Workpiece positioning machining method of a CNC machining center

Abstract

The application discloses a workpiece positioning machining method of a CNC machining center. The process of positioning clamping to machining clamping is divided into multiple clamping stages, the same stage reference point and stage monitoring point are measured before and after each clamping action, the increment rigid body deviation of adjacent stages is deducted, and the stage local deformation increment corresponding to the clamping action is obtained; when the increment exceeds the limit, the subsequent clamping is stopped and the current clamping stage is returned to adjust. After all clamping is completed, the main contribution stage of the final over-limit deformation is identified according to the increments of each stage; when it is qualified, the increments of each stage are respectively interpolated and superimposed to form a local clamping deformation field. After rough machining and / or semi-finish machining, the clamping is kept and re-measured, the material removal local deformation field is obtained and superimposed with the local clamping deformation field, and then finish machining and in-machine verification are performed. The application can prevent intermediate excessive deformation from being offset and covered by subsequent clamping, and reduce machining errors caused by elastic preloading release.

Description

Technical Field

[0001] This invention belongs to the field of computer numerical control (CNC) machining and in-machine measurement technology, specifically involving clamping deformation identification, clamping process control and machining compensation technology for machining centers. Background Technology

[0002] Computer Numerical Control (CNC) machining centers typically restrict the workpiece's degrees of freedom using fixtures and establish a workpiece coordinate system through alignment, tool setting, or in-machine measurement. For regular blanks and workpieces with high rigidity, a relatively stable machining datum can be obtained by relying on locating surfaces, locating pins, and fixed clamping force. However, for castings, forgings, welded parts, thin-walled parts, and irregularly shaped workpieces, there are differences in blank allowance, initial shape, and local stiffness. When the workpiece switches from a lightly clamped state to an actual machining clamped state, in addition to overall translation and rotation, local elastic deformation may also occur at clamping points, support points, and weak areas.

[0003] Existing technologies have proposed using on-machine probes to measure the zero-point datum of the workpiece and automatically set the coordinate system. For example, CN113009881A discloses a method for automatic coordinate system setting and online measurement in CNC machining, CN104139321B discloses an automatic alignment method for in-situ measurement of large structural parts, and CN104476109A discloses a positioning machining method that first sets a rough datum and then determines the finishing datum through online measurement. The above solutions mainly focus on measurement compensation for the overall position of the workpiece or the machining result.

[0004] Furthermore, CN104759942A discloses an in-machine measurement and deformation compensation method for thin-walled parts after machining, CN113245884A discloses a system and method for rapid workpiece changeover using zero-point positioning technology, and CN204075878U discloses a device for completing secondary datum conversion by detecting secondary machining datum points. Existing clamping and positioning schemes can also use dial indicators to observe whether the workpiece undergoes significant deformation during clamping, but this is usually only used to confirm the final state or for manual adjustment, and does not establish a correspondence between a single clamping action, stage-based local deformation, and subsequent machining compensation.

[0005] For workpieces subjected to multiple clamping actions in sequence, different clamping actions may cause local deformations in opposite directions. If only the final state data before and after clamping are compared, the excessive deformation caused by the previous clamping action may be partially offset by the reverse deformation of the subsequent clamping action, making the final displacement surface meet the requirements, but the workpiece still maintains a large elastic preload. After rough machining removes material and changes the local stiffness, this elastic preload is released, which can easily cause contour springback and mismatch in finishing allowance. It is also difficult to determine which clamping part should be adjusted if only a unified adjustment or compensation is made after clamping is completed. Therefore, it is necessary to perform rigid body separation, stage-by-stage judgment, and contribution tracing for the local deformation increment caused by each clamping action, and to continue to use the obtained stage data in the machining path compensation. Summary of the Invention

[0006] The purpose of this invention is to provide a workpiece positioning and machining method for CNC machining centers, in order to solve the problems in the prior art where it is difficult to identify the local deformation contribution of a single clamping action, excessive deformation in the intermediate stage may be offset or masked by subsequent clamping, it is difficult to determine the main adjustment object when the final state exceeds the limit, and it is difficult to accurately compensate for the elastic preload release and workpiece stiffness changes after rough machining.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A workpiece positioning and machining method for a CNC machining center is applied to a computer numerical control (CNC) machining center equipped with a probe and a workpiece fixture, wherein the workpiece fixture can switch between a positioning clamping state and a machining clamping state.

