Bend Radius in Sheet Metal & Roll Forming

1. Definition

The bend radius (usually the inside bend radius) is the radius of the inner surface of a formed bend. It is commonly expressed as a multiple of sheet thickness t (for example 1t, 2t). Too small a radius overstretches the outer fiber and risks cracking; too large a radius changes geometry, fit-up, and sometimes structural performance.

2. Inside vs Outside Radius

• Inside radius (r_i) — what tooling and drawings usually specify
• Outside radius ≈ r_i + t (for a simple bend through the thickness)

Outer-fiber tensile strain rises as r_i/t falls. That is why minimum radius charts are written as multiples of thickness.

3. Minimum Bend Radius Concept

The minimum bend radius is the smallest inside radius a given material, temper, thickness, and bend orientation can accept without unacceptable cracking or surface rupture. It depends on ductility, strength, grain direction, edge quality, and process (air bend vs coin vs progressive roll).

There is no single universal number for “steel.” Supplier data and plant trials beat generic internet charts when the grade is high-strength or coated.

4. r/t Guidelines (Rules of Thumb)

Fabrication references often publish thickness-banded thumb rules (illustrative, not a substitute for supplier data):

Some press-brake articles cite ~1t for many steels under ~6 mm, rising toward 1.5t and 2–3t as thickness grows. Treat these as starting points for RFQ discussion, then prove on the actual coil.

A reduction-of-area based rule of thumb also appears in heavy-bending literature (e.g. relating tensile reduction % to a radius multiplier). Use supplier values when available.

5. Grain Direction

• Across the grain (bend line perpendicular to rolling direction) — usually allows tighter radii
• With the grain (bend line parallel to rolling) — higher crack risk; often needs a larger radius

Coil-fed roll forming typically bends along the length, so grain orientation relative to each bend is fixed by how the strip was rolled and slit. Designers who ignore grain on laser-cut blanks pay in scrap; roll-form plants feel grain mainly through coil quality and slit-edge condition.

6. Material and Temper Effects

• Higher yield / lower elongation → larger minimum r/t
• Full-hard / structural high-strength coils need gentler corners than soft commercial quality
• Aluminum and stainless families have their own charts—do not copy mild-steel r/t blindly

7. Roll Forming Flower Practice

In roll forming, the finished inside radius is produced by the roll profiles across stations, not by a single V-die. Practical notes:

• Distribute angle gradually; do not ask one station for an extreme tight bend on hard coil
• Match roll corner radii to the drawing—undersize rolls force smaller r_i than the steel can take
• Edge cracks on ribs often mean r/t too aggressive for the heat’s elongation
• Increasing station count can reduce peak strain rate / per-pass severity even at the same final radius

8. Press-Brake Context

On a press brake, punch tip radius and V-die opening strongly influence the resulting inside radius (air forming vs bottoming/coining). Fabricator articles note that air forming cannot produce arbitrarily sharp insides regardless of punch tip. Roll-forming buyers who quote brake experience should still re-validate radii on the roll set.

9. Link to Springback

Larger radii and higher yield both interact with springback control. A tighter bend that just avoids cracking may still spring open more on high-yield coil. See Springback and Yield Strength pages.

10. Coated Strip

Galvanized and pre-painted strip can show coating cracks or microcracks on the outer fiber before the base metal fully splits. Minimum radius for appearance may be larger than the structural crack limit of the steel substrate. Specify cosmetic criteria explicitly for visible building panels.

11. Common Mistakes

1. Copying a 0.5t radius from soft CRS onto G550 coil.
2. Ignoring grain on blanked parts mixed into a roll-form program.
3. Assuming tool radius equals finished radius after springback without check.
4. Blaming “bad steel” when the drawing demands an impossible r/t.
5. Using only hardness to judge bendability.

12. Shop-Floor Symptoms

• Orange-peel or cracks on outer bend fibers
• Paint fracture lines along ribs
• Sudden crack onset after coil change at same tool radii
• Roll marks when operators over-close gaps to “force” a sharper corner

13. Boundaries

• Bend radius ≠ FLD (different failure map for stretch paths).
• Drawing radius ≠ guaranteed formable radius for every heat.
• This page does not publish a universal ZTRFM machine capability chart.

14. Buyer / Engineer FAQ

What r/t should I put on a purlin drawing?

Start from supplier recommendations for the grade/thickness, then confirm with flower design and trials. Structural sharpness and formability often conflict—decide which constraint wins.

Can more roll stations fix a too-tight radius?

They can reduce per-pass severity and improve quality, but they cannot create ductility the steel does not have. Final r_i still has a material limit.

Is inside radius the same as punch radius?

Not always—especially in air bending. In roll forming, finished radius follows the calibrated roll geometry and springback.

Why did coated panel crack but base metal look OK?

Coating ductility / adhesion limits were exceeded. Increase radius, improve lube, or change coating system.

Relation to elongation?

Higher elongation generally supports smaller r/t; still verify because edge quality and strain path matter.

15. Design Checklist

1. Specify inside radius on the profile drawing.
2. State material grade, thickness, and coating.
3. Check r/t against supplier / internal minimums.
4. Align flower station plan with radius severity.
5. Trial the hardest expected production temper, not only the softest sample coil.

