Mechanical Fastening Processes Brazing. Brazing – joining process where just the filler material’s heat is raised to its melting temperature to join materials First used as far back as 3000 to 2000 B.C. Filler material has a lower melting temperature than components that you are joining.
Highest:Brazing, bolts, nuts, and other fasteners.
Intermediate:Riveting and adhesive
Low:Seaming and crimping
High End Convection CAB Brazing Furnace
High End Vacuum Furnace
33.2 Surface Structure and Integrity
R a = ( a + b + c + d + ….)/ n
R q = sq rt [(a2 + b2 + c2 + d2 + ….)/n]
Some symbols used to represent surface finish
When measuring surface roughness, instruments called surface profilometers are used. Profilometers are tipped with a diamond stylus which usually has a 10 micon diameter. The way profilometers are used is they travel over the surface in a straight line recording about 10 to 15 roughness irregularities. The distance travel maybe anywhere from .08 to 25 mm. The most common distance used is .8 mm.
There are other ways of measuring surface measurement. This is done by using either an Optical-interference microscope or an atomic-force microscope.
Another thing to take in to account is that all friction dissipates energy. This energy is converted into heat. Sometimes the heat may soften or even melt the material in use
Plastics generally possess low frictional characteristics. This is better for creating items such as bearings, gears, seals, prosthetic joints. But, an important consideration is that plastics have a low melting point. So, any heat caused by friction must be taken into account. Ceramics on the other hand have generally the same frictional characteristics as metals.
The coefficient of friction is found during the manufacturing process or in laboratory tests using smaller versions of various sizes of the material. One of the common tests used is called the ring-compression test. This is where a flat ring is upset plastically between two flat platens. If the both the diameters of the ring expand outward then the friction is zero. If the inner diameter becomes smaller, then there’s an increase of friction.
Wear changes the shapes of tools and dies, affects the tool life, tool size, and the quality of the parts produced.
Importance of wear is evident in the number of parts and components that continually have to be replaced or repaired.
i.e. dull drill bits, worn cutting tools and dies
Running In period removes the peaks from asperities.
Under controlled conditions, wear may be regarded as a type of smoothing or polishing process.
A tangential force is applied and shearing takes place either at
a) the original interface
b) along a path below or above the interface
*adhesive bonds often are stronger than the base metals.
Schematic illustration of (a) two contacting asperities, (b) adhesion between two asperities, and (c) the formation of a wear particle.
Caused by a hard rough surface sliding across another surface. Microchips and slivers are produced, leaving grooves or scratches on the softer surface.
*processes: filing, grinding, ultrasonic machining and abrasive jet machining act in this manner
4-Regimes of Lubrication in Manufacturing Ops.
1)Thick-film: surfaces separated completely by a film of lubricant. Results in dull, grainy surface appearance after forming operations
2) Thin-film: Load between the die and work piece increases, or the speed and viscosity of fluid decrease, the lubricant becomes thinner raising friction and results in slight wear
3)Mixed: a significant portion of the load is carried by the physical contact of the two surfaces. The rest is carried by the fluid film trapped in the valleys (asperities).
4) Boundary: load supported by contacting surfaces covered with boundary film of lubricant. The lubricant is attracted physically to the metal surfaces, thus preventing direct metal-to-metal contact and reducing wear.
Oils are very effective in reduction of friction and wear and have low thermal conductivity but they do not conduct away the heat generated by friction effectively.
*It is difficult and costly to remove oils from component surfaces that are to be painted or welded, and is
difficult to dispose of.
Emulsion: mixture of oil and water in various proportions along with additives. aka water-soluble oils or coolants
1)direct: mineral oil dispersed in water in very small droplets
Important in metalworking b/c the presence of water gives them high cooling capacity.
Synthetic Solutions: chemical fluids that contain inorganic and other chemicals dissolved in water (contain NO mineral oils) Oil found @ Pepboys
Soaps: reaction products of sodium or potassium salts with fatty acids. Effective boundary lubricants. Alkali soaps are soluble in water but metal soaps are generally insoluble
Greases: solid or semisolid lubricant that consists of soaps, mineral oil, and additives. Highly viscous and adhere well to metal surfaces.
Waxes: Less greasy and more brittle. Limited to metalworking operations
Metalworking fluids usually blend with various additives:
Graphite: effective at elevated temps, however friction is low only in the presence of air
Molybdenum disulfide: lamellar solid lube, similar appearance to graphite however has a high friction coefficient in an ambient environment. Oils commonly used with MoS2.
Metallic & polymeric Films: Thin layers of soft metals and polymer coatings. Suitable metals include lead, indium, tin, silver, PTFE(Teflon).
Considerations of several factors:
Potential hazards may be involved by contacting or inhaling some fluid. Improper disposal of fluids may cause adverse effects on the environment.
Much progress has been made in developing environmentally safe fluids, technology, and equipment for their proper treatment, recycling, and disposal.