Large rejections of sheet metal components occur when a sheet is misapplied, i.e., when the demand for sheet ductility is far too high when compared to what the properties of the raw material can meet. Hence, it is essential to prescribe a grade of sheet metal that is suitable for forming during part design and establishes the process design for this material.
Critical components are those wherein the (liberal) variations in material properties permitted by the standards cannot be tolerated. Hence, despite having a sheet conforming to standards, good performance in the press shop cannot be ascertained. For a shopfloor engineer, therefore, it will be helpful if a tool could tell him as to whether a given coil of material should go into the press shop or not, thereby avoiding repercussions if the sheet does not perform.
Often, merely saying that a given grade of sheet will not work, might not be enough. This is because; one might wonder what specification of the sheet would work with certainty. In such a scenario, if a handy tool is available, particularly to deal with the rejections in critical components, cost cutting can be effectively attempted. In the absence of such a tool, press shops resort to using higher grade of sheet metal, which is expensive.
Consider another scenario, where a new component is designed, and it is time for material specification. In this case, one might be in a position to prescribe tolerances on certain key properties that restrict variations. Moreover, this is a viable option, where one need not prescribe a higher grade of material, while making sure that the essential properties are as desired. Such a tool can therefore be used to arrive at material specifications.
Sheet metal working and deep drawing
Sheet metal working offers a wide spectrum of attractive features such as lightweight, high strength to weight ratio and ease of manufacturing. Hence, it is used in various industries such as automobile, aerospace, furniture, household appliances, electrical and electronics, etc.
The deep drawing process is one of the several processes, which are widely used to manufacture a product from sheet metal. During the deep drawing process, an initially flat blank is clamped between the die and the blank holder, after which the punch moves down to deform the clamped blank into the desired shape. The final shape of the product depends on the geometry of the tools, the material behaviour of the blank and the process parameters. The above process is called deep drawing only when the cup is deeper than half the cup diameter; otherwise it is called shallow drawing. Useful tools to study the influence of these parameters are numerical models like the finite element method.
Simulation of forming by finite element method is time consuming and one, therefore, needs a tool that will give a quick decision as to whether the material properties are adequate to form the part.
The material properties of the blank play a very important role in defining the product quality. The objective of any practical work dealing with the manufacture of products is to produce components that will adequately perform their designated tasks. Meeting this objective implies the manufacture of components from selected material with the required geometrical shape, precision and accuracy without any defects. Successful deep drawing depends on many factors. Ignoring even one of them during die design and build can prove to be disastrous.
Yield strength of the work material:
Yield strength is the point at which permanent change of shape occurs in a metal. A low yield strength is desirable for drawing operation so that it can begin without high tearing loads near the Punch radius. The strength can be lowered by annealing the sheet or by passing ace-hardened sheet through temper or flex rollers.
Anisotropy of work material:
With a random structure, before any deformation, properties will be equal in all directions and the material is said to be isotropic. As sheet metals are subject to a rolling process, the material will have some directionality or anisotropy.
There are two types of anisotropy: normal anisotropy and planar anisotropy.
In normal anisotropy, the properties of the material in the direction of thickness differ from those in the plane of the sheet. In planar anisotropy, the work material properties vary in the plane of the sheet. Average plastic strain ratio R is the measure of normal anisotropy, and r is he measure of planar anisotropy. Materials with high R give deeper cup. Planar anisotropy causes undesirable caring of work material during drawing. Between the ears of the cup are valleys in which the material has thickened under compressive hoop stress rather than elongating under radial tensile stress.
This thickened metal experiences most of the blank holder pressure, allowing the metal in the relatively thin areas near the ears to wrinkle.
Effect of punch and die radius:
Forces that are recorded show that there is no change in punch force with increasing punch radius. This is due to the fact that punch radius affects only the small initial bending force when forming the cup bottom.
As the die radius was changed, a significant change in punch force of 35 per cent was recorded. Any increase in the die radius beyond 10 t for the die radius is practical. Larger radii may allow wrinkling while smaller radii are more likely to tear.
Effect of punch-to-die clearance and punch travel:
The selection of punch-to- die clearance depends upon the requirements of the drawn part and on the work metal. Since there is a decrease and then a gradual increase in the thickness of the metal as it is drawn over the die radius, clearance per side of 7-15 per cent greater than stock thickness helps prevent burnishing of the sidewall and punching out of the cup bottom. The drawing force is minimal when the clearance per side is 15- 20 per cent greater than stock thickness. The drawing force increases as the clearance decreases, and a secondary peak occurs on the force stroke diagram where ironing occurs. Redrawing operation requires greater clearance. As the tensile strength of the stock decreases, the clearance must be increased.
Blank holding force:
Circumferential compressive forces on the metal in the area beyond the edge of the die cause the work metal to buckle. If this buckled or wrinkled metal is pulled into the die during the drawing operation, it will increase the strain in the area of punch nose to the point at which the work metal would fracture soon after the beginning of the draw. Blank holder force is used to prevent the buckling and subsequent failure. The amount of blank holder force required is usually one third of the force required during drawing.
Thickness of work metal must also be considered when simple shapes are being drawn. Thinner the work metal, more blank- holder pressure is required while for thicker sheets generally no holding force or very less force is required.
The experimental work showed that the increase in blank holder force has no effect on the punch force. Change in normal force does not change friction force or the coefficient of friction when good lubricants are used and metal surfaces remain separated. If the die or sheet-metal interface surfaces are rougher and a lighter lubricant is used, the blank holding force could have some effect.
When two metals are in a sliding contact under pressure, as with the dies and the work in drawing, it is likely to cause galling (pressure welding) of the tools and the work metal. When extreme galling occurs, drawing force increases and becomes unevenly distributed, causing fracture of the workpiece. Lubricant is primarily used to prevent galling, wrinkling or tearing during the drawing. Selection of lubrication is influenced by ease of application, removal and corrosive action.
The maximum punch force changes significantly as better compounds are used. The 26 per cent reduction in punch force describes the importance of lubrication in deep drawing. As one moves from lighter lubricant towards the heavier one, generally, the maximum possible reduction is increased but the blank holding force also has to be increased. This is due to the fact that the heavier lubricant keeps the blank holder away from the blank and may allow wrinkles to form.
Percentage of reduction:
The larger blanks require more compression of metal (cup diameter remains the same for all blanks), i.e., more punch force is required for large percent reductions. The cup tore when 53.6 per cent reduction was attempted.
Strain exponent (n):
It is simply the slope of the plastic region of a stress- strain curve.