16. Related Pages

• Elongation; Yield Strength; Work Hardening
• Springback; Cracking in Roll Forming
• Pass Design / Flower Pattern

17. Drawing Callout Examples

• “Inside bend radius 2.0 mm min, material t = 1.5 mm (r/t ≥ 1.33), across-grain where blanked.”
• “Roll-formed corners per flower; finished r_i = 3 mm ± 0.5 mm after springback.”

Ambiguous “sharp corner” notes are not manufacturable specifications.

18. Escalation Path When Cracks Appear

1. Measure actual finished inside radius.
2. Pull MTC elongation and yield.
3. Inspect slit edge.
4. Compare to design r/t.
5. Only then change rolls or coil temper.

Extended Practice Notes (1)

When explaining this topic to non-specialists, lead with the decision it affects (buy, form, inspect, or redesign), then introduce the technical definition. Keep commercial claims out of the article body.

For RFQ language, prefer measurable acceptance criteria over adjectives. “Good ductility” is not a specification; a named test method and numeric window is.

When two heats of the same grade behave differently, pull certificates and process logs before changing tooling. Most mystery forming issues are heat-to-heat property or lubrication shifts.

Cross-check companion encyclopedia pages before closing a CAPA: mechanical properties overview, MTC practice, and the specific defect page when cracks or springback dominate.

Document the evidence class for any numeric claim you paste into a customer email: mill certificate, plant trial, or published standard table. Do not invent universal machine kW or m/min values on materials pages.

• Keep units consistent (MPa vs ksi; mm vs in).
• Record gauge length / test method when citing elongation or n/r values.
• Attach heat numbers to scrap photos.
• Separate design minimums from actual forming windows.
• Update internal SOPs when a new grade family is introduced.
• Train operators to stop on out-of-window certificates.
• Keep lubrication logs with coil IDs on critical runs.

1. Define the question the page answers in one sentence.
2. List the three certificate fields that matter most.
3. List the three shop symptoms that should trigger a re-read of this page.
4. Name the next specialist page to open.
5. File the lesson learned in the plant knowledge base.
6. Re-verify after any grade or coating change.

Training tip: have new engineers highlight the Boundaries section first so they do not over-apply the concept outside its domain.

Quality tip: auditors respond better to traceable certificate fields than to verbal grade nicknames.

Process tip: after any coil change that moves yield by a large step, expect springback and load changes even when thickness is identical.

Design tip: write inside radii, thickness, and grade on the same drawing note block so formability constraints travel with the geometry.

Simulation tip: material cards need more than a single UTS number; hardening, anisotropy, and friction assumptions drive outcomes.

Maintenance tip: rising forming loads at constant thickness often signal harder coil or dry friction—not only worn bearings.

Sales tip: do not promise a flower proven on soft coil will run unchanged on a high-yield structural coil without a written process window.

Receiving tip: match heat numbers on tags to the MTC before the coil enters production stock as approved.

Extended Practice Notes (2)

When explaining this topic to non-specialists, lead with the decision it affects (buy, form, inspect, or redesign), then introduce the technical definition. Keep commercial claims out of the article body.

For RFQ language, prefer measurable acceptance criteria over adjectives. “Good ductility” is not a specification; a named test method and numeric window is.

When two heats of the same grade behave differently, pull certificates and process logs before changing tooling. Most mystery forming issues are heat-to-heat property or lubrication shifts.

Cross-check companion encyclopedia pages before closing a CAPA: mechanical properties overview, MTC practice, and the specific defect page when cracks or springback dominate.

Document the evidence class for any numeric claim you paste into a customer email: mill certificate, plant trial, or published standard table. Do not invent universal machine kW or m/min values on materials pages.

• Keep units consistent (MPa vs ksi; mm vs in).
• Record gauge length / test method when citing elongation or n/r values.
• Attach heat numbers to scrap photos.
• Separate design minimums from actual forming windows.
• Update internal SOPs when a new grade family is introduced.
• Train operators to stop on out-of-window certificates.
• Keep lubrication logs with coil IDs on critical runs.

1. Define the question the page answers in one sentence.
2. List the three certificate fields that matter most.
3. List the three shop symptoms that should trigger a re-read of this page.
4. Name the next specialist page to open.
5. File the lesson learned in the plant knowledge base.
6. Re-verify after any grade or coating change.

Training tip: have new engineers highlight the Boundaries section first so they do not over-apply the concept outside its domain.

Quality tip: auditors respond better to traceable certificate fields than to verbal grade nicknames.

Process tip: after any coil change that moves yield by a large step, expect springback and load changes even when thickness is identical.

Design tip: write inside radii, thickness, and grade on the same drawing note block so formability constraints travel with the geometry.

Simulation tip: material cards need more than a single UTS number; hardening, anisotropy, and friction assumptions drive outcomes.

Maintenance tip: rising forming loads at constant thickness often signal harder coil or dry friction—not only worn bearings.

Sales tip: do not promise a flower proven on soft coil will run unchanged on a high-yield structural coil without a written process window.

Receiving tip: match heat numbers on tags to the MTC before the coil enters production stock as approved.