As the requirements of industrial technology become higher and higher, the deformation and fullness of the pipe are also getting higher and higher. From the original sand-filled pipe bending to the current mandrel pipe bending, the fullness of the pipe bending is pursued. Slowly, there are several kinds of mandrels.
Ordinary mandrel:
When the CNC pipe bending machine bends a pipe with a diameter of 6 or 8 mm, due to the small diameter and the relatively large supporting force of the mandrel, the mandrel can be omitted, and the ovality of the bending part is guaranteed to be within 5% of the diameter. When the diameter of the pipe increases, in order to prevent the ovality of the bent pipe from increasing, a mandrel should be used.
When the pipe is pulled forward, due to the action of the mandrel, the material outside the center line of the pipe will be supported at the tangent point, stretched, and hardened to ensure its shape and non-deflation.
The position of the mandrel should be at the bending point, and the position of the tangent point affects the ovality of the pipe and the rebound of the pipe.
If the mandrel is too forward, the material outside the center line of the tube is easily stretched and thinned, the length increases, and the rebound becomes smaller, and the ovality is also small, but there is a goose head phenomenon; sometimes it is accompanied by ripples.
If the mandrel is too backward, the material outside the center line of the tube is not stretched enough, and the tube collapses, the ovality increases, and even wrinkles are generated on the inside.
Therefore, when the bending radius increases, the mandrel should be appropriately forward.
The mandrel should have an advance amount (over the tangent point). There are many factors that affect it, such as: the size of the front R of the mandrel; the size of the bending radius; the gap between the inner diameter of the tube and the mandrel; and so on. The appropriate position of the mandrel should first be roughly estimated based on the experience of bending pipes, and then adjusted through experiments to make it in the right position, and ensure that the tube has no wrinkles, a small goose head, and an ovality within 5% of the outer diameter of the tube.
In order to avoid ripples on the material outside the center line of the tube, in addition to the correct design and adjustment of the mandrel, the mandrel rod should be sufficiently rigid to avoid ripples caused by shaking, and use appropriate lubricants.
In order to avoid scratching the inner wall of the tube, the mandrel should have sufficient rigidity and be carefully deburred and polished; the inner cavity of the tube should also be carefully cleaned and wet sandblasted if necessary.
The radial clearance between the mandrel and the inner diameter of the tube should not be greater than 0.15 mm. If the clearance is too large, wrinkles may occur even if the mandrel is adjusted forward.
Due to the large tolerance of the tube wall thickness, in order to ensure the clearance, the mandrels should be grouped and selected according to the inner diameter of the tube.
When using the mandrel, the material outside the center line of the tube becomes thinner due to stretching, but the thinning amount shall not exceed 10% of the wall thickness. Mild wrinkles are allowed, but shall not exceed 25% of the wall thickness.
Ball mandrel:
In the case of bending thin walls, large diameters, and small bending radii, a ball mandrel should be used. The ball of the ball mandrel can swing in any direction.
The use of a ball mandrel is the same as that of an ordinary mandrel. The function of the ball mandrel is to keep the tube supporting the surroundings of the tube after leaving the mandrel so that the tube does not collapse.
The adjustment of the ball core rod is the same as that of the ordinary core rod, except that the ball part should be placed in the front, and the straight rod part at the back should be adjusted according to the ordinary core rod.
Boost adjustment
When bending, the booster is used to push the die to move actively in the longitudinal direction to offset the resistance during bending. This is a feature of the machine bending.
Using the booster can prevent the tube wall from being excessively thinned during bending, reduce the rebound of the tube, and offset the resistance at the same time, push the tube to complete the bending, reduce the burden on the clamping die, and is beneficial to bending. Generally speaking, the use of the booster can reduce the clamping length and the minimum bending radius.
The speed of the booster can be adjusted by the speed regulating valve installed on the booster cylinder. It should be adjusted slightly greater than the bending speed to obtain a certain side thrust.
The small core rod is really a great force. The increase of the core rod has both advantages and disadvantages, and it should be adopted according to the actual situation.
