserts? • Insert has better properties than molded or cast material. • Insert geometry is too complex or intricate to incorporate into mold. • Examples of applications: – Internally threaded bushings and nuts – Externally threaded studs – Bearings – Electrical contacts Powerpoint Templates Page 77

Integral Fasteners • Components are deformed so they interlock as a mechanically fastened joint. • Methods include: – Lanced tabs – Seaming – Beading Powerpoint Templates Page 78

Lanced Tabs To attach wires or shafts to sheetmetal parts. Figure 33. 18 (a) lanced tabs to attach wires or shafts to sheet metal. Powerpoint Templates Page 79

Seaming Edges of two separate sheetmetal parts or the opposite edges of the same part are bent over to form the fastening seam. Figure 33. 18 (c) single‑lock seaming. Powerpoint Templates Page 80

Design for Assembly (DFA) • • Keys to successful DFA: 1. Design product with as few parts as possible. 2. Design remaining parts so they are easy to assemble. Assembly cost is determined largely in product design, when the number of components in the product and how they are assembled is decided. – Once these decisions are made, little can be done in manufacturing to reduce assembly costs. Powerpoint Templates Page 81

DFA Guidelines • Use modularity in product design. – Each subassembly should have a maximum of 12 or so parts. – Design the subassembly around a base part to which other components are added. • Reduce the need for multiple components to be handled at once. Powerpoint Templates Page 82

More DFA Guidelines • Limit the required directions of access. – Adding all components vertically from above is the ideal. • Use high quality components. – Poor quality parts jams feeding and assembly mechanisms. • Minimize threaded fasteners. • Use snap fit assembly. Powerpoint Templates Page 83

The End. . Any Questions? Powerpoint Templates Page 84