Discharge ablation auxiliary grinding machining method for grinding depth hierarchical control

Abstract

The invention discloses a discharge ablation auxiliary grinding machining method for grinding depth hierarchical control. The discharge ablation auxiliary grinding machining method comprises the steps that firstly, the numerical values of the first-layer grinding depth delta 1, the reference grinding force F0, the pure mechanical grinding depth delta pure, the pure mechanical single-layer grinding depth delta single, the rotating speed of a grinding wheel, the transverse feeding speed of the grinding wheel and the index constant n are set; secondly, first-layer discharge ablation auxiliary grinding is carried out; 3, comparing the average grinding force F1 of the first layer machining with the F0 to obtain the grinding depth delta 2 of the second layer; fourthly, second-layer discharge ablation auxiliary grinding is carried out; 5, comparing the average grinding force F2 of the second layer machining with the F0 to obtain the grinding depth delta 3 of a third layer; and sixthly, the process is repeated till the remaining machining allowance is smaller than the pure mechanical grinding depth delta pure, and pure mechanical grinding is conducted. According to the method, the grinding depth of a single layer is regulated and controlled through real-time feedback of the grinding force, pure mechanical grinding is conducted when the residual machining allowance is smaller than a set value, it is ensured that a molten and solidified layer is completely removed, and efficient and stable machining can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of discharge ablation assisted grinding, and specifically relates to a method for discharge ablation assisted grinding with constant rotational speed and constant feed rate of a grinding wheel suitable for discharge ablation assisted grinding, and a grinding depth layer control based on the detection of the average grinding force value for discharge ablation assisted grinding. Background Art

[0002] In the fields of aerospace, precision instrument manufacturing, etc., the application of difficult-to-machine metal materials such as titanium alloys and superalloys is becoming increasingly widespread. Due to the characteristics of high strength, high hardness, etc. of such materials, severe challenges are posed to the precision machining of their components. Traditional mechanical grinding processes face problems such as large grinding forces, severe grinding wheel wear, and low machining efficiency during the machining process; while electrical discharge machining can handle high-hardness materials, it has defects such as low erosion efficiency and easy formation of a melting and solidification layer on the machining surface, and it is difficult to meet the requirements of high-efficiency machining.

[0003] The discharge ablation assisted grinding technology is a composite machining method that combines electrical discharge induced ablation and mechanical grinding. The basic principle of this technology is as follows: First, a plasma channel is formed between the electrode and the workpiece through a pulsed power supply, and the high temperature generated instantaneously in this channel activates the metal on the workpiece surface; then the activated metal undergoes a violent combustion reaction with the oxygen between the electrodes, a large amount of metal is heated to the molten state, and is removed under the action of medium flushing. Part of the molten material that is not discharged solidifies again on the machining surface to form a melting and solidification layer, and the grinding grains of the grinding wheel grind and remove the material surface. Since the hardness of the melting and solidification layer and the metal softened by heating below it is lower than that of the original matrix material, the grinding force is lower than that of conventional pure mechanical grinding.

[0004] The existing discharge ablation assisted grinding layering is based on the average layering idea of mechanical grinding, that is, the grinding depth of each layer of grinding is the same. Since the discharge ablation machining state is related to the discharge state between the electrodes, the amount of material removed and the softening effect on the surface are different for each layer of machining: when the ablation effect of a certain layer is less, the grinding force of the next layer will increase, which is likely to cause damage to the workpiece surface. In order to avoid damage to the workpiece due to excessive grinding force, the grinding depth is usually set relatively small, resulting in low machining efficiency; when the ablation effect of a certain layer is more, the grinding force of the next layer will decrease, and at this time, the grinding depth can be appropriately increased to improve the machining efficiency. However, the prior art cannot reasonably regulate the grinding depth. Therefore, the fixed grinding depth machining used in existing discharge ablation assisted grinding is difficult to achieve high-efficiency discharge ablation assisted grinding machining. Summary of the Invention

[0005] The object of the present invention is to provide a method for controlling the grinding depth in layers in the discharge ablation assisted grinding process, aiming at the lack of dynamic hierarchical control of the grinding depth in the existing discharge ablation assisted grinding technology, which makes it difficult to adapt to the dynamic changes during the material removal process. The method of the present invention adjusts the grinding depth of a single layer in real time through the feedback of the grinding force, and switches to pure mechanical grinding when the remaining machining allowance is less than the set pure mechanical grinding depth, ensuring that the melted and solidified layer is completely removed at the end of the machining, so as to achieve efficient and stable machining.

