How to Master Rolling Machine in Short Time

Through the rotating roller, the method of bending the sheet under the action and friction of the roller is called rolling. In production, the most commonly used is the three-roller bending machine.

Basic principles of rolling machine

The basic principle of rolling is shown in the figure below. If the blank is placed on the lower roller at rest, its lower surface is in contact with the highest points b and c of the lower roller, and the upper surface is in contact with the lowest point a of the upper roller. At this time, the vertical distance between the upper and lower rollers is exactly equal to the material thickness. When the lower roller does not move, the upper roller descends, or the upper roller does not move, and the lower roller rises, the distance is less than the material thickness. If the two rollers are continuously rolled, the blank will be smooth in all the rolling ranges. Because the two ends of the blank cannot be rolled, they are still straight. When forming parts, we must try to eliminate them.

Basic principles of rolling machine

The curvature of the blank after rolling depends on the relative position of the roller shaft, the thickness of the sheet, and the mechanical properties. As shown in the figure below, the relationship between them can be approximately expressed by the following formula:

The relative distances H and B between the rollers are adjustable to meet the needs of the curvature of the part. Since it is more convenient to change H than to change B, you generally get different curvatures by changing H. Since it is difficult to calculate and determine the rebound amount of the sheet material in advance, the above relational expression cannot accurately mark the required H value, which is only for reference during the initial roll. In actual production, the test method is mostly adopted, that is, after the position of the upper roller is roughly adjusted based on experience, the paper is gradually tested until the required curvature is reached.

Operation of rolling machine

The steps to operate the three-axis rolling machine are as follows: first, raise the upper roller and adjust the distance between the lower rollers according to the thickness of the blank. The distance between the lower rollers should be as small as possible when the bending force of the upper roller is permitted. Generally, it is reasonably fixed according to the thickness of the blank. When the thickness is 4mm, the spacing is 90~100mm, and when the thickness is 4~6mm, the spacing is 110~120mm. Place the blank on the lower roller, cover the two lower rollers, and then lower the upper roller according to the bending radius requirements, and bend the blank locally, and then turn on the roller bed to rotate the roller, and the blank is automatically sent to bend and form. Lift, go up the rollers, and finally remove the parts.

On the symmetrical three-axis rolling machine, by changing the mutual position of the three rollers, four typical parts of equal curvature simple shape, variable curvature simple shape, equal curvature cone, and variable curvature cone can be rolled, as shown in the following figure show. When bending, one-time forming should be avoided as much as possible to prevent excessive bending. It will cause difficulties in repeated operations. After each bending, the lowering distance of the upper roller is generally about 5~10mm. The main points of various shapes of roll bending operations are as follows.

⒈Rolling of equal-curved cylindrical parts

When rolling cylindrical (cylindrical) parts with equal curvature, it can be achieved as long as the upper roller does not move up and down during the bending process, and the three rollers are parallel to each other. The curvature needs to go through several trial rolls from small to ground before finally reaching the requirements. It is worth noting that the blank must be placed upright when it is fed, otherwise, the rolled-out parts will be distorted, as shown in figure (b). It is best to draw a reference line when bending. When bending, make the reference line coincide with the axis of the upper roller before starting the bending, as shown in figure (a). This is particularly important for the bending of large thick sheets. Because the repair of this kind of parts afterward is not only large but also quite difficult.

Rolling of simple parts with equal curvature

⒉Rolling of variable-curvature simple parts

During the rolling process, the three rollers remain parallel to each other, and the up and down positions of the upper rollers can be changed at any time to roll out parts with varying degrees of curvature. For the cylindrical part shown in the rolling diagram, R1>R2>R3>R4>Rs in the figure. The method used in the production is to approximate this part as consisting of several cylindrical shapes with different radii R, press The radius R is divided into sections, which are rolled successively according to the bending radius from large to small. The steps of the whole operation are as follows.

Rolling of variable-curvature cylindrical parts

The process I: Adjust the position of the upper roller with R1, and roll the blank from end a to end f, so that the bending radius of section ef meets the requirements.

