2 Assembly Processes Mechanical Assembly –Threaded fasteners – hardware components with external or internal threads –Screws, bolts, and nuts –Screw is externally threaded – assembled into a blind threaded hole –Bolt is also externally threaded – inserted through holes in the parts and “screwed” into a nut on the opposite side. –Nut is internally threaded – threads of a nut match those on bolts of the same diameter, pitch, and thread form Types of nuts
3 Assembly Processes Mechanical Assembly –Washers – used with threaded fasteners to ensure tightness –Common types – (a) plain (flat) washer; (b) spring washers, to dampen vibration or compensate for wear; and (c) lock washer, to resist loosening –Washers serve various functions: Distribute stresses at the bolt or screw head and nut Distribute stresses at the bolt or screw head and nut Provide clearance for large holes Provide clearance for large holes Protect part surface Protect part surface Seal the joint Seal the joint Resist inadvertent unfastening Resist inadvertent unfastening Types of washers
4 Assembly Processes Mechanical Assembly –Rivets – unthreaded, headed pin – used for achieving a permanent mechanically fastened joint –Primary fastening processes in the aircraft and aerospace industries –Types – geometries telling how the rivet will be upset to form the second head – (a) solid, (b) tubular, (c) semi tubular, and (d) bifurcated. –Other mechanical assembly techniques – stitching, stapling and sewing Types of washers
5 Assembly Processes Thermal Assembly –Involves the use of heat – permanent joining process –Commonly used processes – welding, brazing and soldering –Welding – two or more parts are coalesced by heat and /or pressure –Usually performed on parts made of the same metal –Advantages and disadvantages of the welding are: AdvantagesDisadvantages It’s a permanent jointExpensive in terms of labor cost Welded joint can be stronger than the parent material Involves high energy and is inherently dangerous It is economical in terms of material usage and fabrication cost The permanent bond does not allow for convenient disassembly It can also be accomplished in fieldQuality defects reduce the strength of joint
6 Assembly Processes Thermal Assembly –The welding operation most common welding process is fusion welding – a filler metal is added to facilitate the process and provide bulk and strength to the welded joint –Some types of fusion welding are: Arc welding – an electric arc is maintained between an electrode and the workpiece – electrode may provide the filler metal Arc welding – an electric arc is maintained between an electrode and the workpiece – electrode may provide the filler metal Resistance welding – an electric current passes through two pieces of metal under pressure (examples are spot and seam welding) Resistance welding – an electric current passes through two pieces of metal under pressure (examples are spot and seam welding) Beam welding – supplying heat by bombarding the workpiece with a concentrated beam of (a) electrons (electron beam welding) or (b) high-energy light beam (laser welding) Beam welding – supplying heat by bombarding the workpiece with a concentrated beam of (a) electrons (electron beam welding) or (b) high-energy light beam (laser welding) Gas welding – a combustible gas (oxyacetylene) is burned with air or oxygen to heat the parent metal and the filler Gas welding – a combustible gas (oxyacetylene) is burned with air or oxygen to heat the parent metal and the filler –Types of weld joint – butt joint, corner joint and lap joint
8 Assembly Processes Thermal Assembly –Brazing – filler metal is melted and distributed between the fusion surfaces of metal parts –Melting point of the filler metal is below the melting point of the base metal –Strength is less than that of a welding –Brazing temperature for: aluminum alloys – 600 o C, silver alloys – 730 o C and nickel alloys – 1120 o C. –Advantages – (a) dissimilar metals (b) less heat and power –Soldering – similar to brazing, melting point of the filler metal does not exceed 450 o C –Soldering is associated with electronics assembly, automobile radiators, food containers –Most solders are alloys of tin and lead.