[0006] The technical solution of the present invention is as follows:

[0007] A method for controlling the grinding depth in layers in the discharge ablation assisted grinding process, comprising the following steps:

[0008] Step 1: Set the grinding depth δ1 of the first layer, the reference grinding force F0, the pure mechanical grinding depth δ 纯 、the pure mechanical single-layer grinding depth δ 单 、the rotational speed of the grinding wheel, the transverse feed speed of the grinding wheel, and the value of the exponential constant n, where 0 < n < 1;

[0009] Step 2: Perform the discharge ablation assisted grinding of the first layer; taking the grinding time of the first layer as the sampling period and the grinding force during the first layer grinding as the sampling signal, after the first layer grinding is completed, use the processor to calculate the average grinding force F1 of the first layer machining;

[0010] Step 3: Compare the average grinding force F1 of the first layer machining with the set reference grinding force F0 to obtain the proportional factor K1, and multiply the grinding depth δ1 of the first layer by the proportional factor K1 to obtain the grinding depth δ2 of the second layer; where the proportional factor K1 = (F0 ÷ F1) n ;

[0011] Step 4: Perform the discharge ablation assisted grinding of the second layer with the obtained grinding depth δ2 of the second layer; taking the grinding time of the second layer as the sampling period and the grinding force during the second layer grinding as the sampling signal, after the second layer grinding is completed, use the processor to calculate the average grinding force F2 of the second layer machining;

[0012] Step 5: Compare the average grinding force F2 of the second layer machining with the set reference grinding force F0 to obtain the proportional factor K2, and multiply the grinding depth δ2 of the second layer by the proportional factor K2 to obtain the grinding depth δ3 of the third layer; where the proportional factor K2 = (F0 ÷ F2) n ;

[0013] Step 6: And so on, until the remaining machining allowance is less than the set pure mechanical grinding depth δ 纯 , turn off the oxygen and the pulse power supply, and switch to pure mechanical grinding; during the pure mechanical grinding process, use the set pure mechanical single-layer grinding depth δ 单Continue with layer-by-layer grinding to remove the melted and solidified layer on the workpiece surface.

[0014] Compared with the prior art, in the process of discharge ablation assisted grinding of the present invention, the grinding wheel feeds at a constant rotational speed and a lateral speed, efficiently ablates and removes the workpiece material, while grinding to remove part of the melted and solidified layer. Taking the machining time of this layer as the sampling period and the real-time grinding force of this layer as the sampling signal, the average grinding force of this layer is obtained. The average grinding force is compared with the set reference grinding force F0 to obtain a proportionality factor as the basis for adjusting the depth of cut, and the grinding depth of the next layer is adjusted. When the sampled average grinding force is greater than the set reference grinding force F0, the grinding depth of this layer is multiplied by the proportionality factor (less than 1 at this time) to reduce the grinding depth of the next layer and avoid excessive pure grinding phenomenon during the next layer grinding process. When the sampled average grinding force is less than the set reference grinding force F0, the grinding depth of this layer is multiplied by the proportionality factor (greater than 1 at this time) to increase the grinding depth of the next layer and improve the material removal rate. Finally, the actual grinding force gradually tends to the set grinding force, making each layer of discharge ablation assisted grinding in the desired machining state, and switching to pure mechanical grinding when the remaining machining allowance is less than the set pure mechanical grinding depth, ensuring that the melted and solidified layer is completely removed at the end of machining, thereby achieving efficient and stable machining.

[0015] Furthermore, during machining, appropriate grinding wheel rotational speed and grinding wheel lateral feed speed are determined according to the workpiece material and machining requirements. Preferably, in the discharge ablation assisted grinding method with grinding depth layer control described above, the set grinding wheel rotational speed is in the range of 50 - 3000 rpm, and the set grinding wheel lateral feed speed is in the range of 1 mm / s - 50 mm / s.

[0016] Furthermore, in the discharge ablation assisted grinding method with grinding depth layer control described above, the set first layer grinding depth δ1 is in the range of 1 - 20 μm. During machining, the value of the first layer grinding depth δ1 is specifically determined according to the material and machining requirements.

[0017] Furthermore, preferably, in the discharge ablation assisted grinding method with grinding depth layer control described above, the set pure mechanical grinding depth δ 纯 is in the range of 40 - 60 μm. If the pure mechanical grinding depth δ 纯 is too small, it may occur that the thickness of the melted and solidified layer is greater than the machining allowance, while if the pure mechanical grinding depth δ 纯 is too large, it will affect the machining efficiency. Setting the pure mechanical grinding depth δ 纯 in the range of 40 - 60 μm can better balance the machining quality and machining efficiency.

