Pre-Bending and the Flat End Problem in Cylinder Rolling

Look closely at the weld seam of a shell produced by cylinder rolling, and you will often notice a section that isn't quite round. This flat section left at the two ends of the plate is known in the industry as the flat end, and it is the most fundamental technical challenge in cylinder rolling. In products where geometric accuracy is critical — pressure vessels, silos, or pipe shells — this section creates a direct quality problem. This article covers why the flat end forms and how pre-bending eliminates it.

Why Does the Flat End Form?

The source of the problem lies in how a cylinder rolling machine operates. Bending happens as the plate passes between three rolls: two lower rolls support the plate, while the upper roll descends to apply the bending force. Bending only occurs when the plate is in contact with all three points at the same time.

When the leading edge of the plate first enters the machine, it has not yet reached both lower rolls at once. That means one of the support points isn't engaged yet, and no bending moment can form. The same thing happens as the plate exits: bending ends the moment the trailing edge leaves the last lower roll. The result is that an unbent flat section remains at both ends of the plate, roughly the length of the distance between the lower rolls.

This is not caused by a poorly calibrated machine or operator error — it is a natural and unavoidable consequence of three-point bending geometry.

Why Does the Flat End Cause Problems?

The flat section lands exactly where the cylinder is closed — at the weld seam. This creates the following issues:

- Gap and misalignment at the weld root
- Failure to meet the cylinder's roundness tolerance
- Stress concentration at the weld zone
- Compliance risk with pressure vessel code requirements
- Centering problems during flange or head assembly

In critical applications such as pressure vessels and pipelines, roundness tolerance is subject to inspection; a flat end can lead to outright rejection of the product.

Solution 1: Pre-Bending

The real solution to the flat end problem is pre-bending. In this process, the ends of the plate are prepared for bending in a separate step before the main bending operation begins.

The application proceeds as follows: the end of the plate is positioned in the machine, and the rolls are positioned and clamped to bring that short section close to the target radius. The same process is repeated for the other end of the plate. Once both ends have been prepared this way, the full plate is bent normally, producing a uniform curvature from end to end.

Four-roll cylinder rolling machines are designed for this operation. These machines have a separate clamping roll that holds the plate; this allows the end section to be pre-bent in a single pass without the plate slipping. Pre-bending is possible on three-roll machines too, but it requires more operator skill and time.

Solution 2: Leaving Scrap Allowance

When pre-bending is not possible, a practical fallback method is to cut the plate longer than needed and trim off the flat ends afterward. This method gives a geometrically clean result but comes with two costs: material scrap and an additional cutting operation.

The scrap allowance approach is acceptable for single-piece or prototype production. But in series production, the scrap repeated on every part quickly becomes a significant cost item. This is why, as quantity increases, the advantage of equipment capable of pre-bending becomes more pronounced.

Solution 3: End Preparation with a Press

For thick plates and large-diameter cylinders, bending the ends separately with a press brake or a special die may be preferred. This method comes into play when the cylinder rolling machine's capacity falls short for the end section. It requires the correct die geometry and careful positioning; otherwise, a distinct kink line forms at the end instead of the desired radius.

Points to Watch in Pre-Bending

If pre-bending isn't applied correctly, it can create problems of its own:

- Over pre-bending: if the end section is bent beyond the target radius, an inward kink forms when the cylinder is closed
- Under pre-bending: the flat end isn't fully eliminated, and the problem partly persists
- Failing to account for springback: the pre-bent section also springs back; it must be bent slightly beyond the target radius
- Surface marks: the high pressure of the clamping roll can leave marks on sensitive surfaces such as stainless steel

The correct amount of pre-bend is usually calibrated by measuring against a template on a sample plate, and this setting is then fixed for series production.

The Effect of Material

The length of the flat end and the pre-bending requirement vary by material. In high-strength steels, springback is much more pronounced and requires greater compensation during pre-bending. In stainless steel, work hardening increases the force required. In aluminum, low force is sufficient, but surface marking risk is high. As thickness increases, both the required tonnage and the importance of pre-bending increase.

The Right Approach: Plan at the Design Stage

The most economical solution to the flat end problem is deciding on the method before production begins. When cylinder diameter, plate thickness, material type, required roundness tolerance, and production quantity are evaluated together, it becomes clear whether pre-bending, scrap allowance, or press support is needed. When this decision is made after the fact, either scrap increases or tolerance is missed.

Precision Cylinder Rolling with DMK Makina

DMK Makina plans the entire process, including pre-bending, on cylinder rolling projects at its Lüleburgaz facility, securing roundness tolerance from the outset. Every step, from plate cutting to rolling, welding, and surface protection, is carried out under one roof. We are here for your cylindrical shell needs in the Thrace and Marmara region.