Points 1#: Operating Parameters
That means that even a small increase in thickness can make a difference in the way the rolls will perform.
Begin with selecting the correct bending rolls to roll the part on. Modern plate rolling machines are usually cambered (crowned) at 50% of the full-rated value of the machine. Therefore, a 2-inch machine is cambered to roll a 1-inch plate. All plate bending machines are really designed to function best when used to half their working value.
Problems arise when the upper limits of any plate rolling machine are approached, be it roll former or press brake. That means that even a small increase in thickness can make a difference in the way the rolls will perform.
Points 2#: Right-size Roller
Almost all plate rolling machines achieve precise measurements working at 50% of the full rated value of the roll.
- You can roll a sheet or plate at 1 1/2 times the upper roll of the 3-roll or 4-roll plate bending machine’s diameter. If the top roll has a 10-inch diameter, the minimum best practice “rollable” inside diameter will be 15 inches.
- Plate Rolling Machines that incorporate planetary guides keep roughly 50% more area under bend pressure at any one time. This design can achieve ratios of one-to-one with the upper roll diameter.
Points 3#: Pre-Bending
In the initial-pinch 3-roll plate rolling machine, the pinch rolls provide the “grabbing” force, while the bending roll’s position determines the forming geometry.
The plate rolling machine needs to grab the material during rolling. That’s why, in any rolling plate operation, there is a narrow unbent flat section on the plate’s leading and trailing edges. An operation known as pre-bending reduces the length of these unbent flat sections. These flat sections should be no more than two to three times the material thickness of most rollers. Pre-bending the plate’s leading and trailing edges before performing the final rolling can remove these flats.
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.
Using most double-pinch and four-roll systems it’s possible to pre-bend the leading plate edge without removing the sheet or plate from the roller before performing the final rolling.
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.
Read More: Pre-bending
Points 4#: Consistency
Consistent roll bending involves controlling the forming roll pressure and location. Softer materials require less pressure than harder materials. The yield strength of soft material will cause it to start forming sooner than a high-yield strength material will. That being said, different types of materials with identical thicknesses and rolled to identical diameters have will dramatically different positions of the rolls.
Every time the “forming roll” is raised, lowered, or moved in, or out, the roll changes. The more you raise, lower, or move the rolls the more pressure you add or remove from the process.
A point of note, the closer the “forming roll” gets to the pinch rolls, the greater the required pressure but also the closer to the edge the radius will get.
Points 5#: Springback
As in any forming operation, rolling, too, has to deal with spring back, a property of the material in rolling sheet metal that may require an overlap, sometimes by inches. But, just like spring-back on a press brake, when you release the part from the pressure it will spring back. If rolled to the correct diameter, a closed and appropriate gap will be the result.
Points 6#: Anti-Deflection
One common anti-deflection method is for the rolls to have slightly larger diameters in the middle of the rolls than at the ends.
The roller is rigid near the ends and deflects in the center. The roller can remain cylindrical but is no longer straight, this deflection is a normal part of machine operation.
The rollers deflect in the same way the ram of the press brake does under load; it “opens” or “flexes” in the center, deflection, which has the effect of lessening the pressure from the forming roll on the bend at the center. So, many plate rollers employ some form of anti-deflection device or crowning system.
A roller supported from one only side is considered a cantilevered roller which will deflect in increasing amounts the further away from the supported end the workpiece gets. A roller supported from both ends will have maximum deflection at the center of the roller.
Points 7#: Crowning
If there is not enough crowning the effect will be a “barreling”. This is where the ends of the cylinder hold a tighter diameter than the center. Too much crowning will produce the opposite effect, the cylinder will have an hourglass shape.
There are two possibilities for the barreling effect: there isn’t enough crowning in the center of the “forming roll” or the “pinch rolls” are not set correctly.
Points 8#: Roll Distortions
High-yield strength materials can handle more pressure. But soft, low-yield strength metals such as H-series aluminum, increasing the pinching pressure can distort the material. The material can thin and distort under excess roll pressure so some caution is required.
- Every change in crowning pressure changes the amount of deflection in the center of the roll. A slight barrel shape confirms the center of the roll is deflecting away from parallel.
- There may also be thinning or distorting of the edge of the workpiece. If this is the case, try decreasing the pinch roll pressure slightly. Allow the middle of the roll to deflect just enough that the plate rolling surfaces return to the parallel line of pressure that should be present across the width of the workpiece during rolling. This will not eliminate the barreling effect; in fact, it will most likely make the barreling worse. With inefficient machine crowning shimming may also be required.
- If you have excessive crowning a slight hourglass shape is a result. The line of pressure across the roll now looks like a frown. The frown turns into a straight line of pressure when the crowning is reduced.
Points 9#: Shimming for Crown
At some point, you may need to shim a workpiece to create a
crown. Placed in the center of the workpiece and in the center of the roller, the shims themselves can be pieces of cardboard, plastic, or even light gauge sheet metal, etc. This is a manual way to add crowning effectively. Just as shimming press brake dies start with a wide shim and gradually build to a center.
It is not a good practice, nor will you have good results if you try to place a thick narrow shim dead center.
It is also possible to shim along the edges of the workpiece to compensate for any excessive crowning and hourglassing that may be present.
Points 10#: Additional support systems
Bend past 180° and gravity will pull the sheet inward.
In thin-gauge rolling, you also have to worry about how the rolled cylinder will behave as it climbs up and over the centerline of the roll. Thick plates have the structural mass to maintain their shape throughout the rolling process. Thin sheet metal can bow under its own weight changing the radius and distorting the part. Gravity can change the point of the bend! At first, it forces the material away from the “pinch rolls” and then toward the “forming roll”.
For large or tight-tolerance work an additional overhead support system will be required; and if the material is structurally weak, the sides of the roll may need to be supported and guided through the rolling process as well.
Points 11#: Plate Rolling Capacities
The narrower the width of the plate entering the machine, the thicker the plate that can be rolled. For example, the plate bending machine can roll a 2″ thick, 12′ wide steel plate, or it can roll a 12″ thick, 3′ wide steel plate. The bending rolls can also roll thick material to very tight diameters—for example, 2″ thick, 10″ wide material can be rolled to a 19″ diameter.
The type and grade of the material determine both the largest dimensions that can be rolled and the minimum radii that can be rolled. For harder materials, maximum heavy plate rolling dimensions become smaller and minimum radii become larger.