Question

Discuss the multiple factors which affect/control the maximum drawing ratio per pass in drawing processes?

Answer 1

DRAWING PROCESS:

The drawing is a metal forming process mostly used to manufacturing of wire (small diameter metal rods than the bar) from the round bars, except instead of pressure from back end. The wire (wire drawing) is pulled from front side where it is emerges from circular die. It is effective method for producing the wire, based on cross sectional die required. Thus dies are made from hardened tool steels or tungsten carbine materials. Drawing and extrusion are may be hot (stock heated with high temperature) or cold (stock not heated).

Multiple factors which affect/control the maximum drawing ratio per pass in drawing processes:

Measurement of the amount of drawing performed on a sheet metal blank can be quantified. This can be done with the drawing ratio. The higher the drawing ratio, the more extreme the amount of deep drawing.

Due to the geometry, forces, metal flow and material properties of the work, there is a limit to the amount of deep drawing that can be performed on a sheet metal blank in a single operation.

Drawing ratios can help determine the maximum amount of deep drawing possible. The drawing ratio is roughly calculated as,

DR = D_b/D_p.

D_b is the diameter of the blank and D_p is the diameter of the punch.

For shapes that are noncircular the maximum diameter is sometimes used, or occasionally drawing ratio is calculated using surface areas. The limit to the drawing ratio for an operation is usually 2 or under. Actual limits to the amount of drawing possible are also dependant upon the depth of drawing, punch radius, die radius, anisotropy of the sheet and the blank's material.

The smaller the initial area, the smaller the reduction per pass percentage that is typically used

• Another way to express drawing ratio is the reduction (r). Reduction is measured using the same variables as drawing ratio. Reduction can be calculated by

r = (Db - Dp)/(Db).

Db and Dp being blank and punch diameters respectively.

Reduction should be .5 or under. Often expressed as the percent reduction r = (Db - Dp)/(Db) X 100%. In this case the reduction should be 50% or under.

Maximum reduction per pass:

Increase in reduction, increase the draw stress. If the reduction is large enough, draw stress will exceed the yield strength of the material. Then the wire will just elongate rather than new material being drawn into the die hole. To have a successful wire drawing operation, drawing stress should be less than yield strength of the drawn metal.

Assume a perfectly plastic material (n = 0), no friction and redundant work, then, which means that This gives a condition that the maximum possible reduction, rmax is rmax = 0.632 (theoretical maximum limit)

This analysis ignores the effects of friction and redundant work, which would further reduce the maximum value, and strain hardening, which would increase the maximum reduction because of the stronger wire than the starting metal. Reductions of 0.5-0.3 per pass seem to be possible in industrial operations.

Or

Number of drawing steps or passes required are more if larger reductions are desired. If the required reduction is attempted in a single step, the draw stress required may be too high. If the draw stress applied reaches the material’s yield strength, then instead of getting drawn, the material will start yielding. The material will start elongating locally instead of getting drawn out. Therefore, the draw stress should not be allowed to reach the yield strength of the work material. For maximum drawing, we can say that the draw stress is just equal to the material yield strength