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Plate rolling: basics and tips for THE product engineer

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What is plate rolling?

roll bending process

Plate rolling is one of the most complex metal fabrication processes and a unique process whereby a plate of steel, aluminum, stainless or other material is formed into circular or other shapes by force through a set of powered rollers. Machines consisting of 2 to 4 roll sets press, engage, and form the material through force of electrically, mechanically, or hydraulically driven means.

In the plate rolling process, a flat plate of metal is formed into a specific curve or radius.  The plate is placed between rollers, which turn in opposite directions, and which have a gap smaller than the thickness of the metal plate.  As the plate moves between the rollers it is compressed or made thinner.  As the work is performed, the plate is eventually molded into a cylindrical shape that can then be used as part of a construction project. In the plate rolling process, spring-back is a major factor, as is the material’s chemical composition and its work-hardening characteristics.

Plate rolling involves the rolling of flat sheet metal to create cylindrical items such as pipes, tanker trailers, buckets, and pressure vessels.

What is Spring Back Plate Rolling

When rolled into a ferrule or arched shape, the sheet or plate will spring back a certain amount, and that amount depends on myriad factors. In fact, as highlighted by studies and tests carried out in various parts of the world, the precise amount of workpiece spring back can only be determined experimentally.

How to eliminate the spring back in the rolling process

The tensile strength and thickness of material can cause materials to have a lot of spring-back. Springback is a term in bending and curving of steel which refers to the way the material will attempt to open back up towards its original position after being formed. We think of steel as something stiff and rigid, but there are still elastic properties at work that cause the material to relax a little after bending.

When roll bending a plate metal we must overbend the material to get the desired radius. The unique properties of some plate metals ( such as AR400 or AR360) cause the material to have a lot of spring-back. For these abrasion-resistant materials, they must be bent to a tighter initial radius than standard mild steel to end up with the same finished radius.

Pre-bending in the plate rolling process

With a pre-bent plate, the rolling process can again provide uniform plate curvature with minimal material waste and no need for extra trimming.

What is pre-bending?

A common question regarding steel plate rolling is whether the raw material, the steel blank, should be cut to length or provided with extra material to be trimmed during and/or after the rolling process.

Another question related to the blank size is whether the plate to be rolled can be provided cut to length with the ends beveled.

The answer is that if the plate ends can be bent prior to rolling, then no extra material is required. This process is called “pre-bending.” Pre-Bend is the initial bend a plate roll operator must make before actually rolling the material, it can be done with a radius die in a press brake or done in a three-roll, plate roll. How thick a plate you can pre-bend is controlled by the maximum pressure that can be applied by the top roll of the machine.

More specifically, the capacity of a three-roll plate roll machine is defined by the maximum thickness and the minimum radius combinations that can be pre-bent for a given plate of maximum width. Pre-bending saves time. You do not have to cut then roll, cut then roll, etc. Pre-bending also eliminates flat spots and material waste.

Per-bending process

The per-bending process involves pinching the plate material firmly between two of the rolls and then using the side or lateral roll to force the material into an initial bend before rolling commences. This process must be repeated on each end of the plate being rolled to avoid large flattened sections at the mating ends of the cylinder.

Pre-Bend capacity is always less than that of the rated bending capacity so it is important to ensure the rolling system you are looking at can properly pre-bend the material you are working with.

4 Reasons Pre-Bending is Crucial in plate rolling process

  1. Eliminates Waste: The pre-bending process helps minimize waste by creating an optimum geometrical formation so that both ends of the material can get in touch after rolling as perfectly as possible. Practicing the pre-bending process while having experience and being a skilled plate rolling machine operator plays a very critical role in eliminating waste.
  2. Eliminates the Need for Extra Trimming: The resources used on extra material trimming in plate rolling are nothing to be ignored for any efficiently and effectively functioning fabrication shops or operations. The need for extra trimming can easily be avoided through pre-bending by forming an optimum alignment of both ends of plate metal after rolling the material.
  3. Saves Time: Especially for high-volume production shops, wasting a minute out of the production time means wasting money and it can add up real fast over time. Since pre-bending helps an operator to form the desired shapes faster, the operating time per plate metal on a plate bending machine diminishes, which means rolling more parts with less time.
  4. Smooth Bending Surface and Uniform Curvature / Thickness: The pre-bending process is one of the most important practices in plate rolling when it comes to being able to get the results that were initially intended to get, which is simply rolling the material correctly. Properly rolled material will inevitably result in smooth material surface and uniformity in material curvature and thickness.

Crown the Plate Roll in the rolling process

Steel plate rolls–whether they have two or three bottom rolls–all have a top roll. The top roll can be sized to roll plate into cylinders or cylinder segments to radii close to the diameter of the top roll. However, relatively small top rolls can deflect in the center under the pressure of a curving steel plate. Alternately, larger-diameter top rolls deflect less but limit the machine to rolling only larger diameters.

Benders and rollers, those who specialize in curving steel plates among other steel products, are asked to roll a variety of plate widths, lengths, and thicknesses. Some of the possibilities will cause deflection of the top roll in the rolling process. The result will be a plate cylinder with a barrel form and ends that are not parallel.

To compensate for this deflection in a plate roll, the top roll is “crowned.” A “crown” is the barrel shape of the top roll that is needed to obtain a uniform distribution of the pressure required for the rolling of steel plates. A plate roll has supports at each end and the top roll deflects in the middle when the plate is in motion. This affects the parallelism on the edges of the rolled part.

  • Top rolls are recommended to be crowned for the material that is run the majority of the time.
  • Benders and rollers do not run one-size-thickness, width, and diameter plates only, so they cannot crown their top roll to be one-size-fits-all.
  • If you have a plate that you need to roll using a plate roll that has the wrong crown amount, you may be able to use a shim to artificially add thickness to correct for crowning. The most common shim is cardboard, but you can also use metal and wood shims.

7 keys steps in the plate rolling process

  1. Operator: The operators have a minimum of one to two years’ experience. They also need to know maintenance requirements and the operation of the equipment. The forming process requires one or two persons depending on the weight of the part. One experienced operator or a helper. This is a machine that can be run by one or two persons. If by two, both persons communicate with each other to perform the task.
  2. Equipment: Before use, the operator must inspect the equipment and check for proper operation. The machine needs to check for service (greasing, hydraulic fluid, and leaks).
  3. Drawing review: The operator must review the drawing to understand the task at hand. The operator knows the following:
    • Material type and wall thickness
    • Blank Size of plate (width and length)
    • Radius or diameter
    • Weight
    • Special instructions
    • The following be known to perform the task at hand.
  4. Determine Capacity: Estimating capacity requirements are determined by the operator and management. Management will use the section modules to determine capacity. There are manufacturing charts available. This is important for the safe production of the part.
  5. Performing Ends: The plate is put into the roll and each end is pre-formed to the specified radius. Each end must be pre-formed correctly and checked with a template.
  6. Wrapping: After pre-forming continue to roll the plate into a cylinder and take multiple passes. Once rolled to butt fit the seam and tack. The operator can reroll after tacking.
  7. Rerolling: After welding, the operator may have to reroll the cylinder. The roll is opened and the cylinder is placed into the roll. The roll turns and the operator measured roundness in several places and applies pressure where ever needed.
    Take notes of your setup procedure for future use. This will help later on and add to the bottom line