ise 311 sheet metal forming lab cup drawing introduction basic principles of drawing drawing is a...

ISE 311

Sheet Metal Forming Lab

Cup Drawing

in conjunction with

Section 20.3 in the text book

“Fundamentals of Modern Manufacturing”

Third Edition

Mikell P. Groover February 4th, 2008

2

Outline

• Introduction

• Mechanics of Drawing

• Engineering Analysis of Drawing

• Defects in Drawing

• Objectives of the Lab

• Sheet Metal Forming (Material and Equipment)

• Sheet Metal Forming (FE simulations)

• Summary

3

Introduction

Basic Principles of Drawing

Drawing is a sheet metal forming operation used to

make cup-shaped, box-shaped, or other complex-curved,

hollow-shaped parts. It is performed by placing a piece

of sheet metal over a die cavity and then pushing the

sheet into the opening with a punch. The blank is held

down flat against the die by a blankholder.

4

A blank of diameter Db is drawn into the die by means of a punch

of diameter Dp. The punch and die have corner radii Rp and Rd,

respectively. The sides of the die and punch are separated by a

clearance, c, which is about 10% greater than the sheet thickness.

The punch applies a downward force, F, to deform the metal

while the downward holding force, Fh, is applied by the

blankholder.

Mechanics of Drawing

5


As the punch proceeds towards its final

position, the workpiece experiences a

complex sequence of stresses and strains

as it is formed into its final shape.

In step 1, the blankholder force, Fh, is

applied and the punch begins to move

towards the sheet material.

In step 2, the sheet material is subjected

to a bending operation. The sheet is bent

over the corner of the punch and the

corner of the die.

In step 3, as the punch continues moving

down, a straightening action occurs in the

metal that was previously bent over the

die radius. Metal from the flange is

drawn into the die opening to form the

cylinder wall.

6

In step 4, As the metal in the flange moves toward

the center, it is subjected to the following state of

stress:

1- Compression in the circumferential direction

(the outer perimeter becomes smaller)

2- Tension in the radial direction

3- A relatively small compression in the thickness

direction

Since the volume of metal remains constant, and

because the circumferential stress is relatively

large, the sheet will thicken as it moves in the

flange area. (this is why the clearance between the

punch and die is higher than the sheet thickness by

about 10%)

In order for the material to be drawn:

1- Friction between the sheet material and surfaces

of the blankholder and die must be overcome.

2- Deformation energy should be provided.


7

The downward motion of the

punch results in a continuation of

the metal flow caused by drawing

and compression. Some thinning

at the cylinder walls occurs as

well. Step 5 shows the completed

drawing process.


8

The next slide illustrates the deep drawing tooling used in the lab.

The blank is placed between the die and the blank holder. To

center the blank, the student should make sure that it is touching

the two pins. The blank holder is attached to three pneumatic

cylinders which move it up and down and also provide the

required holding force (by controlling the air pressure inside the

cylinders). The punch is directly attached to the ram of the

mechanical press. Three guides are used to make sure the tools

remain concentric during the process.

Deep Drawing Tooling

Deep Drawing Tooling

9

Die Punch

Pins (2)

Blank holder Pneumatic cylinder (3) Guide (3)

Blank holder

10

The drawing ratio, DR, gives an indication of the severity of the

drawing operation: the higher the ratio, the greater the severity.

The drawing ratio is defined as:

Where, Db = blank diameter and Dp = punch diameter. This

value is dependant upon punch and die corner radii, friction

conditions, draw depth, and material properties of the sheet

metal.

p

b

D

DDR

Engineering Analysis of Drawing

11

The drawing force required to perform a drawing operation can

be roughly estimated by the following formula:

; where

F = Drawing force

t = Original blank thickness

TS = Tensile strength

Db = Blank diameter

Dp = Punch diameter

Increasing the DR will increase the punch force, and this will result in

excessive thinning or even fracture in the cup wall.

