Processing and error control method of high-precision hollow screw

Abstract

The application relates to the technical field of hollow screw processing, in particular to a high-precision hollow screw processing and error control method, which has the technical scheme that first and second aging heat treatments are respectively carried out after deep hole processing and after one-time outer circle processing, residual stress is effectively released, boring or honing is combined with an online measurement device, the inner hole straightness and roundness are monitored in real time, and the tool feeding is dynamically compensated, so that the inner hole precision reaches the design standard, subsequent outer circle processing takes the processed high-precision inner hole as a reference, hydraulic or elastic expansion sleeve gapless positioning is adopted, eccentric errors caused by traditional centers or chucks are avoided, the workpiece deflection curve is actually measured, elastic deformation and gravity deflection are distinguished by combining finite element analysis, the actual axis posture is decoupled, error compensation grinding is realized through multi-axis linkage or tail seat deflection compensation, and the inner hole straightness data and the final thread lead accuracy are spatially corresponded and coded, and are stored in a two-dimensional code or an RFID chip.

Description

Technical Field

[0001] This invention relates to the field of hollow screw machining technology, and in particular to a method for machining and error control of high-precision hollow screws. Background Technology

[0002] Hollow lead screws are precision transmission components with a through hole along the axial direction, based on traditional lead screws. They are widely used in high-speed CNC machine tools, industrial robots, aerospace actuators, and semiconductor manufacturing equipment. However, hollow lead screws have a central through hole with a length-to-diameter ratio greater than 20:1. During deep hole machining (such as gun drilling), significant cutting heat and residual stress are generated, causing the workpiece to bend or twist, affecting subsequent machining accuracy. Due to the poor rigidity of the hollow structure, traditional machining methods cannot guarantee the coaxiality of the inner hole, outer circle, and thread raceway. Especially during multiple clamping processes, inconsistent references can lead to cumulative errors. When grinding the outer circle and threads, the workpiece undergoes elastic deformation due to its own weight and clamping force, causing the actual axis to deviate from the theoretical axis, resulting in inaccurate thread pitch diameter and affecting transmission accuracy. Simple heat treatment cannot completely eliminate machining stress, and stress redistribution during subsequent machining can cause the workpiece to deform again, affecting the final accuracy. Traditional machining methods do not link the inner hole machining data with the final product accuracy, making it impossible to achieve accuracy traceability and subsequent assembly selection, lacking full life-cycle accuracy traceability capabilities.

[0003] Therefore, we propose a high-precision hollow screw machining and error control method to solve the existing problems. Summary of the Invention

[0004] The purpose of this invention is to provide a method for machining and error control of high-precision hollow lead screws, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for machining and error control of a high-precision hollow lead screw, wherein the hollow lead screw has a central through hole extending along the axial direction, comprising:

[0006] S1. Blank preparation and reference pretreatment: Provide solid bar stock, perform preliminary machining on its outer circle and two end faces, and drill center holes on the two end faces;

[0007] S2. Deep hole machining and internal stress pre-release: A central through hole is machined in a solid bar stock by deep hole drilling or gun drilling to form a hollow tubular structure. Then, the first aging heat treatment is performed immediately to eliminate the internal stress generated by deep hole machining.

[0008] S3. Internal hole machining and online measurement: Using the center hole as a reference, the center through hole is bored or honed, and an online internal diameter measuring device is used to monitor the straightness and roundness of the internal hole in real time during the machining process until the internal hole reaches the design accuracy.

[0009] S4. First machining of outer circle and second stress release: Using the machined center through hole as the positioning reference, clamp the hollow lead screw and perform the first machining of the outer circle, leaving a secondary machining allowance. Then, perform the second aging heat treatment to release the stress again.

[0010] S5. Secondary machining of outer circle and pre-forming of thread raceway: Using the machined center through hole as a reference, perform secondary machining of outer circle again, and grind or turn thread raceway.

[0011] S6. Decoupling and adjustment of clamping posture based on actual internal hole measurement: The workpiece is hoisted onto an external cylindrical grinder or a thread grinder, and the internal hole is positioned and clamped by inserting a hydraulic or elastic expansion sleeve into the central through hole; the actual deflection curve of the workpiece due to gravity and residual stress is measured at this time and compared with the theoretical design axis, and the clamping deformation is decoupled and calculated.

[0012] S7. Error Compensation Grinding: Based on the calculation results of step S6, the grinding trajectory is adjusted by multi-axis linkage of the machine tool, or reverse deflection compensation is performed by the tailstock of the machine tool, and the grinding of the outer circle and thread raceway is completed in one clamping.

[0013] S8. Finished Product Inspection and Data Traceability: Conduct comprehensive precision inspection on the processed lead screw, and perform correlation analysis with the inner hole data in step S3 to establish a full life cycle precision archive for the lead screw.

[0014] Furthermore, in S3, the online inner diameter measuring device includes a pneumatic measuring head or a laser displacement sensor, whose measurement data is fed back to the CNC system in real time to dynamically compensate for the feed rate of the boring tool.

[0015] Furthermore, in S4 and S5, the specific implementation method of using the machined central through hole as the positioning reference is as follows: a flexible expansion sleeve with tapered or cylindrical surfaces at both ends is inserted into the interior of both ends of the central through hole, and the expansion sleeve is radially expanded by a hydraulic or mechanical tensioning mechanism, thereby fitting against the inner hole wall to achieve gapless centering.

[0016] Furthermore, in S6, the method for measuring the actual deflection curve of the workpiece due to gravity and residual stress is as follows: multiple non-contact displacement sensors are evenly distributed along the screw axis to detect the radial runout or bending deformation of the outer cylindrical surface under static conditions.

[0017] Furthermore, in S6, the process of decoupling and calculating the clamping deformation includes: establishing a finite element analysis model, inputting the actual geometric shape error data of the inner hole, and distinguishing between elastic deformation caused by clamping force and flexural deformation caused by the material's own gravity.

[0018] Furthermore, in S7, adjusting the grinding trajectory through multi-axis linkage of the machine tool specifically involves generating a grinding trajectory curve equidistant from the theoretical helix based on the actual axis posture after decoupling, so that the mean diameter of the ground thread has a constant radial distance relative to the bending axis in the final working state.

[0019] Furthermore, both the first and second aging heat treatments are a combination of cryogenic treatment and artificial aging processes, specifically including: first cooling to below -80°C at a first cooling rate and holding at that temperature, then heating back to room temperature at a first heating rate, followed by heating to a second temperature and holding at that temperature, and finally cooling with the furnace.

[0020] Furthermore, after S3, an ultrasonic rolling strengthening process is included to improve the surface roughness of the inner hole and form a compressive stress layer.

[0021] Furthermore, in S8, establishing a full lifecycle accuracy file for the lead screw specifically includes: spatially encoding the straightness data of the inner hole and the lead accuracy data of the final external thread, storing them via QR code or RFID chip, for selection or predictive maintenance during subsequent assembly or use.

[0022] Furthermore, it is applicable to the machining of ball screws or roller screws, wherein the length-to-diameter ratio of the hollow screw is greater than 20:1.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. This invention effectively releases residual stress and reduces subsequent machining deformation by performing first and second aging heat treatments after deep hole machining and after the first machining of the outer circle, thereby achieving multi-stage stress release and improving dimensional stability.

[0025] 2. This invention employs a boring or honing process combined with an online measuring device to monitor the straightness and roundness of the inner hole in real time and dynamically compensate the tool feed, ensuring that the inner hole accuracy meets the design standards and realizing high-precision machining and online measurement compensation of the inner hole.

[0026] 3. All subsequent external diameter machining of the present invention is based on the machined high-precision inner hole, and hydraulic or elastic expansion sleeve is used for gapless positioning to avoid the eccentricity error caused by traditional center or chuck clamping, so as to achieve the unification of internal and external references and improve coaxiality.

[0027] 4. This invention distinguishes between elastic deformation and gravitational deflection by measuring the workpiece deflection curve and combining it with finite element analysis, thus decoupling the actual axis posture; then, through multi-axis linkage or tailstock deflection compensation, error compensation grinding is achieved to ensure that the thread pitch diameter line is equidistant from the bending axis.

[0028] 5. The ultrasonic rolling of the present invention strengthens the surface of the inner hole, improves the surface roughness and introduces a compressive stress layer, thereby improving fatigue resistance and increasing fatigue life.

[0029] 6. This invention spatially corresponds and encodes the inner hole straightness data with the final thread lead accuracy, and stores it in a QR code or RFID chip, supporting subsequent assembly selection, predictive maintenance and quality traceability, and realizing full life cycle accuracy archives and data traceability.

[0030] 7. The method of the present invention is particularly applicable to hollow ball screws or roller screws with a length-to-diameter ratio greater than 20:1, and solves the problem that traditional processes cannot guarantee the accuracy of workpieces with high length-to-diameter ratios. Therefore, it is suitable for the processing of ultra-long hollow screws. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the workflow of a high-precision hollow lead screw machining and error control method according to the present invention. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0033] Example 1: Machining of a precision ball screw with a length-to-diameter ratio of 25:1

[0034] This embodiment provides a method for machining and error control of high-precision hollow screws, applicable to the machining of hollow screws for a certain type of precision ball screw auxiliary. The screw material is 38CrMoAl high-grade nitrided steel, the blank specifications are φ120mm×3000mm, the design requirements are a final outer diameter of φ100mm, an inner diameter of φ50mm, a length-to-diameter ratio of 25:1, and a precision grade of P2 (according to ISO 3408 standard, lead tolerance ±5μm / 300mm).

[0035] like Figure 1 As shown, a method for machining and error control of a high-precision hollow lead screw includes:

[0036] Step S1: Blank preparation and reference pretreatment

[0037] Solid bar stock is provided and first subjected to quenching and tempering treatment, with the hardness controlled between HB280-320. The bar stock is then hoisted onto a CNC lathe, and both end faces are machined to ensure a total length of 3050mm (with a 50mm allowance for process chucks). Type B center holes are precision drilled on both end faces, and the conical surfaces of the center holes are ground to ensure a roundness ≤0.002mm and a surface roughness Ra≤0.4μm, serving as the benchmark for subsequent machining. The outer diameter is rough-turned to φ115mm with a depth of cut of 2.5mm and a feed rate of 0.3mm / r to remove oxide scale and defect layers from the surface of the bar stock.

