Initial Pinch Sheet Roll Bending Machines
One of the most common plate rolling configurations, the initial-pinch system entails introducing the plate to the rolls so that roll A and B grip the part, while roll C performs the first prebend. Next, you turn the material 180 degrees and insert the plate to prebend the other edge. Then, you use all three rolls to close the cylinder.
Double Pinch, Three Roll
Three-roll machines generally require prebending the leading end, running the sheet through the machine to prebend the trailing end, and then switching roll rotation direction to roll the cylinder body. In this setup, you first lower roll C and raise roll B, then introduce the plate between A and B. Once the plate is gripped, you raise roll C to form the first prebend. Next, you pass the roll through the machine so that the plate is gripped between rolls A and C, then raise roll B to perform the second prebend. Finally, you position the rolls in a standard pyramid configuration and roll the cylinder. Smaller machines can be mechanical, but most are hydraulic and include drop end yokes for easy extraction of the workpiece. When a cylinder is completely rolled, it is extracted from the top roll. Machines generally are equipped with some type of top roll release mechanism that allows for the cylinder’s extraction. This extraction is accomplished with the help of a forward-tilting or forward-releasing top roll or a removable end yoke.
|Double Pinch, Four Roll|
The four-roll styles have an additional roller underneath the top roll, which constantly pinches the plate during rolling. Four-roll plate rolls have a slight advantage in cycle time because they permit prebending of the leading edge, rolling the cylinder body, and finishing off the trailing prebend, all while rolling in the same direction. These machines are known for their speed, mainly because the workpiece doesn’t have to be removed before rolling to perform the second prebend.
Double-pinch rolls can prebend both plate ends without removal, as is required with single-pinch rolls. You introduce the plate, and the machine grips it between rolls D and C. Roll A then moves up for the first prebend. The system then forms the cylinder, after which roll B raises up to perform the second prebend. When a cylinder is completely rolled, it is extracted from the top roll. Machines generally are equipped with some type of top roll release mechanism that allows for the cylinder’s extraction. This extraction is accomplished with the help of a forward-tilting or forward-releasing top roll or a removable end yoke.
Four-roll plate rolls generally are the only equipment with NCs or CNCs because the fourth roll provides constant pinching action, minimizing the chance for slippage. Automatic controls use an encoder to track movement of the plate through the machine. If the plate slips, the bending roll movements will be out of synch with the rolling movement.
What is the arrangement provided for balancing of top roll when swing arm is opened?
Top roll is counter balanced automatically once the drop end is opened without any jacks or manual arrangement.
What is done to compensate for deflection of rolls during pre-bending and rolling?
All three rolls are crowned (cambering) for compensating deflection.
Is there any method to know the position of bottom rolls?
DRO (Digital Readout) is housed on the operating console which gives the position of the bottom rolls with respect to the top roll.
What is the Capacity for cone bending ?
Cones can be bent up to the edge-bending capacity of the machine. The bottom rolls can be inclined for the required cone angle.
Is the main drive reversible?
Forward and reverse to the main drive is provided. The bottom rolls can be moved up and down individually.
Where are the main machine controls ?
Operating console houses push buttons for all the operations of the machine. The Console is movable so that the operator can keep it at any desired location.
Can we bend hot plates on the machine to get higher thickness capacity ?
Although the standard machine is not meant for hot rolling, occasional hot rolling can be done (a few jobs per month) provided that suitable precautions are taken.
Some Plates have High Yield Point. The machines are rated for 26 kg/mm2. How to find the equivalent thickness ?
Divide the Actual YP by 26. Find square root of this number. Multiply this value by actual plate thickness to get effective thickness.
Cone rolling devices, which permit you to roll a conical shape, are standard on some machines. Lateral material supports and overhead supports are also optional, but less frequently requested. Overhead supports prevent light materials from collapsing when rolled to large diameters. A side support can also assist in preventing light materials from recurving toward the floor if the radius is very large.
