Page 15 - MetalForming Magazine September 2022

Tooling by Design
By Peter Ulintz
Coil Camber
Camber refers to the deviation of a coil’s side edge measured from a straight edge, with the maximum allowable deviation from a straight edge defined in individual ASTM material standards. Measure camber by placing a specific-length straight edge against the concave edge of the coil strip and measuring the maximum distance between the sheet edge and the straight edge in the center of the arc (Fig. 1).
Camber occurs when one edge of the material measures longer than the opposite edge. In relatively wide coils, the longer edge may present as edge wave because the coil is too wide to curve sideways. In narrow strips, camber can be quite common, curving off to one side or even snaking back and forth.
Camber causes great difficulty in keeping the center of the coil strip on the die centerline during feeding, and can make feeding the coil through the die very difficult, if not impossible.
While camber is best dealt with at the source of purchase, stamping dies must still accommodate the maximum allow- able conditions. The accompanying table (Maximum Allow- able Camber) depicts the maximum allowable camber over a given distance based on coil width.
Sources of Camber
During slitting, unbalanced rotary-knife clearances on each side of the slit strip often cause camber. Different clear- ances affect the shear/fracture ratio on each edge of the strip. The edge with more shear depth will force curvature into the strip toward the edge with less shear.
A combination of dull and sharp slitting knives also can cause an uneven shear/fracture ratio, even with a balanced clearance. Sharp slitting knives help prevent camber from being induced into a coil.
Working with Cambered Coils
Coil purchase orders should specify the maximum allow-
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Maximum Allowable Camber
ASTM A682/A CAMBER TOLERANCES
Up to and including 1.5 in. wide
0.5 in. over any 8-ft. length
Over 1.5 in. wide to 24 in. wide
0.25 in. over any 8-ft. length
ASTM A682/M CAMBER TOLERANCES
Up to and including 50 mm wide
10 mm over any 2-m length
Over 50 mm wide to 600 mm wide
5 mm over any 2-m length
Coil Strip Rolling Direction
Fig. 1—ASTM camber-measurement method.
able camber for each die process, or at least those most sensitive to camber. Otherwise, industry standard tolerances apply. If camber reaches beyond the specified limit, reject the coil. Without a specified camber limit on the purchase order, should camber fall within the limits of the applicable material standard, the press shop must learn how to run the coil. The cost and effort to run excessively cambered coils in production easily can exceed the additional cost for coils with restricted camber requirements.
The best method to run coils with camber: Pilot the die. Every time the feed advances, the misaligned material will crowd one of the feed entry guides and the die. Adjust the crowded feed entry guide into the material, not away from it. This will push the coil back into alignment with the tool when the feed roll releases for piloting. The material will relax and recenter rather than accumulate the camber error.
Another option: Coin a shallow chisel-point depression in the top surface of the concaved side of the coil strip in an area outside of the finished part or in the part carrier (Fig. 2). Coining the material will elongate the edge slightly to help even out the camber. The depth and location of the coining can be adjusted to suit the prevailing camber condition.
Sometimes a roller leveler—not to be confused with tra- ditional straighteners—can be adjusted to apply additional
Camber Distance
Straight Edge (8 ft. or 2 m long)
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MetalForming/September 2022
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