Bob Temple - Manufacturing Flashcards
What are the things to think about when choosing manufacturing processes
Cost
Shape (can you make it?)
Materials (can u process them?)
Quality (can u achieve required tolerance, surface finish, integrity
Production quantity (can you produce required quantity? Automation? )
What is casting and what materials is it used with?
1) pour molten material into mould
2) allow it to solidify
3) remove part from the mould
Used with metal, polymers, ceramics
What are the disadvantages of casting?
Dangerous – heat
Skill intensive – must be trained
Prone to errors – if cooled wrong get wrong properties
May not provide suitable material properties
What are the Advantages of casting
Capable of complex shapes with internal cavities
Capable of making large parts in one piece
Capable of processing material is not possible by other means or too expensive
Competitive with other manufacturing processes
Near net shape manufacturing process
Can make repeatable parts easily
What are the two types of casting and what is the difference between them
Expendable mould and permanent mould
A pattern is used to create the cavity in an expendable mould - They can handle larger and more complex parts
In permanent moulds cavity is machined into the mould they are used for high-volume production
What are the factors affecting casting
The mould Filling of the mould Material shrinkage Material cast structure Defects Removal and finishing of the part
What are the different parts of the mould and what do they do
The pattern creates a mould cavity in the shape of the desired casting
The flask is the outer container (Upper half is the cope and lower half is the drag)
Cores are used to create hollow features
What is the parts that is removed from the mould
A rough casting
It requires finishing operations to obtain the final product
What are the main features of the mould
The mould is designed to allow escape of gases and take account of material shrinkage
- The molten metal is poured in through a spray which leads to a narrow gap called the gate that ensures uniform metal flow into the mould through the runners
- A riser is used to allow air to escape indicate a full mould and supply molten metal as solidification shrinkage because event may also be used to allow small amounts of Excess gas to escape
Which features of the mould are removed from the casting during finishing
Risers and the gates
What are the factors in complete filling the mould
Fluidity of the metal
Metal fills mould before it solidifies
Air in the mould can escape as the liquid metal pushes it out
Evolve gases from the liquid metal can escape mould - don’t want slag in the mould
What is the fluidity index of a metal
The length it will flow and a standard spiral passage before solidifying
This depends on the thermal properties of the mould and metal
How can dissolved gases be removed
What’s the difference between gases in solids and in liquids?
Flushing with An inert gas
Melting and pouring under vacuum
More soluble in liquid metals than solid
How does the mould deal with material shrinkage?
Cavity is usually slightly oversized
Risers supply extra metal as it shrinks in the mould avoiding porosity
Casting strength is low when hot- vulnerable to heat tears - mould should not restrain the casting too much as it shrinks
What affects the mechanical properties of the casting?
Metal structure
What influences the metal structure obtained?
Metallurgy of alloy
Thermal properties of alloy and mould (sand takes a while to cool)
What is the structure of a pure metal casting?
Grains nucleate at mould walls (cool)
Chill zone there contains fine, multi-axed grains
Slower cooling rate= coarser grain structure
Bulk grain structure is columnar
Solidification front moves away from the walls with time
Where is the solidification front thinner?
Thinner at concave (corners pointing in) features, thicker at convex features (corners points outside)
What is the mushy zone?
Where the solid component of the alloy forms as dendrites (tree-like) surrounded by liquid
What is the advantage and disadvantage if grey cast iron?
Poor mech properties but easy to cast (high fluidity)
What effect does cooling rate have on the structure of castings?
What else affects the grain structure formed
Slow- coarse grain structure
Fast- finer grain structure
Alloy composition also affects grain structure
What affects the cooling rate of castings?
Mould material (metal is faster)
What properties occur when grain size decreases?
Strength and ductility increase
Microporosity decreases
Tendency to crack during cooling decreases
Lack of uniformity in grain structure causes anisotropic mechanical properties
How can the equiaxed zone of a casting be extended? How does this work?
Using an inoculant
Works by proving nucleation sites throughout the liquid metal
What does higher cooling rates cause for casting?
Reduce grain size
Microsegregation (cored dendrites - more alloy at surface than core)
Macrosegregation (dendritic structures result in lower conc of alloying elements at the centre of the casting)
What are chills and how do they avoid porosity?
Increase cooling rate at critical points
Internal ones become part of the casting, external ones can be removed
Where does porosity occur and what property does this cause?
