Impeller and main shaft end face tooth electric spark pair grinding finishing method

Abstract

The application discloses a kind of impeller and main shaft end face tooth electric spark pair grinding finishing method, this method is respectively to the rough machining and semi-finishing of impeller end face tooth and main shaft end face tooth first and reserves pair grinding allowance, then impeller and main shaft are coaxially clamped, corresponding end face tooth is kept tooth profile correspondence and non-contact discharge gap, and is respectively connected into the two output poles of pulse power and forms processing electrode pair;Subsequently, pulse discharge etching is carried out on the relative tooth surface by using rough machining discharge parameter and finishing discharge parameter in turn, and the pair grinding process is controlled by circumferential pairing position locking, axial gap adjustment, discharge area migration, polarity alternation and processing termination determination;After processing is completed, polishing treatment is carried out on the discharge processing surface;The application can directly pair finishing for the final mating tooth surface, reduce the mating deviation caused by separate processing, and reduce the influence of mechanical cutting factor on end face tooth finishing.

Description

Technical Field

[0001] This invention relates to the field of precision machinery manufacturing technology, and more specifically to an electrical discharge machining method for achieving high-precision fit between blades / impellers and main shaft end face teeth in high-end equipment such as industrial steam turbines. Background Technology

[0002] When the impeller and main shaft of an industrial steam turbine are connected by end face teeth, the end face teeth not only bear the circumferential torque transmission, but also affect the centering state after the impeller and main shaft are assembled. Therefore, the tooth profile consistency and actual contact state between the impeller end face teeth and the main shaft end face teeth are directly related to the assembly quality of the rotor assembly. For impellers and main shafts made of high-strength and high-hardness materials, the end face teeth usually need to be formed by roughing, semi-finishing and finishing processes, and then the finished impeller and main shaft are paired and assembled.

[0003] Currently, prior art with publication number CN104128670A discloses a method for machining the end face teeth of an impeller and the end face teeth of the gear shaft that mate with it. This method designs machining fixtures for the end face teeth of the impeller and the end face teeth of the gear shaft separately, and processes both in steps on the same CNC boring and milling machine. In the finish milling stage, the same double-angle milling cutter is used to process the end face teeth of the impeller and the end face teeth of the gear shaft separately to reduce the errors caused by repeated positioning and tool differences, so that the tooth profile angles of the two are consistent. Prior art with publication number CN116275304A discloses a method for machining the end teeth of a compressor impeller, which uses a machining center to perform layer milling on the impeller end teeth, and points out that there are problems in the existing end face tooth milling process, such as insufficient versatility of special tools, machine tool backlash, and multi-axis linkage errors affecting the positional accuracy of the end teeth.

[0004] While the aforementioned processing methods can reduce the machining error of individual parts' end face teeth by improving machine tool accuracy, standardizing cutting tools, or optimizing clamping datums, the impeller end face teeth and spindle end face teeth still undergo material removal separately. The two actual mating tooth surfaces are not directly used as the final mating machining datum during the processing. Therefore, when the two parts are machined separately and then assembled, factors such as machine tool positioning errors, tool wear, repeated clamping errors, and the parts' own machining deformation may still be reflected in the actual tooth profiles of the corresponding end face teeth. This can lead to inconsistencies in local tooth surface clearance and uneven contact area distribution even after the two individually sized end face teeth are actually mated.

[0005] Furthermore, existing mechanical milling or grinding methods rely on the mechanical contact between the tool and the end face teeth to remove material. When the impeller or spindle is made of difficult-to-machine materials such as high-strength heat-resistant alloys, the machining process is also easily affected by factors such as tool wear, cutting load, and machining heat. Even if the machining accuracy of a single end face tooth is further improved, it is still necessary to control the error of the impeller end face tooth and the spindle end face tooth relative to the theoretical tooth profile separately, making it difficult to directly match and adjust the actual end face teeth for the final assembly. Summary of the Invention

[0006] The existing impeller end face teeth and spindle end face teeth are usually machined separately. There is no direct pairing machining relationship between the two actual mating tooth surfaces. As a result, the positioning error, tool error and machining deformation generated during the single-piece machining process are easily reflected in the mating tooth surface during the final assembly. At the same time, when high-strength heat-resistant materials are precision machined by mechanical cutting, they are also easily affected by cutting force, tool wear and machining heat.

[0007] The purpose of this invention is to provide a method for precision machining of impeller and spindle end face teeth by electrical discharge machining. By directly forming an electrical discharge machining electrode pair between the impeller and spindle that ultimately need to cooperate, the two actual end face teeth participate in electrical discharge machining while maintaining their corresponding relationship. This replaces the method of independently machining the two parts for the theoretical tooth profile with the actual cooperation relationship, so that the machining process can directly act on the corresponding tooth surfaces in the final assembled state.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A method for finishing impeller and spindle end face teeth by electrical discharge machining includes a pre-machining step, a clamping and alignment step, a rough grinding step, a fine grinding step, and a tooth surface post-treatment step.

[0009] In the pre-processing step, roughing and semi-finishing are performed on the impeller end face teeth and the main shaft end face teeth, respectively. A grinding allowance is reserved on the mating contours of the impeller end face teeth and the main shaft end face teeth, so that the end face teeth form a basic tooth shape before entering the EDM grinding stage. At the same time, a material allowance is reserved for subsequent direct discharge etching of the relative tooth surfaces, thereby avoiding complete reliance on EDM to form the overall contour of the end face teeth and allowing subsequent EDM to focus on the pairing and finishing of the two actual mating tooth surfaces.

