ME 383 Exam 2 - Sheet Metal Working and Non-Traditional Manufacturing Processes

What are the seven types of sheet metal topics covered in ME 383?

1. Cutting operations

2. Bending operations

3. Drawing

4. Other sheet metal forming operations

5. Dies and Presses for sheet metal processes

6. Sheet metal operations not performed on presses

7. Bending of tube stock

What is sheet metal working?

Cutting and forming operations performed on relative thin sheets of metal

(0.4 mm < thickness< 6 mm)

Usually performed as COLD working

What is the thickness range for sheet metal?

0.4 mm (1/64 in) to 6 mm (1/4 in)

Material with thickness greater than 6 mm is considered PLATE stock

What products/applications typically require sheet metal?

1. Automobiles and trucks

2. Airplanes

3. Railway cars and locomotives

4. Farm and construction equipment

5. Small and large appliances

6. Office furniture

7. Computers and office equipment

What are the advantages of sheet metal parts?

1. High strength

2. Good dimensional accuracy

3. Good surface finish

4. Relatively low cost (especially after initial investment)

5. Economical mass production for large quantities (lots of parts per minute)

What is the advantage to sheet metal working being a cold working process?

Work hardening occurs especially near edges where there is grain deformation

What is a punch-and-die?

Tooling used to perform cutting, bending, and drawing

Stamping Press

Machine tool that performs most sheet metal operations

What are stampings?

Sheet metal products

Why are as many operations as possible included in a single die?

To reduce the number of dies needed since they can be expensive and difficult to make

What are the three main operations in pressworking that cut sheet metal?

1. Shearing: to separate large sheets

2. Blanking: to cut perimeters out of sheet metal

3. Punching: to make holes in sheet metal

What is bending?

Straining a sheet around a straight axis

What is drawing?

The forming of a sheet into convex or concave shapes (i.e. kitchen sinks or pots)

What is the difference between progressive dies and transfer dies?

Progressive dies leave the part attached to the stock throughout the process, while transfer dies blank the part completely in the beginning

What does the cutting of sheet metal involve?

Sheet metal is typically sheared between two cutting edges; a punch compresses and penetrates the work causing a smooth cut surface; fracture is then initiated at the opposing cutting edge which separates the sheet

- material fails and separates at the cut location when the ultimate shear strength is exceeded

- two tools are needed; one tool above the sheet and one below

- a small clearance must be present between the edges of the two tools to facilitate fracture

- the effects of shearing on the material change as the cut progresses and are visible on the edge of the sheared material

What is the typical size of the clearance for cutting sheet metal?

The clearance is typically 4-8% of the material thickness

What does the size of the clearance for cutting sheet metal depend on?

The specific shearing process, material, and sheet thickness

What is shearing?

Sheet metal cutting operation along a straight line between two cutting edges;

Typically used to cut large sheets

What are the capabilities of shearing?

- Typical sheet thickness: 0.005-0.25 inches

- Tolerance: +/- 0.1 inches (+/- 0.005 inches feasible)

- Surface Finish: 250-1000 micro-in (125-2000 micro-in feasible)

What is blanking?

Process in which a piece of sheet metal is removed from a larger piece of stock by applying a great enough shearing force;

- the piece removed is the desired part known as the "blank"

- secondary finishing is typically required to remove burrs along bottom edge

- 4-8% of material thickness clearance exists between punch and die

- really fast process (sometimes > 1000 strokes per minute)

What is fine blanking?

Process in which blank is sheared from sheet stock by applying three separate forces;

- produces a part with better flatness

- smoother edge with minimal burrs

- tolerances as tight as +/- 0.0003

- allows for high quality parts with no secondary op needed

What are the three different forces applied during fine blanking?

1. Blanking force

2. Holding force

3. Counterforce

What is the difference in drop pieces for blanking and punching processes?

The drop in blanking is the desired part, while the drop piece in punching is just scrap

What is punching?

Process similar to blanking, except that removed material (aka the "slug") is scrap; removing the slug leaves behind the desired feature in the sheet such as a hole or slot

- can produce holes and cutouts of various shapes and sizes

- secondary finishing operations used to attain smoother edges

What is the typical range for clearances in sheet metal cutting?

4% to 8% of material thickness

What is clearance in sheet metal cutting?

The distance between punch cutting edge and die cutting edge

What happens if the clearance is too small in sheet metal cutting?

Fracture lines pass each other causing double burnishing and a larger force

What happens when the clearance is too large in sheet metal cutting?

Metal is pinched between cutting edges and excessive burr occurs

What is the formula to determine the needed clearance in sheet metal cutting?

c = at

c : clearance

a: allowance (determined according to type of metal)

t: stock thickness

** the harder the material the higher the value of "a"

What is one strengthening benefit of sheet metal cutting?

Work hardening occurs at cut edges

What is the allowance (a) for 1100s and 5052s aluminum alloys?