[0038] Step S2, Deep Hole Machining and Internal Stress Pre-Release

[0039] The solid bar stock is hoisted onto a deep hole drilling machine (using a gun drill), and a φ48mm through hole is drilled using the center holes at both ends as auxiliary supports (leaving a 2mm allowance for subsequent finishing). During the drilling process, high-pressure coolant (pressure 5MPa, flow rate 100L / min) is used for forced chip removal, and the drilling speed is controlled at 60m / min with a feed rate of 0.08mm / r.

[0040] Immediately after drilling, the first aging heat treatment is performed. This heat treatment is a combination of cryogenic treatment and artificial aging, specifically including: a) placing the workpiece in a cryogenic treatment chamber and cooling it to -120°C at a cooling rate of 2°C / min, holding it at that temperature for 4 hours to promote the transformation of retained austenite into martensite; b) allowing it to naturally rise to room temperature (approximately 4 hours); c) transferring it to an aging furnace, heating it to 300°C, holding it at that temperature for 8 hours to eliminate cutting stress; d) cooling it to room temperature with the furnace.

[0041] This step aims to eliminate the severe cutting and thermal stresses generated during deep hole machining as soon as possible, preventing stress release from causing bending during subsequent machining.

[0042] Step S3: Precision machining and online measurement of internal holes

[0043] The workpiece, after its first aging process, is hoisted onto a deep hole boring machine, still using the center holes at both ends as the positioning reference. A floating boring bar is used to rough bore (to φ49.5mm) and finish bore (to φ50H6, i.e., φ50mm) the inner hole of φ48mm. +0.019 mm).

[0044] During machining, a pneumatic measuring head is installed at the head of the boring bar. Compressed air is transmitted through the internal air passage of the boring bar to the nozzle of the measuring head, allowing real-time sensing of changes in the gap between the inner hole wall and the nozzle. The measurement signal is transmitted to the CNC system via a wireless transmission module. When an out-of-tolerance deviation in the straightness of the inner hole is detected (threshold set at 0.005 mm / m), the CNC system uses a piezoelectric ceramic actuator to finely adjust the radial feed of the boring tool in real time, achieving closed-loop control. After precision boring, the straightness of the inner hole is controlled within 0.006 mm / 1000 mm, the roundness within 0.003 mm, and the surface roughness Ra ≤ 0.8 μm.

[0045] Step S4: First machining of the outer diameter and second stress release

[0046] At the start of this step, the positioning reference changes from the "center hole" to the "finished inner hole". A flexible expansion sleeve is used for clamping.

[0047] The workpiece is hoisted onto a heavy-duty CNC lathe, and specially designed hydraulic expansion sleeves are inserted into the inner holes at both ends of the workpiece. The hydraulic expansion sleeves have an outer diameter of φ50mm, an axial length of 120mm, and pressure-equalizing grooves on their outer surfaces. The hydraulic station (pressure 10MPa) is activated, and the pressurized oil pushes the thin-walled sleeve inside the expansion sleeve to expand radially and evenly until it completely contacts the φ50H6 inner hole surface. This method achieves gapless centering, uniform clamping force distribution, and avoids localized deformation caused by traditional chuck clamping.

[0048] Using the inner hole as a reference for positioning, the outer diameter of the workpiece is turned (one-time machining) from φ115mm to φ105mm, leaving a secondary machining allowance (2.5mm on each side). Cutting parameters: spindle speed 300r / min, feed rate 0.3mm / r, depth of cut 2mm.

[0049] After turning, a second aging heat treatment is performed. This heat treatment is also a combination of cryogenic treatment and artificial aging, but the parameters are adjusted according to the stress state after rough machining: the cryogenic temperature is adjusted to -100℃ and held for 6 hours; the artificial aging temperature is adjusted to 280℃ and held for 10 hours. This aging aims to release the newly exposed surface residual stress after rough turning of the outer diameter.

[0050] Step S5: Secondary machining of the outer diameter and pre-forming of the thread raceway

[0051] The workpiece is again clamped and positioned using a hydraulic expansion sleeve within the inner hole (same as step S4). Perform semi-finish turning of the outer diameter to machine the outer diameter to φ100.5mm (leaving a grinding allowance of 0.5mm). Cutting parameters: spindle speed 400r / min, feed rate 0.15mm / r, depth of cut 0.5mm.

[0052] Subsequently, a CNC cyclone milling machine was used to rough machine the thread raceway. The raceway type was R5-4 (ball diameter 5mm, pitch 4mm). A grinding allowance of 0.1mm was reserved on each side during rough machining. Oil mist cooling was used during the milling process to prevent thermal deformation.

[0053] Step S6: Decoupling and Adjustment of Clamping Attitude Based on Actual Inner Hole Measurement

[0054] This is a crucial step in achieving error compensation. The workpiece is hoisted onto a precision external thread grinder (such as the German Klingberg P-series grinder). A hydraulic expansion sleeve is used for internal hole positioning and clamping, with the initial clamping force set at 8 MPa.

[0055] With the workpiece stationary, the measurement program is initiated. Five high-precision laser displacement sensors (Keyence LK-H020, resolution 0.01μm) are arranged along the lead screw axis at 100mm from both ends and at equal intervals in the middle to detect the radial runout of the outer cylindrical surface under static conditions. Simultaneously, eddy current sensors are arranged at the head and tail of the machine to monitor the end face runout.

