Fish Tail / End Flare in Roll Forming

1. Definition and Naming

In roll-forming plants, fish tail (and related shop slang) usually means end flare: the flanges at a cut end open or close relative to the steady mid-length section after cut-off. English literature most often uses end flare / cut-end deformation. This page treats fish tail and end flare as the same defect family for encyclopedia purposes.

Note: “fishtailing” in some welding/can contexts is a different defect. Here the topic is roll-formed cut-end distortion.

2. What It Looks Like

• Mouth of a channel opens at one or both cut ends
• Or closes (flare-in) at the lead end while the trail end flares out
• Assembly holes near ends miss mating parts
• Deep side cutouts amplify local flare

3. Residual-Stress Mechanism

During progressive forming, residual longitudinal and shear stresses build in the flanges. When the continuous profile is cut, those residuals release. Research describes residual longitudinal stresses producing bending moments and residual shear stresses producing torsional moments; their superposition yields the typical end shape change.

Light-gauge channel studies also emphasize reverse bending downstream of roll-center contact as a source of the moments that remain as residual stress until cutting.

4. Lead End vs Trail End

Classic U-profile behavior reported in end-flare research:

• Lead end often tends to flare in (close), with a complex tip turn from moment superposition
• Trail end often flares out (open), typically more than the lead closes

Shop practice matches this: measure both ends; do not average them into one “flare” number without labeling lead vs trail.

5. Tooling / Flower Strategies

Roll-Kraft and similar technical notes describe designing over-form then under-form then finish so stretch/compress cycles reduce residual flare. Industry prevention lists also include minimizing deformation per pass and using side rolls to guide/control the strip.

More progressive station layouts with lower peak strain gradients have been associated with better end geometry versus aggressive few-station flowers in FEA-backed discussions.

6. Fixtures and Conditioners

Dedicated flare fixtures / conditioners (two-roll, three-roll, adjustable vertical rolls on open-seam tubes, etc.) can remove substantial flare after or near the exit. Use them as process tools, not as excuses for an untreated flower when flare is extreme on every length.

7. Process Levers

8. Measurement

• Measure flange opening at a fixed distance from the cut
• Compare to mid-length opening
• Report lead and trail separately
• Wait for cut-end temperature / handling settle on thick sections

9. Not the Same as…

• Springback of angles along the continuous length (related but not cut-release flare)
• Twist of the whole stick
• Edge wave along the length
• Cutoff die crush or burred ends from a dull blade

10. Boundaries

• Fish-tail naming varies by shop—confirm photos with the customer.
• Pre-cut blank flare and post-cut continuous-line flare share residual stress ideas but different tooling paths.
• No universal mm flare limit is asserted here.

11. Buyer / Engineer FAQ

Why is one end open and the other closed?

Lead/trail residual moment superposition commonly produces opposite behaviors.

Will a better cutoff die remove flare?

A good die avoids crush, but residual-stress flare remains a forming issue.

Does AHSS flare more?

Higher residuals and springback often enlarge end errors—design accordingly.

Can CAE predict flare?

Yes, FEA is widely used; still verify because stand stiffness and coil scatter matter.

Deep side notches worsen flare?

Yes—cutouts change local stiffness and residual release paths.

Is oil the fix?

No. Lubrication is not the primary control for residual-stress end flare.

12. Troubleshooting Checklist

1. Photograph lead and trail ends with scale
2. Measure mid vs end openings
3. Check cutoff quality separately
4. Review flower over/under/finish strategy
5. Try fixture correction settings
6. Escalate to redesign if out of fixture range

13. Plant Practice Notes

Train shipping to pad flared ends so bundles do not permanently set worse shapes in storage.

For welded frames, end flare that is “in tolerance” for stick length may still fail robotic clamp—agree functional gauges with the customer.

• Label lead/trail on samples
• Log fixture settings with each job
• Do not grind flanges to fake the opening

1. Quantify flare
2. Separate cutoff damage
3. Apply flower/fixture fix
4. Re-measure both ends on 10 sticks
5. Update control plan

Design reminder: build over/under/finish into the flower early for open channels known to fish-tail.

Quality reminder: define measurement location (e.g. 25 mm from cut) so shifts do not invent different numbers.

14. Additional Engineering Notes

End flare changes after each forming step in research observations—late stations matter. Do not assume early-pass fixes alone erase exit residuals.

When pre-punch patterns exist near the future cut, residual release can interact with hole distortion—inspect both geometry and hole position at ends.

• Archive flare photos with flower revision IDs
• Compare coils when flare suddenly grows at constant setup
• Re-check after intentional springback overbend changes

If trail flare always exceeds lead close by a large margin, that matches published superposition patterns—use it as a sanity check on measurement orientation.