Manufacturing Processes
Sheet Metal Processing

3.4.5.1 Introduction
Over half of total metal production ends up in sheet metal parts - auto bodies, appliance shells, cans, etc. Consequently the processing of sheet metals is of vital importance to a range of industries.
Common operations include:

• Stretching
• Drawing
• Deep drawing - is a combination of drawing and stretching
• Bending, flanging
• Punching / Shearing
• Spinning
• Press forming - is a generic term for sheet metal work carried out on a power press

Many sheet metal parts are large and highly visible and surface appearance is important.

Some defects that can occur with Sheet Products:

• Stretching may give rise to a grainy surface (orange peel). This is because the metals are polychristaline. Finergrain material reduces the effect.
• Initial yielding in some materials may be very localised leading to visible surface bands (Luders lines, stretcher - strain marks).
• Localised necking may occur, reducing load bearing capability. Usually materials are chosen with properties that delay the onset of necking. Once a neck has formed, further deformation occurs by local thinning, until finally fracture occurs.

3.4.5.2 Anisotropy
With a random structure, before any deformation, properties will be equal in all directions and the material is said to be isotropic.

Straining the material rotates slip planes and produces noticeable alignment (preferred orientation) of crystals - texture. As sheet metal will have been subject to a rolling process, the material will have some directionality or anisotropy.

For the volume of a specimen to remain constant, the sum of the 3 principal strains must equal zero.

For a tensile test, length, width and thickness strains occur and the strains in the width and thickness directions need not be equal.

Typical normal anisotropy values:

3.4.5.3 Suitability of Metals for Sheet Metal Processes

Virtually all wrought metals are to some extent suitable for sheet metal working.

Some steels have been specially developed for sheet metal work.

Low carbon steels - up to about 0.15% C - are most suitable.

Killed steels - producing a fine grain - using aluminium additions produces high n and r values which are suitable for severe drawing operations.

High - strength low - alloy steels are becoming more widely used. However these tend to have a high yield strength/Young's modulus ratio which gives large 'springback'. Tool design must take account of this effect. Grain refinement is carried out by small additions of Ti, V or Nb. These form carbonitride precipitates which inhibit grain growth in the austenite condition and thus refine the ferrite formed on cooling from the controlled rolling temperature. Combination of grain refinement and precipitation hardening gives yield strengths of 350 to 550 MPa.

3.4.5.4 Pre Coated Strip
A lot of sheet metal is formed with pre-applied coatings:

3.4.5.5 Forming Limit Diagram (FLD)
Formability is usually defined as the ability of a sheet metal to undergo shape stretching without necking or tearing. The FLD is used to compare different metals. Strips of metal of different widths, covered with a grid of small circles, typically 2.5 to 6 mm diameter, are tested with a very good lubricant over a spherical punch. Sheet wide enough to be clamped on all edges undergoes balanced biaxial tensile strain over the centre of the punch. As the width of the strip is decreased, the minor strain decreases. The minor strain may be +ve or -ve. The major and minor strains from the circle nearest to the tear can be considered to be a point on the boundary between safe and unsafe zones of the FLD.
A typical FLD is shown below:

3.4.5.6 Deep Drawing
Whereas in stretching the edges of the blank are normally restrained, resulting in sheet thinning, in deep drawing the edges of the blank are allowed or encouraged to move in, and the sheet thickness is nominally unchanged. Reasonable radii must be provided over the punch nose and the edge of the die to minimise the risk of tearing occurring at these locations. The force required for the operation must be carried by the base of the cup. This limits the attainable deformation, expressed as a reduction (d_o - D_p)/d_o or as a drawing ratio d_o/D_p

Clearance between the punch and the die are generally 7 - 14% of the sheet thickness.

Drawbeads are often used to control the flow of the blank into the cavity.

Limiting Draw Ration The maximum diameter of circle that can be completely drawn into a given cup size under ideal conditions is termed the LDR:

3.4.5.7 Shearing Process Details
For shearing and punching the clearance between the punch and die should be between 0.04 h and 0.12 h where h is the sheet thickness. This is simple but does not give a clean edge. If this is required then a slightly different process such as precision blanking, counter blanking or shaving a previously sheared part will be needed.

Compound or progressive dies are used for mass production of simple sheet parts from strip.

The shearing force is approximately: P_s = C₁(T_s)hl where l is the length of cut and C₁ = 0.85 for ductile materials and 0.65 for less ductile materials, about 0.7 on average.

3.4.5.8 Bending Process Details
Bending involves stretching the outer surface and compressing the inner. For a given thickness, h, tensile and compressive strains increase with decreasing forming radius, R_b The bend radius must be small enough to bring much of the sheet cross section into a state of plastic flow.

The neutral line maintains its original length. When bending around a small radii the neutral line moves towards the compression side, the centreline is elongated and constant volume is achieved by thinning of the sheet. Increased length of the centreline is usually considered when R_b is less than 2h, by assuming that the neutral line is located at one third of the sheet thickness. When the sheet is relatively narrow (w/h less than 8) there is also a contraction in the width w.

Problems that occur in bending include 'orange peel' which may be remedied by using finer grain material. If the ratio of the minimum bend radius to the thickness is not observed, then localised necking or fracture may occur.

Springback occurs and increases the angle of the bend radius. Springback may be eliminated by overbending, applying plastic deformation at the end of the stroke or subjecting the bend zone to compression.

The elastic zone is larger for a relatively gentle bend and for material with a high yield stress to elastic modulus ratio.

The force for bending to 90^o: P_b = w h²(TS)/W_b where w is the width of the strip (the length of the bend line) TS is the tensile stress and W_b is the width of the die opening.

The minimum bend radius for various materials at room temperature is shown below: