Source: School of Electromechanical and Automation, Huaqiao University, Quanzhou, Fujian Author: Liu Shi 'an
The NC EDM milling technology is studied, and the machining method of large area curved surface is explored. The machining path is directly generated by the general die design software, and the electrode loss compensation is calculated by the uniform incremental compensation method of machining path.
EDM; Keywords; EDM milling; Electrode compensation
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Electrical discharge machining (EDM) is the main way to process mold cavity, and one of the keys to its processing quality is the manufacture of electrode. Because the discharge gap of rough, medium and finish machining is different and the electrode size is different, it is necessary to manufacture multiple electrodes to finally meet the requirements of machining accuracy. Especially when the processing area of the cavity is large, it is sometimes necessary to use the split electrode processing method to complete the processing of each part of the cavity in turn. Thereby increase that manufacturing cost of the electrode. When machining divided electrodes, there will also be seams on the surface of the cavity, and the repeated positioning accuracy of the electrodes during secondary clamping will also affect the quality of EDM.
With the development of numerical control technology, a new technological method-numerical control EDM milling, which uses simple electrodes to generate complex surfaces, has emerged in mold cavity processing. The key of NC EDM milling is the generation of machining path and the compensation of electrode loss. Many electrical machining scholars at home and abroad have done a lot of in-depth and detailed research on this, such as studying the equal loss layered machining model and establishing a special CAM software for machining path generation based on this model, and studying the accurate detection technology of electrode loss and online electrode compensation [1~4].
CNC EDM milling can be used for corner cutting, narrow groove machining and lateral servo machining, but this paper pays more attention to spatial linear servo feed. The main research content focuses on the machining direction of spatial curve trajectory and spatial surface generation, and explores the NC EDM milling process of cavity surface.
In this paper, the process route of metal cutting machining center is quoted, and the machining path is generated by using the general die machining software UG, and the machining code is compiled into the numerical control instructions of specific machine tools. In the aspect of electrode loss compensation, only the compensation in the Z-axis direction is considered, and a method of electrode loss compensation is proposed, that is, along the electrode machining path, the electrode loss compensation increases evenly according to the trajectory distance.
1 CNC EDM milling technology
The milling technology of machining center is very mature, so numerical control EDM milling technology is introduced. Through research and experiments, it is proved that NC EDM milling can solve machining problems such as contour machining, slotting machining, machining along curved surface, trimming and slag removal, that is to say, the problem of machining path generation in NC EDM milling can be solved by general die machining software.
It is worth noting that EDM milling is not equivalent to metal cutting. Due to the existence of discharge gap and electrode loss, the dimensional accuracy of the cavity will be affected. Therefore, the following problems must be paid attention to when programming NC EDM milling:
(1) machining allowance. The minimum value of this parameter is required to be greater than the discharge gap, and the machining allowance is not zero during superfinishing, and the allowance should be reserved for the next working procedure in the previous working procedure.
(2) Treatment methods. You can choose to generate arc program during contour machining or grooving. When machining along the curved surface, the linear machining mode must be selected, including cutting-in and cutting-out procedures, that is, the program section must be a spatial micro-linear section, which is also conducive to the calculation of electrode loss compensation.
(3) Machining accuracy. The higher the machining accuracy, the higher the approximation degree of the chord to the spatial curve, and the more the spatial micro-straight line segments, the longer the program. In actual machining, you can choose a lower precision for rough machining to reduce the number of program segments.
(4) Residual peak height. This parameter refers to the transverse feed of the tool, and the smaller its value, the smoother the machined surface. This parameter can also be expressed as a percentage of the tool diameter.
(5) electrode size. In this paper, the measured value of electrode diameter is required to be input during each machining programming, so the calculation of electrode loss compensation can only be placed in the Z axis direction.
(6) Compensation of electrical parameters and electrode length. The selection of electrical parameters should refer to the machining allowance, and the positive polarity machining method should be selected in ultra-finishing process, and the discharge trace protrusion should be smoothed by the energy of electrons. The electrode loss compensation value depends on process experience and is related to electrical parameters, electrode material pair and working fluid. The electrode loss compensation value is evenly inserted into the end point of each micro-straight line segment.
The programming route of NC EDM milling (Figure 1) can generate rough, medium and finish machining paths and NC instructions of machine tools by setting parameters according to the above six requirements.
Machining allowance, machining mode, accuracy, residual peak height and actual electrode size.
ingredient
semi-finished product
UG-NX
Tool path compensation software
electrical parameter
Tool length compensation value input
EDM NC milling program
Figure 1 Programming Route of NC EDM Milling
A space curved surface with the word "EDM" is designed by using the mold software UG. In order to reflect the machining ability of CNC EDM, all processes adopt CNC EDM. Ф Ф14 mm electrode rough machining, layered processing is adopted according to excavation, and the transverse feed is 80% of the electrode diameter; The Ф 8 mm and Ф 4 mm end electrodes used for dielectric finishing are machined by vector and along curved surface, and the transverse feed is 8% and 2.5% of the electrode diameter respectively. Figure 2 shows the tool path of medium finishing.