9 Assembly Processes Adhesive bonding is probably the first of the permanent joining methods Adhesive is usually a polymer e.g. epoxy, silicon and urethane Adhesive bonding – a joining process in which a filler material is used to hold two (or more) closely spaced parts Used for joining similar and dissimilar materials such as metals, plastics, ceramics, wood, paper Curing time or setting time is the time required to change the physical properties of an adhesive from a liquid to a solid state, usually by a chemical reaction Major applications – automotive, aircraft, building products, and packaging industries; other industries include footwear, furniture, bookbinding, electrical, and shipbuilding
10 Conditioning Processes Conditioning processes change internal properties of a material – magnetizing a piece of steel Classification – mechanical, chemical and thermal conditioning Mechanical conditioning is a process in which we use a mechanical force Example – hammering a piece of metal makes it harder and its crystal structure changes, getting longer and thinner Chemical conditioning is a process in which a chemical reaction takes place to change the internal proprieties and structure of a material Example – mixing plaster and water, heat is given off and the plaster hardens Thermal conditioning is the other name of heat treatment in which we change the internal characteristics through controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape
11 Conditioning Processes Thermal conditioning is often associated with increasing the strength, obtaining different hardness/softness levels Example – raise the temperature of a piece of steel to red-hot, in a furnace and then –Keep the piece in there for a long time after switching off the furnace, the metal to gain the most uniform crystal structure and become quite soft – annealing –Cool the piece in water, it becomes harder – hardening –Heat the piece again, not quite as hot, and cool it quickly, the steel becomes less brittle – tempering –Allow the piece to cool very quickly, for example in ice-cold water, the crystals have no time to gain a proper structure and the material becomes most brittle – quenching Other common examples of conditioning may be Light exposing photographic film Baking a cake Cooking an egg Making butter from cream Boiling water Freezing water Using radiation on a tumor Making wine from grape juice
12 Properties of Materials Materials are chosen for their characteristics or properties Examples – glass for windows, plastic for dishes, copper for electrical wires Common properties – strength, hardness, appearance, ability to conduct electricity, and resistance to corrosion etc. Mechanical Properties –Mechanical properties determine the behavior of a material when subjected to mechanical forces –Strength is the ability of a material to keep its own shape when a force is applied –There are four kinds of forces –Tension is a force that pulls on a piece of material, e.g. pulling a spring –Compression is a force that pushes on or squeezes a material, e.g. squeezing s sponge –Torque or torsion is the twisting force, e.g. using a wrench to turn a bolt –Shear force acts on a material like a pair of scissors. One part of the material slides in one direction and the other part slides in the opposite direction –Toughness is the ability of a material to absorb energy without breaking, e.g. leather is tough
13 Properties of Materials Mechanical Properties Four types of mechanical forcesToughness test of a car
14 Properties of Materials Mechanical Properties –Hardness is the ability of a material to withstand penetration forces, e.g. the teeth of a circular saw blades are often made of tungsten carbide –Malleability is the ability of a material through which it can be shaped or extended, e.g. thin wires are drawn from rods –Ductility is the ability of a material through which it can be bent without breaking, e.g. the material of pots and pans has to be ductile enough –Elasticity is the stiffness of a material –Elastic materials are those that come back to its original shape and size after bending, e.g. a rubber band –Plastic materials stay bent after bending, e.g. modeling clay is a plastic material –Yield strength marks the transition of a material from elastic to plastic state Response of a material upon bending forces
15 Properties of Materials Electrical Properties –Materials that offer very little resistance to the flow of electricity are conductors, e.g. silver and copper –Materials that resist the flow of electricity most strongly are insulators, e.g. plastic and rubber –Materials that can be attracted by a magnet are called magnetic materials, e.g. iron, nickel etc –Electromagnet a piece of iron becomes a magnet when current flows in a wire around it Thermal Properties –Properties related to the ability of a material to conduct heat – thermostat and thermos bottle thermometer –Copper and aluminum are the two best conductors of heat –Insulation is a layer of material used to prevent the movement of heat
16 Properties of Materials Optical Properties –These properties refer to a material’s ability to transmit or reflect light, e.g. window –Plastic is used making contact lenses –Other scientific applications of these properties are fiber optics, telescopes, microscopes, headlights and flashlights etc.