A new design for a problem fixture on a machining center
Keywords: machining center, fixture, design, hydraulic control, defect
In the machining of dual-clutch gearbox valve body workpieces, 9 sets of 0P10 stations with solenoid valve processing lines, actuator processing lines, and valve body processing lines distributed horizontally in the valve body area are used. The hydraulic fixture used is Matching machine tools. However, in mass production of products, equipment defects have a certain impact on the quality of processing, and the design of fixtures is particularly prominent.
Fixture problems during production line operation
The appearance of the hydraulic clamp is shown in Figure 1. A machine tool is equipped with 2 fixtures, each fixture can hold 2 workpieces, each component of the fixture is the same, the workpiece is installed horizontally, manual loading and unloading of the workpiece, and the hydraulic control of the fixture's actuator components. According to the description of the production site personnel, the main problem of the fixture is that the movement order of the actuator is incorrect.
When the workpiece is clamped, the ejection of the clamp-assisted support precedes the clamping action of the platen cylinder, which causes the workpiece to bend and deform after clamping. After processing, the workpiece elastically recovers after being relaxed. After measurement, the flatness of the milling is extremely poor; When the workpiece is being loaded, the clamping deviation often occurs, which causes severe damage to the tool during processing; when the processing is completed, the workpiece is often ejected by the auxiliary support when it is loosened and falls off the fixture. Operators have adopted methods such as replacing the spring in the auxiliary support and moving the auxiliary support cylinder installation position back overall, but the effect is not satisfactory.
Defect analysis of fixture design
Take the actuator processing line fixture as an example for analysis, as shown in Figure 2. The positioning and clamping methods and fixture actuators of the other line processing products are similar to those of the actuator processing line fixture.
1. Analysis of positioning and clamping schemes
(1) Workpiece positioning
The positioning of 3 points on the plane restricts the Z-axis movement, X-axis rotation, and Y-axis rotation of the workpiece; the clamp has 1 limit in the X direction and 2 limits in the Y direction, which limits the X and Y movements of the workpiece And Z axis rotation, there are 1 positioning cylinders in X direction and Y direction respectively, pushing the workpiece to the limit of X direction and Y direction on the fixture. The six degrees of freedom of the workpiece are completely limited.
(2) Workpiece clamping
There is a platen cylinder at each of the three points on the plane for clamping the workpiece. In order to prevent workpiece deformation and reduce vibration during machining, 4 auxiliary supports are provided on the fixture. Due to the horizontal installation of the workpiece, in order to prevent the workpiece from falling during installation, a spring clamping assembly is respectively installed in the x direction and the l direction.
2. Analysis of hydraulic control scheme
This fixture belongs to the OP10 station. As the first process of machining, the workpiece is positioned using a blank surface, which is different from the positioning of the subsequent process (the latter process often uses one side and two pins for positioning). Usually, a positioning cylinder is used on the OP10 clamp. The positioning of the workpiece pushed to the limit position is more critical. The positioning sequence of the positioning cylinder and the clamping platen cylinder has the special characteristics of the OP10 station fixture: When performing workpiece clamping, first perform the movement of the platen cylinder. After the workpiece is clamped by the pressure plate, the working pressure of the pressure plate cylinder is relieved to a low pressure, and the positioning cylinder is pushed out to position the workpiece. After the positioning cylinder is finished, the pressure plate cylinder is regained the working pressure required for clamping the workpiece. The purpose of the sequence is to prevent the positioning cylinder from being easily deformed due to the movement of the positioning cylinder. The layout of the hydraulic pipeline components on the specific back of the clamp is shown in Figure 3. The fixture uses 3 fluid channels, which are called Lu, Spa, and LDe.
The Lu channel is used on this fixture to monitor whether the clamping action of the platen cylinder is completed. When the workpiece is clamped, the Spa will enter the oil and Loe will return to the oil; when the workpiece is relaxed, the Loe will enter the oil and the Spa will return to the oil. The passage of hydraulic oil in and out of the channel Spa and Loe is controlled by a three-position four-way solenoid valve on the fluid panel of the machine tool fixture. According to the pipeline, the actuator and the fixture fluid panel of the machine tool, the hydraulic schematic diagram is drawn, as shown in Figure 4.