[0008] Step S1: Import the nominal model of the workpiece, set the reference point set and deformation monitoring point set, and establish the nominal workpiece coordinate system and initial machining path.

[0009] Step S2: Place the workpiece in the workpiece fixture, measure the reference point set and deformation monitoring point set under the positioning and clamping state, obtain the first reference data and the first monitoring data, determine the initial pose of the workpiece based on the first reference data and check the remaining machining allowance.

[0010] Step S3: Divide the clamping process from positioning clamping state to machining clamping state into multiple clamping parts. A clamping phase, ≥2, and perform clamping actions in sequence from the side closer to the main positioning reference to the side farther away from the main positioning reference; in the first Before and after each clamping action, the same subset of stage reference points and stage monitoring points are measured to obtain data for adjacent clamping stages.

[0011] Step S4: Calculate the first clamping stage based on the stage reference data of the adjacent clamping stages. The incremental rigid body deviation is calculated by subtracting the incremental rigid body deviation from the stage monitoring data of adjacent clamping stages to obtain the first stage. Each clamping action corresponds to a stage of local deformation increment; when the stage of local deformation increment exceeds the limit, the subsequent clamping action is stopped, the current clamping stage is returned to and adjusted, and the next clamping stage is entered only after the stage of local deformation increment is qualified.

[0012] Step S5: After all clamping stages are completed, measure the reference point set and deformation monitoring point set under the machining clamping state, and obtain the total rigid body deviation and the final local clamping deformation; when the final local clamping deformation exceeds the limit, determine the main contributing clamping stage according to the contribution of the local deformation increment of each stage in the direction of the excess limit and return to adjustment; when the final local clamping deformation is qualified, update the workpiece coordinate system according to the total rigid body deviation, interpolate the local deformation increment of each stage respectively and vector superimpose to form the local clamping deformation field of clamping action decomposition, and correct the machining path accordingly.

[0013] Step S6: Perform roughing and / or semi-finishing according to the updated workpiece coordinate system and the corrected machining path.

[0014] Step S7: After roughing and / or semi-finishing, while maintaining the machining clamping state, measure the same set of reference points and deformation monitoring points again, separate the updated rigid body deviation after material removal and the local deformation increment after material removal, interpolate the local deformation increment after material removal to form the local deformation field after material removal and superimpose it with the local clamping deformation field; update the workpiece coordinate system and machining path accordingly, perform finishing and perform on-machine verification.

[0015] Furthermore, the reference point set includes at least three main reference measurement points for fitting the main positioning reference surface and at least two orientation measurement points for determining the workpiece orientation; the stage reference point subset includes at least three non-collinear reference points that can be measured in each clamping stage. The stage monitoring point subset includes at least monitoring points near the current clamping location and monitoring points near the area to be finished corresponding to the current clamping location, so as to simultaneously reflect the local displacement of the clamping input end and the machining response end.

[0016] Furthermore, taking the minimum distance residual between corresponding reference points of adjacent clamping stages as a constraint, the rigid body rotation matrix for the k-th stage is obtained. and rigid body translation vector For the first There are several monitoring points at each stage, and the measurement coordinates of adjacent stages are as follows: , and Then the incremental local deformation caused by the k-th clamping action is Therefore, the local deformation increment at each stage has already deducted the overall translation and rotation caused by that stage.

[0017] Furthermore, the stage deformation evaluation value can be the maximum absolute component of the stage local deformation increment along the nominal machined surface normal, or the vector magnitude of the stage local deformation increment. For monitoring points where the final local clamping deformation exceeds the limit, the unit vector of its exceeding deformation direction is used. As a direction for contribution evaluation, the contribution of the k-th clamping stage is: , The main contributing clamping stage is determined based on the magnitude of the contribution, thereby avoiding aimless and repeated trial adjustments between multiple clamping points.