0.045

What is the allowance (a) for 2024ST and 6061ST aluminum alloys, brass, soft cold rolled steel, soft stainless steel

0.060

What is the allowance (a) for cold rolled steel (half hard), stainless steel (half hard and full hard)

0.075

Piercing

typical punching operation, in which a cylindrical punch pieces a hole into the sheet

Slotting

punching operation that forms RECTANGULAR holes in the sheet; sometimes described as piercing even though shape is different

Perforating

punching a close arrangement of a large number of holes in a single operation

Notching

punching the edge of a sheet to form a notch in the shape of a portion of the punch

Nibbling

punching a series of small overlapping slits or holes along a path to cutout a larger contoured; eliminates the need for a custom punch and die, but requires secondary operations to improve the accuracy and finish of the feature

Lancing

punching process in which a partial cut is made in the sheet, so that no material is removed; the material is left attached and bent to form a shape, such as a tab, vent, or louver

Slitting

punching operation in which straight lines are cut into the sheet; no scrap material is produced

Trimming

punching away excess material from the perimeter of a part; i.e. trimming the flange from a drawn cup

Parting

separating a part from the remaining sheet by punching away the material between parts

What is the equation for the punch and die size for a round BLANK of diameter Db?

Punch diameter: Db - 2c

Blanking Diameter: Db

c: clearance

What is the equation for the punch and die size for a round HOLE of diameter Dh?

Punch diameter: Dh

Die Diameter: Dh + 2c

c: clearance

What governs to the dimensions of blank diameter versus hole diameter?

Blank diameter is determined by the size of the die, while hole diameter is determined by the size of the punch

*Why do dies have angular clearances?

To allow slug or blank to drop through the die; rolled metal expands when cut due to internal compressive stresses; angular clearance ensures the part falls out rather than getting stuck in the die

- typically 0.25 to 1.5 degrees on each side

What is the equation for cutting force?

F = StL

OR

F = 0.7(TS)tL

S: shear strength

t: stock thickness

L: length of cut edge (perimeter length)

TS: tensile strength (multiplied by 0.7 when shear strength is not known

*Equation helps determine press size (tonnage)

What does the force vs. penetration curve look like for a cutting process?

Increases at a decreasing rate to a peak then decreases (distorted upside-down parabola);

**Main idea: Force is NOT constant during the process

**Force equation is a simplification, so make sure to use a FOS

Sheet Metal Bending

straining sheet metal around a straight axis to take a permanent bend

What is the only length that does not change during bending?

The length of the neutral axis

What role does rolling direction play in bending?

Part must be bent in the same direction as the rolling direction or elongated inclusions within the material may result in cracking at the outer surface of the bend

What is the bend allowance?

The length of the arc of the neutral axis between tangent points of a bend

Be able to label the sheet metal bending diagram.

Be able to label the sheet metal bending diagram.

What is V-Bending?

Sheet metal bending performed with a

V-shaped die;

- low production

- performed on a PRESS BRAKE

- V-dies are SIMPLE and INEXPENSIVE

What is Edge Bending?

Sheet metal bending performed with a WIPING die;

- high production

- pressure pad required

- dies are COMPLICATED and COSTLY

What are the remedies for springback?

1. Bottoming: increases plastic deformation that occurs as material is forced to bottom of die

2. Overbending: bend the part past the desired angle so that springback results in the actually desired angle

When does stretching occur during bending?

when the bend radius is small relative to stock thickness;

*Amount of stretching must be estimated so that the final part length does not exceed the desired dimension

How is the flat pattern length of a bent part determined?

By taking the sum of both flange lengths and the bend allowance

What is the Bend Allowance Formula?

A_b: bend allowance

alpha: bend angle

R: bend radius

t: stock thickness

K_ba: stretching factor

** R < 2t : K_ba = 0.33

** R >= 2t : K_ba = 0.5

What is springback

Increase in included angle of a bent part relative to included angle of forming tool after tool is removed;

When bending pressure is removed, elastic energy remains in the bent part, causing it to recover partially toward its original shape

What is the typical springback range?

2 degrees - 4 degrees

What is the equation for bending force?

F = K_bf * TS * w * t^2 / D

F: bending force

TS: tensile strength

w: part width in direction of bend axis

t: stock thickness

D: die opening

K_bf for V-BENDING: 1.33

K_bf for EDGE BENDING: 0.33

Drawing

sheet metal forming to make cup-shaped, box-shaped, or other complex-curved, hollow-shaped parts;

process involves placing a sheet metal blank over a die cavity and then using a punch to push metal into the opening;

aka "DEEP DRAWING" to distinguish from wire and bar drawing

What are some common products made using deep drawing?

1. Beverage cans

2. Ammunition shells

3. Automobile body panels

By what process are bullets made?

Drawing

What is the clearance used in drawing?

c = 1.1t

- t: material thickness

- clearance is about 10% greater than the stock thickness

What are the three tests used to determine the feasibility of a drawing process?

1. Drawing Ratio

2. Reduction

3. Thickness-to-Diameter Ratio

These three requirements being met does not guarantee that a process will work, but failing ANY of these processes guarantees that the process will not

What is the drawing ratio?

