Compared with the swing beam shear, the guillotine shear avoids the existence of clearance in structure and can adjust the shear angle, so it has higher efficiency, precision and reliability.
However, when the guillotine shears are used to cut wide and heavy plates or high strength thin plates, there are still some problems such as machine deformation affecting the cutting accuracy.
Most of the existing literature focuses on the influence of shear parameters on shear accuracy, the design and automation transformation of the shear control system, or the simulation of the shear process with discrete points, but few on mechanical property analysis and structure optimization of guillotine shear.
In this article, through the analysis of static and dynamic characteristics of the machine tool, the shearing process of the plate shears is simulated, the continuous shearing data is obtained, and the optimization scheme is given.
2. Static analysis
Taking a 6 × 3200 NC guillotine shear as an example, its structural model is shown in Figure 1.
When working, the backstop device driven by the roller screw adjusts the shearing length, and the pressing cylinder will be compressed by the shearing plate.
After setting the cutting parameters such as blade clearance and shearing angle, the hydraulic cylinders at both ends drive the upper and lower blades to move relative to complete the shearing of the plate.
In the simulation analysis, the transition fillet and threaded hole are ignored, and the simplified three-dimensional model is imported into the finite element analysis software.
The two sides of the upper tool rest are fixed constraints, and the binding contact is set to simulate the welding and thread fixation of rest parts in the upper tool carrier.
Fig. 1 Structural model of 6 × 3200 NC guillotine shear
According to Norsali’s formula:
In the formula:
- σb – Strength limit of the plate to be cut;
- δs – Ductility of the sheet to be cut;
- h – Thickness of the sheet to be cut;
- α – Shear angle;
- x、y、z – The bending force coefficient, the relative value of the side clearance of the cutting edge and the pressing coefficient.
According to formula (1), the vertical shear force P1 = 212.8 kN, the horizontal shear force is 30% of the vertical shear force, and the horizontal shear force P2 = 63.8 kN.
The upper turret is loaded with vertical and horizontal shear forces, which are solved by ANSYS Workbench.
The deformation of each direction at the beginning, middle and end of shearing is compared and analyzed.
From Table 1, the deformation of the upper carriage in Y direction is larger, and compared with Y direction, the deformation in X and Z directions can be ignored.
The starting and ending positions of Y-direction deformation are approximately equal, and far less than the middle position.
The deformation of the upper tool holder presents a concave trend in the process of shearing.
Table 1 Maximum equivalent stress and maximum deformation in X, Y and Z directions of upper tool holder
|Shear position||Maximum equivalent stress/MPa||Maximum deformation in X direction/mm||Maximum deformation in Y direction/mm||Maximum deformation in Z direction/mm|
The maximum equivalent stress in the middle and the maximum deformation in Y direction are shown in Fig. 2 and Fig. 3.
Fig. 2 Maximum equivalent stress of upper tool carrier
Fig. 3 Maximum deformation of upper tool carrier in Y direction
Combined with Table 1 and Figure 2 and 3, it can be seen that the equivalent stress of the upper tool carrier is the largest at the shear start position.
And it is located at the position of the hydraulic cylinder, with the size of 137.7 MPa, which is less than the yield strength of the upper tool rest of 235 MPa.
In the middle position of shearing, the maximum deformation of the upper tool holder in the Y direction is located at the back support plate B, with the size of 1.183 mm.
But the deformation of the blade in the Y direction is 0.346 mm, less than 1 mm, which meets the use requirements.
3. Transient dynamic analysis
In the dynamic simulation, a slider is set at the initial position of the upper blade, which is always subject to the shear force, and a certain speed is given.
The movement of the slider is used to simulate the shearing process of the plate shearer.
Since the slider only transmits shear force, it is set to frictionless contact with the upper blade.
When cutting the sheet metal, the contact between the upper blade and the sheet metal to be cut is shown in Fig. 4, and the contact length (s) is 8:1
Fig. 4 Cutting diagram
In order to simplify the calculation, it is considered that the shear force is evenly distributed in the rectangular area with the length s and the width (t) of the blade, so the slider is set as a cuboid with the length (s) and the width (t), as shown in Figure 5.
