With the advances in Fluid Technology this has changed dramatically. It is now possible to buy a four-roll machine for only about 20% more than a three-roll double pinch machine.
Is this an important development to plate roll users? To answer this question, let's briefly review the functions of the single initial pinch roll, double pinch roll and the basic pyramid roll.
Pyramid Roll:
Unable to pre-bend, must either live with a large flat area at joining point of metal, or use a press brake to pre-bend prior to rolling. Its primary advantage is that it is inexpensive; however, unless you can live with a large flat area on cylinder, it ends up costing you more in terms of secondary equipment and material handling. It is also very difficult to roll cones.
Initial Pinch Roll:
Has pre-bend capability and material can be introduced horizontally. The disadvantage is that it is difficult to do cone bending, and plate has to be removed from machine and rotated 180 degrees in order to pre-bend trailing edge. This is a serious disadvantage as it requires that the plate is squared completely once again, and it is during the squaring operation that many of the mistakes are made that lead to bad parts. It is also responsible for many shop accidents when the plate is being rotated 180 degrees.
Double Pinch Pyramid:
It can pre-bend both ends of a plate without removing plate from machine. The disadvantage is that it takes six different positioning of the rolls to complete a cylinder. Because of the pyramid design, it cannot pre-bend as close to the edge as an initial pinch or a four-roll machine. It is able to roll cones, but with difficulty.
For the purpose of comparison, I will be comparing against the initial pinch and the double pinch rolls.
The pyramid rolls, while they have a place in the market, are not considered when pre-bending is a prerequisite.
The purpose of this document is to demonstrate that the four-roll has all the desirable features of the other two without new era hats the disadvantages - plus, adds additional advantages the other two do not have.
MG Four-Roll Advantages
Simplicity:
The single biggest advantage that four-roll machines have over the other two machines is simplicity.
In order to obtain a perfectly bent pipe with a three-roll double pinch, it is necessary to do three different operations:?
- It is necessary to pre-bend the leading edge of material. This is done by pinching plate between one of the side rolls and top roll.
You must also lower the opposing side roll to create the proper geometry for the pre-bend. Because of this it is impossible to load and roll material in the horizontal position. It also requires a much larger area in the shop as the material must pass all the way through the machine in its stretch-out condition for the pre-bending so it requires at least equal distance on both sides of the machine.
It is necessary to completely change roll position and move plate back to center of machine and position side roll at correct position to achieve required diameter.?
This sounds difficult because it is. Remember, every release of rolls is an opportunity for misalignment of plate.
To roll a given diameter on a four-roll is extremely simple. You introduce plate into the roll touching it off to the opposing outboard roll for quick and accurate squaring of plate. You then raise lower central pinch roll and the plate is locked into position with no possibility of slipping. After this you back plate up to near tangent point of central rolls, and then raise the left or right outboard roll to the correct position to achieve your diameter and begin rolling. When the back edge gets close, simply release the left outboard roll and bring up the right outboard roll until it touches the plate and finish the pipe in one new era hats pass.
By comparison, a very simple operation. Because the plate is automatically squared and always pinched and not released until the pipe is complete, the net result of this difference is that the four-roll requires 66% less positioning and much less experience on the operator's part.
In most cases, it is difficult to determine the correct position of roll to achieve a given diameter. The operator takes his best conservative guess and moves up from there; however, given the fact that on a three-roll double pinch he must have 6 positioning to achieve a diameter (even a wrong one) it becomes very time consuming with a risk of scraping the material before correct diameter is reached. A four-roll machine, which requires only two positionings, never releases metal and arrives at correct diameter in less than half the time, with much less risk of a scraped piece.
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