Ratio that indicates the severity of a drawing operation

Db: Blank Diameter

Dp: Punch Diameter

What is the upper limit for the drawing ratio?

DR

What is the equation for reduction (r) in drawing?

r = (Db - Dp) / Db

Db: blank diameter

Dp: punch diameter

What is the upper limit for reduction (r)

r should be less than 0.5

What happens if the die is not precise for a drawing process?

Wrinkling or tearing will occur

What is the thickness-to-diameter ratio?

the thickness of starting blank divided by blank diameter

ratio = t/Db

t: material thickness

Db: blank diameter

What is the desired value for thickness-to-diameter ratio?

It is desirable for the t/Db ratio to be greater than 1%

The tendency for wrinkling INCREASES as the ratio DECREASES

What is the equation for drawing force?

F = pi * Dp * t * TS * (Db/Dp - 0.7)

F: drawing force

Dp: punch diameter

t: material thickness

TS: tensile strength

Db: blank diameter

0.7: Friction correction factor

What is the behavior of drawing force throughout the punch distance?

the force varies throughout the downward movement of the punch;

the force reaches its maximum value at 1/3 of the punch stroke length

What is the relationship between drawing force and holding force?

Holding force is typically about one-third of the drawing force

What is the equation for holding force?

Fh = 0.015*Y*pi*[Db^2-(Dp+2.2t+2Rd)^2]

Fh: holding force

Y: yield strength

t: starting stock thickness

Rd: die corner radius

Db: blank diameter

Dp: punch diameter

How is blank size determined for drawn shapes?

Db is found by setting the starting sheet metal volume equal to the final product volume (volume remains constant)

- assume negligible thinning of the part wall

- an incorrect blank diameter will result in incorrect part dimensions

What are some common shapes formed using drawing other than cylindrical cups?

- Square or rectangular boxes (i.e. kitchen sinks)

- stepped cups

- cones

- cups with spherical rather than flat bases

- irregular curved forms (i.e. automobile panels)

* each shape presents its own unique technical problems in drawing

What are some other sheet metal forming operations performed on conventional presses?

1. Operations performed with metal tubing

2. Operations performed with flexible rubber tooling

*What is spinning?

Metal forming process in which an axial symmetric part is gradually shaped over a rotating mandrel using a rounded tool or roller

What is a mandrel?

a tapered or cylindrical bar, spindle, axle, or core used to hold, support, or shape a workpiece during manufacturing

*What are the three types of spinning processes?

1. Conventional spinning

2. Shear spinning

3. Tube spinning

*What is the difference between conventional spinning and non-conventional spinning?

Conventional spinning involves the force only being applied in one direction; Non-conventional spinning involves the force being applied in two directions (both the shape and the material thickness change)

What separates non-traditional machining processes from traditional machining processes?

Non-traditional machining processes do NOT use a sharp cutting tool

What are the four types of nontraditional machining and thermal cutting processes considered in ME 383?

1. Mechanical Energy Processes

2. Electrochemical Machining Processes

3. Thermal Energy Processes

4. Chemical Machining

What are nontraditional machining processes?

A group of processes that remove excess material by various techniques involving mechanical, thermal, electrical, or chemical energy (or combinations of these energies);

*Do NOT use a sharp cutting tool in conventional sense

When and why were nontraditional processes developed?

Developed during World War II in response to new an unusual machining requirements that could not be satisfied by conventional methods

Why are nontraditional processes needed?

1. To machine newly develop metals and non-metals with special properties that make them difficult or impossible to machine by conventional methods

2. For unusual and/or complex part geometries that are difficult to accomplish with conventional machining

3. To avoid surface damage that often accompanies conventional machining

What classifies a nontraditional process as MECHANICAL?

typical form of mechanical action is erosion of work material by a high velocity stream of abrasives or fluid (or both)

What classifies a nontraditional process as ELECTRICAL?

electrochemical energy is used to removed material (reverse of electroplating)

What classifies a nontraditional process as THERMAL?

thermal energy is applied to small portion of work surface, causing that portion to be fused and/or vaporized

What classifies a nontraditional process as CHEMICAL?

chemical etchants are used to selectively remove material from portions of work part, while other portions are protected by a mask

What are the 4 types of mechanical energy processes?

1. Ultrasonic Machining (USM)

2. Water Jet Cutting (WJC)

3. Abrasive Water Jet Cutting (AWJC)

4. Abrasive Jet Machining (AJM)

What does USM stand for?

Ultrasonic Machining

What does Ultrasonic Machining involve?

Abrasive contained in a slurry are driven at high velocity against work by a tool vibrating at lot amplitude and high frequency;

- tool oscillation is perpendicular to surface

- tool is fed slowly into work

- SHAPE OF TOOL is formed into part

What materials is USM best suited for?

HARD & BRITTLE work materials such as ceramics, glass, and carbides; also works on certain metals (i.e. stainless steel and titanium)

What shapes is USM typically used to make?

Holes and holes along a curved axis

What does WJC stand for?

Water Jet Cutting