[0010] In the clamping and alignment steps, the pre-machined impeller and spindle are coaxially mounted on the EDM equipment. By adjusting the axial relative position of the impeller and spindle, the corresponding end face teeth are placed in a non-contact state with corresponding tooth profiles and a discharge gap between the relative tooth surfaces. The impeller and spindle are then connected to the two output poles of the pulse power supply, so that the impeller end face teeth and spindle end face teeth directly form a machining electrode pair for mutual discharge. Therefore, instead of using separate tool electrodes to copy or modify the tooth profiles on both sides, the two actual end face teeth used in the final assembly become mutual discharge objects, allowing the actual geometric deviation of the tooth surfaces on both sides to be directly reflected in the discharge gap between them, providing a machining basis for subsequent material removal based on the actual paired state.

[0011] In the rough grinding step, the impeller end face teeth and the spindle end face teeth are placed in an insulating working fluid. While maintaining the tooth profile correspondence between the impeller end face teeth and the spindle end face teeth, pulsed discharges are generated between the opposing tooth surfaces using roughing discharge parameters to discharge and erode the grinding allowance of the impeller end face teeth and the spindle end face teeth. Since material removal is completed through the discharge action between the two opposing tooth surfaces, it is not necessary for the tool to apply a mechanical cutting load to the end face teeth. This reduces the interference of cutting force and tool wear on the tooth surface morphology during the machining of high-strength materials. Furthermore, by removing the large pairing allowance between the opposing tooth surfaces in the rough grinding stage, a smaller tooth surface trimming amount is retained for the fine grinding stage.

[0012] In the fine grinding process, the roughing discharge parameters are switched to the fine grinding discharge parameters, and the discharge gap between the impeller end face teeth and the main shaft end face teeth is adjusted accordingly. The discharge erosion continues through pulse discharge between the relative tooth surfaces, so that the remaining local fit deviation after rough grinding continues to be removed by material removal under low discharge energy conditions. Without changing the actual correspondence between the two end face teeth, the pairing profile of the relative tooth surfaces is further refined, avoiding the introduction of new relative position errors by re-clamping or re-establishing the machining datum.

[0013] In the post-processing step of the tooth surface, after the fine grinding is completed, the impeller and the main shaft are separated, and the EDM surfaces of the impeller end face teeth and the main shaft end face teeth are polished to remove the recast layer and microcracks formed on the EDM surface. This allows the paired tooth shape formed by the EDM grinding to be preserved, while reducing the impact of surface defects formed by the discharge heat on the subsequent load-bearing state of the end face teeth.

[0014] Furthermore, in the pre-machining step, the grinding allowance reserved on the mating contours of the impeller end face teeth and the spindle end face teeth is 0.1 to 0.2 mm on each side. This allows the pre-machining stage to form a basic tooth shape close to the final contour and provides a continuous material removal allowance for subsequent EDM rough grinding. This avoids the grinding allowance being too small to fully correct the original machining differences between the two actual mating tooth surfaces, and also avoids transferring a large amount of material removal to the subsequent fine grinding stage.

[0015] Furthermore, in the rough grinding step, the roughing discharge parameters include a discharge gap of 0.1–0.2 mm, a discharge current of 15–30 A, and a pulse width of 30–50 μs. By using a larger discharge gap and corresponding pulse discharge energy, the reserved grinding allowance is etched away, enabling the rough grinding stage to undertake the main task of removing the allowance and establishing a relatively stable mating profile for subsequent tooth surface refinement and finishing using lower discharge energy.

[0016] Furthermore, in the fine grinding process, the fine grinding discharge parameters include a discharge gap of 0.05–0.1 mm, a discharge current of 6–12 A, and a pulse width of 3–6 μs. By reducing the discharge gap and decreasing the effect of a single pulse discharge, the removal of discharge material is transformed from the removal of excess material in the rough grinding stage to fine finishing of the relative tooth surfaces, thereby reducing the secondary disturbance of the already formed paired tooth surfaces caused by the strong discharge effect.

[0017] Furthermore, the clamping and alignment steps include a pairing phase locking strategy. This strategy determines the circumferential pairing position of the impeller end-face teeth and the main shaft end-face teeth based on the tooth sequence correspondence during the final assembly of the impeller and the main shaft. After determining the circumferential pairing position, it restricts the relative circumferential rotation between the impeller and the main shaft. This circumferential pairing position remains unchanged during rough and fine grinding processes, allowing only axial relative position adjustments between the impeller and the main shaft. By maintaining consistency between the tooth sequence correspondence during processing and the tooth sequence correspondence during final assembly, each impeller end-face tooth is always paired with a predetermined main shaft end-face tooth for discharge. This avoids reintroducing geometric differences in the fit relationship due to changes in the tooth sequence correspondence during processing, ensuring that the tooth surface finishing result formed by EDM corresponds to the final actual assembled tooth pair.

[0018] Furthermore, the rough grinding process includes an axial clearance adjustment strategy. This strategy acquires the gap voltage and discharge current between the impeller end face teeth and the spindle end face teeth, and determines the discharge state between the relative tooth surfaces based on the gap voltage and discharge current. When the discharge state is an open circuit, the impeller and spindle are controlled to move closer to each other axially until the relative tooth surfaces enter a pulsed discharge state. When the discharge state is a short circuit or continuous arc, the impeller and spindle are controlled to move away from each other axially until the pulsed discharge state is restored. During the pulsed discharge state, the relative axial positions of the two surfaces are maintained or gradually adjusted. This allows for dynamic maintenance of the working gap between the two relative tooth surfaces based on the actual discharge state, avoiding excessively large gaps that cause continuous open circuits and excessively small gaps that cause short circuits or continuous arcs, enabling the actual end face teeth to continuously and effectively discharge and erode under non-contact conditions.