[0018] Furthermore, a set of allowance evaluation points is set up, and the remaining machining allowance is checked based on the measured distance between the blank surface and the nominal machined surface. When the allowance is insufficient, the translation adjustment amount and / or rotation adjustment amount of the workpiece are output.

[0019] Furthermore, a local deformation field is constructed for each clamping stage that passes the stage deformation determination. For the tool position... , No. The local deformation field during the clamping stage is represented as follows: In the formula, To participate in the The number of deformation monitoring points interpolated during each clamping stage; According to the first Deformation monitoring points and tool placement points The spatial distance between them is determined and normalized to a weight that satisfies: , For the first The local deformation increment generated at each deformation monitoring point during the k-th clamping stage. The local deformation fields of each clamping stage are unified to the same workpiece coordinate system and superimposed according to the same deformation direction definition to obtain the local clamping deformation field: After roughing and / or semi-finishing, the local deformation field after machining is constructed based on the retest data. This is then superimposed on the local clamping deformation field to obtain an updated local deformation field for finishing path correction: .

[0020] Furthermore, a process datum group, including a process datum surface, a first process datum hole, and a second process datum hole, is machined in the low deformation allowance area of ​​the first clamping, and the datum transformation relationship between the process datum group and the workpiece coordinate system is recorded so as to reconstruct the workpiece coordinate system at the time of the first clamping after changing the surface or re-clamping.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention measures the same stage reference point and stage monitoring point before and after each clamping action, and first deducts the incremental rigid body deviation of adjacent stages, so that the local deformation caused by a single clamping action can be separated from the overall posture change and establish a corresponding relationship with the specific clamping part.

[0022] 2. This invention sets a threshold for the incremental local deformation at each stage. When the increment exceeds the limit, subsequent clamping is stopped and the current stage is adjusted. This can prevent excessive deformation in the intermediate stage from being offset or masked by subsequent reverse clamping, thereby reducing the risk of hidden elastic preload and springback after material removal from the source.

[0023] 3. When the final local clamping deformation exceeds the limit, the contribution of the local deformation increment in each stage in the direction of the excess is used to determine the main contributing clamping stage, so that the final state detection results can be traced back to the specific clamping action, reducing aimless trial adjustments.

[0024] 4. The incremental local deformation at each stage is used for both clamping process determination and contribution tracing. It is also interpolated to form the stage deformation contribution field and superimposed as the local clamping deformation field. The same set of stage data connects clamping control and processing compensation, forming an interconnected technical chain, rather than independent measurement, adjustment and interpolation steps.

[0025] 5. After roughing or semi-finishing, maintain the established clamping state for retesting. Superimpose the local deformation field of the material removal and the local clamping deformation field decomposed by the clamping action in the same coordinate system. This can adapt to elastic preload release, internal stress release, local stiffness changes and machine tool thermal state changes, and improve the targeting of the finishing path.

[0026] This invention can also reconstruct the original workpiece coordinate system after face changing or reclamping using a process datum set, reducing the cumulative transfer error of the datum during multiple clamping operations. Other features and advantages of this invention will be further described in the following detailed embodiments. Attached Figure Description

[0027] Figure 1 This is a flowchart of the workpiece positioning and machining method of the present invention.

[0028] Among them, S1, S2, S3, S4, S5, S6 and S7 respectively establish the nominal model, measurement point set and initial machining path, measure and check the allowance under the positioning and clamping state, implement clamping and measurement in stages, separate incremental rigid body deviation and stage local deformation increment, adjust clamping or construct local clamping deformation field, perform roughing and / or semi-finishing, and re-measure and update compensation, finish machining and on-machine verification. Detailed Implementation

[0029] The computer numerical control (CNC) machining center used in this embodiment is equipped with an on-machine probe that can be mounted on the spindle or tool magazine, a CNC system, and a workpiece fixture. The workpiece fixture has multiple independently adjustable clamping parts and auxiliary support parts, which can switch between positioning clamping state and machining clamping state, and can adjust the clamping action sequence, clamping amount, and auxiliary support state. The positioning clamping state is used to eliminate obvious gaps and maintain measurement stability, while the machining clamping state is used to meet the stability requirements under cutting loads.