DR < LDR ; where

LDR = the Limiting Drawing Ratio

7.0

p

b

pD

DTStDF

Engineering Analysis of Drawing

12

Defects in Drawing

A number of defects in drawing can occur, which include:

(a) Wrinkling in the flange occurs due to compressive buckling in the circumferential

direction (blank holding force should be sufficient to prevent buckling from occurring).

(b) Wrinkling in the wall takes place when a wrinkled flange is drawn into the cup or if the

clearance is very large, resulting in a large suspended (unsupported) region.

(c) Tearing occurs because of high tensile stresses that cause thinning and failure of the

metal in the cup wall. Tearing can also occur in a drawing process if the die has a sharp

corner radius.

(d) Earring occurs when the material is anisotropic, i.e. has varying properties in different

directions.

(e) Surface scratches can be seen on the drawn part if the punch and die are not smooth or

if the lubrication of the process is poor.

13

Objectives

This lab has the following objectives:

• Become familiarized with the basic processes used in

sheet metal forming.

• Analyze a cup drawing operation and attempt to select

the best process parameters.

14

Sheet Metal Forming – Cup Drawing

• Test Materials and Equipment

– Robinson (Open Back Inclinable- OBI) press

• Model A3

• 25-ton capacity

• Safety Equipment and Instructions

– Wear safety glasses.

– Conduct the test as directed by the instructor.

– Do not use hands to put or remove specimens on the die –

use the supplied tongs.

– Turn off the OBI press whenever you need to adjust its

setting.

15


Procedure: • Obtain specimens to be

tested and record the material data onto your data sheet.

• Choose one sample and place it in the tooling using the tongs.

• For the first specimen, set the air pressure for the blankholder cylinders to approximately 30 psi.

• Step on the foot pedal to cycle the press one time.

16


Procedure (continued):

• If the part does not fall out of the bottom on its own, try the following:

Case 1: The part is completely formed but stuck in the die cavity

* Step the pedal again. If the part did not fall, then

* Put a small wooden cylinder in the die cavity and step the pedal.

Case 2: The part is torn (you can see the flange on surface of the die)

* Use a wooden stick to raise the sample up and remove it through the gap between the die and blank holder

Case 3: The part is stuck around the punch

Ask the lab instructor for help

Note: BE SURE the flywheel has completely stopped and the holding

pressure is zero before attempting to retrieve the part and NEVER put

your hands between the die and the blank holder (always use the supplied

wooden stick)


Procedure (continued):

• Inspect the formed part and record any observed defects. If the part is not ideal, adjust the blank holding pressure (or other process parameters) to improve the part quality.

• Select another specimen of the same material and try your new process parameters. Again, record any defects and speculate appropriate adjustments to the process parameters.

• Adjust the parameters again and form your third specimen. Inspect the material and record any observed defects.

• Repeat steps 4-9 for the remaining materials.

17

18

Finite Element Analysis (FEA) and

Simulations

With FEA it is possible to emulate the metal flow (the cup shape, the thinning in the cup wall, the stresses and strains in the deforming cup material) during deep drawing.

The next few slides illustrate the simulation of a cup drawing process generated by FEA that simulates the actual deformation of a steel sheet specimen.

19

Cup Drawing Simulation

Steel 1010: (BHF: 1,000 Lbs)

Stage A (before drawing)


Steel 1010: (BHF: 1,000 Lbs)

Stage B

20


Steel 1010: (BHF: 1,000 Lbs)

Stage C

21


Steel 1010: (BHF: 1,000 Lbs)

Stage D

22


Steel 1010: (BHF: 1,000 Lbs)

Stage E

23


Steel 1010: (BHF: 1,000 LBS)

Load-Stroke Curve

Each point on the curve represents one of the stages from A to E

24

A

B

C

D E

25

Summary – Sheet Metal Forming Lab

This lab preparation material introduced:

• The basic principles of shearing, bending and deep

drawing, as well as terminology used

• The objectives of and the expected outcomes from the

evaluation of experimental trials

• The testing equipment and the test procedure

ise 311 sheet metal forming lab cup drawing introduction basic principles of drawing drawing is a...

Documents