Peter Ulintz
Technical Director

# Understanding Horizontal Forces in Stamping Dies

March 25, 2020

The downward motion of a press ram applies a vertical force onto a die assembly mounted in the press as well as the workpiece inside of the die. The vertical force is expressed in terms of tons-force (tonnage) or kiloNewtons (kN).

It is common practice to estimate vertical forces in a die assembly before starting a die design. Knowing these forces allows the die designer to determine thicknesses for die shoes, forming pads and dies steels in order to minimize deflections in the die. Estimating these forces also helps determine the size, quantity and spacing for any parallels required to achieve the desired feedline and die shut heights.

Before the die components can be designed, considerable knowledge is required about the forces acting on each of the individual components. The vertical forces must be analyzed to determine if horizontal forces or side thrusts, also called force vectors, are created during the cutting and forming processes. Horizontal forces inside of the die often cause many alignment-related problems.

A force vector represents a force with magnitude and direction. This contrasts with simply giving the magnitude of the force, known as a scalar quantity. For example, instead of saying that the force is 2 tons (scalar), we say that the force is 2 tons vertically toward the press bed (vector).

Horizontal Forces in Cutting Operations

A simplified example of a force vector in a stamping die is depicted in Fig. 1. Triangle abc represents the physical relationship between a punch and die where the maximum shear load occurs before the slug fractures. Side ac represents the punch-to-die clearance. Side bc represents the sheet metal thickness through the fracture zone (i.e., the thickness of the sheet minus the punch-penetration depth). This same triangle also represents the force distribution in the cutting process. Side bc represents the vertical cutting force and side ba the resultant horizontal force.

Acquiring the vertical force from a strain gauge (force monitor) or calculating it with known shear strengths, a ratio can be used to find the horizontal force:

Clearance          =    Horizontal Force
Fracture Length             Cutting Force

Assume that Fig. 1 depicts a 1.00-in.-dia. hole in 0.100-in.-thick mild steel, with a shear strength of 35,000 psi. The punch penetration equals 35 percent of material thickness (0.035 in.) and punch-to-die clearance measures 10 percent of material thickness per side (0.010-in.).

The cutting force required to produce the 1.00-in.-dia. hole can be found through the following:

Cutting force = hole circumference x penetration depth x shear strength

Cutting force = 3.1416 in. x 0.035 in. x 35,000 psi

Cutting force = 3848 lb.-force or 1.924 tons

Substituting, we find the horizontal cutting force:

0.010/0.065 = Horizontal force/3848

Horizontal force = (0.010 x 3848)/ 0.065

Horizontal force = 592 lb.-force or 0.296 tons

Given precise die alignment and equal cutting clearance around the hole, the opposing horizontal forces balance out. A punch misaligned by 0.0015 in. would result in a horizontal force on one side of the punch of 503 lb.-force (0.25 tons) and 681 lb.-force (0.34 tons) on the other. Should the force differential be great enough, the punch point could deflect toward the lower-force side of the hole, worsening the misalignment condition.

If we replace the 3.1416-in.-circumference hole with a straight-line trim of equal length, the entire 592 lb.-force of force acts on one side of the punch and attempts to push the die component(s) side-ways. The die designer must understand the impact of horizontal forces acting on individual die components in order to control these forces in the die design.

Other Horizontal Forces

Horizontal forces exist in dies besides the one illustrated for cutting. Force vectors are present during right-angle bending, nonsymmetrical forming and drawing operations. Using cams also generates large side thrusts. Cams with 45-deg. driving angles (Fig. 2) produce horizontal forces equal to the vertical force acting on them.

The press also applies horizontal forces, especially when the ram is not parallel to the bed or guided properly in its gibs. In this case, side thrusts of very high magnitude act on die components and the die assembly. Dies that are somewhat self-aligning—draw dies, for example—will attempt to overcome this situation. Sometimes, larger guide pins and additional heel blocks are employed to move the ram into a parallel position. Unfortunately, the forces required to shift or guide the press ram into proper alignment likely are much greater than can be handled by the die-alignment components, causing deflection of these components.

A well-constructed, symmetrical blanking die, with no shear and a well-aligned press ram, will result in the minimum horizontal loads.

Significant side loads in dies often have multiple causes, including poor alignment of die components during die construction; misalignment resulting from a mis-hit or die crash; angular contact between surfaces, such as angular form steels; nonsymmetrical forms or draws where the punch and die are loaded off-center at initial contact; the use of shear or angular cutting faces to reduce cutting forces; or cutoff, trim, bending and flange dies where forces act on only one side of the die steel.

Historically, dies have been constructed based on experience, intuition and rules of thumb. Various components in dies are designed and used without a real understanding of their function or the forces acting on them. A better understanding of the purpose and function of each die component from an engineering perspective (e.g., loads, deflections, vector angles, bending moments, etc.), is required to improve die life and accuracy of the stamping, and to reduce die maintenance cost. Otherwise, it could cost more to maintain the die than it did to build it. MF

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