Basic Principles Of Sheet Metal Stamping

Theory of Stamping

The cutting of sheet metal in a stamping work is a shearing process. As the punch touches the sheet metal and travels downwards, it pushes the material into die opening. The material is subjected to both compressive and tensile stresses. These stresses will be highest at the edges of die and punch and material will stressing beyond its elastic limit and start cracking there.Shearing Action press tool

3 Steps of shearing or fracture in a cutting operation.

  • Stressing the material beyond its elastic limit.
  • Plastic deformation due to reduction in area
  • fracturing starts in a reduced area.

Forces Disposition

The forces developed in the shearing operation is represented by a triangle as shown in the below-given figure. The vertical shearing force and horizontal lateral force are represented by and  H respectively. The resultant force is represented by R.

Press tool shearing force

Vertical Force

The value of Vertical Component V depends upon the area to be sheared and shear strength of the material to be cut. Shear Area is a multiple of the length of cut and sheet thickness.

Horizontal Force

The value of horizontal or lateral force H depends upon the die clearance. The horizontal force can be stated in terms of vertical force percentage. This percentage is the same as the die clearance percentage.

For perfect shearing/cutting of sheet metal, clearance must be in proper amount.

What is Clearance?

The amount of space between punch cutting edge and die-cutting edge is known as clearance. For better understanding, “Clearance is the amount of extra space required in the hole of the die to allow the punch to pass through to punch a hole in the material” .Press tool clearance

Importance of Proper Clearance

Clearance has a significant role in all cutting operations and forming operations. The die clearance depends upon the work material, it ranges from two to ten percent of the thickness of the worksheet. Ductile material should have lesser die clearance otherwise soft material would be drawn into the gap and harder material needs more die clearance for good shearing action.

Excessive clearance causes more burr on the sheared sheet while less clearance reduces the burr but it also damage the edges of die and punch. This results in frequent resharpening of die and punch and decreases the press tool life.

The range of die clearances for various materials for stamping work:

Note that the die clearances are specified in the percentage of Sheet thickness. For a close cutting profile, there would be a die clearance between the die and punch all around in profile.

Die Clearance for various materials

Clearance for materialsclearance graph

Note: It is not possible to get tolerances than die clearance on components so it is necessary to perform an additional shaving operation for high precision work.

Die clearance for punching tool

The hole pierced in the sheet is a tapered one, with minimum opening equal to punch size. The maximum size of the hole at the bottom of the sheet depends upon the width of the die opening. As the minimum size is important in piercing/punching, the punch is made equal to the hole size.

The Die clearance on the die cut out is bigger than the size stated on the component drawing.

For Example

Piercing a hole of ∅ 20 mm out of 2 mm thick MS sheet.

Punch diameter will be same as hole size i.e 20 mm.

Die clearance at 2.5% of sheet thickness = 2.0 x 0.025

⇒                                                          = 0.05 each side

⇒             Die bore   = 20 + 2 x 0.05 = ∅ 20.10 mm

Die Clearance for Blanking tool

The blanked profile sheet is also tapered one, with minimum size at the bottom and maximum at the top. The maximum size of the hole at the bottom of the sheet depends upon the die opening. As the minimum size is important in piercing/punching, The punch is made equal to the hole size.

The maximum dimensions of the blanks should not exceed the sizes stated in components drawing. So in blanking the die cutout is made equal to die profile hole and punch must be lesser on every side by the clearance size.

For example:

Blanking a  of ∅ 20 mm blanked sheet out of 2 mm thick MS sheet.

Die diameter will be same as hole size i.e 20 mm.

clearance at 2.5% of sheet thickness = 2.0 x 0.025

⇒                                                           = 0.05 each side

⇒         Punch Size   = 20  – 2 x 0.05 = ∅ 19.90 mm

Note: What is the difference between punching and blanking tools ?

Ans: Blanking and Punching tools both are similar however the workpiece  in blanking tool called blank and used in further operation whereas in punching tool the piece falling through the die is scrap.

Bending Tools Principles

The bending process changes the shape of flat blank to make it angular, curved or both without much change in its thickness. It is a very common process for changing sheets and plates into channels, drums tanks, etc. During the bending operation, the outer surface of the material is in tension and the inside surface is in compression. The strain in the bent material increases with a decreasing radius of curvature.

Bend Radius

The minimum radius to which a blank can be bent without cracking depends upon the material and its hardness. The minimum inside radius also depends upon the direction of rolling. The strips should be cut in such a way that the bend lines lie at the right angle to the grain direction.

A sheet is more vulnerable to cracking in bends across the grain direction. So the minimum radius of cracking across the grain is about four times the minimum radii for bend along the grain direction. Usually, the direction of grains is parallel to the longer side of the full uncut sheet.

Minimum radii for bending various materials along the grain directions.Minimum bend radius

Where T= Thickness of material

Bend Allowance

When the metal sheet folded or bent, the metal around the bend is deformed and stretched.  As this happens it gains a small amount of total length in the stamped part. The Bend Allowance is defined as the added length to the actual leg lengths of the part in order to develop a flat pattern.

The leg length is the length of the flange which is outside the bend radius.

Blank Size or Developed length

It is a necessary stamping principle to calculate the length of the blank before bending because a component is usually blanked before bending.

During Bending the metal layers adjacent to the inner radius are compressed while the metal layers adjacent to outer radius are stretched. Some layers of sheet neither compress nor stretch during bending operation called Neutral Plane.

Note: Neutral Plane lies along middle of sheet thickness for radius more than twice thickness. For radii less than twice of sheet thickness approximately one third thickness from inner radius surface of the bend.

While Calculating the blank length or developed length, shifting of the neutral plane should be considered. Bending Blank calculation

Developed length Should be calculated along the neutral plane by following formula.

 \boxed{L &= \frac{\pi}{180} \cdot A \left ( R + 0.5 T \right )} \;  where\; R\geq 2T

 \boxed{L &= \frac{\pi}{180} \cdot A \left ( R + 0.33 T \right )} \;  where\; R< 2T

T= Sheet Thickness

R = Inside bend radius

L= Developed Length

A = Angle of bend in degree

Example: Calculate the developed length or blank length for the following components.

Bending

Solution:  The developed length of the component has two straight parts l1′  l2, and curved part l3.

Given:

Inner radius : 3mm

Sheet thickness : 2 mm

So

l1 =  25 – ( Sheet thickness + Inner radius )          =25 – 5            =20 mm

l3 =  45 – ( Sheet thickness + Inner radius )          =45 – 5            =40 mm

As the inside radius of the bend is less than 4 ( twice the sheet thickness )

l2 =  L &= \frac{\pi}{180} \cdot A \left ( R + 0.33 T \right ) \;  where\; R< 2T   

= (π/180)  × 90 × ( 3+0.33 × 2 )

= 5.749 mm

Now Total developed Length

=  l1  +  l2  +  l3

=  20 + 5.79 +40

65.79 mm

 

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