Metal Working

Question 1

Metal forming

Answer 1

Plastic deformation to change shape of metal work pieces. Uses stresses that exceed yield strength

Question 2

How does temp impact metal forming

Answer 2

Ductility is increased and yield strength reduced when temp is raised

Question 3

Two categories of metalworking

Answer 3

Bulk deformation
Sheet megalworking

Question 4

Bulk deformation

Answer 4

Usually categorised by significant deformation and massive shape changes, and the surface area to volume ratio is relatively small.

Question 5

Operations in bulk deformation

Answer 5

Rolling- compressive deformation process where thickness in reduced by two opposing cylindrical tools called rolls

Forging- workpiece is compressed between two opposing dies, so that die shapes are impacted to the work. Usually hot working processes but can be cold

Extrusion- compression process where thickness work metal is forced to flow through a die opening, taking shape of opening as crosssection

Drawing- diameter of a round wire or bar is reduced by pulling through a die opening

Question 6

Sheet metal working processes (press working)

Answer 6

Forming and cutting operations on metal sheets, strips and coils. High surface area to volume ratio . Part produced is often called a stamping. Usually occurs at room temp or warm if metal brittle

Question 7

What tools are ued for sheet working

Answer 7

Punch and die. Punch is positive portion and die is negative portion

Question 8

Sheet metal operations

Answer 8

Bending- straining of a metal or plate to take an angle along a straight axis

Drawing- forming a flat metal sheet into a follow or concave shape, like a cup, by stretching the material. A blank holder is used to hold down blank while punch pushes down metal

Shearing- cuts work using punch and die (not a forming process)

Question 9

Sheet vs plate

Answer 9

Referred to plate when thickness more than 6mm

Question 10

How are sheet or plates produced

Answer 10

Flat rolling. Usually low carbon metal used as cheap, good formability and sufficient strength

Question 11

Cutting operations

Answer 11

Cutting of sheet metal accomplished by shearing action between two sharp cutting edges. As punch begins to push plastic deformation occurs. As punch moves downwards penetration occurs in which punch compresses sheet and curing into the metal. Penetration zone usually 1/3 thickness of sheet. As punch continuous, fracture is initiated in the work at the two cutting edges. Two fracture lines should meet resulting in clean separation

Question 12

Rollover

Answer 12

During dutting operation is is the depression made by pinch in work before cutting.

Question 13

Burnish

Answer 13

In cutting operation it results from penetration of punch before fracture.

Question 14

Fractured zone

Answer 14

In cutting operations, it is the rough surface cut edge where continued downward movement of punch caused fracture

Question 15

Burr

Answer 15

In cutting operation, it is a sharp corner edge cause by elongation of metal during final separation of two people

Question 16

Mpmost improtant operation in pressworking

Answer 16

Shearing, blanking and punching

Question 17

Shearing

Answer 17

Sheet metal cutting operations along a straight line between two cutting edges. Upper blade often inclined to reduce force required

Question 18

Blanking

Answer 18

Cutting the sheet metal along a closed outline in a single step to sperate piece from surrounding stock. Cut out pruduct is called blank

Question 19

Punching

Answer 19

Similar to blanking except that it produces a whole and the separate piece is scap, called the slug.

Question 20

Bending of sheet metalwork

Answer 20

The straining of the metal around a straight axis. Metal inside the neural plane is compressed and metal on outside is stretched. Metal is plastically deformed

Question 21

V-bending

Answer 21

Sheet metal is bent between a v-shaped punch and die. Angled range from very obtuse to very acute. Usually for low production operations. V-dies are usually relatively simple and inexpensive

Question 22

Edge bending

Answer 22

Involves cantilever loading of the sheet metal. A pressure pad is used to apply a force to hold base of the part against the die, while the punch forces the part to yield and bend over the edge. More costly than v dies due to pressure pad so used for high production

Question 25

Spring back

Answer 25

When bending pressure is removed at the end of the deformation operation, elastic energy remains, causing it to recover partially towards original shape. It is the increase in included angle of the bent part relative to the included angle of the forming tool after tool is removed

Question 26

Equation for spring back

Answer 26

(Included angle of sheet metal minus included angle of tool) divided by angle of tool.

Question 27

What two methods can accommodate for spring back

Answer 27

Overbending- punch angle and radius fabricated slightly smaller so that final form springs back to desired value Bottoming- squeezing the part at the end of the stroke, thus plastically deforming it in the bend region

Question 28

Drawing for sheet metal forming

Answer 28

Used to make complex curved and concave parts like cup shape. Performed by placing a piece of sheet metal over die cavity and then pushing the metal into opening with a punch. Blankholder used to hold it down against die

Question 29

Why should the punch and die sides not have corners for drawing

Answer 29

Would become a punch operation

Question 30

Mechanical Stages of standing

Answer 30

As punch begins, metal is subject to bending. Sheet is bent over corner of punch and the die. Outside perimeter of blank moves towards center As punch continues, a straightening action occurs in metal originally bent over punch. Embalming has been downwards with the punch. More metal pulled into clearance from outer edge of blank. Metal flows through constricted space

Question 31

How does friction impact stamping. How to reduce frictional forces

Answer 31

Friction between sheet metal and the fur ages of the blank holder and die must be overcome. Initially, static friction is involved until the metal starts to slide where it become dynamic. Lubricants or drawing compounds may be used to reduce frictional forces

Question 32

How does compression impact stamping

Answer 32

Occurs at outer edge of the blank. As this part of the metal draws towards centre, the outer perimeter becomes smaller so metal is squeezed and becomes Whicker as metal volume is constant. This results in wrinkling of the remaining flange of the blank

Question 33

What increases chance of wrinkling during stamping

Answer 33

When thin sheet metal is drawn or blankholder force is too low.

Question 34

What occurs if blankholder force is too large for stamping

Answer 34

Metal can’t flow properly towards cavity, causing stretching and possible tearing.

Question 35

What are the opposing forces of the metal during the force of stamping

Answer 35

Deformation and friction. Some deformation is stretching and thinning