Manufacturing Processes Sheet Metal

Shearing Definition

Before a sheet-metal part is made, a blank of suitable dimensions is first removed from a large sheet (usually from a coil) by shearing.

This sheet is cut by subjecting it to shear stresses, generally using a punch and a die.

The edges are neither smooth nor are they perpendicular to the plane of the sheet.

Shearing generally starts with the formation of cracks on both the top and bottom edges of the workpiece (at points A and B, and C and D.

The major processing parameters in shearing are?

The shape of the punch and die

The speed of punching

Lubrication

The clearance, c, between the punch and the die.

Punch Force Definition

The force required to punch out a blank is basically the product of the shear strength of the sheet metal and the total area being sheared along the periphery. As the clearance increases, the punch force decreases, and the wear on dies and punches also is reduced.

Shearing Operations

Punching: The sheared slug is scrap

Blanking: The slug is the part to be used and the rest is scrap

Shearing operation in die cutting is?

Perforating: punching a number of holes in a sheet

Parting: shearing the sheet into two or more pieces

Notching: removing pieces (or various shapes) from the edges

Lancing: leaving a tab without removing any material.

Slitting Definition

Shearing operations can be carried out by means of a pair of circular blades similar to those in a can opener

The blades follow either a straight line, a circular path, or a curved path

A slit edge normally has a burr, which may be folded over the sheet surface by rolling it (flattening) between two rolls

If not performed properly, slitting operations can cause various distortions of the sheared edges.

What are Tailor Welded Blanks?

Regular sheet-metal-forming processes : the blank is usually a one-piece sheet of one thickness cut from a large sheet

Laser-beam butt welding of two or more pieces of sheet metal with different shapes and thicknesses

The strips are welded to obtain a locally thicker sheet or add a different material and are then coiled

Bend Allowance Definition

Used to determine the length of the blank for a part to be bent

The bend radius is measured to the inner the blank for a part to be bent.

What is minimum bend radius?

The smallest radius a sheet metal can be bent without cracking or damaging the material. It is influenced by the material type, thickness, and bending method.

Springback Definition

Materials have a finite modulus of elasticity; plastic deformation is always followed by some elastic recovery when the load is removed

Occurs in flat sheets and plates, solid or hollow bars, and tubes of any cross section

Final bend angle after springback is larger than the angle to which the part was bent

The final bend radius is larger than before springback occurs.

When does Springback Increase?

R/T ratio and the yield stress, Ys, of the material increase

Elastic modulus, E, decreases

Work/strain hardening increases

What is some compensation for Springback?

Overbending: uses a presetting technique, where the part is initially bent to a greater angle than required, allowing for the anticipated springback during the final forming process.

Coin the bend area by subjecting it to highly localized compressive stresses between the tip of the punch and the die surface = bottoming the punch

Stretch bending: In stretch bending, the sheet metal is first stretched, and then bent around a die. This stretching process helps to increase the material's tensile strength and hardness. When the part is bent, the stretched material resists the compression on the inside of the bend and the tensile force on the outside, reducing the amount of elastic deformation and therefore the amount of springback. 

Bending Die Types

V-die

Wiping Die

Air Bending

Roll Bending

Bending Force Definition

Bending force for sheets and plates: can be estimated by assuming that the process is one of simple bending of a rectangular beam, as described in texts on mechanics of solids

Bending force is a function of the strength of the material, the length, L, of the bend, the thickness, T, of the sheet, and the die opening, W. Friction is neglected

Roll Bending Definition

Plates are bent using a set of rolls. By adjusting the distance between the three rolls, various curvatures can be obtained

Flexible and is used widely for bending plates for applications such as boilers, cylindrical pressure vessels, and various curved structural members

It can be bent by a strip with a compliant roll made of polyurethane, which conforms to the shape of the strip as the hard upper roll presses upon it.

Beading Definition

Periphery of the sheet metal is bent into the cavity of a die

The bead imparts stiffness to the part by increasing the moment of inertia of that section

Beads improve the appearance of the part and eliminate exposed sharp edges that can be hazardous

Flanging Definition

Process of bending the edges of sheet metals, usually to 90°

Shrink flanging : the flange is subjected to compressive hoop stresses that, if excessive, can cause the flange periphery to wrinkle

Wrinkling tendency: increases with decreasing radius of curvature of the flange

In stretch flanging: flange periphery is subjected to tensile stresses that, if excessive, can lead to cracking along the periphery

What is Hemming?

A process where the edge of a sheet of metal is folded over itself to create a finished edge. This technique is often used to eliminate sharp edges, improve structural integrity, and enhance appearance. If r = 50, the minimum bend radius is zero = the sheet can be folded over itself

How can you increase the bendability of metals?

You may increase their tensile reduction of area either by heating or by bending in a high-pressure environment (which improves the ductility of the material)

Equation for Bend Allowance

𝐿𝑏= α(𝑅 + 𝑘𝑇)

α : bend angle( in radians)

R: bend radius

T: Sheet Thickness

K: constant, 0.33 (for R < 2T) to 0.5 (for R > 2T)

K: in ideal cases, the neutral axis is at the center of the sheet thickness, i.e. k=0.5

Minimum Bend Radius Equation

R = T ((50/r) - 1)

r: tensile reduction of area of the sheet metal

Bending Force Equation

P = (kYLT²)/W

P: the maximum bending force

L: Length of bend

T: Thickness of sheet

W: Die opening

Y: Yield strength of material

k: a factor ranges from about 0.3 for a Wiping die, to about 0.7 for a U-die, to about 1.3 for a V-die

Bending Force for V-Die Equation

P = ((UTS)LT²/W

P: the maximum bending force

L: Length of bend

T: Thickness if sheet

W: Die opening

UTS: Ultimate strength of material

Punch Force Equation

𝐹 = 0.7𝑇𝐿 ∗ 𝑈𝑇𝑆

T : sheet thickness

L : total length sheared (such as the perimeter of a hole)

UTS: ultimate tensile strength of the material.