[0030] During the machining preparation stage, the nominal model of the workpiece (CAD), process dimensions, and tolerance requirements are imported into the measurement and machining program. A nominal workpiece coordinate system is established based on the design datum or the machining datum specified by the process, and a set of datum points, a set of deformation monitoring points, and a set of allowance evaluation points are set on the nominal model. The set of datum points includes the main datum measurement points and orientation measurement points; deformation monitoring points are preferentially arranged near the clamping area, auxiliary support area, thin-walled area, and area to be finished.

[0031] The workpiece is placed in the workpiece fixture and kept in a positioned and clamped state. The on-machine probe's measurement reference point set and deformation monitoring point set are invoked to obtain the first reference data and the first monitoring data, and based on these, the initial rotation and translation relationship of the workpiece relative to the nominal workpiece coordinate system is determined. The allowance evaluation points are measured, and their coordinates are transformed to the nominal workpiece coordinate system. The distance between the actual blank surface and the target surface is calculated along the normal of the nominal machined surface. If the allowance is insufficient, the workpiece position is adjusted and the measurement is repeated.

[0032] After the allowance condition is met, the clamping process is divided into: Each clamping stage corresponds to a clamping part or a group of clamping parts moving synchronously. The clamping sequence proceeds from the side closer to the main positioning reference to the side farther away from the main positioning reference. Stage 0 is the positioning clamping state; the next stage is executed... Before the clamping action, retain the first... Phase data, after execution, measures the same subset of phase baseline points and subset of phase monitoring points to form the first phase data. Phase data.

[0033] The preferred subset of stage reference points includes at least three non-collinear reference points that are not obscured by the clamping elements throughout the clamping process. According to the... Phase and the Stage baseline data acquisition and Then the first Phase monitoring point coordinate transformation to the first Stage coordinate frame, and with the first By comparing the coordinates of the monitoring points at each stage, we can obtain... This calculation ensures that the local deformation increment for each clamping action does not include the overall rigid body displacement during that stage.

[0034] If the normal component or vector magnitude of the local deformation increment in any stage exceeds the preset stage deformation threshold, the next clamping action is not executed. Instead, the current clamping action is released or reduced, the order of the current action and subsequent actions is changed, adjacent or relative auxiliary supports are adjusted, or local supports are added. Then, the workpiece is returned to the stage before the current clamping action and remeasured. Only after the current stage passes the judgment will the next stage begin. Therefore, even if two clamping actions produce deformation increments in opposite directions, the excessive deformation in the previous stage cannot be masked by the reverse displacement of the later stage.

[0035] After all clamping stages pass the judgment, the workpiece fixture is in the machining clamping state. The complete set of reference points and deformation monitoring points are measured again to obtain second reference data and second monitoring data. The total rigid body deviation and final local clamping deformation from the positioning clamping state to the machining clamping state are calculated. If the final local clamping deformation still exceeds the preset total deformation threshold, the direction of the exceedance is determined at the exceedance monitoring point. Calculate the contribution at each stage The system will prioritize returning to the clamping phase that contributes the most to make adjustments; the adjusted phase and its subsequent phases will be re-executed.

[0036] Once the final local clamping deformation is deemed acceptable, the total rigid body deviation is written into the workpiece coordinate system offset or coordinate transformation parameters. For local deformation, the final state difference value of the second monitoring data is not directly interpolated all at once, but rather calculated for each stage that passes the judgment. Establish separately Then, vector superposition is performed to obtain The processing path can be based on Correct the tool position of the component in the nominal machining surface normal direction, or adjust the finishing allowance of the corresponding area.