[0056] The detected deflection curve data is input into the computer to establish a finite element analysis model (corresponding to claim 5). The model input includes the measured and saved internal hole straightness data from step S3 (as the initial geometric defect), the material's elastic modulus (210 GPa), and density (7850 kg / m³). 3 Clamping force parameters (radial contact pressure distribution corresponding to 8MPa) and deflection curve measured by the sensor.

[0057] Static analysis was performed using ANSYS Workbench, and combined with a reverse identification algorithm, which decoupled the total deformation as follows: elastic deformation caused by clamping force accounts for about 8% of the total deformation (local flattening at both ends), natural deflection caused by the material's own gravity accounts for about 65% of the total deformation (maximum deflection of 0.12 mm in the middle), and bending deformation caused by residual stress imbalance accounts for about 27% of the total deformation (S-shaped bending).

[0058] Finally, the true axis attitude curve of the workpiece under the current clamping state is determined.

[0059] Step S7, Error Compensation Grinding

[0060] Based on the actual axis attitude decoupled in step S6, error compensation grinding code is generated. The specific implementation method is as follows:

[0061] In the CNC system of a machine tool, the theoretical helical parametric equation is convolved with the actual axis attitude curve. When the grinding wheel moves along the Z-axis, the X-axis performs real-time micro-motion based on the radial offset of the actual axis, and the C-axis rotates synchronously based on the phase offset. That is, the pitch diameter of the ground thread always maintains a constant radial distance relative to the actual bending axis of the workpiece, ensuring that the nut runs smoothly in a horizontal working state (considering its own weight).

[0062] Using a CBN grinding wheel (120# grit), the following steps are completed in a single clamping operation (without disengaging the hydraulic expansion sleeve): a) Fine grinding of the outer diameter: Grind the outer diameter to φ100. 0.015 mm; b) Thread raceway fine grinding: single-pass infeed grinding is adopted, divided into rough grinding (0.08mm allowance), fine grinding (0.02mm allowance), and finishing grinding (no feed 2 times).

[0063] During the grinding process, the above-mentioned compensation trajectory is achieved through the coordinated operation of the X, Z, and C axes of the machine tool. Grinding parameters: grinding wheel speed 4500 r / min, workpiece speed 8 r / min, grinding depth 0.005 mm / cycle.

[0064] Step S8: Finished Product Testing and Data Traceability

[0065] After processing, a laser lead screw dynamic measuring instrument (such as Renishaw XL-80) is used to perform comprehensive accuracy testing on the finished product, including: the actual measured thread lead error is ±3.8μm / 300mm, which is better than the P2 grade requirement; the thread pitch diameter runout is 0.005mm; and the outer roundness is 0.002mm.

[0066] The detection data is spatially mapped and encoded to the inner hole straightness data saved in step S3. The specific implementation method is as follows:

[0067] The data matrix is ​​established as follows, with each 100mm segment along the lead screw axis:

[0068] 0-100 +2.1 +1.2 15 100-200 +1.5 +0.8 22 ... ... ... ... 2900-3000 -1.8 -1.1 345

[0069] A QR code is generated and laser-marked onto the non-working end face of the lead screw using a fiber laser marking machine. The QR code contains the following information: product model, production date, accuracy class, and cloud database index URL. By scanning the QR code, users can access the lead screw's full lifecycle accuracy profile, which can be used for subsequent precision matching with nuts or for predicting accuracy trends during equipment maintenance.

[0070] Example 2: Adding ultrasonic rolling reinforcement

[0071] This embodiment is basically the same as embodiment 1, except that an ultrasonic rolling strengthening treatment of the inner hole is added after step S3.

[0072] After the inner hole is precision bored to φ50H6, the hole wall is treated with an ultrasonic rolling tool. The equipment used is an HS-30 ultrasonic rolling device, with the following parameters: ultrasonic frequency of 28kHz, static pressure of 300N, rolling speed of 0.2m / min, rolling times of 2 passes, and rolling head diameter of φ49.98mm (interference allowance of 0.02mm).

[0073] Testing revealed that the surface roughness of the inner hole decreased from Ra0.4μm to Ra0.08μm. X-ray diffraction (XRD) analysis showed that a residual compressive stress layer with a depth of approximately 0.5mm (compressive stress value of approximately -650MPa) was formed on the surface, with the maximum compressive stress located 0.1mm below the surface layer.

[0074] This compressive stress layer effectively offsets the tensile stress that may be generated during subsequent external cylindrical grinding, further reducing the risk of deformation of the thin-walled lead screw. In this batch of 20 parts, the yield rate increased from 95% (19 / 20) in Example 1 to 100% (20 / 20), and the dimensional stability of the lead screw was better. After storage for 3 months, the accuracy was found to be unchanged upon re-inspection.

[0075] Example 3: Machining of a roller screw with a length-to-diameter ratio of 30:1

[0076] This embodiment is basically the same as embodiment 1, except that the type of screw processed is a roller screw with a length-to-diameter ratio of 30:1 (φ80mm×2400mm) and the material is 18CrNi4A high-strength steel.