Regions of larger section thickness- surrounding thinner regions solidify first
Reduces ductility impairs surface finish and makes casting permeable
What can be done to speed up post casting shrinkage?
Annealing
How is the metal heated and prepared for casting?
Gas/electric furnace
Crucibles made from ceramic (withstand high temps)
Induction heating makes eddy currents in iron and iron alloys-molten metal reactive with O2/dissolve atmospheric gases
Metal is de gassed before pouring otherwise the metal will be porous
What affects the time taken for the casting to solidify?
Volume
Surface area
T= C(V/A)^n
T=time C= constant for process and alloy N= constant between 1.5 and 2.5 V=volume A=SA
What is the equation to determine solidification time? Why is it useful?
Chvorinovs rule
Useful in designing castings to avoid shrinkage porosity and for feeding systems eg riser size and location
What are the examples of expendable mould casting?
Sand
Investment (lost wax)
Evaporative pattern (lost foam )
What are the types of permanent mould casting?
Gravity die
Pressure die
Centrifugal
Describe sand casting
Most used
Use pattern to make desired cavity in sand
Mould broken when casting removed
Cores also made from sand- broken for casting removal- must be supported by core prints and chaplets in mould
Sand bonded to form mould (still permeable for gases to escape)
What is the most common sand casting mixture?
Green moulding sand- sand with clay and water
What are the 4 important characteristics of sand for moulding?
Refractoriness (withstand high temps)
Cohesiveness (retain given shape)
Permeability (allow gas escape)
Collapsibility (allow metal to shrink, and free the casting)
What is the sequence of operations for sand casting?
1) mech drawing of part used to generate design for pattern
2) is patterns mounted on plates with pins for alignment
3) core boxes produce core halves which are pasted together
4) cope half of mould assembled by securing cope pattern plate to flask with aligning pins and attaching inserts to form the sprue and risers
5) flask rammed with sand and plate inserts removed
6) drag half produced - bottom board placed below drag and aligned with pins
7) all inverted, pattern withdrawn, leaving imprint
8) core set in place within drag cavity
9) mould closed, held closed
10) metal poured, metal solidifies and casting is removed
11) sprue and risers cut off, casting cleaned, inspected and heat treated
What are draft angles?
Sloping walls used to ease removal of the pattern from the mould (1-3 degrees)
What is a metal match plate?
Used for high volume manufacture of smaller castings
What should be taken into account when choosing the pattern material?
Used repeatedly so must be durable for expected no of castings
Low cost can be made from wood but aren’t v durable
More robust materials are hard to machine
Weight
Susceptibility to water attack
What do core prints do?
Hold and align core in the cavity?
What are chaplets?
Metal supports that can be used to provide additional stability to the core
What are the 2 advantages of sand casting?
Wide variety of metals and shapes- no limit to part size
Relatively cheap moulds- flexible, allow changes to mould if necessary
What are the disadvantages of sand casting?
Difficulties in meeting exact requirements eg tolerances, surface quality, internal soundness
More suited to lower production volumes except where mechanised mould making is employed
What is investment casting?
Pattern is surrounded or ‘invested’ with a ceramic coating that becomes the mould
Describe the steps of investment casting
Produce master pattern if desired casting
Produce master die
Produce wax patterns.
Assemble wax patterns on a common sprue (tree)
Coat ‘tree’ with initial investment material
Finish coat- layers applied increasingly coarser particles to increase permeability of walls
Allow investment to harden
Melt out wax pattern
Fire investment to finish hardening process and burn off residual wax
Pour molten metal into mould cavity
Allow metal to solidify
Remove castings
Post processing
What’s the diff between an investment cast rotor and conventionally cast ones?
Investment cast has finer and more uniform grain
What are the advantages of investment casting?
Only method producing precision shapes castings and high mp materials
Parts require minimal finishing
High production rates and one off products possible
Flexible, range of alloys processable, fine surface detail, good dimensional tolerances
What are the disadvantages of investment casting?
V expensive as extra steps better for small products not big
What is evaporative pattern casting?
Similar to investment casting- pattern made from expanded polystyrene tho
Flask vibrated to pack sand before pouring in molten metal
What are permanent moulds?
2 part moulds made from durable and thermally resistant materials eg cast iron, steel, bronze, graphite, refractory alloys
Moulds not porous so air vents must be included
Moulds not broken up so parts must be designed to be removable from mould
What are the advantages of permanent moulds?
Reusable
Good surface finish
Good dimensional accuracy
Temp of mould controlled
What are the disadvantages of permanent moulds?