[0037] Perform roughing or semi-finishing according to the updated workpiece coordinate system and the corrected machining path. After material removal, maintain the machining clamping state and wait for the measurement conditions to stabilize, then measure the complete set of reference points and deformation monitoring points again. Compare the remeasured reference data with the second reference data to obtain the updated rigid body deviation; transform the second monitoring data according to the updated rigid body deviation and compare it with the remeasured monitoring data to obtain the local deformation increment after material removal, and construct... .Will and Superposition Based on this, update the finishing toolpath.

[0038] After finishing, the dimensions, positions, and / or relative positions of holes, slots, planes, and contours are measured using an on-machine probe and compared with the nominal dimensions. Machining ends when the verification deviation is within the process tolerance; if the verification deviation exceeds the process tolerance but there is still remaining allowance for cutting, a supplementary corrective toolpath is generated based on the verification deviation and machining is repeated; for deviations that cannot be corrected by material removal, an anomaly flag is output.

[0039] For workpieces requiring face-changing machining or re-clamping, a process datum set can be machined in the low deformation allowance area of ​​the first clamping. This process datum set includes a process datum surface, a first process datum hole, and a second process datum hole. After machining, the process datum set is measured, and its datum transformation relationship with the current workpiece coordinate system is recorded. After re-clamping, the process datum set is measured again to reconstruct the workpiece coordinate system from the first clamping. Then, the stage clamping determination, local deformation field construction, and post-machining update steps are executed.

[0040] This invention is not limited to three-axis vertical machining centers, but can also be used in horizontal machining centers, four-axis machining centers, or five-axis machining centers. The number of clamping stages, the number and location of measurement points, the stage deformation threshold, the total deformation threshold, the interpolation weight, and the compensation method can all be determined based on the workpiece material, structural rigidity, machining tolerances, and fixture type.

Claims

1. A workpiece positioning and machining method for a CNC machining center, characterized in that, include: S1. Set up the reference point set and deformation monitoring point set, and establish the workpiece coordinate system and machining path; S2. Measure and obtain the first reference data and the first monitoring data under the positioning and clamping state; S3. The process of switching to the processing clamping state is divided into at least two clamping stages corresponding to different clamping parts. The clamping is performed sequentially from the side closer to the main positioning reference to the side farther away from the main positioning reference, and the same stage reference point and stage monitoring point are measured before and after each clamping action. S4. Determine the incremental rigid body deviation based on the stage reference data of adjacent clamping stages, and subtract the incremental rigid body deviation from the changes in stage monitoring data to obtain the stage local deformation increment of the corresponding clamping action; when the stage local deformation increment exceeds the preset stage threshold, stop the subsequent clamping, adjust the clamping action and / or auxiliary support of the corresponding clamping part, and then re-execute the clamping stage. S5. After all clamping is completed, the reference point set and the deformation monitoring point set are re-measured under the machining clamping state to determine the total rigid body deviation and the final local clamping deformation. When the final local clamping deformation exceeds the limit, the main contributing clamping stage is determined according to the contribution of the local deformation increment at each stage and returned for adjustment. When it does not exceed the limit, the workpiece coordinate system is updated according to the total rigid body deviation, and the local deformation increment at each stage is constructed as a local clamping deformation field to correct the machining path. S6. Perform roughing and / or semi-finishing; S7. After roughing and / or semi-finishing, keep the machining clamping state and retest to determine the updated rigid body deviation and the local deformation increment caused by material removal. Construct the local deformation increment of material removal into a local deformation field of material removal. Update the workpiece coordinate system and the machining path according to the updated rigid body deviation, the local deformation field of material removal and the local clamping deformation field, and then perform finishing and on-machine verification.

2. The workpiece positioning and machining method of a CNC machining center according to claim 1, characterized in that, The reference point set includes at least three main reference measurement points for fitting the main positioning reference surface and at least two orientation measurement points for determining the workpiece orientation; the stage reference point subset includes at least three non-collinear reference points that can be measured in each clamping stage; the stage monitoring point subset includes at least a first monitoring point and a second monitoring point, wherein the spatial distance between the first monitoring point and the current clamping part is not greater than the preset point distance, and the second monitoring point is located in the area to be finished corresponding to the current clamping part.

