People’s concern about the electric spark wire cutting plus T-members, includes machining accuracy (size accuracy and shape accuracy) and surface quality (surface roughness, cutting stripe and surface texture change layer) of the processing part. Although many factors affecting the WEDM processing accuracy and surface quality have been discussed in detail in the 3rd chapter, how to deal with it in production practice is still difficult, and often causes a variety of problems due to mishandle of certain process problems.
6.2.1 Several Common Processing Quality Problems in Wire cut EDM using molybdenum wire
1 workpiece deformation, size over tolerance
According to theory, when the geometric accuracy and control accuracy of on-line cutting machine tools can be guaranteed, as long as the operator reasonably arranges the processes, and performs the necessary wire diameter compensation and clearance compensation, it should be able to cut out the precision parts that people need. When the workpiece is seriously deformed and the size is out of tolerance, the operator should carefully check whether the following issues have been correctly considered and dealt with during processing:
(l) The internal stress formed by the heat treatment of the workpiece material and its effect on the precision of the EDM wire cutting with molybdenum wire electrode.
(2) How does the workpiece clamping method, cutting direction and path, etc. help to avoid the influence of internal stress in the material?
(3) When setting compensation 一 Whether or not to pay attention to the influence of the loss of the electric wire loss, ie, the calculated value of the compensated amount in programming, whether the wire diameter is the actually measured value during machining.
(4) Whether the discharge gap size compensated is consistent with the actual one. Because the discharge gap size is related to the selected processing parameters, see Chapter 3 (3-9) of this book:
8 =k0 u ; – k ,t-; ” + k l{旷 4 + Amm
(5) Whether influences on the machining size by the tolerance zone is taken into consideration during programming。
2. Difficulties in matching punch and die
Wire cutting punch and die, usually using wire (molybdenum wire or brass wire) cutting method respectively. However, it is very difficult to fit the male die and the female die during assembly. Even if the clearance between the male die and the female die is large, the gap between the male die and the female die is very difficult to achieve a uniform gap. This phenomenon does not mean that the machining size error is too great, but it ignores the necessary process measures to solve the following problems:
(I) In the process of WEDM using molybdenum cutting wire, the circular arc radius of the lobe is often smaller than the radius of the reentrant, because the minimum R of clearance of the die is constrained by electrode radius and unilateral discharge gap but that of punch can be very made small.
(2) The flexible wire is affected by the discharge force, and the actual position will generally be delayed by the servo feed position. Therefore, when it is processed to the corner, it will cause a clear collapse phenomenon. If the corresponding corner processing technology is not adopted, it is difficult. Make sure that the clean corners of the punch and die are the same size.
(3) How to ensure that the position of the punch and die is the same, and that the upper die can move smoothly along the guide post up and down after assembly.
3. The machined surface roughness is not good enough and is not uniform
With the development of mold technology, the processing quality of the mold face is getting higher and higher. It is generally hoped that the precision of line cutting will increase from the original taxi (0. 0 l – 0.005) mm to the standard ( 0 . 005 -0 In 002) mm, the surface roughness increases from Ra (0.5-2) μm to Ra (O.I-0.5) μm. The surface roughness of WEDM using Molybdenum cutting wire or brass wire is limited by the cutting speed, and the surface quality of the cut can hardly meet the needs of mold manufacturing. In particular, the high-speed wire cutting machine not only has a large surface roughness value, but is also generally noticeable in black and white stripes, and is an urgently required problem for the mold manufacturing industry. In order to obtain a better surface quality, operators should seriously consider:
(1)When using a single cut, it is not advisable to use too large a pulse size (large pulse width, high peak current) for machining in order to pursue cutting speed. However, according to different surface roughness requirements, a reasonable selection of smaller pulse width (narrow pulse width, small peak current) is required to ensure smooth servo feed.
(2)Adopt multiple cutting processes, that is, the first cut does not consider the surface quality wipe to ensure the cutting speed, and the second and third times cutting gradually repair the light. Note that not all high-speed Wire cut EDM machines using molybdenum cutting wire as electrode can effectively perform multiple cuttings to obtain a good surface quality.
(3)Take effective measures to improve the appearance of surface stripes in high-speed wire cutting using molybdenum cutting wire (see Section 6.1, “wire-cut surface stripes”).
4. The service life of the wire cutting die is not stable
It has sometimes been found that the precision of the WEDM (using molybdenum cutting wire) tool and the stampings during the trial die can meet the design requirements. However, after one week of use, the quality of the stampings begins to decline, and the accuracy of the inspection molds is also somewhat poor. This phenomenon occurs generally in the case that the on-line cutting surface roughness value is large. Because the surface of the mold after WEDM has a layer of melted solidified layer (see Chapter 3, Figure 3 – 33), this layer of melted and solidified layer has high hardness and good wear resistance, but it has lower adhesion to the substrate and is not resistant to shock. That is, it is easy to fall off during the stamping process; therefore, changing the mold clearance and surface hardness will affect the service life.
Research found that this layer of molten condensate is very thin, generally not up to 10 times Ra, as long as the surface roughness of a little wire-cutting is increased, it should have little effect on the service life of the mold. For the low speed wire-wound wire EDM machines, which using brass wire instead of molybdenum wire electrode , the surface roughness value of the machined surface is guaranteed to be Ra <0.5μm, and the thickness of the molten solidified layer is only 0.002mm – 0.003mm, which should have little effect on the precision of the die. For the general high-speed wire-cut electric discharge machine using molybdenum cutting wire, the rough surface roughness of the working surface must be Ra5μm or more, that is, the thickness of the melted solidified layer can be more than 0.225mm, and the influence on the progress of the die cannot be ignored. The solution is:
(1) During wire-cutting, leave a little bit of polishing residue and remove the molten solidified layer by other methods.
(2) Using multiple cutting techniques, after the cutting and shaping, repair the light once or twice, so that the surface roughness of the wire-cutting machine can be reached Ra l . 25 μ m or less.
(3) When using a high-speed wire-wound wire EDM for molds with a large gap of several thousand, try to use a slightly smaller gauge, so that the machining quality is controlled at a machining accuracy of 0.01 mm, with a rough surface. Degree Ra :;;;;2 . 5 μ,m within.
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