Better for low mp alloys eg Al and Zn
Mould costs can be high
Wear of mould
What affects the mould life of a permanent mould?
Temp of alloy being poured Mould material Mould temp Thermal shock Mould configuration
What is gravity did casting?
Head of liquid and atm pressure applied to liquid
Feeder has higher level of metal in it than the casting
Feeder removed after casting has solidified and cooled
What are the advantages of gravity die casting?
Long die life 25000+ cycles
No expensive machines or equipment required
Dies May be cheap- rough cast and minimal machining
What are the disadvantages of gravity die casting?
Limited capabilities in part shape, complexity and fine detail
Production rates slow
What alloys can be used for gravity die casting?
Al, Zn, Mg, Cu, brass
What is pressure die casting?
Metal injected under action of external pressure
Greater capital expenditure incurred- high production rates, machines can inject and close dies, process semi/fully automated, need min 10000 production runs
Low labour costs
What are the properties of pressure die casting dies
Heat resisting alloys used
Higher surface finish applied
Die wear increases with temp of metal
Multiple cycles can cause surface cracking of the dies
Water based lubes with graphite or other suspensions help reduce die temp
Life cycle of properly constructed and maintained can be 1/2 million cycles
Very rapid production
What are the steps of pressure die casting?
Lube dies Close and lock dies Molten metal forced into die cavity Held under pressure till it solidifies Die opens Casting is ejected
What’s the hot chamber process? (Pressure die casting)
Metal in molten pot
Piston traps done and forced it into die cavity
Lower mp alloys cast (Zn)
Die cooled by circulating fluid
What’s the cold chamber process? Pressure die casting?
Metal poured into injection cylinder
Cylinder not heated
Higher mp alloys cast (Al, Mg)
Large as high clamping force needed
What are the advantages of pressure die casting?
Dimensions accurate Fine surface feature reproduction Thin walls Little finishing required Capital costs outweighed if production runs are high enough
What are the disadvantages to pressure die casting?
Limited to metals with lower mp so mould isn’t destroyed
Part size limited
High tooling costs
What’s a misrun and how can it be prevented?
When molten metal solidifies before it has filled the mould
Ensure high enough temp for fluid to melt
Add fluidising additives
Control conditions- no dendrites formed
Ensure thick walls to allow material flow
Remove sharp corners to promote flow
Run metal in at a few places at the same time
Use chvorinovs rule to balance solidification times
How is porosity avoided?
Use rises to supply extra metal during shrinkage
Vents to allow has escape
Permeable mould materials
Degas casting metal
Use chills at thicker sections
Avoid thickness variation in the casting
Avoid hot spots- big thickness changes in wall
What are the reasons for choosing a casting process?
Size of parts Configuration (complexity, finish etc) Production quantity Tolerances Metals castable
What is the ranking order in terms of part size of castings?
Biggest
Sand
Centrifugal
Gravity die, investment, evaporative
Pressure die
Smallest
What’s the configuration ranking order?
Best
Investment Evap pattern, sand Gravity die Pressure die, Centrifugal
Worst
Which has the best finish?
Pressure die casting?
What’s the production costs ranking?
Perm mould- high mould costs, low labour- low cost in high volume
Sand- cheap moulds, moderate labour costs, low cost for low to medium volume
Investment casting- high labour costs
What’s the tolerance ranking?
Good
Squeeze casting Pressure die Investment Gravity die Centrifugal Sand
Poor
General disadvantages of casting?
High forces required- large and expensive equipment, heavy duty equip, tooling expensive and needs replacing when wears out
Generally only justified for large scale production
How much force is required for casting?
Flow force
Friction- can be needed eg in rolling
Shape/tooling forces (redundant work)
What’s working?
Plastically deforming a metal to change us shape or properties
What are the advantages of hot working?
Lower flow stress Improved ductility Lots of deformation possible Improved mech properties- crystal structure refined Directional flow lines- more strength Better machining properties than cold working Promotes internal difFusion Pores and vacancies removed
What’re the disadvantages of hot working?
Heating costs Poor surface condition Handling problems Poor dimensional tolerances Distortion on cooling and residual stress possible
What’re the advantages of cold working?
No heating required Good surface finish Good dimensional accuracy Strength, fatigue, wear properties improved Contamination problems reduced Good reproducibility
What’re the disadvantages of cold working?
