Sheet forming

Question 1

What are some basic processes in sheet metal forming

Answer 1

Punching, Cutting, Blanking, stretching and drawing

Question 2

Describe the process of blanking

Answer 2

Use a punch to cut out a piece of a sheet which is laying on a die

Question 3

What are the deformities on the edges of sheets formed from blanking called

Answer 3

Burs

Question 4

Burs are …………….. worked materials

Answer 4

Cold

Question 5

What is the Bur height dependant on

Answer 5

Increases with ductility of sheet, dullness and shape of punch, Increasing punch speed and decreasing clearance decrease the size of Burr

Question 6

What does the size and shape of material effect

Answer 6

Formability of sheet

Question 7

What effects the quality of sheared edge

Answer 7

Shape and speed of punch, lubrication and clearance

Question 8

How can the bendability of a sheet be estimated

Answer 8

Using the reduction in area

Question 9

What is the equation for reduction in area

Answer 9

r=((Af-Ao)/Ao)x100

Question 10

How can the Minimum bend radius be roughly approximated

Answer 10

R=T((50/r)-1)

Question 11

How can rolling direction effect the bendability of a sheet

Answer 11

Rolling in the transverse direction will make the sheet more likely to fail.

Question 12

When does Spring back occur

Answer 12

After plastic bending

Question 13

How can you tell weather a sheet buckles or compresses

Answer 13

It will undergo whichever process minimises the systems energy. This depends on the sheets cross section and material properties

Question 14

What sheets are more likely to buckle

Answer 14

Thinner sheets

Question 15

How would you go about trying to stop a sheet from wrinkling

Answer 15

Use a thicker sheet
Stretch in the radial direction
Use a blank holder to constrain the sheet to keep it flat while forming

Question 16

What do metals require to be sheet formed

Answer 16

To be Ductile

Strain Hardening

Question 17

What does a gradual strain Hardening lead to

Answer 17

Less necking

Question 18

What sort of grain size is good for forming

Answer 18

A fine grain size

Question 19

What test would you use to test the formability of a sheet

Answer 19

Cupping test (Erichsen test)

Question 20

Describe the cupping test

Answer 20

Hemispherical punch used to bend the sheet

Question 21

How does increasing the thickness of the blank effect its formability and bendability

Answer 21

shifts formability curve upwards, makes it more deformable but makes it less bendable so therefore more likely to tare when bending

Question 22

What happens the material in stretch forming

Answer 22

It gets thinner

Question 23

What stops the blank from slipping in stretch forming

Answer 23

The bead

Question 24

What is the main determinant of weather a metal alloy forms well

Answer 24

Strain Hardening coefficient

Question 25

What is the equation for true stress

Answer 25

Stress= Force/(current area of specimen)

Question 26

What is the equation for strain hardening

Answer 26

Strain= ln(L/Lo)

Question 27

How does increasing the strain hardening effect the formability

Answer 27

increases the formability

Question 28

What is the equation for the line on the true stress strain graph

Answer 28

Log(stress) = nlog(strain) + log(K)

Question 29

What is the difference between deep drawing and stretch forming

Answer 29

In deep drawing the blank is not held in place, it is allowed to slide

Question 30

How would you create a constant thickness across the sheet after deep drawing

Answer 30

The thickness at top of drawn part is greater than bottom so an iron is used to create constant thickness

Question 31

What is the equation for Limiting Draw Ratio

Answer 31

(max blank diameter)/punch diameter)

Question 32

What is the normal anisotropy (R)

Answer 32

The best indication of weather a metal sheet can be deep drawn into a round cup shaped part

Question 33

What is the equation for normal anisotropy

Answer 33

(True strain width)/(true strain thickness)

Question 34

What is the equation for average normal anisotropy for rolling

Answer 34

Ravg= (Ro+2R45+R90)/4