You often will see capacities for both prebending and rolling for any plate rolling machine. Prebending is performed on the plate roll at the leading and trailing edges of the sheet and eventually the seam. A sheet cannot physically be bent right to the edge, and thus what remains is referred to as the unbent flat. The minimum flat you canexpect is 1.5 times the material thickness and often 2.5 to 3.5 times the material thickness for heavier plate. If the prebending is done correctly, the seam should come together nicely.
It is the prebending operation, in attempting to minimize the unbent flat, which takes the most power. That’s why prebending ratings are lower than rolling capacities for any given machine. You must be mindful when reviewing machine capacities that the maximum rolling capacity is usually expressed with the basic requirement of multiple rolling passes and very long unbent flats. You also must note the material thickness and width and equipment characteristics such as cylinder diameter, machine type, yield, and diameter of rolls. Operator proficiency also should be taken into consideration.
Automated controls are recommended for high-volume cylinder or shell production and to roll complex shapes that are not easily reproduced using standard manual controls. Multiple bends, variable-radius bends, and ovals are common examples of these complex shapes.
Some machines have extended roll shafts that protrude through the machine frame. Section or pipe dies can be fitted on these stub shafts, but it is not practical to roll angle iron on a plate roll. Angle tends to twist when rolled, and plate rolls do not have outboard, adjustable, lateral material guides to prevent this twist. You should consider using a section rolling machine or angle roll for that type of bending. In general, section dies on plate rolls are good for bending flat bar the hard way, rods, or small pipe.
Compar?s?on Of 3 & 4 Rolls Mach?nes (+ Advantages, – D?sadvantages)
3-Rolls Double Pinch Machines
+ Additional open height allows you to roll smaller channel,beam or bar in the roll.
+ Able to bump form easily.
+/- Ability to prebend material on both sides of machine without removing plate.However prebending on a 3 roll double pinch requires more skill than either the initial pinch or 4 roll double pinch.
– Machine requires more shop floor space when rolling larger diameter cylinders.During the prebending process the material will be hanging all the way out one side and then the entire sheet will need to be fed through the machine to prebend on the other end.
– Difficult to square plate without having a roll to press against and will need to be re-squared after rotating the plate through to the other end for the 2nd prebend.
– Difficult to add side supports feed tables or set the machine into a pit for a lowerplate entry height because plate is fed into the machine at an angle.
– Requires the highest level of skill to operate since there is no definite pinch to hold the plate from shifting during the rolling process.
4-Rolls Double Pinch Machines
+ Easiest machine to operate since the material is clamped during the squaring process and held during the entire rolling process.
+ Able to prebend roll and post bend the entire piece without releasing the pinch.This allows you to prebend and roll the entire cylinder in a single pass.
+ Because the plate is clamped through the entire bending cycle, a rotary encoder can be utilized to indicate how much plate has been fed through the machine,allowing it to be fitted with an optional CNC Control.
+ Material is fed into the machine parallel to the floor,easily fitted with feed tables,side supports or placed into a pit for ease of operation.
+ Requires less floor space because the part is rolled up and over right after the first prebend.
+ Machine can be fed from either direction.
– Higher initial cost than the initial pinch or double pinch three roll.
– Harder to bump bend on the machine since the pinch rolls are directly above and below each other.
Influence of Material Characeteristics On selection Of Machine:-
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 springback can only be determined experimentally. Regardless, some basic factors usually apply:
Thinner sheet—more springback
Larger radius—more springback
Smaller radius-to-thickness ratio—less springback
Bending sheet in fewer steps (when rolls move to produce a different radius)—less springback
High yield strength—more springback
High elastic modulus—less springback
Work-hardening factors into the equation as well, especially when rolling material like stainless steel. When a sheet of this material is rolled, its yield strength increases proportionally to the number of bending steps, until the yield strength reaches a value close to 360 MPa (50,000 PSI). This is why such work-hardening (or self-hardening) phenomena should be taken into account when evaluating the rolling capacity of a particular machine.
The material’s chemical composition influences yield strength and hardness. A brief look at a sheet’s certificate can tell a fabricator a lot about how the metal will behave in the plate roll. Key elements to observe include carbon (C), nickel (Ni), chromium (Cr), manganese (Mn), molybdenum (Mo), and silicon (Si). More carbon makes steel harder. So does more chromium, the alloying element in stainless steel. Both manganese and molybdenum can increase hardenability. And so can silicon, which can also decrease elongation characteristics.