[0019] Furthermore, the rough grinding process includes a discharge region migration strategy. This strategy, while maintaining the circumferential pairing position of the impeller and spindle, adjusts the axial relative position to allow regions with smaller local discharge gaps on the relative tooth surfaces to first enter a pulsed discharge state and be eroded by discharge. As material is eroded from these regions, the corresponding discharge gap increases, and the axial relative position of the impeller and spindle is further adjusted to allow other regions with smaller local discharge gaps to sequentially enter a pulsed discharge state. Thus, the originally uneven local gaps between the two actual tooth surfaces can directly control the discharge location, allowing material removal to preferentially act on relatively protruding or smaller local gaps, and gradually shifting to other local areas as material is removed. This ensures that the grinding process unfolds continuously around the local geometric differences of the actual paired tooth surfaces, rather than uniformly removing material according to a pre-set theoretical tooth profile.

[0020] Furthermore, the fine grinding process includes a polarity alternation strategy. This strategy divides the fine grinding process into multiple consecutive electron discharge cycles. Between adjacent electron discharge cycles, the connection between the impeller and the main shaft and the two output poles of the pulse power supply is interchanged, causing the impeller end face teeth and the main shaft end face teeth to alternately change their discharge polarity. Within each electron discharge cycle, the circumferential pairing position of the impeller and the main shaft remains unchanged. By alternating the two final mating tooth surfaces to different discharge polarities, it avoids the continuous bias of material erosion towards one side of the tooth surface caused by prolonged fixed polarity. This allows both the impeller end face teeth and the main shaft end face teeth to participate in the micro-adjustment of the mating tooth surfaces, maintaining a one-to-one correspondence between the two actual tooth surfaces.

[0021] Furthermore, the fine grinding process includes a discharge energy switching strategy. This strategy acquires the discharge voltage, discharge current, and corresponding discharge duration between the impeller end face teeth and the spindle end face teeth during the current discharge cycle. Based on these parameters, it determines the cumulative discharge energy for the current discharge cycle. When the cumulative discharge energy reaches a preset switching condition, the connection relationships between the impeller and spindle and the two output poles of the pulse power supply are swapped. After the connection relationships are swapped, the discharge energy for the next discharge cycle is re-accumulated. Compared to polarity switching only at fixed intervals, this method determines the polarity switching timing based on the actual discharge effect generated in each discharge cycle. This prevents the time spent in open circuits, short circuits, or other situations where normal material removal does not occur from being simply equated with the effective processing time. Consequently, the material removal effect between the two polarity stages has a more direct correspondence with the processing process.

[0022] Furthermore, the fine grinding process includes a processing termination determination strategy. This strategy acquires the axial position adjustment of the impeller and spindle within multiple consecutive electron discharge cycles, as well as the discharge state between the relative tooth surfaces. It determines the stability of the fine grinding process based on the changes in axial position adjustment and pulse discharge state between adjacent electron discharge cycles, and stops the fine grinding process when the axial position adjustment and pulse discharge state meet preset stability conditions across multiple consecutive electron discharge cycles. As smaller gap areas in the relative tooth surfaces are gradually eroded, the axial position adjustment required to maintain effective discharge tends to stabilize, and the corresponding discharge state changes also tend to stabilize. Therefore, using the aforementioned processing parameters to jointly determine the processing termination time avoids insufficient pairing and finishing caused by stopping processing only according to a fixed processing time, or excessive material removal caused by continuous discharge. This ensures that the processing termination condition corresponds to the current pairing discharge state of the two actual end face teeth.

[0023] The beneficial effects of this invention are as follows: 1. By connecting the impeller and main shaft, which are ultimately assembled, to the two output poles of a pulse power supply, the impeller end face teeth and the main shaft end face teeth directly form a machining electrode pair for mutual discharge. Rough grinding and fine grinding are then performed while maintaining the tooth profile correspondence. Compared to the method of completing the two end face teeth separately using theoretical tooth profiles as machining references before assembly, this invention allows the geometric differences between the two actual mating tooth surfaces to be directly reflected in the electrical discharge machining process. This allows the material to be removed and unfolded towards the actual tooth surface for final assembly, thereby reducing the superposition of errors formed by the separate machining of the two end face teeth after assembly.

[0024] 2. The removal of the pre-reserved grinding allowance is completed by pulse discharge erosion method. The non-contact discharge state is maintained between the impeller end face teeth and the spindle end face teeth. There is no need to apply a continuous cutting load to the end face teeth through mechanical tools. Therefore, the influence of tool wear, mechanical cutting force and machining thermal deformation on the finishing state of the end face teeth can be reduced. It is especially beneficial for the fine machining of end face teeth of high strength, high hardness and heat-resistant materials.

[0025] 3. By using a pairing phase locking strategy, the circumferential pairing position is determined based on the tooth sequence correspondence of the impeller and the main shaft during final assembly. The relative circumferential rotation between the two is restricted during subsequent rough and fine grinding processes, so that the same impeller end face tooth is always paired with the corresponding main shaft end face tooth for electrical discharge machining. This avoids changing the tooth sequence correspondence during machining and reintroducing geometric deviations between different tooth surfaces, so that the tooth surface finishing result formed by grinding can directly correspond to the final assembly relationship.

[0026] 4. Adjust the axial relative position of the impeller end face teeth and the main shaft end face teeth according to the gap voltage and discharge current to maintain a working gap suitable for pulse discharge. At the same time, take advantage of the difference in actual discharge gap between different tooth surface positions to allow the area with smaller gap to enter the discharge state first. After the material in this area is eroded and the local gap increases, continue to adjust the axial position to allow other smaller gap areas to enter the discharge state in sequence. This allows the discharge area to gradually migrate with the actual tooth surface error distribution, which helps to reduce local protrusions and local gap differences, and gradually coordinates the relative contours of the two actual mating tooth surfaces.

