The total resistance a part shows against being separated from the strip material during stamping is called the cutting force (shear force). The cutting force depends on the specific shear resistance, which varies according to the dimensions of the stamped part and the material type.
If the die’s cutting face and the punch tip are ground flat (the die’s cutting face parallel to the punch’s cutting face), the maximum cutting force is obtained. (Figure a below). Here, since the die and punch cutting faces are flat, the cutting depth is zero. When the punch tip is given a taper equal to 1/3 of the strip material thickness, the punch isn’t strained until it has cut through 1/3 of the strip material thickness (Figure b below). For this reason, the forming force in Figure b is lower compared to Figure a.
In Figure c above, the single-sided taper given to the punch tip equals the sheet material thickness. Here, the cutting height equals the strip material thickness. The punch cuts single-sided by the strip material thickness (T), and the total forming force is even lower compared to the others.
When stamping rectangular-shaped parts, the die’s cutting face is ground concave (Figure a above). Round washer-shaped parts can be produced with this type of die without deforming.
The die’s cutting face can also be ground convex (Figure b above). In dies shaped this way, the stamped parts don’t deform.
As in Figures c, d, and e above, when the punch’s cutting face is ground concave, V-type, or with a single-sided taper, the stamped parts deform. However, the profile given to the die’s cutting face or the punch tip reduces the total cutting (forming) force.
Calculating the Cutting Force in Dies
The total cutting or forming force is calculated assuming no taper is given to the die and punch cutting faces. To reduce the forming force, punches of different lengths can be used (Figure 7.24), or a taper is given to the punch tip by an amount found through testing. The total cutting (forming) force is found with the formula below.
To find the total cutting (forming) force, you need to know the strip material thickness, the length of the cut or pierced perimeter, and the strip material’s shear resistance. The table below gives the shear resistance of sheet materials and the shear resistance of materials whose tensile strength is known.
To find the safe forming force, a press safety factor is applied. The press safety factor is usually taken as N = (1.5 — 4).
Accordingly, the safe forming force is;
Pem = P . N , kg
Pem = Safe forming force, kg
N = Safety factor
In a forming operation done with a die and punch that have a flat cutting face, the work done by the die is written from the formula as follows.
Work(die) = P( 1 — % Z . B .O )T , ton.mm, or,
When the forming force (P) is taken in kg;
Work(die) = P(1 — % Z . B . O)T , kg.m gives the work done by the die.
Factors Affecting the Cutting Force
1 — The amount of taper given to the punch tip and the dimensional accuracy of the stamped part, 2 — The ratio of length to width on the stamped part,
3 — The condition of the die and punch cutting faces,
4 — The die clearance (die tolerance),
5 — Lubrication of the die,
6 — The distances between holes, notches, and slots on the stamped part
7 — The condition of the punch’s cutting face;
a) — Ground,
b) — Not ground,
c) — The direction of the grinding lines,
d) — Whether the punch is coated or uncoated.
8 — The number of holes on the stamped part,
9 — The amount of strip material cutting allowance (scrap material),
10 — The material type and its shear resistance,
11 — The friction coefficient between the punch guide plate and the punch.
Related Posts
