As cone bending is very difficult on an initial pinch 3 roll plate bending machine it has, up until the last few years, been accepted that the best method to bend cones is with a double pyramid pinch roll. However, it is not an easy process with a double pyramid roll at all. In fact, it is not uncommon for jobbers, as well as manufacturers, to own a double pinch roll and still choose to bump out their cones on a press brake. The only machine capable of bending a cone properly is a four-roll plate bending machine.
To roll cones on a three-roll plate rolling machine is very difficult. First, one must realize that a cone has to be developed by rolling a plate at two different speeds at the same time. This is a difficult situation to achieve. Both the three-roll and the four-roll plate rolling machines are capable of inclining the side rolls in a positive attitude, and both have a hardened contrast die to control and slow down the speed of the small diameter. This is an equal comparison as far as it goes; but, by guiding the small diameter and inclining the roll (both of which are necessary to roll cones) you have still created an unnatural situation for rolling cones. Why? Because on a three-roll double pinch plate bending machine all three rolls are driven, which makes it very difficult for the contrast die to be able to retard the rotation on the small diameter while making the large diameter move faster. This causes lamination and scarring on the plate and the roll.
So, why can a four-roll, which also has inclinable side rolls and a hardened contrast die do this difficult function better than a three-roll plate rolling machine? The answer is this: the four-roll (lower central pinch roll) can be inclined in a negative attitude and is also capable of adjusting the force at which it pinches which allows the roll to grip the cone only on the large diameter which needs to turn faster and only with enough force to turn the part. This allows the small diameter to be slowed down more easily.
To sum it up, rolling cones properly absolutely requires a lower central pinch roll (fourth roll) capable of a negative inclination and adjustable pinch pressure. Only four-roll machines have this capability but, be careful, not all four-roll machines have it. Be sure to ask the builder about this feature. Also, make sure when inquiring about four-roll machines that the side rolls move independently so that one can be used as a squaring gauge.
“4-Roll Plate Rolls Vs 3-Roll Plate Rolls – Advantages and Disadvantages”, Ezine Articles, Visited 20 Aug, https://ezinearticles.com/?4-Roll-Plate-Rolls-Vs-3-Roll-Plate-Rolls—Advantages-and-Disadvantages&id=5542745