The process Ⅱ: Adjust the lower roller with R2, roll from end a to e, so that the bending radius of section de meets the requirements. When the upper roller approaches point e, it rises slowly and moderately to make a smooth transition to prevent edges and corners from appearing between R1 and R2.

From a to d, from a to c, from a to b to complete the other process III to process V.

For mass production, to improve efficiency, after the procedures of the entire batch of workpieces are completed, the subsequent procedures are carried out. It is best to inspect each part of each process according to the template or mold tire, so as not to affect the subsequent process.

⒊Rolling of tapered parts Theoretically speaking, during the bending process, the two lower roller shafts are kept parallel, and the upper roller shaft is inclined and does not move up and down so that the tapered parts with equal curvature can be rolled out. The two lower roller shafts are kept parallel, and the upper roller shaft is inclined and moved up and down to roll out tapered parts with varying degrees of curvature. It is necessary to make the two ends of the blank feed between the rollers at different speeds to roll out the conical parts with equal or variable curvature that meets the requirements. This is because the curvature of the two ends of this kind of part is different, and the unfolding length is also different. Therefore, when bending, it is required to have different bending speeds at both ends. The speed at the end with the larger curvature should be slower, and the speed at the end with the smaller curvature should be faster. Since the sheet material is subjected to the rolling pressure of three rollers at the same time during the bending, and the rollers are generally cylindrical, it is impossible to obtain several different speeds at the same time. To solve this problem, the blank is required to be in the bending direction Divide into several areas, carry out segmental bending.

The commonly used methods of rolling tapered parts in production mainly include the rectangular feeding method, partitioned rolling method and rotary feeding method, smallmouth deceleration method, and so on. The figure below shows the rectangular feed roll bending method for tapered parts. During operation: First, feed the material according to the AEFD rectangular center line OH shown in Figure (b) and roll out the cylindrical shape on both sides, so that the middle section rolls out of the straightness of the bus bar. At this time, the four corners are expanded, especially the two places A and D. To highlight, as shown in figure (c). Then roll both sides with AB and CD positioning and feeding, so that the two sides are rolled in, and the straightness of the generatrix is rolled out so that the tapered part is rolled out, as shown in Figure (d). In essence, it is rolled in three areas. When rolling this kind of part, the blank should be placed at the same position as the length of the roller. If it moves left and right, the curvature of the rolled part will not meet the requirements.

Rectangular feed rolling of tapered parts

The figure below shows the zone rolling method of tapered parts. Operation: First, the slab of the rolling cone is divided into sections as shown in the figure. When rolling, first align the upper roller to the 5-5' line to bend until the big end reaches 4; then roll-up. Align the wheel with the 4-4' line to roll, until the big end reaches 3, and finally follow the above steps to complete the roll bend in each zone.

Partition rolling of tapered parts

The purpose of the aforementioned segmentation is to reduce the difference in the length of the curve at both ends of the segment so that the tapered part can be rolled similar to a cylindrical part, and then the blank is rotated between each part to compensate for the speed difference between the two ends to ensure rolling out. The accuracy of the part. The practice has proved that the smaller the area, that is, the more times the blank rotates during rolling, the better the quality, but it is not necessary to divide it too much. It should be determined according to the size of the part and the size of the taper.

⒋The figure below shows the device for rolling a conical surface by the rotary feeding method. To roll the fan-shaped blank material into a conical surface, the blank must be rotated and fed around 0 o'clock, and the centerline of the side rollers must be adjusted to tilt it. For this reason, in the T-shaped groove of the additional worktable in front of the plate bending machine, a guide wheel arranged in an arc shape is installed to force the fan-shaped material to rotate around the O point. The function of the end guide wheel is to make the end part of the material detach from the front guide wheel and still be able to rotate and feed in and roll into a cone.

Schematic diagram of rotary feeding device

The figure below shows the device for rolling a conical surface with a small mouth deceleration method. Adjust the upper roll to an inclined position, and add a deceleration device at the smallmouth end to increase the feed resistance of the smallmouth end of the blank, so that the feed speed of the smallmouth is reduced, and the fan-shaped blank rotates and rolls while feeding.

