Design and Manufacture 1

advantages and disadvantages of basic permemant mould casting

advantages - close dimensional control, good surface finish, finer grain structure

disadvantages - limited to lower melting point metals, simple part geometries, high cost of mould

What is the definition of metal forming?

processes that utilise plastic deformation to change the shape of metal workpieces

How do rotating rolls pull works into the gap?

via friction

Definition of forging (bulk deformation)

work is compressed between two dies, and via either impact or gradual pressure a part is formed

What kind of components are produced by forging?

High strength components - crackshafts, gears, connecting rods

Advantage of hot/warm working?

Significant deformation can be achieved

Advantages and disadvantages of extrusion

Advantages - large variety of shapes are possible, close tolerances are possible, in some cases little to no waste

Disadvantages - part must have a cross section that is uniform through the length

Advantages and disadvantages of machining

advantages - good dimensional accuract and surfact finish, typically performed after a different manufacturing process to provide the final geometry, dimension and finish

disadvantages - waste of material, time consuming

what is the viscosity of lubricant highly dependant on?

temperature

expressed as a percentage of the critical speed, what is a safe operating speed for a system?

~ 50% the critical speed

What are some causes of gear failure?

- bending stress fatigue, number of teeth, pressure angle, bearing fatigue on tooth surfaces

what are the two different kinds of fasteners? Give examples of both

Permenant and non permenant

pemenant - welding, brazing, soldering, adhesives, rivets

non permenant - screws, nuts, bolts, pins

what are some Design considerations of Fasteners

strength, corrosion, maintenence, weight, magnetic properties, assembly, cost and life

Why is the mould opened IMMEDIATELY after basic permenant mould casting concludes?

to prevent cracks from developing

What is the flow stress of a material?

the instantaneous value of stress required to keep defomring the material

What temp is cold, warm and hot working performed at?

cold - RT or below o.3Tm

warm - >o.3Tm but below 'recrystalization'

hot - 0.5Tm < T < 0.75Tm

Tm = melting temp in kelvin

What equipment is used in forging?

Impact forging - forge hammer

Press forging - forge press

What are the different classifications of forging processes?

Open die forging - work is compressed between two flat dies, allowing the metal to flow laterally

impression die forging - die surfaces contain cavity or impression that is imparted onto the work

flashless forging - work is completely constrained in the die, no excess flash is created

Definition of extrusion (bulk deformation)

the work is forced to flow through a die opening to produce the desired cross-sectional shape

What are the two different types of extrusion?

Forward and backward extrusion / direct and indirect

Definition of machining processes

the family of shaping operations that is defined by the removal of material from a starting work part in order to achieve the desired geometry

What machine performs the manufacturing process of drilling?

Drill press

What is the difference between a rough cut and a finishing cut with regards to the cutting speed, feed speed and depth of cut?

rough cut - high feed speed, large depth of cut, low cutting speed

finishing cut - low feed speed, shallow depth of cut, high cutting speed

Definition of welding

two or more parts - typically metals - are coalesced at their contact point by the application of heat and/or pressure

Advantages and disadvantages of welding

Advantages - permentant joint is provided, can be done "in the field", most economical way to join parts

DIsadvantages - Needs to be performed manually, expensive in terms of labour, inhernetly dangerous, inconvenient disassembly, quality defects

Advantages and disadvantages of powder metallurgy?

Advantages - net or near net shape, wastes little material, can be made with specfic levels of porosity, can be automated

Disadvantages - High tooling and equipment costs, expensive powders, difficulty storing/handling the metal particles as they degrade over time and pose a fire risk

What are the different kinds of lubrication? What is the differences of their applications?

Boundary lubrication - lubricant film does not completely seperate the sliding surfaces, allowing for metal to metal contact

Full film lubrication - moving surfaces are completely seperated by a layer of lubricant

How are some components self lubricating?

Some metallic components have a high porosity, lubricant is impregnated within the sliding layer of the bearing and is then released through pores in the sliding layer

What is the definition of PV factor for boundary lubricated bearings? What happens at the limiting PV value?

PV factor is an indicator of a boundary lubricated bearings ability to withstand frictional energy dissipated in the bearing

at the limiting PV value, failure occurs due to unstable temperatures

How are rolling element bearings classified?

classified by the loading conditions (radial vs thrust bearings) AND by the configuration of the rolling elements (ball, roller, needle)

What are some of the design considerations with shafts?