[0027] 5. During the fine grinding process, the discharge polarity of the impeller and the spindle is alternately changed so that the two end face teeth are in different electrode states in different discharge cycles, so that both actual mating tooth surfaces can participate in discharge dressing. Furthermore, the cumulative discharge energy is determined based on the actual discharge voltage, discharge current and discharge duration in each discharge cycle, and the polarity switching is controlled accordingly. This ensures that the polarity switching corresponds to the actual discharge machining process, reducing the difference in machining amount caused by abnormal discharge states such as open circuit and short circuit when using fixed-time switching, which is conducive to maintaining balanced dressing between the two mating tooth surfaces.

[0028] 6. By acquiring the axial position adjustment amount and the discharge state between the relative tooth surfaces during the continuous discharge cycle, the machining stability during the fine grinding process is judged, and the machining is stopped when the axial adjustment and discharge state tend to be stable. This makes the machining end condition no longer dependent on the preset fixed machining time, but can reflect the changes in the actual pairing discharge process of the two end face teeth. This helps to avoid excessive material removal caused by insufficient dressing or continuous discharge, and improves the consistency and controllability of the entire pairing fine machining process. Attached Figure Description

[0029] Figure 1 This is an overall flowchart of the present invention; Figure 2 This is a flowchart illustrating the rough grinding process, the fine grinding process, and the post-treatment process of the tooth surface in this invention. Figure 3 This is a schematic diagram of the electrical discharge machining clamping method in this invention; Figure 4 This is a development diagram of the end face teeth in this invention; Auxiliary markings: 1. Impeller; 2. Main shaft; 3. Impeller end face teeth; 4. Main shaft end face teeth. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Identical components are denoted by the same reference numerals. It should be noted that the terms "front," "rear," "left," "right," "upper," and "lower" used in the following description refer to directions in the accompanying drawings, and the terms "bottom surface," "top surface," "inner," and "outer" refer to directions toward or away from the geometric center of a specific component, respectively.

[0031] Example 1 This embodiment provides a method for precision machining of impeller and main shaft end face teeth using electrical discharge machining (EDM), applicable to the matching precision machining of impeller end face teeth and main shaft end face teeth in industrial steam turbines. During machining, the impeller and main shaft, which ultimately need to be assembled, are treated as a single machining combination. Instead of using separate tool electrodes to perform final tooth surface shaping on both, the impeller end face teeth and main shaft end face teeth form an EDM electrode pair, and rough and fine grinding are completed while maintaining the final assembly tooth sequence relationship.

[0032] The electrical discharge machining equipment implementing this method has two coaxial clamping positions for mounting the impeller and the spindle, respectively. At least one clamping position can move axially along the common axis of the impeller and the spindle, and is controlled by an axial servo mechanism to move closer to or away from the other clamping position. The two clamping positions have circumferential positioning and locking functions, which are used to restrict the relative circumferential rotation of the impeller and the spindle after the circumferential pairing position is determined. The electrical discharge machining equipment is also equipped with a pulse power supply, an insulating working fluid supply unit, a gap voltage acquisition unit, a discharge current acquisition unit, a polarity switching unit, and a controller, wherein the controller is connected to the axial servo mechanism, the gap voltage acquisition unit, the discharge current acquisition unit, the pulse power supply, and the polarity switching unit, respectively.

[0033] like Figure 1-4As shown, before machining begins, the impeller end face teeth and the main shaft end face teeth are rough-machined and semi-finished respectively, so that the impeller end face teeth and the main shaft end face teeth form a basic tooth shape corresponding to the design contour. However, the two end face teeth are not machined to the final mating contour at this stage. Instead, a 0.1mm grinding allowance is retained on each side of the mating contour. In this way, the roughing and semi-finishing processes are responsible for forming the basic contour of the end face teeth, while the final material related to the correspondence between the two actual mating tooth surfaces is removed and retained for the subsequent EDM grinding stage.

[0034] After pre-machining, the impeller and spindle are installed in two clamping positions on the EDM machine. The two clamping positions are adjusted to ensure that the impeller axis and the spindle axis are coaxial. Specifically, the spindle can be installed in a fixed clamping position, and the impeller in a clamping position with axial movement freedom. Alternatively, the impeller can be fixed while the spindle moves axially. The key is to change the axial distance between the impeller end face teeth and the spindle end face teeth while maintaining their coaxiality and circumferential correspondence.

[0035] When establishing the circumferential pairing relationship between the impeller and the main shaft, the circumferential positioning datum used during the final assembly of the impeller and the main shaft is used as the alignment basis. For impellers and main shafts with predetermined assembly marks, the datum end face teeth are first determined according to the assembly marks, and then the datum end face teeth and the remaining end face teeth are aligned one-to-one according to the tooth sequence relationship during final assembly. For impellers and main shafts whose assembly positions are determined by process datums, the pairing position of the end face teeth is determined according to the corresponding circumferential process datum. After completing the circumferential alignment, the relative circumferential rotation between the impeller and the main shaft is restricted by the circumferential locking mechanism of the clamping position.

[0036] After circumferential locking is completed, the tooth sequence correspondence between the impeller and the spindle will not be changed during subsequent rough and fine grinding processes. Only the axial relative position between the two is allowed to be changed through the axial servo mechanism. Therefore, a tooth on the impeller end face always corresponds to the tooth on the spindle end face that it mates with during the final assembly throughout the entire grinding process, ensuring that the subsequent tooth surface dressing relationship is consistent with the final assembly relationship.

[0037] The axial servo mechanism is controlled to gradually bring the impeller and spindle closer together until the impeller end face teeth and spindle end face teeth reach a state where their tooth profiles correspond but their relative tooth surfaces do not contact each other. In this state, the initial discharge gap required for EDM is maintained between the two end face teeth. The impeller and spindle are connected to the two output poles of the pulse power supply, respectively, while the machining areas containing the impeller end face teeth and spindle end face teeth are immersed in an insulating working fluid, thereby forming a machining electrode pair that discharges between each other.