Higher force to initiate and complete deformation
Heavier and stronger equip needed- more expensive
Strain hardening occurs- may need annealing
Residual stresses may be present
Surface must be clean and scale free
Elastic memory may be present
Warm working vs cold working? Discuss
Reduced loads on equip and tools
Increased metal ductility
Reduced straining allows more processing without annealing
Less residual stress induced
Discuss warm working vs hot working
Less scaling, oxidation, de-carbonisation Better dimensional control Smoother finish possible Less need for finishing processes Lower energy consumption in heating Longer tool life (reduced thermal shock)
What’s rolling?
What does it do to ingots?
Squeezing metal between 2 rolls to reduce thickness and increase length
Breaks down coarse brittle and porous structure of ingots Ito wrought structure then cold rolling improves strength hardness and finish
Describe flat rolling
Rolls pull material between them through frictional force
Fr on exit must be higher than entry on the no slip point
What’s draft and how is the max calculated?
Diff between initial and final thickness
ho - hf = mue^2 R
R is radius
ho- hf is draft
List the defects in flat rolling
Wavy edges
Zipper cracks
Edge cracks
Alligatoring
What’s doll bending?
Bending of straight cylindrical rolls, caused by rolling force
Produces strip with uniform thickness
What’s forging?
Forming of discrete metal parts by application of compressive forces using tooling and dies
Good strength and toughness
Not usually high precision
Can be hot or cold
Force supplied by presses or hammers
What’s the forging force equation?
F= Yf pie r^2 (1+(2mue r /3h)) Yf- flow stress of material R- radius of part H- height of part Mue - coefficient of friction
What’re the steps of forging?
Heat blank Descale blank Lubricate Forging -preforming, finish Trimming etc
Why’s lube required for forging?
Wear reduction
Help metal flow
Thermal barrier
Parting agent
Hot-graphite, molybdenum disulphides, glass
Cold- soap, oil, conversion coatings
What’s forgeability?
And what’re the tests for it?
Ability of a material to deform without cracking
Upset test- max height reduction in upsetting before surface cracking occurs
Hot twist test- number of turns before failure
What’re the factors affecting forging die design?
Parting line should be a flat plane through centre of forging
Adequate draft required
Generous fillets and radio reduce wear and help with filling
Keep ribs wide and low
Balance section changes to help flow
Take advantage of fibre flow lines
Keep tolerances as large as poss
What’re the die material requirements?
Strength and toughness resistance Hardenability Mech/thermal shock resistance Wear resistance Tool and die steels containing Cr, Ni, Mo, V
What’s isothermal/ hot die forging?
Dies heated to same temp as forging
Maintains ductility and malleability of the workpiece during forging
Complex parts with high dimensional accuracy can be made
High cost, low production rate
What’s upset forging? (Cold heading)
What are parts limited by?
Cold working operation
Used for wire and rod mostly <50mm
High speed production
Parts limited by
- limiting length of unsupported metal that can be upset is 3 times bar diameter
- lengths greater than 3X Diameter May be upset if die cavity is <1.5x bar diameter
What is extrusion?
Forces a metal volley through a die
Part produced has uniform thickness
Metal is forced through the die using a ram to either push the metal through the die or the die through the metal
What does drawing do?
Produces v small diameter parts eg wire and rod where the original billet would buckle if it were compressed
What is the drawing ratio and what’s the normal value for it?
Final area/initial area
Generally about 0.4- giving reduction in area per mass of 60%
What material properties are affected by elongation and how?
High elongation for formability
Large strain hardening component
High strain rate sensitivity
High uniform elongation desired as materials are being stretched
What material properties are affected by grain size?
Coarser the grain, rougher the surface
Affects mech properties
Surface appearance- orange peel affect
What is shearing?
Cutting metal between rotating blades or punch and die
Metal sheet fractured along a line between the die and punch
What affects quality of cut edge in shearing?
Clearance
Low- excessive tool wear
High- poor edge quality but long tool life
Tool wear shows an increase in clearance
If too small- line of failure from punch to die is irregular
Too large - excessive plastic deformation occurs- deformed edge without parallel sides and plus tear and blurred top surface
What’re the characteristics of a sheared edge?
Small region of parallel sided plastic deformation- called burnished zone
Fractured surface
What’s the approx optimum separation value for a shearing operation?
K x sheet thickness
More ductile 0.04
What’s the equation for shearing force and what affects it?
Shearing force = 0.85to0.65x UTS x thickness x cut length
Usually less tho (average only 70% hence the 0.65 to 0.85)
What does a rake angle do in shearing?
