Research on Rolling Process of Special-shaped Bodies by Four-roll Plate Rolling Machine

Abstract

The four-roll plate rolling machine, with its four independently controllable rolls, has significant advantages over three-roll plate rolling machines in forming special-shaped components. It can complete the rolling and forming of special-shaped parts such as conical cylinders, variable-curvature cylinders, polygonal special-shaped cylinders, and S-shaped curved plates. With a single clamping, pre-bending of both ends of the plate can be achieved, and the residual straight edge is small. This paper elaborates on the technical points of rolling special-shaped bodies with four-roll plate rolling machines from aspects such as forming mechanism, classification of special-shaped parts, complete process flow, common defects and control measures, and CNC process optimization, providing process reference for the processing of special-shaped shells in industries such as pressure vessels, environmental dust removal, and steel structure equipment.

Keywords
: Four-roll plate rolling machine; Special-shaped body; Roll bending forming; Pre-bending; Conical cylinder; Variable curvature; Springback compensation

I. Introduction

Conventional cylindrical rolling is the most basic process in plate rolling processing, while rolling special-shaped plates, including conical cylinders, variable-diameter transition sections, multi-arc composite parts, polygonal cylinders, etc., is a difficult process in plastic forming of plates. When three-roll plate rolling machines roll special-shaped parts, there are limitations such as easy deviation of the plate, difficulty in pre-bending the ends, and poor forming accuracy of conical cylinders. The four-roll plate rolling machine consists of an upper roll, a lower roll, and two left and right side rolls. The upper and lower rolls clamp the plate, and the two side rolls can be independently lifted and tilted for adjustment. The plate is clamped stably and is not easy to slip or deviate, so it is increasingly widely used in the field of special-shaped part processing. With the popularization of CNC four-roll plate rolling machines, by programmatically controlling the tilt angle of the side rolls, the pressing amount, and the feed speed difference, various special-shaped shells can be stably produced in batches, greatly reducing the number of trial rolling times and improving forming consistency.

II. Forming Mechanism of Special-shaped Bodies by Four-roll Plate Rolling Machine

In ordinary cylindrical rolling, the plate is subjected to uniform force along the width direction, and the bending curvature is consistent everywhere. The core characteristics of special-shaped body rolling are:
Different positions in the transverse direction of the plate require different bending curvatures, and there are differences in feed speed.
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1. Working principle of the four-roll plate rolling machine: the upper roll presses down to provide the bending moment, the lower roll supports the plate, and the left and right side rolls tilt and lift to apply a lateral bending force to the plate; the plate is clamped by the upper and lower rolls, with stable friction to prevent slipping. This is the basis for rolling conical cylinders and variable-diameter parts.
   

2. Cone forming mechanism: the circumference of the large end of the cone is greater than that of the small end. During rolling, the large end of the plate requires a faster feed speed, while the small end requires a slower feed speed. By installing the side rolls at an angle, the lifting amount on the small-end side of the side rolls is greater, creating a feed difference between the two sides of the plate and achieving conical surface forming.
   

3. Variable-curvature special-shaped part forming: change the downward pressure of the side rolls in segments along the length of the plate, control the bending radius segment by segment, and achieve multi-segment arcs, S-shaped surfaces, and multi-sided cylinder forming. The springback amount is calculated separately for each segment, and the over-bending amount is compensated segment by segment.
   

4. Springback effect: after the plate is rolled and unloaded, elastic springback occurs. For special-shaped parts, different positions have different curvatures, so the springback amount is inconsistent. Springback compensation must be calculated in advance based on the material yield strength, plate thickness, and target curvature, and over-bending must be performed during rolling so that the dimensions meet the drawing requirements after springback.
   