3. Problems with hydraulic control schemes
Combined with the analysis of the hydraulic principle diagram, the design of the hydraulic control method of the fixture has the following unreasonable phenomena:
① The action of all actuators on the fixture is controlled by the same set of superimposed hydraulic valves. The pipeline leading from the hydraulic valve is connected to the Spa and Loe channels of the fixture. This design method makes it impossible for the pressure plate cylinder and the positioning cylinder to achieve the order when the workpiece is clamped. The sequence of operations cannot complete the switching process of first releasing pressure and then increasing pressure after the platen cylinder is finished. The positioning cylinder and the platen cylinder move at the same time, which can easily cause workpiece deformation and positioning failure, and even the workpiece is directly ejected by the positioning cylinder and the workpiece is clamped out of the workpiece, which causes damage to the flat milling cutter during processing.
② The action of clamp-assisted support is usually designed after the workpiece is fully positioned and clamped. The fixture attempts to limit the oil input to the auxiliary support through a one-way throttle valve, delaying its movement, and making the ejection of the auxiliary support slower than the action of the platen cylinder and the positioning cylinder. But in fact, unless the throttle valve is completely closed, as long as it is slightly opened, the oil will enter the auxiliary support cylinder, and because the volume of the oil chamber inside the auxiliary support cylinder is small, it can be ejected with a small amount of hydraulic oil. This design results in auxiliary support when clamping the workpiece
Prior to the action of the pressure plate cylinder, the workpiece was ejected by the auxiliary support, which could not be positioned correctly in the plane, and was deformed under the opposite force of the auxiliary support and the pressure plate, which affected the processing quality.
③ When the workpiece is relaxed, because the actuator uses the same fluid channel Spa to return the oil, the positioning cylinder retracts, the platen cylinder releases the workpiece and the auxiliary support returns at the same time. It is easy for the return oil of the platen cylinder and the positioning cylinder to return in short time. A back pressure is formed in the pipeline of the auxiliary support cylinder, which causes the auxiliary support to rush forward, and the workpiece is ejected by the auxiliary support and dropped from the fixture.
④ Since the same set of hydraulic valves are used for control, the working pressure of the positioning cylinder is equal to the working pressure of the platen cylinder for workpiece clamping. From a design perspective, if the pushing force of the workpiece positioning is too large, it will easily cause the workpiece to deform and affect the processing quality. Therefore, the control pressure of the two actuators needs to be controlled separately.
⑤ The pressure reducing valve on the auxiliary support pipeline should be cancelled. The structure of the auxiliary support is shown in Figure 5. The working process is as follows: the hydraulic rod pushes the piston rod to move, the spring l inside the auxiliary support cylinder is compressed, and at the same time, the support rod moves forward with the piston rod driven by the spring 2 at the front of the piston rod. After the support rod contacts the workpiece, the piston rod continues to move until the end of the stroke of the cylinder. The spring 2 at the front of the piston rod is compressed. Hydraulic oil cannot directly act on the workpiece. The force on the workpiece is the force generated by the deformation of the spring 2. The hydraulic oil continues to enter the auxiliary support cylinder, and the support rod is firmly locked by the internal tension sleeve to support the weak part of the workpiece and prevent the processing vibration. The higher the pressure of the hydraulic oil entering the auxiliary support, the higher the degree of holding of the auxiliary support rod, and the better the anti-vibration effect. When the hydraulic oil in the auxiliary support is drained, the piston returns to its original position under the action of the deformation recovery of the spring 1, and the support rod returns to the initial position.
Improvement of fixture hydraulic control method
Modification of the hydraulic control method of the fixture: adding hydraulic control valve groups, modifying the hydraulic components of the fixture, rearranging the hydraulic pipeline, and increasing the use of the fluid channel of the fixture. The improved hydraulic principle of the fixture is shown in Figure 6.
After the improvement, the pressure oil circuit of the pressure plate cylinder and the positioning oil cylinder is separated when the workpiece is clamped. The channel Spa pressure oil is used for the action of the pressure plate cylinder and the switching of pressure relief and pressure when the workpiece is clamped. The paths commonly use the increased fluid channel z. Add a tube sequence valve to the pipeline of the auxiliary support cylinder to ensure that the ejection of the auxiliary support is after the workpiece is fully positioned and clamped. The pressure reducing valve is no longer used on the pipeline of the auxiliary support cylinder, the working pressure of the auxiliary support is increased, and the anti-vibration effect is enhanced when the workpiece is processed. A pressure reducing valve is used to reduce the working pressure of the positioning cylinder, so the driving force for positioning the workpiece can be adjusted accordingly.
After the improvement, the fixture realizes the correct action sequence of the actuator when the workpiece is clamped and loosened. The process deviation caused by the bending and deformation of the workpiece, the scrap caused by the workpiece installation deviation, and the drop after the workpiece processing has not occurred. Quality stability is guaranteed. Hope that when facing similar design problems, the above sharing can provide some reference for everyone.