Page 15 - MetalForming magazine • December 2022

Tooling by Design By Peter Ulintz
Punch Chipping and Wear
Cutting Punch
As mentioned in last month’s Tooling by Design column, I recently conducted an infor- mal survey of die makers and tooling engineers, asking them: “What are the most troublesome problems that you experience in your press shop?” This month, I’ll discuss one of those most-cited prob- lems: tool chipping and wear.
radius = 0
radius = 0.002 in. max.
In general, the beneficial properties of higher-strength sheet materials used in automotive applica- tions—high yield strength, high tensile strength and high ductility—also con- tribute to degraded tool life and pro- mote premature failures.
Higher-strength sheet material includes medium- and high-carbon steels; most stainless steels; high- strength low-alloy steels; dual-phase and other advanced high-strength steels (AHSS); titanium sheet; and nick- el-based superalloys.
Tool-failure mechanisms common with these higher-strength materials include chipping, cracking and abrasive wear for cutting operations, with galling representing the more predominant mechanism during forming operations.
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
Chipping and Cracking
Due to the higher stresses required to penetrate stronger materials as com- pared to lower-strength mild steel, higher-strength materials require addi- tional cutting clearances between the punch and die components. Too-tight cutting clearances bring an increased tendency for chipping and wear due to the high stripping and frictional forces acting between the work mate- rials and tooling surfaces.
Chipping represents the most com- mon failure mechanism when cutting, punching and blanking higher-strength materials. Chipping occurs when process stresses become high enough to cause fatigue of the tooling material, an indi- cation that the material lacks toughness. Toughness failures may result from improper material selection or several other factors, including heat treatment, EDM fabrication or improper operating conditions (alignment, feed, etc.).
Excessive cutting clearances may result in cracking problems. Cracking results spontaneously when the process stresses exceed the tensile strength of the tooling material. Punch cracking is a greater concern with AHSS materials due to their very high tensile strengths. The tensile strength of some AHSS materials can approach that of the tool steels working on them.
Besides increasing cutting clear-
Sharp corner carries entire load (high stress)
Radius edge increases contact area (reduced stress)
Fig. 1—A small radius on the punch edge prolongs tool life.
Cutting Punch
ances, another method to reduce edge chipping: Add a very small radius to the punch edge (Fig. 1). Accom- plish this with a single light pass over the cutting edge using a hand-held India oil- stone. The result: a 0.001 to 0.002-in. radius that helps to reduce edge stress.
Stress is a measure of an external force acting over the cross-sectional area of an
object (Fig. 2). Generally, the cutting force (F) is similar with or without a small radius, but the additional surface area of a small radius reduces edge stress.
Two Types of Wear
Tool wear can be characterized as abrasive or adhesive.
Abrasive wear occurs when the tool material wears away, typically by fric- tion. Reduce abrasive wear by increas- ing the tooling steel’s hardness, but note that this often reduces its tough- ness and resistance to adhesive wear.
Adhesive wear (galling) results from microscopic welding at localized contact points between the tool and sheet metal surfaces. Tool extraction shears these microwelds. Shearing occurring on the tool surface removes small fragments, leading to a gradual loss of material.
Reduce adhesive wear by increasing tool-steel toughness—dropping the Rockwell hardness a couple of points— or by reducing the friction between the tooling material. As increasing and decreasing Rockwell hardness entails a balancing act between toughness, abrasive wear and chipping, also con- sider the proper selection and use of lubricants, tool steel and tooling sur- face treatments.
Thick and high-tensile-strength materials in blanking operations can subject tool steels to the highest
12 MetalForming/December 2022
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