Typically, the proper punch-to-die clearance in the punching station is the largest clearance that produces a high-quality shear band and a clean break edge without producing a burr. These engineered clearances—as much as 10 to 15 percent/side, depending on the material thickness and type—optimize tool life and minimize work hardening inside of the hole.

Conversely, shave clearances will be very tight, generally 1 to 1.5 percent of sheet thickness per side. Excess clearance in the shave station results in shearing and refracturing of the hole. Small cutting clearances in the shave station result in a shaved edge measuring slightly less than 100 percent of material thickness. Edges that must be sheared to 100 percent of material thickness require special processes, such as fine blanking.

Assume that 0.032-in.-thick mild steel requires a shaved hole with a finished diameter of 0.3750 in. What size should the punched-hole diameter be before shaving? 

Start with the finished-hole diameter and work backward. When specifying the shave-punch diameter, remember that the resulting hole size will measure slightly smaller than the actual punch size—usually 0.0002 to 0.0004 in. smaller—due to elastic recovery after shaving. For this example, we will use 0.3752 in. as the shave-punch diameter.

Fig. 1 depicts the next steps to determine the die-matrix and punched-hole diameters: 

1. The die-matrix opening should measure 1.25 to 1.50 percent of sheet metal thickness per side larger than the shave punch: 0.032 in. x 1.5 percent = 0.0005-in./side. Thus, the final matrix diameter measures 0.3762 in. The matrix also should be designed with a 0.25-in. land before transitioning into 0.5-deg. taper.
2. The die-matrix diameter for the punching station should equal the shave-matrix diameter: 0.3762 in.
3. Subtract the cutting clearance (10 percent per side) from the die-matrix diameter.
4. The result provides the punch-point diameter: 0.3698 in.

Very close tolerances, or when shaving thick materials with pronounced die break or taper, may require more than one shaving operation. When applications require two shave steps, the amount of material shaved in the second step should total one-half that of the first step. As an example, for a total shaving of 0.009 in. of material, remove 0.006 in. via the first shave punch and 0.003 in. via the second.

A concave surface on the shave-punch face can improve surface finish and extend punch life. Establishing the proper back angle for each application is a black art that requires trial-and-error development. Some brass and bronze alloys do not require any back angle; mild steels, depending on hardness, may require 10 to 20 deg.; and some aluminum alloys may need as much as 40 deg. of back angle.

Due to the close cutting clearances and high cutting forces associated with shaving operations, a guided stripper plate may be required to adequately support the punches and to maintain accurate punch-to-die alignment. This can reduce punch chipping greatly (a common problem), and cause premature wear or shearing of the cutting edges.

Edge-Shaving Parameters

When shaving free edges, the parameters for punched holes also apply. In addition, the free-edge shave punch must be heeled and properly supported to prevent deflection due to the unbalanced cutting forces. When possible, consider cutting and shaving opposite sides of the die strip to provide balanced loading side-to-side in the strip (Fig. 2).

Scrap Retention

Retention of the shave scrap can be troublesome, especially in progressive dies. One solution: Use a commercially available venturi-type slug-extraction device, also called a “slug sucker.” 

Another option: Design the punching and shaving stations such that the punched-hole slug retains the shave slug. Fig. 3 depicts a method using a single-step punch design, with the shave diameter determined by step 1 in Fig. 1. The punch-point diameter is determined by step 2, the point length restricted to one-half of the sheet metal thickness. This allows the hole to be punched and shaved in a single station with good control of the shave scrap. A major disadvantage here is difficulty in resharpening the punch.

Fig. 4 illustrates a more traditional two-step method. The exception here: The slug is retained in the die strip through the application of counterpressure, limiting punch entry to 80 percent of material thickness. In the next station, the hole is shaved, and the slug ejected with the shave scrap attached.
Finally, a good lubricant should be chosen whenever shaving. Depending on the type of material and its thickness, a heavy-duty EP lubricant typically used for fine blanking, broaching or in-die tapping operations may be necessary. MF

Technologies: Tooling

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