Sheet forming Flashcards
What are some basic processes in sheet metal forming
Punching, Cutting, Blanking, stretching and drawing
Describe the process of blanking
Use a punch to cut out a piece of a sheet which is laying on a die
What are the deformities on the edges of sheets formed from blanking called
Burs
Burs are …………….. worked materials
Cold
What is the Bur height dependant on
Increases with ductility of sheet, dullness and shape of punch, Increasing punch speed and decreasing clearance decrease the size of Burr
What does the size and shape of material effect
Formability of sheet
What effects the quality of sheared edge
Shape and speed of punch, lubrication and clearance
How can the bendability of a sheet be estimated
Using the reduction in area
What is the equation for reduction in area
r=((Af-Ao)/Ao)x100
How can the Minimum bend radius be roughly approximated
R=T((50/r)-1)
How can rolling direction effect the bendability of a sheet
Rolling in the transverse direction will make the sheet more likely to fail.
When does Spring back occur
After plastic bending
How can you tell weather a sheet buckles or compresses
It will undergo whichever process minimises the systems energy. This depends on the sheets cross section and material properties
What sheets are more likely to buckle
Thinner sheets
How would you go about trying to stop a sheet from wrinkling
Use a thicker sheet
Stretch in the radial direction
Use a blank holder to constrain the sheet to keep it flat while forming
What do metals require to be sheet formed
To be Ductile
Strain Hardening
What does a gradual strain Hardening lead to
Less necking
What sort of grain size is good for forming
A fine grain size
What test would you use to test the formability of a sheet
Cupping test (Erichsen test)
Describe the cupping test
Hemispherical punch used to bend the sheet
How does increasing the thickness of the blank effect its formability and bendability
shifts formability curve upwards, makes it more deformable but makes it less bendable so therefore more likely to tare when bending
What happens the material in stretch forming
It gets thinner
What stops the blank from slipping in stretch forming
The bead
What is the main determinant of weather a metal alloy forms well
Strain Hardening coefficient
What is the equation for true stress
Stress= Force/(current area of specimen)
What is the equation for strain hardening
Strain= ln(L/Lo)
How does increasing the strain hardening effect the formability
increases the formability
What is the equation for the line on the true stress strain graph
Log(stress) = nlog(strain) + log(K)
What is the difference between deep drawing and stretch forming
In deep drawing the blank is not held in place, it is allowed to slide
How would you create a constant thickness across the sheet after deep drawing
The thickness at top of drawn part is greater than bottom so an iron is used to create constant thickness
What is the equation for Limiting Draw Ratio
(max blank diameter)/punch diameter)
What is the normal anisotropy (R)
The best indication of weather a metal sheet can be deep drawn into a round cup shaped part
What is the equation for normal anisotropy
(True strain width)/(true strain thickness)
What is the equation for average normal anisotropy for rolling
Ravg= (Ro+2R45+R90)/4
