Analysis and Countermeasures of Bottlenecks in Wire EDM Machining with Large Angles of Automotive Mould Parts

Wire EDM technology is the application of negative polarity processing principle to electro-erosion processing of metal parts. It can process conductive materials of any material, hardness and density. It has a wide range of processing and occupies an important position in the processing field with broad application prospects.

Wire EDM machine tools are mainly divided into fast wire, medium wire and slow wire. The machining accuracy, cost and efficiency increase in order. According to the precision requirements of the parts, the corresponding wire EDM machine is selected for processing.
The wire EDM machine tool has been upgraded many times, and the machining accuracy and efficiency are gradually improving. Due to the limitation of the design principle of the wire EDM machine tool, the large-angle precision surface or precision hole of many mold parts is still being processed by the wire EDM. Limitations: If forced processing, these large-angle precision surfaces are relatively rough and cannot meet the requirements of use. For example, Japan's DUO64 Makino Slow Wire EDM wire cutting machine can process a high-precision surface with a maximum angle of 3°. After 3°, the roughness of the machined surface cannot be guaranteed; It reaches 30°, but the roughness cannot be guaranteed; it can be seen that the greater the WEDM machining angle, the more the accuracy cannot be guaranteed, and even the parts will have WEDM knife marks, so it is more important to choose a suitable WEDM process .

1. Problem description

Nowadays, the appearance of automobiles is becoming more and more novel, peculiar and beautiful, which makes mold design more difficult. Mold design needs to use many complex mold parts to realize the injection molding and demolding of automobile products, and there are also requirements for the processing of mold parts. As a result, some mold parts with large-angle precision surfaces are difficult to process, such as the processing of 18° T-slots (see Figure 1) for fixed mold sliders. How to realize the WEDM machining of such parts while ensuring the processing quality has become the primary consideration target of the WEDM operators.

2. Reason analysis

When the wire EDM of large-angle surface of mold parts is cut, the upper and lower heads of the wire EDM machine need to be placed at the corresponding angle. When the wire EDM wire does not swing at the angle, it will not be affected by the weight of the wire. Once the angle is swing Later, the electrode wire will be affected by gravity, and the arc will appear larger or smaller (the radian is determined by the tension of the wire). In addition, after the electrode wire swing angle, the chip removal effect of the cutting fluid will be greatly reduced. The above factors directly affect The flatness of the wire-cut EDM surface. In order to minimize the influence of the electrode wire by gravity, most WEDM machine tool manufacturers reduce the influence of gravity by increasing the tension of the electrode wire, that is, the auxiliary roller pressure mechanism is used to increase the tension of the electrode wire, which will increase The tension of the entire wire transport system increases the pressure and friction between the electrode wire and the conductive block, causing serious wear and loosening of the electrode wire. At the same time, the increased tension of the entire wire transport system will increase the working pressure of the bearing. , Resulting in increased rolling friction of the bearing, reducing the service life of the bearing; in addition, due to the swing angle of the electrode wire, it will cause poor chip removal, which will affect the processing efficiency and processing accuracy.
In summary, when wire EDM is cutting large-angle mold parts, try not to use the wire swing angle to process the parts. It is best to place the parts at a corresponding angle so that the large-angle side of the part that needs wire EDM is placed in a Vertical direction, and then use EDM wire cutting for processing, this will not only help chip removal, reduce the tension of the wire transport system, increase the service life of the machine tool, but also more easily ensure the quality of parts processing.

3. Solutions

For small mold parts, angle clamping fixtures for WEDM can be made for part clamping and WEDM, as shown in Figure 2. Since the wire EDM clamping fixture is prone to stress deformation during machining, the first wire cutting track (rough machining) must have a margin of 1mm on one side during machining. After the stress is released, the machining is in place, and then the angle ruler After measuring and confirming that the angle is correct, the clamping parts are processed.
The 30° clamping fixture is used to clamp the guide block as shown in Figure 3. Use glue to bond the bonding surface of the guide block and the clamping fixture. After the bonding is firm, use the pressure plate to compress the clamping fixture. It can be seen from Figure 3 that after the clamping is completed, the holes that need to be cut by EDM have become vertical, and then use the dial indicator to correct the mold parts to ensure that the reference edge of the mold parts and the X axis and Y of the machine tool reference coordinate system The axes are parallel. The X direction is centered according to the width of the part and the zero position is set. The Y direction is set with the reference edge to establish the part coordinate system (that is, the programming coordinate system), and then program and thread the guide hole.
When the parts are large (>5kg), it is not recommended to use angled clamping fixtures for processing. The glue bonding strength is limited, and the parts will be impacted by the cutting fluid during processing, which is easy to cause bonding failure. Therefore, it is not recommended to use angled clamping fixtures for processing large mold parts; large-angle surface electric discharge lines for large mold parts It is recommended to use a sinusoidal magnetic suction table for clamping
The shaft centers of the two cylinders in the sine magnetic suction table are on the same horizontal plane, and the shaft center distance is 100mm, as shown in Figure 5. In this way, it is relatively convenient to calculate the angle of placement. Now specifically introduce the use of the sinusoidal magnetic suction table.
The 18°T-shaped slot of the EDM slide block is composed of two sine magnetic suction tables with a cylindrical axis distance of 100mm, a cylinder R=10mm, and a swing angle of 18°. Combined with the Pythagorean theorem, the gauge block that needs to be cushioned can be calculated The thickness is:
H = 100sin18° = 30.902 mm
After placing the gauge block, lock the sinusoidal magnetic suction table, and then clamp the mold parts. With the aid of the dial indicator, the reference edge of the mold parts is clamped parallel to the machine coordinate axis, and then the magnetic force is turned on through the magnetic knob, so that the mold parts It is adsorbed on the sine magnetic suction table, and then you can set the zero position of the coordinate system of the mold part, and program the wire cutting program to cut the 18° T-slot. The offset selection should refer to the actual wire cutting thickness of the part.