Page 15 - Metalorming Magazine January/February 2023

Tooling by Design
By Peter Ulintz
Shaving Operations
Stamping applications
requiring square-cut
edges and close dimensional tolerances
often make good candi-
dates for shaving. Shave
operations most common-
ly associate with punched
openings such as round
holes, but free-edge features also can be shaved.
Hole-Shaving Parameters
Generally, the amount of material to be removed by shaving totals about 10 percent of sheet thickness for most applications. This provides good edge conditions and allows enough material in the “scrap ring” for slug retention— an important point as slug retention represents a common problem with shaving operations.
The amount of material removed by shaving can relate to the size of the cut-edge fracture zone (die break). Diemakers and tool engineers some- times mistakenly believe that reducing the length of the fracture zone—by reducing punch-to-die clearance in the punching station—will help reduce shaving-related problems by reducing the amount of material being shaved.
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
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 per- cent of material thickness require spe- cial processes, such as fine blanking.
Assume that 0.032-in.-thick mild steel requires a shaved hole with a fin- ished 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. small- er—due to elastic recovery after shav- ing. For this example, we will use 0.3752 in. as the shave-punch diameter.
Fig. 1 depicts the next steps to deter- mine the die-matrix and punched-hole diameters:
a. 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.
Component
(Step 2) Hole Punch
(Step 1) Shave Punch
Punch-point size
0.3698
0.3752
Punch-to-die clearance (total)
0.0064
0.0010
Die-matrix (button/bushing) size
0.3762
0.3762
Fig. 1—Establishing the pre-shaved punch-hole diameter.
Unfortunately, this approach usually causes more problems than it solves.
Reducing punch-to-die cutting clearance produces a larger shear band and a correspondingly smaller fracture zone. However, a larger shear band also produces a larger cold-worked zone that makes shaving more difficult due to the increase in cutting force, friction and process heat, leading to punch fatigue, chipping and increased punch wear. The burr resulting from tighter punching clearances also interferes with accurate part positioning over the shave-die opening. And, removing the burr by shaving alone proves difficult, as a smaller scrap-ring web results, which hampers scrap shedding.
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
Shave Notch
Blank Shave Notch
Feed
12 MetalForming/January/February 2023
www.metalformingmagazine.com
Fig. 2—Balancing cutting and shaving forces in the die strip.