[0038] During rough grinding, in this embodiment, the initial discharge gap between the impeller end face teeth and the spindle end face teeth is adjusted to 0.1 mm, the discharge current is set to 25 A, and the pulse width is set to 42 μs. After starting the pulse power supply, a pulse voltage is applied between the two opposing tooth surfaces. The insulating working fluid forms a discharge channel at the location where the breakdown condition is met, and the instantaneous heat generated by the pulse discharge erodes the material at the corresponding tooth surface location.

[0039] During the rough grinding process, the controller continuously acquires the gap voltage output by the gap voltage acquisition unit and the discharge current output by the discharge current acquisition unit, and identifies the discharge state of the relative tooth surfaces based on the changes of the two during the pulse application.

[0040] When a pulse voltage is applied and a high gap voltage is maintained between the relative tooth surfaces without a corresponding discharge current, it indicates that the current local gap has not yet reached the effective breakdown condition, and the current state is judged as an open circuit state. When the gap voltage breaks down during the pulse action, and a discharge current corresponding to the current discharge parameters is formed, and the insulation state is restored after the pulse ends, the current state is judged as a normal pulse discharge state. When the gap voltage remains in a low voltage state and is accompanied by a continuous current, or when the normal insulation state is not restored between adjacent pulses, the current state is judged as a short circuit state or a continuous arc state.

[0041] The range of each of the above states is pre-stored in the controller and can be determined by adjusting the processing equipment based on the output characteristics of the pulse power supply, the insulating working fluid used, and the roughing discharge parameters. It is not required that a single fixed voltage value or a single fixed current value be applicable to all processing conditions.

[0042] When the controller detects that the relative tooth surfaces are continuously in an open-circuit state, it controls the axial servo mechanism to bring the impeller and the main shaft closer together axially, gradually reducing the discharge gap. During the approach process, the gap voltage and discharge current are continuously monitored until the relative tooth surfaces re-enter the normal pulse discharge state. When a short-circuit state or a continuous arc state is detected, the controller controls the axial servo mechanism to move the impeller and the main shaft away from each other axially, widening the gap between the relative tooth surfaces and allowing the insulating working fluid to re-enter between the relative tooth surfaces until the abnormal discharge state disappears and normal pulse discharge resumes.

[0043] When the relative tooth surfaces maintain a normal pulse discharge state, the controller maintains the current axial position, or keeps the impeller and main shaft moving closer together axially according to a small feed amount that the axial servo mechanism can stably execute. After each axial adjustment, the discharge state is reconfirmed based on the gap voltage and discharge current, so that the axial position adjustment and the actual discharge gap state form a closed loop relationship.

[0044] Because the impeller end face teeth and the main shaft end face teeth still have their own actual contour deviations after roughing and semi-finishing, when the two end face teeth correspond to each other according to the final assembly tooth sequence, the actual gap between different positions of their relative tooth surfaces is not completely consistent. When the impeller and the main shaft approach each other axially, the positions where the local tooth surfaces protrude or the two tooth surfaces are closer to each other have smaller discharge gaps. Therefore, this area reaches the breakdown condition of the insulating working fluid first and generates pulse discharge.

[0045] After continuous pulsed discharge, the material at the corresponding tooth surface location in the smaller gap area is gradually eroded away, reducing the original local protrusion and increasing the local discharge gap at that location. As the gap at this location increases, the conditions for preferential discharge in this area gradually weaken. Based on the overall discharge state, the controller continues to drive the axial servo mechanism to move slightly closer axially, gradually bringing other local areas with slightly larger gaps into the gap range capable of generating pulsed discharges.

[0046] Therefore, with the circumferential pairing position of the impeller and the main shaft remaining unchanged, the discharge area can gradually shift from the original smaller gap area to other smaller gap areas as material is eroded and the axial position is adjusted at each local location. This migration of the discharge area does not depend on the circumferential relative movement between the impeller and the main shaft, nor does it require an external machining tool to scan along the tooth surface according to a preset trajectory. Instead, it utilizes the local gap difference formed by the two actual mating tooth surfaces themselves to determine the positions at which they enter the effective discharge state sequentially.

[0047] By continuously performing the aforementioned axial clearance adjustment and discharge area migration process, the main grinding allowance retained in the pre-machining stage is gradually eroded away. During the rough grinding process, the impeller and spindle always maintain their original clamping state and circumferential pairing position, avoiding re-clamping or reallocation of the gear sequence correspondence during the machining process.

[0048] After rough grinding, without disassembling the impeller and spindle, and without releasing their circumferential locking, the EDM equipment is switched from roughing discharge parameters to finishing discharge parameters. In this embodiment, the discharge gap between the impeller end face teeth and the spindle end face teeth is adjusted to 0.05mm, the discharge current is adjusted to 10A, the pulse width is adjusted to 5μs, and a non-contact discharge state adapted to the finishing discharge parameters is re-established through the axial servo mechanism.

[0049] After entering the fine grinding stage, a lower discharge current and shorter pulse width are used to continue the discharge erosion of the corresponding tooth surfaces after the rough grinding, so that subsequent material removal is concentrated on the remaining local contour differences between the two actual mating tooth surfaces. During the fine grinding, the circumferential mating position of the impeller and the main shaft remains unchanged, so that the tooth surface correspondence formed by the rough grinding continues into the fine grinding stage.

[0050] To avoid maintaining a fixed discharge polarity throughout the entire fine grinding process, this embodiment divides the fine grinding process into multiple sequential discharge cycles. During each discharge cycle, the impeller and spindle remain connected to one of the two output poles of the pulse power supply. After the current discharge cycle is completed, the connections between the impeller and spindle and the two output poles of the pulse power supply are interchanged.