- stepped diametres along the shafts to accomodate elements such as gears, sprockets and pulleys

- keys to prevent rotation relative to the shaft

material, stiffness, stress and strength , fatigue

What are the two different kinds of couplings?

rigid and flexible couplings

What is the aim of seals?

to provide a complete physical barrier to leakage flow

What are some of the qualities of "o-ring" elastomeric seals?

- simple, cheap and versatile

- they fall under both the static and dynamic categories

- large standard range of size and materials

What is a unique consideration when regarding seals for food related items?

must be able to seal against bacteria which are very small

What are the two different kinds of dynamic seals?

rotary and reciprocating

list 3 kinds of dynamic seals

V-ring seals, lip seals, mechanical face seals, labyrinth seals

What are the 3 different classifications of gears?

- parallel axes

- non-parallel co-planar

- non-parallel non-coplanar

Advantages and disadvantages of Helical cut gears

- can be designed for all shafts, quieter than spur gears

- more expensive than straight cut gears, produce (generally unwanted) axial force

What is a gear train? What are the two types of gear trains?

"one or more pairs of gears operating together to transmit power"

You can have simple and compound

How does number of teeth impact gear design?

For a given diameter, the greater the number of teeth, the finer the pitch and hence the weaker they are

What are the notable characteristics of a bolt vs a screw

a bolt may or may not have a section of plain shank

a machine screw will generally be threaded throughout its full length

a screw will generally have a taper and a thread designed to cut into the material

What are the advantages and disadvantages of using adhesives as a fastener?

Advantages - uniform distribution of stresses over bonded area, stiffer structures, smooth surface finish, fatigue resistant

disadvantages - lower strength, decreasing strength with temp. oxidation and hard to dismantly

What is metal casting?

"the process in which molten metal flows by gravity into a mould where it solidifies into the shape of the mould cavity"

List the advantages and disadvantages of metal casting

Advantages - Complex part creation, can make external and internal shapes, parts are net or 'near net' shape, can produce large parts, some methods are suited to mass production

Disadvantages - limitation on mechanical properties, poor dimensional accuracy and surface finish for some processes, safety hazard, environmental problems

Why must a mould by slightly oversized?

to allow for metal shrinkage during solidificaiton and cooling

What are the two different classifications of metal casting?

expendable and permenant

List the parts involved in the sand casting process and their uses

Pouring cup - the "funnel" by which molten metal flows into the mould

Riser - a resevoir in the mould that serves as a source of liquid metal to compensate for the shrinkage during solidification and cooling

Mould - consists of "cope" and "drag" (upper and lower half respectively) contained in a box and seperated by the parting line

Cavity - defines the external surfaces of a cast part, formed by packing sand around a pattern

Core - defines the internal surfaces of a cast part, placed inside the mould to define the interior geometry

What must be considered during riser design

it must be designed to freeze AFTER the main casting

What is the difference in the freezing of pure metals vs the freezing of an alloy?

pure metals freeze at a constant temp, alloys freeze over a temp range

What does a higher V/A ratio mean in regards to cooling and solidification?

slower cooling and solidification

What are the benefits of small, large and irregular grain sizes of sand in sand casting?

small - better surface finish

large - gases can escape due to better porosity

irregular - strengthens the mould due to interlocking

what are the different types of sand that can be used?

Green sand mould - contains moisture when poured|

dry sand mould - organic binders and is baked

skin dried mould - dried to a depth

Advantages and disadvantages of investment casting

Advantages - high precision pieces, close dimensional control, good surface finish, wax can be reused, net shape, all metals can be investment cast

disadvantages - relatively expensive and small size

What are the two different types of die casting?

Hot and cold chamber

Advantages and disadvantages of die casting?

Advantages - economical for large production, close dimensional control, good surface finish, thin sections are possible, rapid cooling

Disadvantges - limited to lower melting point metals, part geometry must account for the fact that the mould needs to be opened

What are the different kinds of casting defects?

Misrun - a portion of the casting solidifies before it fills the mould completely

cold shut - two portions of metal dont fuse due to premature freezing

cold shot - metal spatters during pouring, solid globules form

shrinkage cavity - depression in surface or internal void in the casting

microporosity - network of small voids caused by localised solidification shrinkage or gases

hot tearing - casting is restriced from contraction after cooling

What are the two categories of metal forming and their subgroups?

