Plate rolling defects can be one of the biggest challenges to overcome when you are using a plate roll machine especially at the upper and lower limits of the capacity of the machine. There is camber built into the rolls to compensate for the deflection that occurs when the machine is working under load. All plate roll machines have a range somewhere in the middle of their capacity where they roll perfectly without any defects.
Normally this camber is calculated for 60% of the machines rolling capacity unless the customer requests a specific camber for the material they will be rolling most. At 60% of the machines rated capacity the roll is deflected to the point where the bending force on the plate is even across it’s entire width. This means that any material that is rated at or near 60% of the machines capacity should be rolled with a perfectly straight seam (no defects). As soon as the material starts to move away from this 60% capacity you will start to see some defects along the seam of your rolled part.
They will be small at first and gradually get bigger the further you move away from 60%. With thicker material the rolls are deflected past straight creating a situation where the force on the outer edges of the plate is greater then the force in the center. This causes the plate to roll open in the middle and more closed on the ends. When using thinner plates the effect is the opposite. There are 4 basic kinds of plate rolling defects. Some of the defects are caused by the cambering of the rolls while the rolls being out of parallel can cause others.
Types of Plate Rolling Defects
The 4 basic types of plate rolling defects are:
This defect happens when you are rolling material that is thicker then what the rolls are cambered for. Because the plate is thicker it deflects all of the camber out of the roll while you are bending the plate causing the rolls to pinch the plate tighter on the outside edges and looser in the middle. The result is a seam that is closed more on the ends of the plate and is open in the center. To compensate for this you can try to use a lower pinching pressure so when you pinch the rolls together the lower pinching pressure deflects the rolls less. The down side to this is that it can require you to leave a longer flat on the pre-bend. The other option to correct this defect is called shimming. You have to use some type of material (cardboard for example) and place it along the edge of the plate in the center of the plate. Usually the middle third of the material. This cardboard (or shim) placed on top of the material essentially changes the camber of the roll making it bigger in the center so when the roll is deflected during bending you get a straighter seem as the shim fills in the space left by too much deflection of the roll. The thickness and width of the shim can be adjusted depending on the severity of the defect.
This defect is the opposite of the barrel defect. It happens with material that is thinner then what the rolls are cambered for. Because the plate does not deflect all the camber out the material it is pinched tighter in the center and looser on the ends. To compensate for this type of defect you can try to increase the pinching pressure to deflect more of the camber out of the rolls. If the material being rolled is very thin it is possible to use too much pressure and cause a rippling effect on the plate. If this happens we can again use shimming to correct the defect. This time instead of putting material along the edge of the plate in the center we will put it out towards the outer edges. It works the same way by essentially making the outer edges of the rolls a larger diameter.
When a plate rolls skewed the seam is normally straight but instead of the edges of the plate lining up they are shifted to the left or right. This normally happens if the plate is not put into the machine square or possibly not cut square but it can also happen if the side rolls are out of parallel (described below).
The conical defect is when one side of the plate rolls tighter than the other side. It can be caused by the material not being loaded directly into the center of the machine or, it can be caused by the lower or side rolls being out of parallel (described below).
Avoiding Plate Rolling Defects
The skewing and conical defects can normally be corrected by making sure that your side and lower rolls on your plate roll are parallel. It is one of the easiest adjustments you can make. How they are set will greatly influence how well the ends of your plate match up after you have finished rolling your part. Normally the rolls are set to parallel when the machine is assembled but they may need to be adjusted once the machine has been bolted down to your shop floor. After this final adjustment they should be ok for the life of the machine.
There are some other solutions for correcting plate rolling defects such as purchasing a second or even third interchangeable top roll with different cambers. This is a very good solution but not practical for many shops. Some people will also use some material to wrap their existing rolls with to make the diameter larger in the center or on the ends. This method also works well but generally the wrappings don’t last long and have to be replaced. It takes a little effort to find the solution that works best for you.
This article was contributed by Brian Hill at MG srl, one of the world’s leading manufacturers of plate rolling machines