[0051] In practice, during the first discharge cycle, the impeller is connected to the first output terminal of the pulse power supply, and the main shaft is connected to the second output terminal. After the polarity switching condition for the current discharge cycle is met, the controller stops the pulse power supply output and confirms that the discharge current between the impeller end face teeth and the main shaft end face teeth has disappeared. Subsequently, the polarity switching unit is controlled to change the connection relationship, connecting the impeller to the second output terminal and the main shaft to the first output terminal, and then starting the next discharge cycle. After the next discharge cycle ends, the polarity is reversed again in the same way, thus causing the impeller end face teeth and the main shaft end face teeth to alternately change the discharge polarity during the fine grinding process.

[0052] The aforementioned polarity switching only changes the electrical connection between the impeller and the main shaft and the pulse power supply; it does not change the clamping position or circumferential pairing relationship between the impeller and the main shaft. Before switching polarity, the pulse power supply output should be stopped, and the switching should be performed after the discharge current disappears. Then, the pulse voltage should be reapplied to avoid directly changing the output polarity connection while the discharge current is present.

[0053] For each electron discharge cycle, the controller continuously acquires the discharge voltage, discharge current, and duration of each normal pulse discharge between the impeller end face teeth and the main shaft end face teeth, and uses the above data to determine the cumulative discharge energy in the current electron discharge cycle.

[0054] Specifically, the controller first identifies each pulse according to the aforementioned discharge state determination method, and only pulses identified as normal discharge pulses are used as valid pulses for accumulating discharge energy. For each valid pulse, the discharge energy corresponding to that pulse is determined based on the collected discharge voltage, discharge current, and the actual duration of the valid pulse, and the discharge energy corresponding to each valid pulse is continuously accumulated according to the pulse occurrence sequence. Open-circuit pulses that do not generate normal discharge current and abnormal pulses identified as short-circuit or continuous arc states are not used as the basis for accumulating the normal discharge processing amount of the current discharge cycle.

[0055] In one specific data processing method, the controller synchronously collects the discharge voltage and discharge current during the effective discharge pulse according to the set sampling interval. The voltage and current values ​​obtained at the same sampling moment are combined with the sampling interval to determine the electrical energy within the corresponding time period. The electrical energy corresponding to each sampling time period during the duration of an effective pulse is accumulated sequentially to obtain the discharge electrical energy corresponding to the effective pulse. Subsequently, the discharge electrical energy corresponding to each effective pulse in the current discharge cycle is accumulated to form the cumulative discharge energy of the current discharge cycle.

[0056] For different polarity connection states of the impeller and main shaft, corresponding polarity switching conditions are preset in the controller. These conditions can be determined through preliminary process experiments based on the impeller material, main shaft material, finishing discharge current, pulse width, and discharge erosion characteristics under the corresponding polarity conditions. When the impeller and main shaft are made of the same material, corresponding switching conditions can be set according to the processing characteristics of the same material under different polarity states; when the impeller and main shaft are made of different materials, different cumulative discharge energy switching conditions can be set for each of the two polarity connection relationships.

[0057] When the controller detects that the accumulated discharge energy of the current discharge cycle has reached the preset switching condition corresponding to the current polarity connection, it stops the current discharge cycle, performs the aforementioned polarity swap, and restarts the accumulation at the beginning of the next discharge cycle. Therefore, the switching of the discharge cycle is determined based on the actual normal pulse discharge process occurring under the current polarity state, rather than solely on the fixed time elapsed since the start of the discharge cycle.

[0058] During the fine grinding process, a machining termination determination is simultaneously executed. To ensure that the termination determination reflects whether the axial clearance control still requires frequent adjustments and whether the discharge state between the relative tooth surfaces has stabilized, the controller records the adjustment status of the axial servo mechanism and the corresponding discharge state in each discharge cycle.

[0059] For axial position adjustment, whenever the controller issues an axial approach or axial departure command to the axial servo mechanism based on open circuit, normal pulse discharge, short circuit, or continuous arc state, the direction and displacement of that adjustment are recorded. At the end of a discharge cycle, the actual displacement corresponding to each axial adjustment within that cycle is summarized to characterize the degree of axial position adjustment required to maintain normal pulse discharge in the current discharge cycle. This axial position adjustment is calculated using the actual adjustment amounts from each instance, rather than solely using the net position difference between the start and end positions of the discharge cycle, to avoid multiple approaches and departures within a single cycle canceling each other out and failing to reflect the actual degree of axial adjustment.

[0060] For the discharge state, the controller statistically analyzes the occurrence of normal pulse discharge state, open circuit state, short circuit state, and continuous arc state for each discharge cycle. Based on the prevalence of normal effective discharge pulses in the current discharge cycle and the changes in abnormal discharge states, the controller determines the stable discharge state of the current cycle. The above statistics do not require using the state at a single instant as the basis for processing termination; rather, they observe the state changes between multiple consecutive discharge cycles.

[0061] In the initial stage of fine grinding, there are still local gap differences between the two opposing tooth surfaces that require further adjustment. As different local areas successively enter the pulse discharge state, the axial servo mechanism needs to be adjusted axially closer or further away relatively frequently. At the same time, the open circuit, normal discharge, and abnormal discharge states in each discharge cycle will change as local material on the tooth surface is eroded away. As the small local gap areas of the two actual mating tooth surfaces are gradually eroded away, the required axial gap adjustment gradually decreases, and the changes in discharge state between different discharge cycles also gradually decrease.

[0062] Therefore, the controller selects multiple consecutive electron discharge cycles as the judgment interval for the stable processing state, and continuously compares the axial position adjustment and discharge state of each electron discharge cycle within the judgment interval. When the axial position adjustment within multiple consecutive electron discharge cycles remains within the preset axial stability range, the change in axial position adjustment between adjacent electron discharge cycles remains within the preset change range, the normal pulse discharge state remains within the preset normal processing range, and there is no continuous short circuit or continuous arc state, the current fine grinding process is determined to have reached the preset stable condition.