[0077] Due to the higher strength and larger length-to-diameter ratio of the material, the aging heat treatment parameters for steps S2 and S4 were adjusted: Cryogenic treatment: the temperature was reduced to -150℃ and the holding time was extended to 8 hours; Artificial aging: the temperature was increased to 320℃ and the holding time was extended to 12 hours, with the furnace cooling rate controlled at ≤20℃ / h.

[0078] In the error compensation grinding of step S7, considering the longer workpiece and greater self-weight deflection (maximum deflection 0.18mm), a multi-round compensation strategy is adopted: a) First round of grinding: remove most of the allowance, leaving 0.1mm semi-finish grinding allowance; b) repeat the measurement and decoupling of step S6; c) update the compensation trajectory according to the new measurement results; d) Second round of fine grinding: complete the final dimension.

[0079] Final test results: Lead error ±4.2μm / 300mm, inner and outer circle coaxiality 0.004mm, meeting design requirements.

[0080] Example 4: In-depth application of full lifecycle data

[0081] This embodiment further deepens the application of data in step S8 based on embodiment 1.

[0082] The QR code stores an index that points to a cloud database. The precision profile in the database contains the following layers: Basic layer: product model, material furnace batch number, heat treatment process parameters (including actual temperature curves); Geometric layer: internal hole 3D point cloud data (approximately 2MB after compression), external thread lead error curve (including phase), radial runout spectrum; Process layer: actual processing parameters for each process (such as peak grinding force, temperature fluctuation curves), operator information; Application layer: error characteristics of recommended matching nuts, suggested preload value, predictive maintenance model parameters.

[0083] During subsequent nut assembly, the assembly worker scans the QR code on the leadscrew, and the system automatically reads the leadcrew's error phase characteristics. Simultaneously, the system scans the QR code on the nut to be assembled to obtain its error data. The matching algorithm employs the "error phase complementarity principle" to calculate the optimal assembly angle, guiding the worker to rotate the nut to the specified phase for assembly. Actual measurements show that using this matching method reduces the overall axial clearance runout after assembly from 0.008mm using the traditional method to 0.003mm.

[0084] After the leadscrew is installed on the user equipment, a distributed fiber optic sensor (Bragg grating) is inserted through its inner hole to monitor temperature, strain, and vibration in real time during operation. The data is uploaded to the cloud via an IoT gateway. The system compares the real-time data with the original accuracy profile and uses a Long Short-Term Memory (LSTM) network to predict the accuracy degradation trend. When the predicted remaining service life is lower than a set threshold (e.g., 2000 hours), a maintenance warning is automatically sent to the user, suggesting replacement of the lubricating oil or adjustment of the preload.

[0085] Example 5: Application of different material systems

[0086] This embodiment verifies the applicability of the method of this application to titanium alloy materials. The material selected is TC4 titanium alloy with specifications of φ150mm×2000mm, and the design requires an inner hole of φ60mm and an outer diameter of φ120mm.

[0087] To address the poor thermal conductivity and low elastic modulus of titanium alloys, the process parameters were adjusted accordingly: Step S2 deep hole machining: drilling speed reduced to 30m / min, feed rate 0.05mm / r, coolant pressure increased to 8MPa; aging treatment (steps S2, S4): cryogenic temperature -150℃, artificial aging temperature 450℃ (stress-relieving annealing temperature for titanium alloys); Step S7 grinding: diamond grinding wheel used, grinding depth reduced to 0.002mm / pass to prevent burns.

[0088] After processing, the test results showed a lead error of ±5.5μm / 300mm and an inner and outer circle coaxiality of 0.008mm, meeting the design requirements. This demonstrates that the method described in this application is also well-suited for difficult-to-machine materials.

[0089] As can be seen from the above embodiments, this application effectively solves the technical problems of easy deformation, poor internal and external coaxiality, and low precision retention in the machining of hollow screws by adopting the process sequence of "internal hole first and internal and external datum unification", combined with the stress control method of "segmented aging + deep cryogenic treatment", and the core technology of "clamping posture decoupling and error compensation grinding based on internal hole measurement".

[0090] According to actual production verification by a precision machinery company, after adopting the method of this application: the first pass rate of products increased from 78% of the traditional process to over 95%; the coaxiality of the inner and outer circles increased from 0.02mm of the traditional process to within 0.005mm; the accuracy retention time of the lead screw at the user's site was extended by more than 30%; and the after-sales maintenance cost was reduced by 40%.

[0091] Example 6

[0092] In S2, high-precision deep-hole drilling machines (such as gun drills) are used for deep-hole machining. A single-sided deep-hole drilling process is employed to drill a central through-hole from one end of a solid bar stock. During drilling, a high-pressure cooling and lubrication system (pressure 5~10 MPa) is used to promptly remove chips and cool the cutting tool, preventing residual stress caused by heat buildup. Drilling parameters are adjusted according to the material (e.g., bearing steel GCr15): spindle speed n = 2000-3000 rpm, feed rate f = 0.02-0.05 mm / r, ensuring the straightness of the through-hole is ≤0.1 mm / m.