Changing the amount of each element alters a sheet’s rolling characteristics, as well as rolling capacity considerations. An ASTM 304 stainless steel the identical size and thickness of a carbon sheet obviously will roll very differently. But even for two sheets of 304 stainless with certificates showing identical chemical makeup, the rolling results still may be different, thanks to different heat-treatment processes.
Explain the hourglass and barreling effects in rolling. How do they occur, and how can they be prevented?
The goal is to achieve a parallel line of pressure. This is easier said than done, though, when you consider the nature of the plate roll. Like a press brake, a plate roll is more rigid near the ends—near the rigid machine frame—than it is in the center. That’s why forming pressure can cause the rolls to deflect more in the middle. So like press brakes, plate rolls employ crowning. In most common applications, rolls have slightly thicker diameters in the middle.
A plate roll may have a level of crowning that will accommodate the majority of workpieces it will process to ensure the operation achieves that imaginary parallel line of pressure across the rolls. At least that’s the ideal situation. Of course, plate rolling shops, especially job shops, thrive on offering diverse rolling capabilities. Plenty of jobs may fall outside the norm, and operators need to know how to compensate both for insufficient and excessive crowning.
Insufficient crowning causes a barreling effect, with the ends of the cylinder rolled to a tighter diameter than the center. Excessive crowning produces the opposite: a cylinder with an hourglass shape.
To correct this, you can either increase pinch pressure or shim. To know which solution to use and to what extent, you first need to know your machine’s rolling capacity and the behavior of the metal you’re rolling. Increasing pinch pressure can work, but you must ensure that the material—considering the grade, thickness, workpiece shape, and diameter—isn’t close to the maximum forming capabilities of your machine.
Next, you need to know how the material will react to increased pinch pressure. Increasing pinch pressure works for some materials, but not others. For most stainless steels, you can add more pressure. But when you’re working with soft metals, such as softer aluminum grades, increasing pinch pressure can end up distorting the material. I call it the “pie-doughing” effect. Like thinning out a piece of pie crust, soft material can thin and distort under excess pinch pressure.
Finally, you can move forward with a plan to compensate for incorrect crowning. If your workpiece emerges and you find it has a slight barrel shape, you have two possible scenarios. Either the pinch roll position was not set correctly and too much pressure was initially applied, or there just isn’t enough crowning in the middle of the roll.
With a crowned roll, each incremental change in pressure will change deflection more in the center than on the ends. When you get a slight barrel shape, the center of the roll is deflecting downward too much. Looking head-on, you can visualize that line of roll pressure resembling a slight smile—higher on the sides, lower in the middle.
In this barreling situation, if you didn’t wrinkle the ends of the part (lucky you), try decreasing the roll pressure slightly. Ideally, you can decrease pressure to the point where the middle deflects just enough so that the roll surface is parallel with the edges. This achieves the desired result: again, that parallel line of pressure across the length of the workpiece.
However, if you have significantly insufficient roll crowning for the job, roll pressure changes may not be as effective. With insufficient crowning, a little pinch pressure reduction on the ends does reduce deflection in the middle to some extent, but not enough to eliminate the barreling effect.
If the workpiece emerges in a slight hourglass shape, you have excessive crowning for the job. If you visualize a line of pressure across the roll, this time it would look like a slight frown—higher in the center, lower on the sides. You can try increasing the roll pinch pressure—only, of course, if your machine can handle the increased tonnage. This increases the deflection in the center, pushing the center downward. The slight frown turns into a straight line of pressure across the workpiece.
If altering roll pressure doesn’t work, you need to resort to shimming. Placed in the middle, the shims themselves—which can be pieces of cardboard, plastic, or even thin-gauge sheet metal—effectively add crowning, ideally to the point where deflection in the center matches the deflection on the ends. Placed on either side of the workpiece, the shims can compensate for excessive crowning.
Exact shim placement can vary from job to job.
4- ROLLS Cnc Sheet Bending Rolls
4- Rolls Cnc Plate Bending Machines