Page 13 - MetalForming November 2022

Tooling by Design
By Peter Ulintz
Solutions for Galling
During my die design and die maintenance seminars, I often take informal surveys by asking attendees, “What are the top five or six die-related problems in your press shop?”
The 10 most-common responses:
• Misfeeds (usually the number-one response)
• Slug pulling (a close second)
• Strip not feeding straight (aligned) through the die
• Galling
• Wrinkles and/or splits
• Dimensional variation
• Tool-steel chipping/wear
• Tool breakage
• Scrap and slivers in the die
• Sensors shutting the press (nui-
sance faults).
In a series of my prior columns,
Improving Progressive Die Perform- ance (MetalForming magazine October through December 2021), I addressed several of these problems.
The most-common problem occur- ring during coil change: coil-stock mis- feed. Improper positioning of the lead- ing end of the coil strip can be responsible for more die damage dur- ing the first 10 strokes of the press than the next 10,000 strokes.
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
Another problem, slug pulling, results from a vacuum created when the perforating punch withdraws from a hole. This usually occurs after increas- ing punch-to-die cutting clearances to improve hole edge conditions, or to reduce stripping forces or cutting pres- sures. Sometimes, die components are replaced and a die-maintenance per- son may increase the cutting clearance slightly to help increase tool life. As a result, a slug previously retained ade- quately with die clearances of 5 to 8 percent per side now may pull out of the die matrix after the clearances are
“To reduce the
likelihood of galling
in draw corners,
additional punch-to-
die clearance must
be machined in the
vertical wall.”
increased to 10 percent per side or greater.
Sometimes, galling can be self- inflicted when die engineers inadver- tently overlook a fundamental principle in deep drawing: In-plane compression of the sheet metal causes an increase in sheet thickness.
Four Rules for Galling Reduction
1. Ensure that corner draw-forming sections have proper punch-to-die clearance for the magnitude of mate- rial thickening occurring in that por- tion of the die.
The arrival of CNC machining and 3D modeling has reduced the amount of time required to design, machine and assemble die components. One
technique having sped up this process: the ability to cut clearance into the punch or die cavity by offsetting the cutter path by maximum material thick- ness. This works well until encountering a deep-drawn formed corner.
The corners in any box or irregularly shaped geometry will form in the same manner as with cylindrical cup draw- ing. The corners feature compressive stress on the metal moving toward the die entry and tensile stress on the metal drawn over the die radius. As a result, thickness changes that initially occurred in the flange move into the vertical wall. Consequently, the corner of the panel lacks maximum materi- al-thickness clearance.
To reduce the likelihood of galling in draw corners, additional punch-to- die clearance must be machined in the vertical wall. To keep sheet metal flow- ing freely into the corner wall, the draw pad also needs additional clearance around the corners so that the thicker flange does not “pinch” the material and restrict flow. This generally requires the use of analytical tools such as metal forming simulations to achieve cost- effective results. The additional corner clearance reduces galling tendency far better—and at lower cost—than adding a surface coating. If tight corner clear- ance is the root cause, adding a surface treatment literally coats the problem —an expensive solution.
2. Polish all die sections for corner draws as if prepping for a surface- coating application.
Surface coatings—very expensive to apply—result in close attention paid to the heat treatment process and the quality of die polishing. Oftentimes, this level of attention greatly exceeds that given to die sections not consid- ered for surface coating. Thus, per- formance improvements sometimes attributed to surface coatings may
10 MetalForming/November 2022
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