Optimization of Machining Margin Allocation in EDM Cavity Machining

Abstract : In the EDM cavity processing, the determination and distribution of the machining allowance have a great influence on the machining efficiency of EDM. This paper presents an optimization model for machining allowance and distribution. The experimental results show that the model obviously improves the machining efficiency.
Keywords: EDM machining allowance optimization

Abstract:In EDM,undersize arrangement for each machining step effects greatly on the efficiency of machining.An optimization model of the undersize and its distribution is proposed in this paper.The experi~ment results show that the machining efficiency is improvedconstrained by the model .
Keywords:EDM the undersize optimization

1 Overview

As we all know, compared with other general machining methods, EDM processes use much longer processing time to remove the same volume of workpiece materials. Therefore, how to improve the machining efficiency of EDM is a research staff engaged in EDM. A goal pursued. In the EDM cavity processing, it is generally a gradual transition from rough machining to machining. Among them, how to determine the machining allowance and the processing amount of each power regulation, that is, how to determine the feed amount in each depth direction of the electrode and the lateral translational amount in the shaking processing have a great influence on the EDM efficiency. Therefore, it is necessary to carry out more in-depth discussion of this issue, establish an EDM allowance and distribution optimization model, so as to provide a valid basis for a reasonable determination of the amount of processing at each step, and then improve the processing of EDM. effectiveness. This article is based on the above point of view, proposed an EDM allowance and distribution optimization model, conducted an in-depth study of the above issues.

2 EDM allowance and distribution optimization model

2.1 The basic principle of the determination and distribution of the machining allowance In the machining of the EDM cavity, it is necessary to go through a number of electrical standards conversion to complete the entire process from roughing to finishing. The rough standard processing effect is that the electrode loss is small, the surface roughness is poor, the processing speed is high, and the discharge gap is large. Therefore, roughing should remove as much of the excess as possible, so that the basic shape of the workpiece. From the middle to the finishing process, the processing speed drops a lot. For this reason, the conversion of each step should be guaranteed to reduce the processing volume as much as possible in order to improve the discharge trace of the discharge gauge before the light is repaired. EDM efficiency. That does not allow "under repair" phenomenon. Otherwise, the accumulation of "unsuccessful" will disturb the entire finishing process and will not even achieve the goal of refinement. At the same time, it can not make the processing volume too large, otherwise it will reduce the processing efficiency. To calculate the feed amount in each depth direction of the electrode and the lateral translational amount in the rocking process, it is only necessary to consider factors such as the inlet and discharge gaps and the electrode loss.
2.2 Commonly Used Methods and Their Insufficiency For EDM operators, it is not easy to determine the amount of processing at each step. They often refer to the range of processing values ​​commonly recommended for overnight to determine the amount of processing at each step.螎 In order to obtain the required surface roughness safely, it is inevitable that the amount of processing at each step of processing and finishing in Chad will be large. This will greatly affect the processing efficiency. Some recommend using formula (1) to calculate the translational momentum:

Si=Gi-1+Ri-Gi+Ei (1)

Where Si-i is the standard translational momentum;
Gi—the spark gap of the ith block;
Gi-1—the spark gap at the i-1 block;
Ri - the maximum value of the surface roughness of the ith plane;
Ei - overhaul.
Using the above formula, the feed amount of the electrode in the depth direction can be calculated similarly, so that the processing amount of each step is also determined. In addition, there is another method that is widely recommended, that is, firstly to obtain an empirical value of machining allowance based on the surface roughness after rough machining, and then, the machining amount of each step is proportional to the surface roughness of each step of the electric power gauge. In this way, the processing volume of each step from the middle machining to the finish machining is distributed.
The above methods can be summarized by using the schematic diagrams of the electrode depth direction feed amount and the lateral translation amount shown in FIG. 1 and FIG. 2 . In the figure, Gi is the discharge gap for each electrical gauge, Ri is the surface roughness formed by each electrical gauge, Hi is the feed rate of the electrode, Ei is the overhaul, and Si is the amount of translation of the electrode. Both of them make all the surface roughness formed by the former rule removed at every step of processing, and also increase an overhaul amount, which is sometimes too large, so that it is too conservative and will greatly reduce the processing effectiveness.

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Fig. 1 Schematic diagram of normal electrode feed

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Fig. 2 Schematic diagram of normal electrode mobility

2.3 Optimization Models for Machining Margin and Distribution In fact, it is out of the consideration of partial safety that all the surface roughness formed in the previous step is removed at every step of processing. Theoretical analysis, in fact, if the current rule formed by the bottom of the discharge mark and the discharge of the previous step to form the bottom of the discharge mark is ideal. In order to ensure a reliable dressing of the discharge trace of the previous step, and considering that the surface heat affected layer is to be removed, an appropriate overhaul amount may be added, so that the required surface roughness can be safely obtained, and in addition As much as possible reduce the amount of processing at each step, thereby effectively improving the processing efficiency. For this reason, this paper proposes an optimization model for machining allowances and distribution. The amount of machining at each step of the electrode can be described by equation (2):

Wi=Ri-1-Ri+a. Ri-1 (2)