Fig. 5 Schematic diagram of slider simulation
By solving the maximum equivalent stress, the maximum deformation in Y direction and Z direction at each position, as shown in Table 2.
The deformation and stress trend of the upper tool holder in the shearing process are shown in Fig. 6.
Table 2 maximum equivalent stress and maximum deformation in Y and Z directions of upper tool carrier under each load step
|Load step||Maximum equivalent stress/MPa||Maximum deformation in Y direction/mm||Maximum deformation in Z direction/mm|
It can be seen from Table 2 and Figure 6 that the deformation of the upper tool holder changes with the change of shear position.
The deformation is large in the middle and small and nearly symmetrical on both sides, which is consistent with the static simulation results.
The maximum deformation of the upper tool holder in Y and Z directions occurs in load step 8, and the deformation is 0.635 mm and 0.478 mm, respectively, which are less than 1 mm.
The maximum equivalent stress occurs in load step 4, which is 159.2 MPa, which is less than the yield strength of the upper tool holder 235 MPa.
Fig. 6 Deformation and maximum equivalent stress of upper tool holder in Y and Z directions
The maximum deformation and the maximum equivalent stress in Y direction and Z direction are shown in Fig. 7, Fig. 8 and Fig. 9 respectively.
Fig. 7 Maximum deformation of upper tool carrier in Y direction
Fig. 8 Maximum deformation of upper tool carrier in Z direction
Fig. 9 Maximum equivalent stress of upper tool carrier
It can be seen from Figures 7, 8 and 9 that the maximum deformation of the upper carriage in Y direction occurs at blade C, and the maximum deformation in Z direction occurs at blade D.
The deformation in Y direction and Z direction is less than 1 mm, which meets the design requirements.
The maximum equivalent stress occurs at the E position of the upper tool carrier under the action of the hydraulic cylinder.
The stress of the blade is small and has good stiffness.
4. Frame modal analysis
In the modal analysis, the four bases of the frame are fixed and constrained.
Block Lanczos method is selected as the mode extraction method, and the expanded mode number is set to 4 to solve the first four natural frequencies of the frame structure.
The four modes of vibration are shown in Figure 10, and the natural frequencies, amplitudes and modal shapes of the four structural modes of vibration are shown in Table 3.
Fig. 10 The four modes of vibration
Table 3 Modal analysis data table of plate shears
|Order||Frequency/Hz||Amplitudes/mm||Mode of vibration|
|1||19.02||1.77||The front panel is bent forward in the X direction|
|2||24.98||0.81||The upper part of the frame bends and swings in Z direction|
|3||28.96||6.09||The lower support beam is bent forward along the X direction|
|4||42.66||1.53||The front panel is bent back and forth in X direction|
According to Figure 10 and Table 3, the vibration deformation of the frame is mainly concentrated in the front panel and the lower support beam, and large vibration and noise will occur.
When the frequency is 19.02 Hz, the amplitude of the front panel is larger, it would affect the shearing process and reduces the shearing accuracy.
The shearing frequency of NC guillotine shear is 9 times/min, which is far less than the first order natural frequency and can meet the requirements of normal work.
At the same time, in the working process, the influence of external vibration sources should be minimized to avoid excessive vibration deformation.
5. Optimization design
Through the above analysis of the static and dynamic characteristics of the shearing process, it can be seen that the deformation of the upper turret presents a concave trend during shearing, which will affect the burr and dimensional accuracy of blanking, and reduce the shearing quality of the shearer.
Now, by adjusting the blade clearance in the cutting parameters, the positioning stiffness of the upper tool carrier is increased, and the cutting quality is improved.
In this article, a kind of dynamic bevel guide type blade clearance adjustment device is designed, as shown in Figure 11.
Fig. 11 Structure diagram of inclined guide rail guillotine shear