Reduces contact area of blade/die/punch this reducing shearing force
Whatre the major processing parameters in shearing?
Shape and materials of punch and die
Speed of punching
Lubrication
Clearance between punch and die
What’re the process categories of shearing?
Spitting
Nibbling
Die cutting
Fine (fein) blanking
What are the key factors in shearing? (8)
Punch determines hole size Die determines blank size Punches should be kept short Weak parts in dies should be inserts Punches are slightly tapered Dies last 2-3 times longer than punches 6% material thickness clearance between punch and die Material thickness < punch diameter
What’s slitting?
Produces straight or curved cut in the workpiece
Pair of circular blades can be used
What’s nibbling?
Small traumatic punch and die is used to rapidly make many overlapping holes
What’s a piercing?
The metal not under the punch
What’s a blank?
The metal not under the punch
What’s fein/ precision blanking?
V ring or gripper is used to keep deformation zone in compression, ensuring the whole region is plastically sheared
Produces v smooth square edges
Need fine Grain material
Clearances about 1%
Requires triple action press with control of punch, pressure pad, and die
How are bending force and weight related?
Bending force is inversely proportional to weight
How does tube bending work?
Need special tubing to avoid buckling and folding
Need internal support to prevent buckling eg mandrels (plug, balls etc, particulate matter (sand)
Depends on thickness of tube
What’s stretch forming?
A die or punch stretches metal
What’s stretch drawing?
Punch and die squeeze metal into shape between them
What’s embossing?
Uses friction between die and sheet to clamp sheet
What’re blank holder in drawing?
Pressure hold down ring
Stops wrinkling
What’re the variables of deep drawing? (8)
Properties of sheet metal Ratio of blank to punch diameter Clearance between punch and die Punch radius Die corner radius Blankholder force Friction Lube
What’s the draw limit ratio eqn?
Original blank diameter/ cup diameter
Varies 1.6-2.4
Large corner radii required on tools
What’s the steps of multistage drawing?
Redrawing- reduces wall diameter
Ironing- wall thickness reduced
Reverse drawing- stress in opposite direction
Whats peen forming? How does it work?
Peeling with cast iron or steel shot
Create smooth and complex surfaces
1) sheet surface subjected to compressive stresses which expand surface layer
2) material below peened surface remains rigid, surface expansion cause sheet to develop curvature
3) process induces compressive residual stresses, improved fatigue strength of material
What’s spinning?
Forming axi-symmetric parts over a mandrel with tools or rollers
Whats tube spinning?
Cylindrical part thickness reduced by spinning them on cylindrical mandrel using rollers
What’s superplastic forming? What materials can be used for it?
Forming into complex shapes with lots of deformation
Fine grained alloys eg Al, Zn Titanium
What’re the advantages of superplastic forming?
Low strength of tooling so low costs
Complex shapes in one price, fine detail, close tolerances and no secondary ops needed
Weight and material savings as high formability
Little/no residual stresses in formed parts
What’re the limitations of superplastic alloys
Material must not be superplastic at room temp
Must be formed slowly
What’s bulging?
Placing tubular, conical or curvilinear part in a split female die and expanding against walls of die
Expansion can be achieved
What’re rubber forming?
One of the dies is made from flexible material eg rubber
Die does not damage or scratch surface of sheet
Process is used for bending and embossing surfaces
Typical pressures about 10Mpa
What’s hydroforming/fluid forming?
Pressure over rubber membrane controlled up to 100 MPa
Allows close control of part, avoiding wrinkling or tearing
Deeper draws obtainable as- high pressure forces cup against punch, friction at punch/cup interface reduces longitudinal tensile stress
Low tooling costs, flexibility and ease of operation, no damage to surface of sheet metal
What’s explosive forming?
Sheet metal clamped over die and assembly lowered into water tank
Air in die evacuated
Explosive charge denoted a distance from sheet, shock waves force material into die cavity
Material needs to be ductile at high rates of deformation
Versatile, no limit to size of workparts, suites to low volume production of large parts
What’s magnetic pulse forming?
Energy in capacitor bank discharged through magnetic coil
Eddy currents induced in part create opposing magnetic field which collapses the part onto die or workpart
Higher electrical conductivity of material=greater forces (don’t need Ferro-magnetic material)
Process suited for shallow drawing on thin sheet, collapsing sheet onto tubes and in bulging and flaring operations