Schematic diagram of small mouth deceleration device

⒌The rolling of parts with a small radius of curvature affects the parts with a relatively small radius of section curvature, and sometimes it cannot be completely roll-formed on a three-axis rolling machine. This kind of part generally requires two processes to bend, as shown in the figure. First, roll out the required curvature on the three-axis rolling bed to make the two sides meet the requirements, and then use the bending die to bend the middle curvature on the press brake to make it finally meet the requirements.

Rolling of parts with small radius of curvature

⒍The rolling spiral staircase side plate of the spiral staircase side plate is a part of the cylindrical shape, and its rolling method is the same as that of the cylinder, but the angle between the placement of the roll on the plate and the roll of the plate before crimping should be the spiral of the spiral staircase. The angle of the rise and the angle of placement during rolling can be measured with a model. The model angle β≈180°-a°, as shown in the figure.

1-Rotating ladder side plate

2-Tilt angle measurement model

When rolling, depending on the length of the side plate of the spiral ladder and the specific conditions of the plate rolling machine, it can be carried out in a single block H or multiple blocks at the same time. The helix angle a is calculated according to a=arctan H/2πr, and the meaning of each symbol in the formula is shown in the figure.

Precautions for the operation of the plate rolling machine

When operating a three-axis bending machine, the following points should be noted.

⒈If the two lower rollers of the rolling bed are the driving shafts, the bite force between the rollers and the blank is small, and the blank is easy to slip and not move, so the curvature of one roll cannot be too large. If the part has a large curvature, it must be repeatedly rolled many times, each time the upper roller is lowered by an appropriate amount, and the curvature of the part is gradually increased. If the three rollers are all driving shafts, a larger curvature can be rolled at one time.

⒉When rolling a thin plate of 4mm or less on an asymmetrical three-axis rolling machine where all three rollers are active shafts, the position of the rollers can be adjusted according to the curvature of the part, and then start to rotate, and then directly send the blank for rolling. , The edge of the blank that is fed first must be higher than the center of the lower roller inside. For this reason, when feeding the material, push it down while pushing it down so that the front end of the blank can be picked up to facilitate biting and rolling.

In batch production, the blank should be placed in the same position of the roller length each time, otherwise, the curvature of the roll will not be the same.

⒊Because the three rollers of the symmetrical three-axis rolling machine are arranged symmetrically, during the rolling, the sheet material cannot be rolled at the entrance or exit end, and there is a straight section with a length approximately equal to half of the center distance of the two lower rollers. This part of the straight line is difficult to eliminate when rounding, so the sheet end should generally be pre-bent, as shown in the following figures (a), (b), due to the use of the mold pre-bending shown in the following figures (a) and (b) A special pre-bending die is required, so in production, it is usually eliminated by adding a backing plate [see Figure (c)], or it can be eliminated by leaving enough margin at both ends of the sheet in advance and cutting after rolling. 

Elimination of bend straight section

Figure (c) shows that the method of adding a pad to eliminate the straight section of rolling is to put a pad on the two lower rollers (to reduce the pressure of the rolling bed, the pad can be rolled in advance), and the thickness of the pad is curved. The blank is thicker, It is best to be about twice as thick, and the length is slightly longer than the bent blank. When rolling, the blank is placed on top of the backing plate, and the backing plate is used to exclude straight sections. For parts with large curvature, the straight section should be eliminated before rolling. If eliminated after rolling, the curvature of the part is already large, and the backing plate is added, it is likely to be blocked by the beam and cannot be rolled. For parts with small curvature, the straight section can be eliminated by the method of backing plate before or after rolling.

⒋When rolling, because the roller has a certain pressure on the blank and friction with the surface of the blank, when rolling parts with high surface quality requirements, the surface of the roller and the blank should be cleaned before rolling. For blanks with adhesive tape and other protective surfaces, also pay attention to removing the metal scraps and glue on the paper surface, and tear off the overlapped part of the adhesive tape, otherwise, the surface quality of the parts will be affected.