3. The workpiece positioning and machining method of a CNC machining center according to claim 2, characterized in that, In step S4, the incremental rigid body deviation is determined by minimizing the distance residual between corresponding stage reference points of adjacent clamping stages. The stage monitoring data of the previous clamping stage is then compared with the stage monitoring data of the next clamping stage after coordinate transformation according to the incremental rigid body deviation to obtain the stage local deformation increment. In step S5, the direction of the ultimate local clamping deformation exceeding the limit is used as the contribution evaluation direction. The contribution of each clamping stage is determined according to the component of the local deformation increment of each stage in the contribution evaluation direction, and the clamping stage with the largest contribution is determined as the main contributing clamping stage.

4. The workpiece positioning and machining method of a CNC machining center according to claim 1, characterized in that, In step S1, a set of allowance evaluation points is set; in step S2, the measured coordinates of the set of allowance evaluation points are transformed to the workpiece coordinate system, and the distance between the actual blank surface and the nominal machined surface is calculated; when the remaining machining allowance of any area to be machined is less than the preset minimum allowance, the translation adjustment amount and / or rotation adjustment amount of the workpiece are output and repositioned.

5. A workpiece positioning and machining method for a CNC machining center according to claim 2, characterized in that, The clamping action adjustment in steps S4 and S5 includes at least one of the following: reducing the clamping action of the current clamping part, changing the action sequence of the current clamping part and the subsequent clamping parts, adjusting the auxiliary support state that is adjacent to or opposite to the current clamping part, and adding local support; after adjustment, the stage reference point subset and the stage monitoring point subset are remeasured.

6. The workpiece positioning and machining method of a CNC machining center according to claim 1, characterized in that, For each clamping stage that passes the stage deformation judgment, the interpolation weight is determined according to the spatial distance between the stage monitoring point and the tool position point to form the stage deformation contribution field corresponding to the clamping stage. Then, the stage deformation contribution fields are vector superimposed to obtain the local clamping deformation field. The material removal local deformation field formed in step S7 and the local clamping deformation field are vector superimposed in the same workpiece coordinate system.

7. The workpiece positioning and machining method of a CNC machining center according to claim 1, characterized in that, The on-machine verification includes measuring the dimensions, positions, and relative positions of holes, slots, planes, and / or contours; when the verification result exceeds the preset machining tolerance and the remaining machining allowance allows for continued machining, a corrected toolpath is generated based on the verification deviation and supplementary machining is performed.

8. A workpiece positioning and machining method for a CNC machining center according to claim 1, characterized in that, In step S4, the evaluation value of the stage local deformation increment is the maximum absolute component of the stage local deformation increment along the nominal machining surface normal or the vector magnitude of the stage local deformation increment, and the evaluation value is compared with the preset stage threshold.

9. A workpiece positioning and machining method for a CNC machining center according to claim 1, characterized in that, In step S7, after roughing and / or semi-finishing, the machining clamping state is maintained and the measurement conditions are stabilized. The complete set of reference points and deformation monitoring points are remeasured. The remeasured reference data is compared with the reference data obtained before roughing and / or semi-finishing in the machining clamping state to determine the updated rigid body deviation. After deducting the updated rigid body deviation, the local deformation increment of the material is determined.

10. A workpiece positioning and machining method for a CNC machining center according to claim 1, characterized in that, During the roughing process of the first clamping, a process datum group including the process datum surface, the first process datum hole and the second process datum hole is machined in the low deformation allowance area of ​​the workpiece, and the datum transformation relationship between the process datum group and the updated workpiece coordinate system is recorded. After the workpiece is changed or re-clamped, the process reference group is measured and the workpiece coordinate system at the time of the first clamping is reconstructed according to the reference transformation relationship, and then steps S2 to S7 are executed.

Images