Plate Roll Pre-Bending: Why Cylinders Have Flat Ends and How to Fix Them

Flat ends remain because the leading and trailing edges cannot pass fully through the effective bending zone between the roll tangent points. Plate roll pre-bending forms those ends before the main rolling passes. The achievable flat length depends on machine geometry, lower-roll spacing, top-roll diameter, plate thickness and yield strength, end-preparation method and the approved rolling sequence.

When a flat remains at both ends, the welded joint can look flattened even though the middle of the cylinder has the correct curvature.

Figure 1. A flat remains at the seam when the plate ends are not pre-bent to the cylinder radius.

Quick Answer: How Do You Reduce Flat Ends?

Define the allowed flat in the drawing, verify the machine’s pre-bending capacity separately from rolling capacity, and select one of four methods: integrated pre-bending on a suitable 3-roll or 4-roll machine, external pre-bending on a press brake or hydraulic press with approved tooling, sacrificial end allowance followed by trimming, or a limited process improvement for low-demand noncritical parts. More roll pressure alone cannot bend an edge that never reaches the effective bending zone.

Why the Straight Section Forms

In a conventional symmetrical 3-roll layout, the top roll bends the plate over two lower rolls. The plate end cannot be supported on both sides after it passes a tangent point, so a length near the edge receives less bending moment. Lower-roll center distance is therefore a major factor, but it is not the only one.

Figure 2. Force diagram for a conventional 3-roll layout. The plate end leaves the supported bending zone before its full length reaches the target radius.

• Machine geometry: roll diameter, lower-roll spacing, movable-axis range and clamping method.
• Workpiece: material yield strength, thickness, width, target radius and plate-end condition.
• Capacity: pre-bending thickness may be lower than rolling thickness for the same model.
• Process: turning/repositioning, end allowance, number of passes, supports and measurement.
• Condition: roll wear, parallelism, deflection and left/right synchronization.

A Model-Specific Straight-End Reference

The source draft states that a conventional W11 8×2500 machine can leave approximately 100-130 mm of straight edge at each end, depending on the roll arrangement.

The public W11 symmetrical 3-roll plate rolling machine page is a model-family reference; the signed configuration and test data must control this flat-end figure.

What Flat Ends Cause

The most visible result is a flattened port or seam area even when the middle curvature is correct.

• Flanges, heads or adjoining shells do not fit the end circumference evenly.
• The seam gap and edge offset vary, increasing fit-up and tack-welding time.
• Forced closure introduces residual stress and can move the cylinder away from its required diameter or roundness.
• Extra pressing, trimming, grinding and post-weld calibration add labor and material cost.
• Batch parts lose interchangeability when flat length changes from one blank or setup to the next.
• A small-diameter cylinder is affected more because the flat is a larger share of its circumference.

For pressure-retaining or code-controlled parts, the drawing, forming procedure, material limits, weld procedure and inspection must be approved under the applicable code by qualified personnel. This article is process guidance, not permission to force or weld a nonconforming shell.

Method 1: Integrated Pre-Bending on the Plate Roll

Integrated pre-bending brings a plate end close to a roll tangent and applies bending force while the plate is held or supported. The sequence differs by machine geometry. Follow the model manual and verify both ends on the actual workpiece.

Asymmetrical 3-Roll

An offset lower/side roll can support one end close to the top roll for pre-bending. The plate is normally reversed or repositioned for the second end. Check how much flat remains after both setups.

Universal Upper-Roll 3-Roll

Controlled movement of the upper roll relative to the lower rolls can provide stronger end forming and adapt to mixed radii. The exact sequence and remaining flat depend on the quoted machine, not the name ‘universal’ alone.

See the published W11S universal plate rolling machine for this machine family.

4-Roll

The top and bottom rolls clamp the plate while a side roll pre-bends the first end. The opposite side roll performs the second end as the plate remains controlled, which reduces handling and supports repeat production.

Figure 3. Four-roll pre-bending: the top and bottom rolls clamp the plate while a side roll bends the end.

Review the current W12 4-roll plate rolling machine and confirm pre-bending thickness separately from rolling thickness.

The 3-roll vs 4-roll plate rolling machine guide compares clamping, turning and one-setup production.

Method 2: External Pre-Bending

For some jobs, both ends can be formed on a press brake or hydraulic press with suitable tooling before the plate is moved to the rolling machine.

This method can give a clear end radius without changing the plate roll, but it adds lifting, alignment and another setup. Tool radius, step spacing, surface protection, springback and transition into the rolled section must be planned. The press must have verified force and tooling capacity for the actual plate.

• Mark the pre-bend zone and tangent transition on both ends.
• Use a controlled series of bends or approved die instead of one uncontrolled local hit.
• Check the pre-bend against a template or radius measurement before moving the plate.
• Protect the formed ends during transport and align the pre-bent arcs with the plate roll setup.

Method 3: Add Allowance and Trim

Extend the blank so the unavoidable flats fall outside the finished length, then trim them after rolling. This avoids pre-bending equipment but consumes material and requires an accurate cutting and edge-preparation step. Include both end allowances, kerf, bevel and final seam gap in the blank plan.

Method 4: Reduce the Effect Through Process Control

Accurate feeding, repeated light passes, correct supports and consistent left/right settings can reduce extra flat caused by poor operation. They cannot remove the tangent-zone flat that the machine geometry cannot reach. Use this approach only where the drawing allows the result and the part is not being accepted to a stricter pressure or structural code.

Choose the Method from the Part Requirement

Use the minimum rolling diameter guide to check whether the target radius is realistic for the plate and roll system.

The plate rolling machine price guide explains the cost of stronger pre-bending, supports and control packages.

Browse BEIGEMA’s plate rolling and profile bending machine range only after the part and flat-end limits are defined.

Frequently Asked Questions

Why does more roll pressure not remove the flat end?

The edge may already be outside the supported bending zone. More force on the middle cannot create bending moment at an unsupported end and may overload or mark the part.

Do all 3-roll machines lack pre-bending?

No. Symmetrical, asymmetrical and variable/universal 3-roll designs use different end-forming methods. The exact model and sequence must be checked.

Is a 4-roll machine guaranteed to leave zero flat?

No machine should be sold on an absolute zero-flat promise without a defined material, thickness, diameter and measurement method. Four-roll clamping generally shortens and controls the flat.

Can I cut off the flat ends after rolling?

Yes, if the part plan includes sacrificial allowance and the added cutting, bevel, material cost and final dimensions are acceptable.

What should the RFQ say about pre-bending?

State material and yield, thickness, width, target diameter, allowed flat at each end, seam preparation, roundness, batch size and whether the machine must form both ends in one setup.

Send those requirements to BEIGEMA for a model-specific pre-bending review.