Where Wi-i is the processing amount in the depth-feed direction or lateral direction, where the length is measured;
Ri-1—the surface roughness formed by the ith barrier;
Ri - the surface roughness formed by the i-1st gauge;
A—belt belt factor.
The above formula can be graphically represented by Figure 3. In this model, Ri is the Rmax used to evaluate the surface roughness in the national standard, which is the maximum height deviation. Each processing only etched off part of the peaks of the surface roughness formed by the previous gauge, and formed the surface roughness of this standard on the remaining part of the gauge, making the roughness of the surface roughness and the previous rule formed by this rule. Quasi-formed surface roughness of the trough's root is flat. For the sake of safety, the concept of a seatbelt coefficient is also proposed in the model so that it can be multiplied by the surface roughness formed by the previous gauge, and the overhaul of this regulation, or seat belt, can be obtained. It is proportional to the surface roughness formed by the previous gauge. Seat belt coefficient a is less than 1, it is related to the electrode and workpiece materials and other factors, according to different circumstances desirable 0.1 ~ 0.3. The bigger a is, the greater the overhaul is, the safer the process is. The proposal of the concept of seatbelt coefficient provides a basis for calculation and adjustment of the overhaul.

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Fig. 3 Optimization of machining quantity

Based on this optimization model, the electrode depth direction feed amount and lateral translation amount can be represented by FIG. 4 and FIG. 5 .

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Fig. 4 Schematic diagram of electrode feed based on optimized model

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Fig. 5 Schematic diagram of the translational momentum of the electrode based on the optimization model

If the electrode loss is taken into account again, the amount of horizontal and vertical feed of the electrode at each step can be expressed by equation (3):

Si (or Zi) = Gi-1-Gi + Ri-1-Ri + a. Ri-1
+EWi. (0.5Ri-1-0.5Ri+a.Ri-1) (3)

Where EWi is the electrode consumption ratio of the ith block, and the rest of the parameters have the same meaning as above. In this way, based on the EDM process database, it is easy to calculate the amount of horizontal momentum and depth direction feed for each step of the electrode.
Using the above model, the back-injection method is used to successively calculate the amount of each step from the finish machining to the middle machining, and then the total machining allowance left after the rough machining is left to the middle machining and the finishing machining, and the reduction of the electrode can be obtained. Provide a reference basis, if the reduction of the electrode is different, you can modify the model according to the reduction of the electrode.

3 Experimental verification results

To verify the optimization model presented in this paper, a comparative experiment was conducted. The experimental conditions are as follows:
Machine Tools: Hanchuan-HCD300K;
Pulse power supply: Hanchuan MD20FZ;
Electrode: copper, Φ14mm cylindrical electrode, end surface and bottom surface fine cars;
Workpiece: Cr12MoV, bottom surface (positioning surface) and finished surface grinding, Ra<6.3μm;
Processing polarity: negative polarity;
Working fluid: kerosene;
Chip removal conditions: no impulse pumping, automatic oil circulation;
Processing requirements: processing depth of 5mm, the final finishing surface roughness Ra is 3.2μm.
Based on the technology database, the choice of electricity standard conversion is C270→C240→C220→C210, where C270 is the rough processing standard and the rest is the medium processing and precision machining standards. The surface roughness Ra that can be achieved by the C210 specification in the process database is approximately 2.7 μm. The experiment was divided into four cases. Among them, two cases were calculated in the former traditional model. 1 is the first mentioned above. According to the surface roughness after rough machining, the intermediate machining and finishing are determined based on the empirical values. The machining allowance is then assigned according to the surface roughness of each step. 2 According to the model shown in Fig. 1 and Fig. 2, the overhaul amount refers to the method using the seatbelt coefficient to determine an appropriate value. In the optimization model shown in Fig. 4 and Fig. 5, two cases are calculated. 1 Seat belt coefficient a=0.1. 2a=0.2. In each case, the electrode depth direction feed amount and the lateral translation amount at each step were calculated, and the calculation results are shown in the following table. Among them, Zi is the distance of the electrode with respect to the processing bottom surface, and Stepi is the radius of the electrode shaking.
Measured processing time and surface roughness, the experimental results shown in Figure 6, Figure 7, where optimization 1 and optimization 2 represent a = 0.1 and a = 0.2.

Table calculation results

Machining allowance Zi Stepi C240 ​​C220 C210 C240 ​​C220 C210 Traditional model case 1 217 208 126 69 198 279 335 Traditional model case 2 110 156 100 69 144 200 230 Optimization model a=0.1 42 122 86 69 110 146 161 Optimization model a=0.2 52 127 88 69 114 153 170

From Fig. 6 and Fig. 7, it can be seen that the conventional model case 1 greatly reduces machining efficiency due to excessive machining allowance. In the optimization model, when a = 0.1 (Optimization 1), the overhaul is small, slightly unsafe, and the surface roughness is slightly larger. When a=0.2 (Optimum 2), it is ideal. Although the surface roughness of the four processing conditions is slightly different, they are all at one level. Therefore, it can be considered that the surface roughness they reached is basically the same, and the processing requirements can be safely achieved, and the processing efficiency of the optimization model is improved a lot.

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Figure 6 Comparison of processing time

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Fig. 7 Comparison of surface roughness

4 Conclusion

In the EDM cavity processing, the determination and distribution of the machining allowance have a great influence on the machining efficiency of EDM. Based on the machining allowance and distribution optimization model proposed in this paper, the machining allowance can be reasonably determined and assigned, which significantly improves the machining efficiency.

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