[0093] In the first aging heat treatment (deep cryogenic treatment + artificial aging), the first aging heat treatment is performed immediately after drilling to eliminate the internal stress generated during deep hole machining. The specific process is as follows: A first cooling rate v... c1 Cool the workpiece to below T1 = −80℃ (e.g., -120℃) at a rate of 5-10℃ / min, holding for t1 = 2-4 hours. The cooling process can be described by a piecewise linear function: T(t) = T0-v c1 ·t, 0 ≤ t ≤ t c Where T0 is room temperature, t c = (T0-T1) / v c1 This refers to the cooling time. The first heating rate is v. h1 The temperature is slowly increased to room temperature at a rate of 3-8℃ / min to avoid sudden changes in thermal stress. The temperature change during the warming phase is: T(t) = T1 + v h1 ·(tt c ), t c ≤ t ≤ t c +t h , where t h = (T0-T1) / v h1 The workpiece is then heated to a second temperature T2 = 150-180℃ and held at that temperature for t2 = 4-6 hours. The heating rate can be controlled by v. h2 ≈ 5℃ / min, temperature holding period T(t) = T2. Cooling to room temperature with the furnace, cooling rate v c2 ≈ 1-2℃ / min.

[0094] This combined process can effectively reduce residual tensile stress caused by deep hole machining and improve the dimensional stability of subsequent machining. According to the thermo-elastic-plastic theory, the amount of residual stress release Δσ is related to the temperature change amplitude and the material yield strength, and can be predicted through finite element simulation.

[0095] In S3, the workpiece is clamped on a precision boring machine or honing machine, using the center holes at both ends of the inner bore as a reference. Multiple processes—rough boring, semi-finish boring, and finish boring—are used to gradually improve the accuracy of the inner bore. During the finish boring or honing stage, an online inner diameter measurement device is introduced, as detailed below:

[0096] A high-precision pneumatic measuring head or laser displacement sensor (0.1μm resolution) is selected and mounted behind the boring bar or on the honing head support, moving synchronously with the machining position. The pneumatic measuring head senses the aperture size through changes in airflow from the nozzle and outputs an analog voltage signal; the laser displacement sensor directly scans the inner hole contour and outputs a digital signal. Measured value D m (z) is collected in real time with the axial position z.

[0097] The measurement data is transmitted to the CNC system in real time and compared with the theoretical aperture value D0 to obtain the deviation signal: e(z) = D m (z)-D0. The CNC system dynamically adjusts the radial feed rate u(z) of the boring tool based on the deviation. A PID control algorithm is used. K p K i K d The PID parameters were tuned experimentally. The actuator is a piezoelectric ceramic micro-feed tool holder with a resolution of 0.01 μm. Closed-loop control keeps the aperture error within ±1 μm.

[0098] Simultaneously, the measurement data is recorded as the straightness and roundness curves of the inner hole. Let the coordinates of the inner hole's center axis be (x...). c (z),y c (z) is obtained by fitting the measured values. Straightness error is defined as: The roundness error is calculated using the least-squares circle of the profile in polar coordinates. Machining is stopped when the straightness of the inner hole is ≤2μm / 100mm and the roundness is ≤1μm to ensure that the inner hole reaches the design accuracy (e.g., IT5 grade).

[0099] In S4, the machined center through hole is used as the positioning reference, and flexible expansion sleeves at both ends are used to achieve gapless centering. The specific structure is as follows: the main body of the expansion sleeve is an elastic sleeve (made of spring steel) with a taper α = 1:10. The inner hole fits with the inner hole of the workpiece, and the outer conical surface contacts the tensioning conical sleeve. The expansion sleeve is inserted into the inner holes at both ends of the workpiece, and the conical sleeve is pulled by a hydraulic tensioning mechanism (or mechanical nut), causing the expansion sleeve to expand radially and fit evenly against the inner hole wall. Let the axial tensile force be F, the average diameter of the expansion sleeve D, the wall thickness t, the elastic modulus E, and the Poisson's ratio v, then the radial expansion amount δ can be approximately calculated as: Where K is the contact stiffness correction factor (usually taken as 0.8~1.2). By controlling F to make δ reach 0.01~0.02mm, sufficient clamping force is ensured while avoiding plastic deformation of the inner hole.

[0100] In the initial machining of the outer diameter, rough turning and semi-finish turning are performed on a CNC external cylindrical lathe, leaving a secondary machining allowance (0.3~0.5mm on each side). Cutting parameters: Cutting speed v c= 100-150m / min, feed rate f = 0.1-0.2mm / r, depth of cut a p = 1-2mm. After machining, the radial runout of the outer circle is checked and controlled within 0.05mm.

[0101] In the second aging heat treatment, a second aging process is performed to release the stress generated by the outer diameter cutting. The process is the same as the first aging (deep cryogenic + artificial aging). However, the parameters can be finely adjusted according to the material condition: cooling rate v c1 = 8℃ / min to T1 = −100℃, hold for 3 hours; after reheating, heat to T2 = 160℃ and hold for 5 hours. This treatment further stabilizes the internal structure of the workpiece and reduces deformation during subsequent processing.

[0102] In step S5, a flexible expansion sleeve is used again for internal hole positioning and clamping for finish turning of the outer diameter. The finish turning allowance is 0.1~0.2mm, ensuring an external diameter tolerance of IT6 and a cylindricity ≤ 3μm. During finish turning, the measured axis of the inner hole recorded in step S3 is used as a reference, and the workpiece bending is compensated by the CNC system. Let the projection of the inner hole axis onto the horizontal plane be x. c If (z), then the finishing tool path should follow this curve to ensure that the outer circle and inner hole are coaxial after machining. The path correction amount is given by the following formula: ΔX(z) = -x c (z), ΔY(z) = -y c (z) means that the tool is offset in the X and Y directions to counteract the bending of the inner hole.

