Draw Depth and Punch Nose Radius in Drawing Dies

The draw depth of the part to be drawn needs to be calculated. However, this calculation is done for flangeless parts and is found with the formula below.

h_n = The draw depth on the final draw, mm

Die Opening and Punch Nose Radius

To make it easier for the part material to flow (advance) inside the die and to prevent the drawn part from tearing, the die opening and punch nose radii are taken as large as possible. However, if the radii are chosen too large, this causes the holding force to decrease and wrinkling at the drawn part’s mouth. If the radii are chosen too small, the part will tear at the start of the draw.

As the ratio of the die opening and punch nose radii to the sheet material thickness increases, the drawing stress factor reaches its maximum value in a single-operation draw. So the largest drawing stress factor is obtained when the ratio of the die opening and punch nose radii to the sheet material thickness is large.

When the die opening radius equals the sheet material thickness, the drawing stress factor Gt should not exceed 1.25. If the drawing stress factor Gt = 1.32, the die opening radius should be taken as twice the sheet material thickness. When the drawing stress factor Gt = 1.5, the die opening radius should be taken as five to ten times the sheet material thickness, and the punch nose radius should be taken as five times the sheet material thickness.

The chart above gives the percentage (%) reduction ratio between diameters, the diameter ratio, the drawing stress factor, and the percentage (%) reduction ratio between areas, based on the type of material being drawn.

The figure above shows the relationship diagram between the drawing stress factor, drawing efficiency, and percentage draw-depth stress.

In blank-holder-less drawing dies, if the length of the drawn part’s material resting on the die seating surface doesn’t exceed 3 times the sheet material thickness (f < 3T), wrinkling won’t occur in the drawn part (figure above).

The length of the drawn part material’s seating surface on the die must not exceed twenty times the sheet material thickness (f1 = r1 < 20T) (figure above).

The die opening is made as a plain radius, an elliptical curve, or angled at 45° to 60° to the horizontal plane (above).

For aluminum materials, the opening radius is generally taken as seven times the sheet material thickness. If the radius value is taken as less than four times the sheet material thickness, material buildup occurs at the part’s mouth. (Figure above)

When the die opening and punch nose radii are increased, the drawing process becomes easier. (Figure above)

The relationship between the die opening radius and the punch nose radius can be explained as shown in the figure above. The distance (A) between the center of the die opening radius and the edge of the part to be drawn should be approximately 3 mm.

The distance (B) between the center of the punch nose radius and the outer surface of the part to be drawn is taken as approximately 3 mm. The center of the punch nose radius (C) on the final two draws is taken on the same axis.