Ф ф8mm covered electrode, E293ф4mm covered electrode, E250
(a) Medium processing (b) Fine processing
Ф 4mm covered electrode, E250 covered electrode Ф 4mm covered electrode, E200
(c) Medium finishing (d) Finishing
Fig. 2 Tool path of EDM medium and finish milling.
In Figure 2d, there is a pink residual area in the lower left corner (the depression with large curvature of the curved surface), which cannot be penetrated by the end knife. It is necessary to use Ф 4 mm finger-shaped R-knife electrode for final finishing, and remove the residual after finishing.
In addition, under the condition of the same machining allowance, it is also required to generate reverse tool path and carry out reverse milling to eliminate the step wave surface caused by electrode loss in the previous process and improve the surface shape accuracy.
2 electrode loss compensation countermeasures
2. 1 Influence of electrode loss
In the process of NC EDM milling, the discharge usually occurs at the sharp corner of the front edge of the electrode tip, with high current density and high discharge concentration, which leads to serious electrode loss. In the initial stage of machining, the amount of workpiece material removal is large; In the last stage of machining, the material removal of the workpiece is the smallest, so the actual machining surface is a "slope", as shown in the surface of Figure 3A. Between surface a and surface b is the untreated area of this process. Obviously, electrode loss affects machining accuracy.
Electrode compensation transition surface c
Ideal machined surface b without electrode loss
Uncompensated machined surface a
H 1 current layer thickness
H2 Next layer thickness
Fig. 3 Reference plane of electrode loss compensation control
2.2 The purpose of electrode loss compensation
On the one hand, it can control the size and shape accuracy of each layer of milling, on the other hand, it can reduce the accumulated burden of milling allowance of the next layer. The electrode loss compensation value should be given according to the principle of no over-compensation, that is, its value should be less than the sum of the machining capacity of this layer and the machining allowance of the next layer.
2.3 Calculation method of electrode loss compensation
When milling along the surface, the machining path is generated in a straight line, and all program segments are spatial micro-straight line segments. Assuming that the electrode loss is evenly distributed along the distance when the machining path is relatively long, and its compensation value is compensated to the end of each spatial straight line along the trajectory, the electrode loss compensation value in the I program section is:
△i=(△/∑Lk) (∑j=0→iLj)
Where: △i is the electrode loss compensation value of the I-th program segment; △ is the expected electrode loss during current layer milling; ∑Lk is the total machining path length of the current layer; ∑j=0→iLj is the machining path length of the electrode in the i-th program segment.
△ value is related to electrical parameters and machining path length, and is mainly used for EDM medium and fine machining; Its value is set to zero during superfinishing.
△i value is used for the electrode loss compensation value in the Z-axis direction of the I-th program segment, which is obtained by off-line compensation calculation method.
3 EDM surface milling process experiment
The process experiment was carried out on RobForm30 three-axis CNC EDM machine. The machining path file is generated by UG software modeling, the machining allowance and electrical parameters generated by expert system are selected, and the NC EDM milling program code is generated by electrode loss compensation.
Table 1 is the processing parameters selected in the experiment. The thickness of workpiece material removed by finishing is 0.0 16mm, and the estimated electrode loss △ is 0.05~0.07mm (experimental value). The actual total length of the machining path is about 45000.00 mm, and according to theoretical calculation, the length per 100mm is 0./kloc-0 ~ 0.16 micron. 18000 programs each compensate 0.0025 ~ 0.0038 micron on average. Therefore, if the calculation is normalized, the cutter electrode will only be compensated in a practical sense after the cutter is machined for a long distance, and the proportion of the program segments with real compensation is very low.
Table 1 EDM milling parameter mm
Electrode compensation of electrical parameters of machining allowance for machining types
rough machining
rough machining
Medium processing
Intermediate finishing
end
superfinish
0.800 E383 0.500
0.400 E373 0.250
0.200 E293 0. 100
0. 150 E250 0.075
0. 134 E220 0.050~0.070
0. 122 E200 0
Note: Electrical parameters are adjusted by RobForm30 EDM machine tool.
During rough machining, electrode compensation depends on the specific situation. First, choose the compensation method for processing, and the compensation value is generally less than the machining allowance. If the electrode loss is large and the fillet of the electrode end face is too large, the electrode should be replaced at this time, and the Z axis should be zeroed again before machining. In superfinishing, only forward and backward machining tool paths need to be generated, and the surface needs to be polished back and forth. In the experiment, contour machining, allowance machining and trimming are also added, and the setting of machining accuracy and the maximum length of micro-straight line segment are considered.
The electrode manufacturing part is an important link, so the on-board grinding device is made by ourselves. According to the tool grinding principle of milling machine, the positioning datum of "electric contact" is designed, which can accurately position and trim the cylindrical surface of electrode and the spherical surface of electrode. However, because the copper electrode is easy to deform under the action of mechanical force to make way for the tool, only the finger electrode with a diameter of φ5 ~ 8mm has been successfully trimmed at present.
Figure 4 is a physical photo of CNC EDM milling, which is a curved surface with an area of about 100 mm× 70 mm ... You can also go to Baidu to have a look!