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Cold Working of Metals
Transcript of Cold Working of Metals
of Cold Rolling
Malleability and Ductility of a metal are necessary for cold working.
Advantages and Disadvantages
Better surface finish
closer dimensional tolerances
superior mehanical properties
better strength to weight ratio
Drawing, Forming and Extruding Metal
Cold forming is less expensive b/c cost of heating metal is eliminated
There is no thermal damage to dies
Warm forming reduces the need for secondary machining and operating personnel
Warm forming can work metals difficult to cold work (titanium, stainless steel) without the high pressures of cold forming
and Wire Drawing
-Hot formed metal bars and tubes are reduced in size by being drawn through a slightly smaller die
-The drawing process hardens the metal and gives it a smooth finish
Also called cold heading or cold forging
Cold Forming Threads
Threads can be machined or cold rolled onto blanks. How do you choose?
-Metal becomes more brittle and less workable.
-Annealing required to continue the process
-Metal may contain residual stresses that can cause warping when machined or welded
-More massive and powerful equipment needed
-Subsequent heating will undo cold working and reduce its strength
The Theory of Cold Working
Metals are stessed below their recrystalization temperature
These stresses must be above the elastic limit (yield strength)
Each time the metal is worked it becomes stronger and harder and more brittle
Manufacturers must be careful to stop cold working before the material fails
If more deformation is needed, a process anneal is
used to restore deformation
Cold working can determine...
toughness, strength, hardness, and ductility of a metal
Some materials work harden - quickly increase
hardness as cold working progresses
plasticity due to compression
plasticity due to tension
If metal is formed by loading it beyond its elastic limit (cold forming) it will be permanently deformed, but will bounce back a certain extent called "spring back" or elastic recovery.
When metal is cold worked, though the grains are stretched in the direction of working...
The atom structures within the grains are not aligned and can react to deformation differently, causing residual stresses
Each grain has a different stress even though the overall object is not loaded, which can be released during machining or welding, warping the object. Recovery anneal is used to get rid of residual stress (heating material below its recrystaliztion temperature
Cutting a flat shape out of a strip of sheet metal. The shape is saved for further operations
If the hole material is scrap, it
Cutting small round holes
into sheet metal
If the holes are small and close together its called
Blanking Punch and DIe
Sometimes mechanical presses are fitted with progressive punching and forming dies
EX. Electrical outlet boxes
Aluminum cans are formed by the "draw and iron" process where a sheet formed by hot rolling then cold rolling is punched. That slug is then drawn to a cup, ironed and trimmed.
Some Advantages of cold/warm forming
Forming a metal while below its phase transformation temperature (melting) but above its recrystalization temperature
Drawing plate, sheet and Foil
-A flat piece of metal is formed into a holow shape by applying force with a punch to the center portion of the metal
Blanking and piercing process can be added to drawing using progressive dies
Wall thickness can be further reduced by ironing
Tube Drawing -Seamless Tubing
Cylinders are drawn into tubes using a mandrel that creates the inner cavityand provides an internal finish
This process is much more expensive then piping that is curled and butt-welded
Process used to manufacture hydraulic cylinders high pressure pipes
Similar to tube drawing, except with smaller diameters, and it is a continuous process done on rotating equipment
Drawing Process Limitations...
The metal work hardens as it is pulled through the die
Annealing may have to take place so that material can regain some ductility to avoid the forces needed to cold work the metal becoming greater than the strength of the metal
Metal upsetting process
Only used for small parts like screws, bolt blanks, rivets, and ball bearing blanks
Upsetting machines can produce up to 36,000 per hour (for small rivets) and 27,000 per hour for screw blanks
No metal is wasted
Cold forming machines can produce final products through series of upsets, extrusions, trimming and piercing/blanking
(Advantages over machining)
The amount of deformation required is directly related to the plasticity of the metal. Some grades of aluminum, gray cast iron, and die-cast metals cannot be rolled because of their insufficient elongation and reduction of area.
Most stainless steels and high manganese steels raidly work harden when cold worked. These metals do not readily lend themselves to cold rolling
Some parts are not adaptable for rolling, for example those with threads that are too close to a shoulder. Some tapered threads should be cut instead of rolled
Rolling is far superior to cutting if the finish is a consideration
The rolling process produces a greater surface hardness that wears well and helps resist metal fatigue
HIgher Speeds and Greater Strength
Generally, thread rolling is a somewhat faster operation than cutting threads with dies
Rolled threads are stronger than cut threads because of grain flow
Critical Thinking Activity
In groups of 2 answer the 3 case problems on page 222 of your book. (use complete sentences, typed)
Develop a rubric for these questions
-A guideline to grade assignments
-Specify the elements of the answer you require and what grade should be received.
and cold roll forming
Roll bending- used to from curved shapes, cylinders or rings. Plates, stocks and structural shapes are bent
Cold -Roll Forming- bending a flat strip into a complex shape through a series of rolls (each performaing an incremental part of the bend).
Straightening or flattening - process to remove unwanted bens from metal sheet or bar stock
Stretching a light gauge sheet metal over a contoured form or die block
Stretch Wrap forming
A Blank is stretched beyond the yeild point and then wrapped around a form block
A shallow stamping process in which bottom and top dies are brought together to form a stretched sheet
Reduces spring back
reduces tooling costs
High Energy Rate
Explosive Forming -
same as explosive forming except uses chemical explosion to produce shockwaves that bend metal
No limit to the size or thickness of plate that can be formed
Reduced spring back
Metal Spinning and Flow Forming
Process in which a disc is rotated and forced against a form (no reduction in thickness)
Flow Forming -same as metal spinning but with a reduction in thickness
Forming Pipe and tubing
Smaller sizes of sheets are rolled together and then the edges are welded together