Bulk deformation - rolling, forging, extrusion, drawing

Sheet metalworking - bending, drawing and shearing (cutting)

what are the ideal material properties for metal forming?

low yield strength

high ductility

Why do we utilise higher temperatures in metal forming?

requires lower forces and ductility increases at higher temps

Definition of rolling (bulk deformation)

reducing the thickness of a work by compressive forces exterted by two opposing rolls

What equipment is used to perform rolling?

A rolling mill

Advantage of cold working?

Strength of material is increased due to strain hardening

what are the three different kinds of machining?

Conventional machining - a sharp cutting tool is involved - turning, milling, drilling

abrasive - material is removed by hard abrasive particles - grinding, sanding

Non traditional processes - energy forms - laser cutting, water jet cutting, electrical discharge

Definition of turning (machining)

a single point cutting tool removes material from a rotating workpiece

what machine performs the manufacturing process of turning?

a lathe

Definition of drilling (machining)

a rotating cylindrical tool with two cutting edges removes material to create a round hole

Definition of milling (machining)

workpiece is slowing fed past a rotating cylindrical tool with multiple cutting edges to generate a piece or to strengthen a surface

What machinie performs the manufacturing process of milling?

Milling machine

What are the two different types of milling? What makes them unique?

Peripheral milling - the axis of the tool is PARALLEL to the surface of the part being machines

Face milling - the axis of the tool is PERPENDICULAR to the surface of the part being machines

Definition of Powder Metallurgy

Parts are produced from the formation of metallic powders

What are the two different steps of powder metallurgy and their definitions?

Pressing - powders are compressed into "green compact"

Sintering - "green compacts" are heated to form the particles into a hard, rigid mass - this is performed BELOW the melting point of the metal

What is the difference between true and bulk density?

True density - true volume of the material, if the powders were melted into a mass

bulk density - density of powders in the loose state after pouring

With reference to the chemistries of the particles, when is blending and mixing used? (powder metallurgy)

blending is used when the powders have the same chemistry, mixing is used when handling powders of different chemistries - in both examples particle sizes can vary

What is the benefit of hot isostatic pressing?

because it is carried out at a high temperature, it presses AND sinters in one step

What are the two different types of bearings?

Sliding bearings (plain, journal) and rolling element bearings

What % of the limiting PV value for a boundary lubricated bearing should you aim for for the practical PV value?

roughly half

What is meant by the term "radial clearance" and "thrust clearance" with respect to rolling element bearings

radial clearance - total movement of the inner ring relative to the outer ring in a direction PERPENDICULAR to the bearing axis

thrust clearance - maximum movement of the inner ring relative to the outer ring in a direction PARALLEL to the bearing axis

What is the dynamic load rating of a bearing?

C - the dynamic load rating is the constant radial load a bearing can endure for 1x10^6 revolutions without evidence of fatigue

What are some of the modes of failure in regards to bearings?

wear, fatigue, static and impact loading, overheating, corrosion

What are the two different kinds of shafts?

Transmission shafts - shafts that transmit power between source and machines

Machine shafts - shafts that form an integral part of the machine

The factors of safety range goes from 1.25 to 4.0. what are some of the differences between a 1.25-1.5 range factor of safety compared to a 3-4 factor of safety?

1.25-1.5 - reliable materials, controlled conditions, loads and stresses that are known with certainty

3-4 - untried materials, average conditions OR well known materials under uncertain loads, stresses and conditions

What is the critical speed of a system?

the rotational speed at which dynamic forces acting upon the system cause it to vibrate at its natural frequency

What are the two different kinds of seals? What are their differences?

Static and dynamic seals

static seals - takes place between two surfaces that do not move relative to one another

dynamic seals - takes place between two surfaces that move relative to one another

What are gaskets?

a material clamped in a joint between two components with the purpose of preventing fluid flow

What are possible methods of seal failure?

- seals operating out with their specific temperatures

- contamination

- misalignment

What are gears? When are they used?

"toothed cylindrical wheels for the transmission of mechanical power from one rotating shaft to the other"

used when the distance between shafts is not large

Advantages and disadvantages of straight cut gears (spur gears)

- simple, cheap and common

- produces noise, and its less compact

Describe a planetary gear system

a system of gears consisting of a central "sun" gear, planetary gears and an outer ring

What are the 4 different kinds of joints that can be made with adhesvies?

Butt joint, lap joint, scarf joint, double strap joint

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