3. Classification and Process Characteristics of Special-shaped Workpieces

3.1 Conical Cylinder (Most Common Special-shaped Part)

Conical shells are divided into uniform-taper conical shells and variable-taper transition conical shells, widely used in cyclone dust collectors, hoppers, and pipe reducer sections. Key process points:

1. Plate blanking and development must be accurate, and the large end and small end must be marked; when loading, strictly center the workpiece so that the centerline remains aligned with the roller axis to prevent deviation.
   

2. The side rolls are set at an inclined angle, and the side roll at the small-diameter end is raised to a greater height to create a stroke difference between the large and small ends.
   

3. Use low-speed rolling and gradual forming in multiple passes; avoid excessive reduction in a single pass to prevent wrinkling and twisting of the plate.
   

4. Both ends must be pre-bent to eliminate straight edges at the ends, ensure the butt weld groove fits closely, and reduce the workload of shape correction.
   

3.2 Variable Curvature / Multi-arc Combined Special-shaped Parts

The workpiece radius changes continuously along the generatrix direction, such as bell mouths, S-shaped bent plates, and composite transition shells. Key process points: the advantages of CNC four-roll are prominent; establish process nodes according to workpiece segments, set the side roll reduction for each segment, dynamically adjust roller positions during rolling, and control the bending radius segment by segment; after completing each segment, measure the arc length and curvature in time and correct parameters for the next segment.

3.3 Polygonal Special-shaped Cylinders (Triangular Cylinders, Square Cylinders)

The polygonal cylinder is composed of straight sections + arc corners, and is formed by four-roll segmented rolling; plastic deformation is concentrated at the corners, while the straight sections remain straight. Key process points: pre-bend the corners separately, control the bending angle, and avoid excessive deformation at the corners; form in segments, and after each segment is formed, measure the side length and diagonal to prevent cylinder twisting.

IV. Complete Rolling Process Flow for Special-shaped Bodies

4.1 Pre-process Preparation

1. Drawing process breakdown: extract workpiece thickness, material, dimensions, taper angle, and curvature tolerance; calculate the plate development dimensions, cutting allowance, and springback compensation. For thick plates and high-strength steel, consider the cold bending forming limit and preheat when necessary.
   

2. Plate pretreatment: level the steel plate and remove burrs and oxide scale; mark the plate centerline, large/small end boundaries, and segment control lines.
   

3. Equipment inspection: check roll surface wear, hydraulic system, and synchronization accuracy; for CNC models, input workpiece parameters and pre-generate the rolling program.
   

4.2 End Pre-bending Process

The key to weld seam butt-joint quality for special-shaped parts lies in end pre-bending. A four-roll plate rolling machine can complete pre-bending at both ends in a single setup without turning the plate over. Feed the plate end between the upper and lower rolls; the side rolls rise to apply pressure, causing plastic bending at the end and eliminating the residual straight edge. The pre-bending down-pressing amount is determined by plate thickness and target radius, with springback compensation added. After pre-bending is completed, use a template to check the curvature.

4.3 Multi-pass Roll Bending Forming

For special-shaped parts, one-time large-pressure forming is strictly prohibited; adopt small-pressure multi-pass progressive rolling:

1. Conical shell: Set the side roller tilt, start at low speed, feed the large end of the plate quickly and the small end slowly; after each pass, fine-tune the side roller stroke difference and measure the diameters of the large and small ends at any time.
   

2. Variable-curvature parts: According to segment nodes, dynamically modify the side roller pressing parameters and form segment by segment.
   

3. After each pass, release the upper roller, take out the template to detect the curvature, and correct the next pass's pressing amount based on the measured value.
   

4.4 Closing, Shaping and Inspection

Form until nearly closed, stop rolling, perform joint alignment inspection, and adjust the misalignment; after closing, perform overall roundness/conicity correction to eliminate cylinder distortion and ovality errors. Inspection items: cross-section profile, generatrix straightness, joint gap, misalignment; mark after passing, then cut and remove the workpiece.

4.5 Post-processing

Welding, flaw detection, shaping; for workpieces with large deformation, flame local shaping may be adopted.