Part 2: Step by Step Roll Pass Design Process

Step 1: Define Final Profile Geometry

Start with a detailed drawing of the finished profile. You need:

• Overall width and height
• Flange lengths (left and right, if asymmetric)
• Inside bend radii (typically 1.5× to 3× material thickness)
• Thickness of material
• Tolerances (e.g., ±0.5 mm on critical dimensions)

Example (symmetric C-channel):
Height = 80 mm, flange width = 40 mm, thickness = 2.0 mm, bend radius = 3 mm.

Example (asymmetric Z-purlin):
Top flange = 60 mm, bottom flange = 40 mm, web = 100 mm (angled), thickness = 2.5 mm.

Step 2: Calculate Developed Width (Flat Strip Width)

The flat strip width is calculated by adding all straight sections and the arc lengths of bends.

Formula:
Developed width = sum of straight lengths + (π × bend angle in radians × (inside radius + k × thickness))

Where *k* is the neutral axis factor (typically 0.33 to 0.5 for cold forming). For simplicity, many designers use: bend allowance = 0.5 × thickness per 90° bend for radii 1.5× thickness.

Example (C-channel 80×40×2 mm, 90° bends):
Straight sections: bottom = 80 mm, two webs (from bottom to flanges) = 40 mm each? Actually careful: For C-channel, flat strip = 2× flange + 2× (web – thickness?) Better to use known formula. But for guide, we skip exact math – focus on process.

Step 3: Determine Number of Forming Stations

A common rule of thumb: one station per 10–15° of total bend per bend line.

For a profile with four 90° bends (total 360° of bending), you need approximately 18–24 stations if each station bends 15–20° per bend line. For asymmetric profiles with different bend angles on each side, you may need more stations because bends cannot be applied simultaneously.

Typical station counts:

• Simple C-channel (12–16 stations)
• Standard sigma channel (18–22 stations)
• Complex asymmetric profile with lips (24–30 stations)

Step 4: Create the Flower Pattern

This is the most critical step. Draw the cross-section at each station (or every 2nd station) showing how the strip gradually bends.

For symmetric profiles:

• Station 1: 15° bend on both sides
• Station 2: 30° bend
• Station 3: 45° bend
• Continue until 90° (final bend angle).
  After 90°, you may add an over-bend station (e.g., 92°) to compensate for springback, then a final sizing station that brings it back to 90°.

For asymmetric profiles:

• Bends are often applied sequentially: first bend the longer flange, then the shorter one.
• The flower pattern becomes unbalanced. You may need additional guide rollers or anti-twist stations to prevent the profile from twisting.

Practical tip: Use roll forming simulation software (COPRA FEA, AutoForm, etc.) to validate the flower pattern before cutting metal for rollers. It saves thousands of dollars in tooling corrections.

Step 5: Design Individual Rollers

Once the flower pattern is fixed, you design the roller geometry for each station. Each roller pair (top and bottom) must:

• Match the cross-section at that station
• Have sufficient clearance (typically 0.1–0.3 mm per side for material thickness up to 3 mm)
• Provide smooth transition to the next station

Roller material and hardness:

• Cr12 or D2 steel, hardened to HRC 58–62
• Surface finish Ra ≤ 0.8 µm to reduce friction and marking
• For high-volume or abrasive materials (e.g., galvanized with zinc spangle), consider chrome plating (25–50 µm thickness)

Step 6: Compensate for Springback

Springback varies with material grade, thickness, and bend radius. General guidelines:

• Mild steel (Q235, ST37): over-bend 0.5–1° per 90° bend
• High-strength steel (Q345, ST52, DP600): over-bend 1.5–2.5° per 90° bend
• Aluminum (6061-T6): over-bend 2–3° per 90° bend
• Stainless steel (304): over-bend 2–3° per 90° bend

Implement over-bend in the last 2–3 forming stations, then add a final sizing station that brings the profile back to nominal geometry. The sizing station typically has little or no bend – just calibration.

Step 7: Add Sizing and Straightening Sections

After the last forming station, the profile may have residual twist or bow. A straightening unit (often called a straightener or leveler) with multiple small rollers (e.g., 7–9 pairs) corrects these defects. The straightening unit is critical for asymmetric profiles.

Step 8: Validate with Simulation or Trial Run

Before manufacturing the rollers, run a Finite Element Analysis (FEA) simulation. If simulation is not available, request the roll forming machine manufacturer to run a sample using soft tooling (e.g., 3D-printed or machined plastic inserts) on a test line.

What to check in validation:

• No edge cracking or excessive thinning (>10% thickness reduction)
• No wrinkling or buckling between bends
• Final dimensions within tolerance (±0.5 mm)
• Twist less than 1 mm per meter

Part 3: Common Mistakes in Roll Pass Design (And How to Avoid Them)

Mistake 1: Too Few Forming Stations

Consequence: Excessive bending per station causes edge cracking, high residual stress, and poor profile straightness.
Solution: Increase number of stations. If limited by machine length, use larger diameter rollers to allow more gradual bending.

Mistake 2: Ignoring Springback

Consequence: Profile opens up after forming (e.g., 90° flange becomes 87°).
Solution: Measure springback on sample material and add over-bend accordingly. Keep records for different material grades.

Mistake 3: Symmetric Flower Pattern for Asymmetric Profile

Consequence: Profile twists severely after exiting the last station.
Solution: Use sequential bending and add anti-twist stations (roller pairs mounted at an angle) or a straightening unit.

Mistake 4: Sharp Bend Radii

Consequence: Cracking on outer fiber, especially for high-strength steel or thick material.
Solution: Minimum inside bend radius should be 1.5× thickness for mild steel, 3× thickness for AHSS, and 5× thickness for aluminum.

Mistake 5: Poor Roller Clearance

Consequence: Scratches on coated surfaces, excessive roller wear, and high power consumption.
Solution: Use clearance of 0.1–0.3 mm per side. For pre-galvanized or painted material, increase to 0.3–0.5 mm to avoid coating damage.