[0103] The thread raceway is then pre-formed: For ball screws, the thread raceway is machined using a CNC milling machine or precision lathe, leaving a grinding allowance of 0.05~0.1 mm. The position of the thread pitch diameter relative to the inner hole axis is ensured by the linkage of the machine tool's C-axis and X-axis, with the initial phase corresponding to the inner hole orientation (aligned through the inner hole keyway or marking point). For roller screws, the thread tooth profile is machined using a forming tool.

[0104] After preforming, the thread lead error Δp(z) is detected and correlated with the inner hole straightness data to provide input for subsequent error compensation grinding (steps S6~S7).

[0105] This embodiment effectively controls the machining accuracy of the inner hole and outer circle through refined deep hole machining, combined aging treatment, online closed-loop boring, and positioning clamping based on flexible expansion sleeves, significantly reducing residual stress and providing a reliable foundation for the high-precision final forming of the hollow lead screw. All machining data (inner hole straightness, outer circle dimensions, thread pre-forming error) are encoded according to axial position and stored in the process database for subsequent processes to access.

[0106] Example 7

[0107] In S6, after the workpiece is hoisted onto the cylindrical or thread grinding machine, a hydraulic or elastic expansion sleeve is inserted into the central through hole for internal positioning and clamping. Due to gravity and residual stress, the actual axis of the workpiece will undergo bending deformation. To accurately compensate for this, the following steps are required:

[0108] 1. Actual deflection curve measurement

[0109] Multiple non-contact displacement sensors (such as eddy current sensors or laser displacement sensors) are evenly distributed along the lead screw axis to detect the radial runout or bending deformation of the outer cylindrical surface under static conditions. Let the sensor be at the axial position z. i The radial offset measured at (i = 1,2,...,n) is δ i The deflection curves u of the actual axis in the horizontal plane (X direction) and vertical plane (Y direction) are obtained by interpolation or fitting. x (z) and u y (z). For example, cubic spline interpolation can be used: , , where ϕ j (z) are basis functions, α j With β j From the measured data δ i The solution is obtained.

[0110] 2. Theoretical design axis definition

[0111] The theoretical design axis is an ideal straight line, usually based on the line connecting the center holes at both ends. Let the theoretical axis be represented in the coordinate system as: r 理想 (z) = [0,0,z] T That is, there is no radial deviation.

[0112] 3. Decoupling calculation of clamping deformation

[0113] Actual axis deviation d(z) = [u x (z),u y (z),0] T It consists of two parts: the elastic deformation d caused by the clamping force. 夹紧 (z) and the deflection d caused by the material's own weight 重力 (z), and satisfying: d(z) = d 夹紧 (z)+d 重力 (z) To decouple the two, a finite element analysis model is established. Input the actual geometric shape error data of the inner hole (from the online measurement in step S3), including the straightness error e of the inner hole. 孔(z) and roundness error. The workpiece is considered as a beam structure with initial geometric defects, whose moment of inertia I(z) varies with the shape of the inner hole. Boundary conditions are applied: the clamping points at both ends of the expansion sleeve are elastically constrained (simulating the radial stiffness k of the expansion sleeve), and a gravity load q (uniformly distributed load) is applied. Through finite element analysis, the theoretical deflection curve under gravity alone is obtained. The clamping deformation is then: d 夹紧 (z) = d(z)- If clamping force F is considered c Local deformation at the point of application can be corrected by introducing a contact mechanics model. The final decoupled actual axis attitude is: r 实际 (z) = r 理想 (z)+d 夹紧 (z)+d 重力 However, during grinding, the total deviation d(z) in the clamping state needs to be compensated because after grinding, when the clamping force is removed, the workpiece will spring back, causing the thread axis to not coincide with the inner hole axis. Therefore, the compensation goal is to make the ground thread axis consistent with the inner hole axis in the free state.

[0114] In step S7, error compensation is performed during the grinding process based on the actual axis deviation d(z) decoupled from step S6. There are two specific implementation methods:

[0115] 1. Multi-axis linkage adjustment of grinding trajectory

[0116] Machine tools (such as five-axis linkage grinders) generate grinding trajectories equidistant from the theoretical helix based on the actual axis orientation. Let the equation of the theoretical thread helix be: Where R is the thread pitch diameter, p is the lead, θ is the angle parameter, and ϕ0 is the initial phase.

[0117] Considering the actual axial curvature, the grinding wheel center trajectory should maintain a constant radial distance R relative to the actual axial direction during grinding. That is, the actual grinding trajectory point P... 实际 (θ) satisfies: P 实际 (θ) = r 实际 (z)+R·n(z,θ), where n(z,θ) is a unit vector perpendicular to the actual axis and pointing in the direction of the thread profile. Since the actual axis is curved, n needs to be calculated based on the local tangent direction. , where e r (θ) = [cosθ,sinθ,0] T A radial unit vector. This is the tangential vector of the actual axis.

[0118] The above trajectory is achieved by interpolation through a CNC system, so that the pitch diameter of the ground thread has a constant radial distance relative to the bending axis in the final working state, thereby making the thread axis coincide with the inner hole axis in the free state.

