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Flashcards in Metal Forming 4 Deck (16)
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1
Q

Differences between bulk deformation processes and sheet metal processes?

A

Bulk deformation has significant deformations and massive shape changes with small SA:VOL ratio. For sheet metalworking the SA:VOL is high and consists of forming and cutting processes.

2
Q

Describe the 4 main bulk deformation metal forming processes

A
  • Rolling: compressive deformation where thickness of slab/plate is reduced by opposing cylindrical rolls.
  • Forging: a workpiece is compressed between two opposing dies so that the die pattern presses into the work. Usually hot working, sometimes not.
  • Extrusion: a compression process where the work metal is forced to flow through a die opening, taking the shape of the opening
  • Drawing: diameter of a round wire or bar is reduced by pulling it through a die opening
3
Q

Describe 3 main sheet metal working techniques

A

1) bending: involves straining metal sheet/plate to take an angle along a (usually) straight axis
2) drawing: forming a flat metal sheet into a hollow/concave shape by stretching. Blankholder holds down blank while punch pushes into sheet. AKA cup/deep drawing
3) shearing: involves cutting rather than forming. A shearing operation cuts the work using a punch and die. Not a forming process.

4
Q

Difference between deep drawing and bar drawing

A

Deep drawing is a sheet metal forming process to make cup shape parts whereas bar drawing is a bulk deformation process used to reduce the diameter of a cylindrical part

5
Q

Dis/Advantages of cold working to warm/hot

A

-greater accuracy
-better surface finish
-higher strength/hardness of the part due to strain hardening
-grain flow during deformation gives desirable directional properties to be obtained in the resulting product
-no heating of the work is required, which saves on furnace and fuel costs and permits higher production rates
DISADVANTAGES
-higher forces/power to perform
-surface needs to be clean before working
-ductility/strain hardening of the metal limit forming that can be done

6
Q

What’s isothermal forming?

A

Forming operations that eliminate surface cooling and the resulting thermal gradients in the part. Done by preheating the tools that come in contact with the part to the same temperature as the work metal.

7
Q

What are the 3 basic types of sheet metalworking operations?

A

1) cutting
2) bending
3) drawing

8
Q

Describe the stages in the shearing process

A

1) just before the punch contacts work
2) lunch begins to push into work
3) punch compressed and penetrates into work causing a smooth cut surface
4) fracture is initiated at the opposing cutting edges that separate the sheet

9
Q

Describe the features of a sheared edge

A

top of the cut surface is called the rollover. This corresponds to the depression from the lunch in the work prior to cutting. It’s where initial plastic deformation occurred. Just below the rollover is a smooth region called the burnish. This comes from punch penetration before the fracture began. Beneath the burnish is the fractured zone, a rough surface of the cut edge where continued downward movement of the punch caused metal fracture. Finally, at the bottom is a burr, a sharp corner on the edge from metal elongation during separation of pieces.

10
Q

Describe each of the two types of sheet metal bending operations: V bending and edge bending

A

In V bending, a simple punch and die with the included angle are used to bend the part. In edge bending, the punch forces a cantilevered sheet metal section over a die edge to obtain the desired bend angle.

11
Q

Dis/advantages of V and edge bending

A

V bending: simple and cheap tools with variety of bend angles possible but low production rate
Edge bending: high production rates possible but higher cost and limited angles

12
Q

For what is the bend allowance intended to compensate?

A

Compensated for stretching of sheet metal that occurs in a bending operation when the bend radius is small relative to the stock thickness. In principle bend allowance = length of bent metal along neutral axis.

13
Q

What is spring back in sheet metal bending?

A

The elastic recovery if the sheet metal after bending, it’s usually measured as the difference between the final included angle of bent part and the angle of the tooling used to make the bend divided by the angle of the tooling

14
Q

Define drawing in the context of sheet metalworking

A

Drawing is used to produce cup/box shaped, or other complex curved, hollow parts. Drawing is accomplished by placing a piece of sheet metal over a die vanity and then using a punch to push the metal into the cavity

15
Q

Why can pipe bending processes with a mandrel achieve a lower bend radius than one without a mandrel?

A

When the tube is bent, the wall on the inside of the bend is in compression, and the other in tension. These stress conditions cause thinning and elongation of the outer wall and thickening/shortening of the inner wall. Therefore the inner and outer walls are forced towards eachother to cause the tube cross section to flatten. A mandrel prevents this pipe collapse.

16
Q

Describe the various stages of deep drawing:

A

1) punch makes initial contact with work
2) bending
3) straightening
4) friction and compression
5) final cup shape showing effects of thinning in cup walls