Sheet forming Flashcards

1
Q

What are some basic processes in sheet metal forming

A

Punching, Cutting, Blanking, stretching and drawing

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2
Q

Describe the process of blanking

A

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

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3
Q

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

A

Burs

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4
Q

Burs are …………….. worked materials

A

Cold

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5
Q

What is the Bur height dependant on

A

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

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6
Q

What does the size and shape of material effect

A

Formability of sheet

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7
Q

What effects the quality of sheared edge

A

Shape and speed of punch, lubrication and clearance

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8
Q

How can the bendability of a sheet be estimated

A

Using the reduction in area

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9
Q

What is the equation for reduction in area

A

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

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10
Q

How can the Minimum bend radius be roughly approximated

A

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

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11
Q

How can rolling direction effect the bendability of a sheet

A

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

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12
Q

When does Spring back occur

A

After plastic bending

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13
Q

How can you tell weather a sheet buckles or compresses

A

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

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14
Q

What sheets are more likely to buckle

A

Thinner sheets

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15
Q

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

A

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

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16
Q

What do metals require to be sheet formed

A

To be Ductile

Strain Hardening

17
Q

What does a gradual strain Hardening lead to

A

Less necking

18
Q

What sort of grain size is good for forming

A

A fine grain size

19
Q

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

A

Cupping test (Erichsen test)

20
Q

Describe the cupping test

A

Hemispherical punch used to bend the sheet

21
Q

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

A

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

22
Q

What happens the material in stretch forming

A

It gets thinner

23
Q

What stops the blank from slipping in stretch forming

24
Q

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

A

Strain Hardening coefficient

25
What is the equation for true stress
Stress= Force/(current area of specimen)
26
What is the equation for strain hardening
Strain= ln(L/Lo)
27
How does increasing the strain hardening effect the formability
increases the formability
28
What is the equation for the line on the true stress strain graph
Log(stress) = nlog(strain) + log(K)
29
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
30
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
31
What is the equation for Limiting Draw Ratio
(max blank diameter)/punch diameter)
32
What is the normal anisotropy (R)
The best indication of weather a metal sheet can be deep drawn into a round cup shaped part
33
What is the equation for normal anisotropy
(True strain width)/(true strain thickness)
34
What is the equation for average normal anisotropy for rolling
Ravg= (Ro+2R45+R90)/4