[0119] 2. Tailstock reverse deflection compensation

[0120] For simpler machine tool structures, compensation can be achieved by adjusting the tailstock center height. Let the radial deviation at the tailstock (z = L) be d(L) = [u x (L),u y (L),0] T Then the deflection of the tailstock in the X and Y directions are respectively: , L 磨削 This refers to the length of the grinding contact area. By hydraulically or manually adjusting the tailstock's offset in the horizontal and vertical directions, the workpiece axis is brought closer to an ideal straight line at the grinding point.

[0121] The above specific embodiments are merely several preferred embodiments of the present invention. Based on the technical solutions of the present invention and the relevant teachings of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A method for machining and error control of a high-precision hollow lead screw, wherein the hollow lead screw has a central through hole extending along the axial direction, characterized in that, include: S1. Blank preparation and reference pretreatment: Provide solid bar stock, perform preliminary machining on its outer circle and two end faces, and drill center holes on the two end faces; S2. Deep hole machining and internal stress pre-release: A central through hole is machined in a solid bar stock by deep hole drilling or gun drilling to form a hollow tubular structure. Then, the first aging heat treatment is performed immediately to eliminate the internal stress generated by deep hole machining. S3. Internal hole machining and online measurement: Using the center hole as a reference, the center through hole is bored or honed, and an online internal diameter measuring device is used to monitor the straightness and roundness of the internal hole in real time during the machining process until the internal hole reaches the design accuracy. S4. First machining of outer circle and second stress release: Using the machined center through hole as the positioning reference, clamp the hollow lead screw and perform the first machining of the outer circle, leaving a secondary machining allowance. Then, perform the second aging heat treatment to release the stress again. S5. Secondary machining of outer circle and pre-forming of thread raceway: Using the machined center through hole as a reference, perform secondary machining of outer circle again, and grind or turn thread raceway. S6. Decoupling and adjustment of clamping posture based on actual internal hole measurement: The workpiece is hoisted onto an external cylindrical grinder or a thread grinder, and the internal hole is positioned and clamped by inserting a hydraulic or elastic expansion sleeve into the central through hole; the actual deflection curve of the workpiece due to gravity and residual stress is measured at this time and compared with the theoretical design axis, and the clamping deformation is decoupled and calculated. S7. Error Compensation Grinding: Based on the calculation results of step S6, the grinding trajectory is adjusted by multi-axis linkage of the machine tool, or reverse deflection compensation is performed by the tailstock of the machine tool, and the grinding of the outer circle and thread raceway is completed in one clamping. S8. Finished Product Inspection and Data Traceability: Conduct comprehensive precision inspection on the processed lead screw, and perform correlation analysis with the inner hole data in step S3 to establish a full life cycle precision archive for the lead screw.

2. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that: In S3, the online internal diameter measurement device includes a pneumatic measuring head or a laser displacement sensor, whose measurement data is fed back to the CNC system in real time to dynamically compensate for the feed rate of the boring tool.

3. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, In S4 and S5, the specific implementation method of using the machined center through hole as the positioning reference is as follows: a flexible expansion sleeve with tapered or cylindrical surfaces at both ends is inserted into the interior of both ends of the center through hole, and the expansion sleeve is radially expanded by a hydraulic or mechanical tensioning mechanism, so as to fit with the inner hole wall and achieve gapless centering.

4. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, In S6, the method for measuring the actual deflection curve of the workpiece due to gravity and residual stress is as follows: multiple non-contact displacement sensors are evenly distributed along the screw axis to detect the radial runout or bending deformation of the outer cylindrical surface under static conditions.

5. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, In S6, the process of decoupling and calculating the clamping deformation includes: establishing a finite element analysis model, inputting the actual geometric shape error data of the inner hole, and distinguishing between elastic deformation caused by clamping force and flexural deformation caused by the material's own gravity.

6. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, In S7, the grinding trajectory is adjusted by multi-axis linkage of the machine tool as follows: based on the actual axis posture after decoupling, a grinding trajectory curve equidistant from the theoretical helix is ​​generated, so that the mean diameter of the ground thread has a constant radial distance relative to the bending axis in the final working state.

7. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, Both the first and second aging heat treatments are a combination of cryogenic treatment and artificial aging processes. Specifically, they include: first cooling to below -80°C at a first cooling rate and holding at that temperature, then heating back to room temperature at a first heating rate, followed by heating to a second temperature and holding at that temperature, and finally cooling with the furnace.

8. The machining and error control method for a high-precision hollow lead screw according to claim 1, characterized in that: Following S3, an ultrasonic rolling strengthening process is also included to improve the surface roughness of the inner hole and form a compressive stress layer.

9. The method for machining and error control of a high-precision hollow lead screw according to claim 1, characterized in that, In S8, establishing a full lifecycle accuracy profile for the lead screw specifically includes: spatially encoding the straightness data of the inner hole and the lead accuracy data of the final external thread, storing them via QR code or RFID chip, for selection or predictive maintenance during subsequent assembly or use.

10. A method for machining and error control of a high-precision hollow lead screw according to any one of claims 1-9, characterized in that: It is suitable for machining ball screws or roller screws, wherein the length-to-diameter ratio of the hollow screw is greater than 20:1.

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