[0018] Further, in order to balance the machining efficiency and machining quality, in the above-mentioned discharge ablation assisted grinding method with grinding depth layer control, the set pure mechanical single-layer grinding depth δ 单 is in the range of 1-5 μm.

[0019] Further, preferably, in the above-mentioned discharge ablation assisted grinding method with grinding depth layer control, the value range of the exponential constant n is ⅓ ≤ n ≤ ⅔. If the value of n is too small, the change in grinding depth will be small, and the machining adjustment will not be obvious, affecting the machining efficiency. If the value of n is too large, the change in grinding depth will be too large, affecting the machining quality. Taking the value of n within the above range can better balance the machining efficiency and machining quality.

[0020] Further, preferably, in the above-mentioned discharge ablation assisted grinding method with grinding depth layer control, the reference grinding force F0 is 60-80% of the grinding force value when the material is not subjected to discharge ablation (i.e., pure mechanical grinding) and the grinding depth is the target grinding depth δ0 under the same grinding parameters; the target grinding depth δ0 is in the range of 1-20 μm. The target grinding depth δ0 is determined according to the workpiece material and machining parameters and is an expected value. According to the characteristics of discharge ablation assisted grinding, since the single-layer grinding depth is small, during the grinding wheel feeding process, the abrasive grains generate a grinding effect on the discharge ablation softened matrix metal and the molten and solidified layer under the action of rotation, achieving effective removal. Therefore, the average grinding force is significantly lower than that of pure mechanical grinding. According to the different machining materials and pulse parameters, the average grinding force varies within a certain range, generally 60%-80% of that of pure grinding. Therefore, it is more appropriate to set the reference grinding force F0 between 60%-80% of the grinding force during pure grinding.

[0021] Further, preferably, in the above-mentioned discharge ablation assisted grinding method with grinding depth layer control, the first-layer grinding depth δ1 can be equal to the target grinding depth δ0. During machining, the first-layer grinding depth δ1 can be taken near the target grinding depth δ0. Stipulating that the first-layer grinding depth δ1 is equal to the target grinding depth δ0 is beneficial to process design.

[0022] Further, preferably, in the above-mentioned discharge ablation assisted grinding method with grinding depth layer control, during the pure mechanical grinding process, the system judges whether the molten and solidified layer has been completely removed according to the real-time grinding force; if the grinding force is significantly lower than the pure grinding force F 止 , it indicates that there is still an incompletely removed molten and solidified layer on the surface, and continue with pure mechanical grinding; when the grinding force continuously approaches the pure grinding force F during one-layer machining 止 , it is determined that the molten and solidified layer has been completely removed, and the machining process is terminated; wherein, the pure grinding force F 止 is the grinding force when the material is not subjected to discharge ablation and the grinding depth is δ under the same grinding parameters 单The numerical value of the grinding force at that time. Thus, the molten and solidified layer on the workpiece surface can be completely removed, and it is easy to implement. Further, in the pure mechanical grinding process, if the deviation of the grinding force in one layer of machining from the pure grinding force F 止 never exceeds 2%, it is determined that the molten and solidified layer has been completely removed. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a flowchart for controlling the grinding depth stratification in the method of the present invention

[0024] Figure 2 is an overall schematic diagram of the experimental system in an embodiment of the present invention;

[0025] Figure 3 is a schematic diagram of the principle of controlling the grinding depth stratification in the method of the present invention;

[0026] Figure 4 is a comparison chart of the machining efficiency of discharge ablation assisted grinding using the traditional average stratification method (comparative experiment 1) and the stratification control method of the present invention (comparative experiment 2);

[0027] Figure 5 is a comparison chart of the morphology of the machined workpiece under a white light interferometer in this embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0028] The technical solution of the present invention will be further specifically described below through specific embodiments in conjunction with the accompanying drawings, but it is not used as a basis for limiting the present invention. The content not described in detail in the following embodiments is common technical knowledge in the art or can be achieved by conventional technical means in the art.

[0029] Embodiment (see Figures 1 - 5 ):

[0030] When machining using the discharge ablation assisted grinding method with grinding depth stratification control, the single-layer grinding depth is calculated and adjusted during machining based on the average grinding force value of the upper layer, and the single-layer cutting depth is adjustable within the range of 1 μm - 20 μm. During the discharge ablation assisted grinding process, the grinding wheel feeds at a constant rotational speed and a transverse speed, efficiently ablating and removing the workpiece material of this layer, and at the same time grinding and removing part of the molten and solidified layer. Then, taking the machining time of this layer as the sampling period and the real-time grinding force of this layer as the sampling signal, the average grinding force of this layer is obtained; the obtained average grinding force is compared with the set reference grinding force F0 to obtain a proportionality factor as the basis for adjusting the grinding depth, thereby adjusting the grinding depth of the next layer.