⒌Roll bending processing is not only used for sheet metal but also profile. The biggest difference between profile rolling and sheet bending is that when profile rolling, rollers need to be designed and manufactured according to the cross-sectional shape of the profile, and the rollers are mounted on the rollers. Rolling is performed by the roller, so every time the same part is rolled, it is necessary to replace the secondary roller. In the process of rolling and bending, the profile is prone to deformations such as distortion and twisting of the cross-sectional shape, and the amount of repair afterward is large. Therefore, it is generally used in small-scale production or completion of auxiliary processes. In batch production, in addition to simple or low-demand parts that are formed by roll bending, most of the small parts are formed by press bending, and the large parts are formed by stretch bending.

Hot roll bending

The steel plate can be rolled at room temperature or after heating. It is generally believed that when carbon steel is cold rolled, its plastic deformation should not exceed 5%, that is, the ratio of the difference between the outer circumference and the inner circumference of the rounded plate to the inner circumference should not exceed 5%. It can be expressed as

Hot roll bending is the bending and forming of the material to be processed after heating. As the heating temperature increases, the deformation resistance of the metal material will decrease and the plasticity will increase. Therefore, it is beneficial to the processing of metal materials that are difficult to deform and make at room temperature. And to improve the scope of use of the equipment. In production and processing, when the processing capacity of the rolling machine is insufficient or the degree of deformation of the processed material is too large, hot rolling can be used.

⒈The heating temperature of hot roll bending is shown in the table for the heating temperature of hot roll bending of commonly used materials.

Material designation	Thermal bending temperature/°C
	heating	termination
Q235A、15、20	900-1050	≥700
15g、20g、22g	900-1050	≥700
16Mn(R)、15MnV(R)	900-1050	≥750
18MnMoNb、15MnVN	900-1050	≥750
OCr13、1Cr13	1000-1100	≥850
1Cr18Ni9Ti、12Cr1MoV	950-1100	≥850
H62、H68	600-700	≥400
1060（L2）、5AO2(LF2)、3A21(LF21)	350-450	≥250
titanium	420-560	≥350
Titanium alloy	600-840	≥500

⒉Precautions for hot roll bending Although the basic principle of hot roll bending is the same as that of cold roll bending, after all, the metal material of hot roll bending is carried out under heating. Therefore, special attention should be paid to the following during the hot roll bending operation matter.

●Hot roll bending does not need to consider the occurrence of bending spring back, but the phenomenon of thinning, elongation and indentation during hot roll bending is more pronounced than cold roll bending. Therefore, full attention must be paid to the heating process design and hot roll bending process.

●Due to the existence of the temperature difference between the metal surface and the inside during heating, the expansion degree of the inside and outside of the metal material is uneven, resulting in thermal stress. During the heating process, the transformation time of the metallographic structure is also different. The structure transformation occurs first and then causes stress between the structures. Therefore, for materials with thicker sections, the furnace temperature should be prevented from being too high when entering the furnace. As a result, the heating speed of the billet is too fast, and the thermal expansion is too large to produce stress cracks; for materials that require annealing or quenching + tempering and other heat treatments, they must be carried out separately after hot rolling.

●For the roll bending of a closed cylinder, roll it to the weld just closed. However, to prevent the simple section from being unloaded prematurely due to high temperature, and deformed due to its weight, it is necessary to continue rolling on the bending machine for cooling. When the curvature of the rolled simple section meets the requirements, the downward pressure of the upper roller on the simple section should be released in time to allow the simple section to run over the coiling machine to prevent the thinning of the hot coil from continuing to occur. According to the hardening performance of the material, appropriate forced cooling measures can be taken, such as air blowing, to speed up the cooling rate. During this stage of rolling, the principle of keeping the radius of curvature of the tube section stable is the principle, and the tube section can be removed only when the temperature of the tube section drops to the point where it is difficult to see the red hot color (<500C) on the surface. The placement of the unloaded tube section should also pay attention to the new deformation due to its weight. After hot roll bending, the reasonable placement method of the workpiece is shown in the table.