[0063] The preset axial stability range and corresponding variation range are determined in advance based on the axial feed resolution of the EDM equipment, the machining accuracy of the teeth on the end face to be machined, and the finishing discharge parameters used. The preset normal machining range corresponding to the normal pulse discharge state is determined in advance based on the discharge state that can maintain stable discharge erosion under the pulse power supply, insulating working fluid, and finishing discharge parameters used. The above parameters can be calibrated through process tests before the formal processing of products of the same specification and stored in the controller.

[0064] To avoid premature termination of processing due to disturbances in the insulating working fluid, localized intermittent discharges, or fluctuations in the acquired signal during a single electron discharge cycle, this embodiment does not use the achievement of a stable condition in a single electron discharge cycle as the termination criterion. Instead, it requires that the stable state be continuously satisfied over multiple consecutive electron discharge cycles. When the preset stable condition is met for multiple consecutive electron discharge cycles, the controller stops the fine grinding process and stops the pulse power supply output.

[0065] The aforementioned termination determination method links the end of the machining process to the actual degree of axial adjustment and the actual discharge state required during the fine grinding process. When there are still significant local differences between the relative tooth surfaces, requiring continued axial compensation and regional discharge erosion, the controller continues the fine grinding process. When the axial position adjustment and pulse discharge state both tend to stabilize during continuous machining, the fine grinding process is then terminated, thus avoiding the need to determine the machining endpoint solely based on a fixed machining duration.

[0066] After the fine grinding is completed, the pulse power supply output is stopped, the axial servo mechanism is controlled to move the impeller and the spindle away from each other, and both are removed from the EDM equipment. The EDM surfaces of the impeller end face teeth and the spindle end face teeth are polished to remove the recast layer and microcracks formed on the tooth surface during the rough and fine EDM grinding processes.

[0067] The polishing process employs a low-material-removal surface treatment method, primarily aimed at removing surface defects formed during electrical discharge machining (EDM). This avoids further significant reshaping of the main body contour of the end face teeth formed by the paired EDM during the polishing stage. After polishing, the impeller and spindle are fitted according to the circumferential mating positions determined during the grinding process. The fit after machining can be confirmed through tooth surface contact checks and end face tooth indexing checks, and the same circumferential tooth sequence relationship is maintained for subsequent assembly.

[0068] Example 2 The pre-processing, clamping and alignment, pairing phase locking, axial clearance adjustment, discharge area migration, polarity alternation, discharge energy switching, and processing termination determination methods in this embodiment are the same as in Embodiment 1. The difference is that a different set of discharge parameters is used for rough grinding and fine grinding.

[0069] In the rough grinding process, the discharge gap between the impeller end face teeth and the spindle end face teeth is adjusted to 0.12mm, the discharge current is set to 28A, and the pulse width is set to 45μs. Under the above processing parameters, the discharge state of the current relative tooth surfaces is continuously judged by the gap voltage and discharge current, and the axial relative position of the impeller and the spindle is adjusted according to the discharge state, so that the local small gap areas enter the pulse discharge state in sequence, and the reserved grinding allowance is removed by discharge.

[0070] After rough grinding is completed, without changing the clamping relationship between the impeller and the main shaft and the circumferential pairing position, the discharge gap is adjusted to 0.06mm, the discharge current is adjusted to 12A, and the pulse width is adjusted to 6μs, and the fine grinding stage is entered. This parameter combination belongs to another implementation method given in the applicant's original disclosure.

[0071] During the fine grinding stage, multiple electron discharge cycles are divided according to the method in Example 1, and the connection relationship between the impeller and the main shaft and the two output poles of the pulse power supply is changed alternately between adjacent electron discharge cycles. The cumulative discharge energy is determined according to the voltage, current and duration of the actual normal pulse discharge in each electron discharge cycle. Polarity switching is performed according to the preset switching conditions, and the axial position adjustment and discharge state of multiple consecutive electron discharge cycles are used to determine whether the fine grinding process has reached the stable termination condition.

[0072] Therefore, the axial clearance adjustment, discharge area migration, polarity alternation, and machining termination control are not limited to the single discharge parameter used in Example 1, but can be implemented in conjunction with different roughing and finishing discharge parameters within the scope of the claims.

[0073] Example 3 This embodiment further illustrates the method for determining the preset switching conditions and processing termination determination conditions during the polarity alternation processing.

[0074] Before batch processing of impellers and spindles of the same specification, a pre-processing test can be conducted using process parts with the same or similar materials, end face tooth structure, and finishing discharge parameters as the impellers and spindles to be processed. During the process test, normal pulse discharge states of the impeller under two different discharge polarities are established. The gap voltage, discharge current, duration, and tooth surface state after processing are recorded for each polarity state. The corresponding control conditions are determined based on the processing range that can maintain stable finishing without continuous short circuits, continuous arcs, and obvious abnormal erosion.

[0075] For polarity switching conditions, an allowable cumulative discharge energy range can be determined for each polarity state, and the corresponding switching conditions can be stored in the controller. When the cumulative discharge energy during formal processing reaches the switching condition corresponding to the current polarity, a polarity switch is executed. Since the impeller and spindle may have different discharge erosion characteristics due to material type, thermophysical properties, and different polarity conditions, the two polarity states are allowed to use corresponding switching conditions respectively, without requiring them to use the exact same cumulative discharge energy threshold.

[0076] For the processing termination criteria, the axial servo adjustment and pulse discharge status are recorded during the process test as the fine grinding process gradually enters a stable state from the initial dressing state. The axial adjustment range that can maintain normal fine grinding is used as the basis for setting the axial stability condition, and the state range where normal pulse discharge is continuously stable and there is no continuous abnormal discharge is used as the basis for setting the discharge stability condition. During formal processing, the controller compares the actual processing state of multiple consecutive discharge cycles with the above preset conditions, and stops the fine grinding process only when the axial position adjustment and pulse discharge status continuously meet the stability requirements.