[0031] See Figure 1 , during machining, the grinding depth δ1 of the first layer is a fixed value within the range of 1 μm - 20 μm, and the grinding depth δ of the (i + 1)-th layer i+1Determined by the processing status of the previous layer, the time from the start of grinding the i-th layer to the end of grinding this layer is the sampling period T i During T i time, the real-time grinding force value is sampled, and the average grinding force value at time T is calculated through the microprocessor n . Through the formula K i =(F0÷F i ) n the scale factor K is calculated i , the exponential constant n = 1 / 3, and the grinding depth δ of the (i + 1)-th layer is adjusted to ki•δ i+1 . i .

[0032] When the overall processing is close to the target size (the machining allowance is less than 50 μm), the oxygen and pulse power supply are turned off, and pure mechanical grinding is switched to. The grinding depth is set to a constant 2 μm for continued layer-by-layer grinding. At the same time, the system judges whether the molten and solidified layer has been completely removed according to the real-time grinding force; if the grinding force is significantly lower than the pure grinding force F 止 (under the same parameter conditions, the grinding force when pure grinding to a depth of 2 μm is obtained through experiments), it indicates that there is still an incompletely removed molten and solidified layer on the surface, and pure mechanical grinding continues; when the grinding force in one layer of processing continuously approaches the pure grinding force F 止 , it is determined that the molten and solidified layer has been completely removed, and the processing process is terminated

[0033] The present invention will be further described below in combination with processing experiments and comparative experiments. The processing parameters are shown in Table 1 below

[0034] Table 1: Processing parameters

[0035] Item Content Workpiece Material Cemented carbide YG8, size 16mm × 16mm × 4.5mm Power Feeding Method Positive Polarity Machining Grinding Wheel 800# CBN, metal copper bond; rotational speed 1500r / min, transverse feed rate 30mm / s Working Medium Water and oxygen mixed medium Pulse Width : Pulse Interval 150μs : 120μs Current 5A Oxygen Pressure 0.5MPa Water Pressure 0.15MPa

[0036] Processing experiment of the present invention

[0037] The discharge ablation assisted grinding method of the present invention is used for processing. The experimental system is as Figure 2 , and the processing parameters are shown in Table 1. The reference grinding force F0 is set to 70 N (the target grinding depth δ0 is 2 μm, and F0 = 70 N is obtained through experiments), the first layer grinding depth δ1 is 2 μm, and the exponential constant n = 1 / 3. During the processing, the system continuously adjusts the grinding depth. When the machining allowance is less than 50 μm, it switches to pure mechanical grinding, and the state of the molten and solidified layer is judged in real time through the grinding force signal before the end of processing, until the deviation of the grinding force relative to the pure grinding force F 止 in one layer of processing does not exceed 2% all the time (the grinding force when pure mechanical grinding to a depth of 2 μm, F 止 = 85 N is obtained through experiments), stop processing. The white light interference pattern of the material surface after processing is as Figure 5 shown in (b).

[0038] Comparative Experiment 1:

[0039] Using Figure 2 the experimental system shown, the processing is carried out with the process parameters shown in Table 1. Specifically: Using the traditional constant-speed feed and constant grinding depth layer-by-layer method for discharge ablation assisted grinding, the single-layer grinding depth is 2 μm. The white light interference pattern of the processed material surface is as shown in Figure 5 (a).

[0040] It can be seen from Figure 5 that there are ablation pits on the processed material surface in Comparative Experiment 1 due to discharge ablation and erosion, and they are not completely removed. However, no ablation pits can be seen on the processed material surface using the method of the present invention, and its surface topography only has grooves generated after mechanical removal by abrasive grains.

[0041] Comparative Experiment 2:

[0042] Comparative Experiment 2 is processed using the same method of controlling the grinding depth layer by layer in the present invention, but there is no pure mechanical grinding link.

[0043] The comparison chart of the processing efficiency of Comparative Experiment 1 and Comparative Experiment 2 is as shown in Figure 4 . The experiment shows that using the method of controlling the grinding depth layer by layer in the present invention can improve the processing efficiency.

[0044] The above general description of the invention involved in the present application and the description of its specific implementation manners should not be understood as a limitation on the composition of the technical solution of the invention. Those skilled in the art can, based on the disclosure of the present application, without violating the constituent elements of the involved invention, add, subtract or combine the disclosed technical features in the above general description or / and specific implementation manners (including embodiments) to form other technical solutions within the protection scope of the present application.

