Because of its good flexibility, the horizontal machining center is easy to form a flexible machining automatic line, and it has become the preferred equipment mother machine for cnc manufacturers. Among them, the machining center with double pallet exchange device is the most widely used, and its dual worktable saves time for workpiece clamping and disassembly.
Since the pallet exchange device needs to be lifted before it rotates, the traditional design in the past is to use the oil cylinder without self-locking function to directly lift the lifting device, and only in the slow running state can it ensure safety and stability, otherwise it will The impact is great and unreliable.
For this reason, in the development of high-speed machining centers, in order to shorten the lifting time of the pallet exchange device, a rapid lifting mechanism was designed, which has strong bearing capacity, small impact force, simple structure, and truly realizes rapid lifting.
The fast lifting mechanism of the double pallet exchange device of the high-speed machining center uses the oil cylinder as the power to drive the moving cam, which accurately realizes the fast, stable, impact-free and low noise of the lifting device in the whole lifting operation. The contour curve of the cam is composed of two straight lines, one oblique line, and four parabolic curves. The contour curve of the cam in the acceleration and deceleration interval is designed as a parabola, and the parabolic acceleration is constant, which minimizes the impact when the lifting mechanism changes speed.
In the machining process of lifting the pallet, in order to reduce the working time, the lifting mechanism should be operated quickly. When lifting, in order to reduce the impact force and noise generated by the lifting device at the moment of activation, the moment of contact with the pallet, and the moment when the stroke reaches the end, it is necessary to reduce the operating speed of these moments.
Therefore, the speed of the lifting mechanism cannot be constant during the entire movement process, but changes. At the same time, considering that the higher the rate of change (acceleration), the greater the impact force. It is necessary to design a characteristic that can adapt to speed changes and has good performance. Curve, so as to achieve the lifting mechanism not only speed change but also smooth and fast movement.
This structure uses a moving cam as the main component, and the contour curve of the cam is formed by two sections of straight lines, one section of oblique lines, and four sections of parabola. Using the curved surface structure of the moving cam to achieve continuous lifting, from zero speed-acceleration-uniform speed-deceleration-uniform speed-acceleration-uniform speed-deceleration-zero speed. The cam profile in the speed change interval adopts a parabola. The acceleration of the parabola is constant, which greatly reduces the rate of speed change, reduces the impact, and makes the movement stable and reliable.
The mechanical structure of the quick-lift mechanism is shown in Figure 1. The mechanism is mainly composed of four parts: drive cylinder; move cam; support roller; push rod.
The specific movement process of the lifting mechanism is: the driving cylinder 1 pushes the moving cam 2 to move forward, and the working contour curve of the moving cam pushes the support roller 3 to move vertically, and the support roller drives the push rod 4 to move vertically through the pin. It shows that the moving cam has moved to the end position and the push rod has reached the highest point.
Work process analysis
The pallet exchange device of the high-speed machining center must lift the pallet quickly before turning. The weight of the two pallets and the fixture on the pallet is about 4 tons, and the pallet exchange device itself weighs about 1 ton.
The whole lifting process is as follows: The lifting device accelerates the lifting and quickly approaches the pallet—slowly touches the pallet and lifts the pallet—accelerates the lifting of the pallet—closes to the end point and decelerates—and enters the load bearing state. The lift stroke is 50mm and the required time is 0.8 seconds.
Distribution of motion trajectories
The first section has a vertical stroke of 0mm and a horizontal stroke of 5mm, which is composed of a straight line.
The second section has a vertical stroke of 12.5mm and a horizontal stroke of 48mm. It is composed of two parabolic sections and is in contact with the rising straight line of the next section. The putter first accelerates by 5.48m and then decelerates by 7.02m. The lifting device has approached the pallet and latched.
The third section has a vertical stroke of 5mm and a horizontal stroke of 80mm. It is composed of a straight line and moves at a slow and uniform speed. The pallet exchange device is slowly contacted with the pallet (to complete the latch) and is ready to lift the pallet.
The fourth section has a vertical stroke of 32.5mm and a horizontal stroke of 112mm. It is composed of two parabolic sections and is in close contact with the next horizontal line. Accelerate and lift the pallet by 18.288mm and then decelerate by 14.212mm to reach the end point.