1. Influence Of Machining Clearance (Side Clearance)
The size of the machining gap and its consistency directly affect the machining accuracy of EDM. Only by mastering the numerical values of the machining gap and surface roughness of each standard, can the size of the electrode be correctly designed, the amount of shrinkage determined, and the standard conversion during processing determined.
2. Surface Roughness
The roughness of the EDM surface depends on the depth of the discharge pits and the uniformity of their distribution. Only when shallow and evenly distributed discharge pits are generated on the machined surface can the machined surface have a small roughness value.
In order to control the uniformity of the discharge pits, it is necessary to use the equal-energy discharge pulse control technology, that is, to detect the falling edge of the gap voltage breakdown, control the discharge pulse current width to be equal, and use the same pulse energy for processing, so that the surface roughness of the machined surface is microscopic consistent.
3. The Influence Of Machining Inclination
During processing, regardless of the hole or the cavity, the side wall has a slope. The reason for the slope is generally due to the uneven loss of the electrode, except for the technical requirements of the electrode side wall itself or the original slope in the manufacturing process. , and ‘secondary discharge’ and other factors.
(1) The influence of the degree of contamination of the working fluid.
The dirtier the working fluid, the more opportunities for ‘secondary discharge’. At the same time, due to the poor state of the gap, the number of electrode recovery will inevitably increase. Both cases will increase the machining slope.
(2) The influence of electrode loss.
The electrode forms a taper due to wear, and this taper is reflected on the workpiece to form a machining slope.
(3) The influence of processing depth.
As the machining depth increases, the machining slope also increases, but not in a proportional relationship. When the machining depth exceeds a certain value, the size of the upper opening of the workpiece will no longer be enlarged, that is, the machining slope will no longer increase.
(4) The influence of oil flushing or oil extraction.
The effect of oil flushing or oil extraction on the machining inclination is different. When machining with flushing oil, the galvanic corrosion products flow out from the machined surface, which increases the chance of ‘secondary discharge’ and increases the machining slope. In the case of oil pumping, the electro-corrosion products are discharged by the suction pipe, and the clean working fluid enters from the periphery of the electrode, so there is less chance of ‘secondary discharge’ on the machined surface, and the machining slope is also small.
Different machining objects have different requirements for machining inclination. In cavity machining, since it requires a certain draft angle, the requirements for machining inclination are not strict. For straight wall dies, the machining slope is required to be strict. As long as the law affecting the machining slope is mastered, the predetermined requirements can be achieved.
4. Reasons And Rules For Rounding Of Corners
The loss of the sharp corners and edges of the electrode is more serious than the loss of the end face and the side. Therefore, with the loss of the electrode edge, the edge is rounded, and the processed workpiece cannot be cleaned. Moreover, as the machining depth increases, the radius of the rounding of the electrode corners increases. But beyond a certain processing depth, its increasing trend gradually slows down, and finally stays at a certain maximum value.
In addition to the loss of the electrode, the reason for the rounding of the corners is the equidistant nature of the discharge gap. Due to the equidistant discharge of the sharp-edged electrode, the workpiece will inevitably have rounded corners; the sharp point of the concave and sharply corrugated electrode does not have a discharge effect at all, but the workpiece will also be rounded due to the accumulation of chips. Therefore, even if the electrodes are completely free of wear, it is still impossible to obtain complete clearing due to the equidistant nature of the gap discharge. If the rounding radius is required to be small, the discharge gap must be reduced.
In general cavity processing, the requirements for clear corners are often not very strict. However, the processing die often requires clearing and cornering, which can be achieved by increasing the penetration depth of the electrode.
The accuracy of EDM is mainly reflected in the machining gap △, the machining inclination tga or the inclination angle a, the corner rounding radius R, and the surface roughness.
The machining gap △ can be expressed by the following formula:
In the formula, δ is the unilateral initial discharge gap; a is the unilateral discharge erosion amount; d is the unilateral loss of the electrode.
The machining inclination tga is the difference between the maximum machining size of the upper part of the workpiece and the minimum machining size of the lower part of the workpiece, divided by the distance h between the measurement surfaces, which can be expressed by the following formula:
Or in terms of slope angle a:
△max: The maximum machining size of the upper part of the workpiece on the measuring surface
△min: The minimum machining dimension of the lower part of the workpiece on the measuring surface
α: Slope angle h: Distance between upper and lower measuring surfaces
The corner rounding radius R indicates the degree of sharp corners that appear in EDM. This is an important indicator for workpieces with sharp corners and fluted edges.
During EDM, there is a certain discharge gap between the electrode and the workpiece. If the discharge gap remains unchanged during the machining process, the discharge gap can be compensated by correcting the size of the electrode to obtain higher machining accuracy. However, the size of the discharge gap actually varies, which affects the machining accuracy.