Claims

1. A method for grinding deep layer-by-layer controlled discharge ablation-assisted grinding, characterized in that, It includes the following steps: Step 1: Set the values of the first-layer grinding depth δ1, the reference grinding force F0, the pure mechanical grinding depth δ 纯 , the pure mechanical single-layer grinding depth δ 单 , the grinding wheel speed, the transverse feed speed of the grinding wheel, and the exponential constant n, where 0 < n < 1; Step 2: Perform the first-layer discharge ablation assisted grinding; take the first-layer grinding time as the sampling period and the grinding force during the first-layer grinding as the sampling signal. After the first-layer grinding is completed, use the processor to calculate the average grinding force F1 of the first-layer machining; Step 3: Compare the average grinding force F1 of the first layer machining with the set reference grinding force F0 to obtain the proportionality factor K1, and multiply the grinding depth δ1 of the first layer by the proportionality factor K1 to obtain the grinding depth δ2 of the second layer; where the proportionality factor K1 = (F0 ÷ F1) n ; Step 4: Perform the second-layer discharge ablation assisted grinding with the obtained second-layer grinding depth δ2; take the second-layer grinding time as the sampling period and the grinding force during the second-layer grinding as the sampling signal. After the second-layer grinding is completed, use the processor to calculate the average grinding force F2 of the second-layer machining; Step 5: Compare the average grinding force F2 of the second layer machining with the set reference grinding force F0 to obtain the proportionality factor K2, and multiply the grinding depth δ2 of the second layer by the proportionality factor K2 to obtain the grinding depth δ3 of the third layer; where the proportionality factor K2 = (F0 ÷ F2) n ; Step 6: By analogy, continue the process until the remaining machining allowance is less than the set pure mechanical grinding depth δ 纯 , turn off the oxygen and pulse power supply, and switch to pure mechanical grinding; during the pure mechanical grinding process, continue to grind in layers with the set pure mechanical single-layer grinding depth δ 单 to remove the melted and solidified layer on the workpiece surface.

2. The method for grinding depth layer control assisted discharge ablation grinding machining according to claim 1, wherein: The set grinding wheel speed is in the range of 50 - 3000 rpm, and the set transverse feed speed of the grinding wheel is in the range of 1 mm / s - 50 mm / s.

3. The method for discharge ablation-assisted grinding with grinding depth layer control according to claim 1, characterized in that: The set first-layer grinding depth δ1 is in the range of 1 - 20 μm.

4. The method for discharge ablation-assisted grinding with grinding depth layer control according to claim 1, characterized in that: The set pure mechanical grinding depth δ 纯 is in the range of 40 - 60 μm.

5. The method for grinding depth layer control of discharge ablation assisted grinding according to claim 1, characterized in that: The set pure mechanical single-layer grinding depth δ 单 is in the range of 1 - 5 μm.

6. The method for discharge ablation-assisted grinding with grinding depth layer control according to claim 1, wherein: The value range of the exponential constant n is ⅓ ≤ n ≤ ⅔.

7. The method for grinding depth layer control-assisted electrical discharge ablation grinding according to claim 1, characterized in that: The reference grinding force F0 is 60 - 80% of the grinding force value when the material is not subjected to discharge ablation under the same processing conditions and the grinding depth is the target grinding depth δ0; the target grinding depth δ0 is in the range of 1 - 20 μm.

8. The method for grinding depth stratified control of discharge ablation assisted grinding according to claim 7, characterized in that: The first-layer grinding depth δ1 is equal to the target grinding depth δ0.

9. The method for grinding depth stratified control of discharge ablation assisted grinding according to any one of claims 1-8, characterized in that: During the pure mechanical grinding process, the system determines whether the melted and solidified layer has been completely removed based on the real-time grinding force. If the grinding force is significantly lower than the pure grinding force F 止 , it indicates that there is still an incompletely removed melted and solidified layer on the surface, and pure mechanical grinding continues. When the grinding force continuously approaches the pure grinding force F 止 during one layer of machining, it is determined that the melted and solidified layer has been completely removed, and the machining process is terminated. Among them, the pure grinding force F 止 is the grinding force value when the grinding depth is δ 单 under the same machining conditions without electrical discharge ablation of the material.

10. The method for grinding depth layer control assisted discharge ablation grinding according to claim 9, characterized in that: During the pure mechanical grinding process, if the deviation of the grinding force in one layer of machining from the pure grinding force F 止 is always no more than 2%, it is determined that the melted and solidified layer has been completely removed.

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