[0077] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for precision machining of the impeller and the end face teeth of the main shaft by electrical discharge machining, characterized in that: Includes the following steps: The pre-machining process involves roughing and semi-finishing the impeller end face teeth and the main shaft end face teeth, respectively, and reserving grinding allowances on the mating contours of the impeller end face teeth and the main shaft end face teeth. Clamping and alignment steps: The pre-machined impeller and the main shaft are coaxially installed on the EDM equipment. The axial relative position of the impeller and the main shaft is adjusted so that the corresponding end face teeth are in a non-contact state with corresponding tooth profiles and a discharge gap between the relative tooth surfaces. The impeller and the main shaft are respectively connected to the two output poles of the pulse power supply so that the end face teeth of the impeller and the end face teeth of the main shaft form a machining electrode pair that discharges to each other. Rough grinding process: Place the impeller end face teeth and the main shaft end face teeth in an insulating working fluid. While maintaining the tooth profile correspondence between the impeller end face teeth and the main shaft end face teeth, use rough grinding discharge parameters to form a pulse discharge between the relative tooth surfaces to discharge and remove the grinding allowance of the impeller end face teeth and the main shaft end face teeth. Fine grinding process: Switch the roughing discharge parameters to the fine grinding discharge parameters, adjust the discharge gap between the impeller end face teeth and the main shaft end face teeth, and continue to perform discharge erosion through pulse discharge between the relative tooth surfaces; The post-processing steps for the tooth surfaces involve separating the impeller and the main shaft after fine grinding. The EDM surfaces of the impeller end face teeth and the main shaft end face teeth are then polished to remove the recast layer and microcracks from the EDM surfaces.

2. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 1, characterized in that: In the pre-processing step, the grinding allowance reserved on the mating contours of the impeller end face teeth and the main shaft end face teeth is 0.1 to 0.2 mm on each side.

3. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 1, characterized in that: In the rough grinding process, the roughing discharge parameters include a discharge gap of 0.1–0.2 mm, a discharge current of 15–30 A, and a pulse width of 30–50 μs.

4. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 1, characterized in that: In the fine grinding process, the fine grinding discharge parameters include a discharge gap of 0.05–0.1 mm, a discharge current of 6–12 A, and a pulse width of 3–6 μs.

5. The method for precision machining of the impeller and the spindle end face teeth by electrical discharge grinding according to any one of claims 1-4, characterized in that: The clamping and alignment steps include a pairing phase locking strategy, which includes: Based on the tooth sequence correspondence of the end face teeth when the impeller and the main shaft are finally assembled, the circumferential pairing position of the impeller end face teeth and the main shaft end face teeth is determined; after determining the circumferential pairing position, the relative circumferential rotation between the impeller and the main shaft is restricted, and the circumferential pairing position is kept unchanged in the rough grinding and fine grinding steps, so that the relative position of the impeller and the main shaft is adjusted along the axial direction.

6. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 5, characterized in that: The rough grinding process includes an axial clearance adjustment strategy, which includes: The gap voltage and discharge current between the impeller end face teeth and the main shaft end face teeth are obtained, and the discharge state between the relative tooth surfaces is determined based on the gap voltage and discharge current. When the discharge state is an open circuit state, the impeller and the main shaft are controlled to move closer to each other along the axial direction until the relative tooth surfaces enter a pulse discharge state. When the discharge state is a short circuit or a continuous arc state, the impeller and the main shaft are controlled to move away from each other along the axial direction until the pulse discharge state is restored, and the axial relative position of the two is maintained or gradually adjusted in the pulse discharge state.

7. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 6, characterized in that: The rough grinding process includes a discharge region migration strategy, which includes: While keeping the circumferential pairing position of the impeller and the main shaft unchanged, the axial relative position is adjusted so that the area with a smaller partial discharge gap in the relative tooth surface enters the pulse discharge state; the area that has entered the pulse discharge state is eroded by discharge, so that the discharge gap of the corresponding area increases, and the axial relative position of the impeller and the main shaft is adjusted again so that other areas with smaller partial discharge gaps enter the pulse discharge state in sequence.

8. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 7, characterized in that: The fine grinding process includes a polarity alternation processing strategy, which includes: The precision grinding process is divided into multiple consecutive electron discharge cycles. Between adjacent electron discharge cycles, the connection between the impeller and the main shaft and the two output poles of the pulse power supply is interchanged, so that the discharge polarity of the impeller end face teeth and the main shaft end face teeth alternately changes, while keeping the circumferential pairing position of the impeller and the main shaft unchanged in each electron discharge cycle.

9. The method for precision machining of impeller and spindle end face teeth by electrical discharge grinding according to claim 8, characterized in that: The fine grinding process includes a discharge energy switching strategy, which includes: The discharge voltage, discharge current, and corresponding discharge duration between the impeller end face teeth and the main shaft end face teeth in the current discharge cycle are obtained. The cumulative discharge energy of the current discharge cycle is determined based on the discharge voltage, discharge current, and discharge duration. When the cumulative discharge energy reaches the preset switching condition, the connection relationship between the impeller and the main shaft and the two output poles of the pulse power supply is swapped, and the discharge energy of the next discharge cycle is re-accumulated after the connection relationship is swapped.

10. The method for precision machining of impeller and main shaft end face teeth by electrical discharge grinding according to claim 9, characterized in that: The fine grinding process includes a processing termination determination strategy, which includes: The axial position adjustment of the impeller and the main shaft and the discharge state between the relative tooth surfaces are obtained within multiple consecutive electron discharge cycles. The stability of the fine grinding process is determined based on the changes in the axial position adjustment and the pulse discharge state between adjacent electron discharge cycles. When the axial position adjustment and the pulse discharge state of multiple consecutive electron discharge cycles meet the preset stability conditions, the fine grinding process is stopped.

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