Page 16 - Metalorming Magazine January/February 2023

Tooling by Design
b. The die-matrix diameter for the punching station should equal the shave-matrix diameter: 0.3762 in.
c. Subtract the cutting clearance (10 percent per side) from the die-matrix diameter.
d. The result provides the punch-point diameter: 0.3698 in.
Die Steel
Shave Clearance
Engineered Die Clearance
1⁄2 Material Thickness
Die Steel
Sharp Corners
Die Steel Punch to bypass die relief
Fig. 3—Retaining shave scrap with a stepped punch.
Very close tolerances, or
when shaving thick mate-
rials with pronounced die
break or taper, may require
more than one shaving
operation. When applica-
tions require two shave
steps, the amount of mate-
rial 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 hard- ness, 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 prema- ture wear or shearing of the cutting edges.
Enter Material 80 percent
Step 1
Engineered Clearance
Enter Die Matrix 0.030 in.
Step 2
Shaving Clearance
Shave scrap remains attached to slug
Pressure Pad
www.metalformingmagazine.com
MetalForming/January/February 2023 13
Fig. 4—Retaining shave scrap using counterpressure in the punching station.
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 punch- ing 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 counter- pressure, 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 thick- ness, a heavy-duty EP lubricant typi- cally used for fine blanking, broaching or in-die tapping operations may be necessary. MF