Micro-screw machining method based on non-center hole reference and composite grinding center

Abstract

The application discloses a micro lead screw machining method based on a center hole-free reference and a composite grinding center, and belongs to the field of precision machining. The method takes the established outer circle of the center hole-free grinding as the only positioning reference, cancels the center hole completely, and does not rely on the center hole for positioning of any process. The steps include: providing an integral hardened round bar; performing center hole-free through grinding to establish a unified outer circle reference; clamping based on the outer circle reference, only applying radial clamping force, and not applying axial thrust; under the same clamping state, sequentially completing outer circle machining, thread grinding and cutting off through multi-axis linkage. The application also provides a special composite grinding center without a center, which comprises a workpiece spindle with indexing function, a B-axis rotary table, an A-axis driving device, a plurality of grinding wheel spindles and an independent cutting-off grinding wheel spindle. The application completely abandons the center hole reference, eliminates reference conversion error and center bending deformation, improves material utilization, and realizes efficient and high-precision batch manufacturing of micro lead screws.

Description

Technical Field

[0001] This invention belongs to the field of precision machining, and more specifically, relates to a method for machining micro lead screws based on a centerless datum and a composite grinding center. Background Technology

[0002] Miniature lead screws are core components of precision transmission systems and are widely used in medical devices, aerospace, robotics, and other fields.

[0003] Currently, the mainstream machining processes for miniature lead screws are, in order: turning the outer shape and threads (soft state), heat treatment and quenching, grinding the outer diameter, and grinding the threads. This process has the following technical drawbacks:

[0004] (i) Reliance on the center hole reference results in severe loss of accuracy.

[0005] Current technologies generally use the center holes at both ends as the positioning reference throughout the entire process chain. However, the center holes deform after quenching and heat treatment, requiring repeated "center hole repair" processes before rough grinding and fine grinding. Even so, the roundness error, taper angle deviation, and coaxiality deviation at both ends of the center hole will still be reflected in the outer circle and thread, becoming the fundamental bottleneck limiting the improvement of accuracy.

[0006] (ii) The bending and deformation of the slender lead screw caused by the two centers holding it in place.

[0007] Traditional processes employ a two-center clamping method. For slender miniature lead screws (especially workpieces with a diameter ≤6mm and a length-to-diameter ratio ≥5), the axial force generated by the center clamping force easily leads to bending deformation of the lead screw. Due to the low bending stiffness of slender lead screws, this deformation results in initial bending of the lead screw before grinding, and further exacerbates tool deflection under the grinding force, ultimately making it difficult to guarantee concentricity, cylindricity, and pitch accuracy. This problem has long existed in existing technologies and has yet to be effectively resolved.

[0008] (iii) Material waste caused by process chucks

[0009] To accommodate "one clamp and one top" or double-top clamping, the blank must have a pre-reserved process chuck. For lead screws with larger nominal diameters, the chuck length accounts for approximately 10%-20% of the total blank length, and material waste is acceptable.

[0010] However, for miniature lead screws with a nominal diameter of less than 1 mm, the center hole cannot be directly machined onto the workpiece end face. A process chuck with a diameter of at least 3 mm (more than three times the workpiece diameter) and an extended length must be reserved at the end of the workpiece. Taking a lead screw with a diameter of 1 mm and a length of 100 mm as an example, a bar stock with a diameter of more than 3 mm is required. The volume of the finished lead screw accounts for only about 10% of the total bar stock volume, resulting in a material waste of up to 90%. At the same time, 50%-70% of the machining time in processes such as turning the outer diameter, drilling the center hole, turning the thread bottom shape, heat treatment, repairing the center hole, and grinding the outer diameter is consumed on the chuck section that will eventually be cut off, resulting in extremely serious waste of materials and machining costs.

[0011] (iv) Technological bias of replacing millstones with carts

[0012] In the field of thread machining, the following technical teaching has long been established: turning has a much higher material removal rate than grinding, therefore, turning should be the preferred method for initial thread forming. Based on this teaching, those skilled in the art generally believe that "replacing turning with grinding" is only applicable to regrinding situations with extremely shallow thread profiles, and not to the initial forming of threads.

[0013] (v) Limitations of existing composite grinding technology

[0014] 1. Chinese Utility Model Patent (CN221538354U)

[0015] Chinese utility model patent number CN221538354U (authorization announcement date August 16, 2024) discloses a device for combined rough and fine grinding of external threads. The device uses a first grinding wheel spindle connected to a multi-line grinding wheel for rough thread machining and a second grinding wheel spindle connected to an external thread grinding wheel for fine thread machining, thus realizing the completion of rough and fine grinding of threads on one machine tool. A third grinding wheel spindle can be configured for external raceway grinding.

[0016] However, this equipment still uses the traditional "one clamp, one tip" clamping method, relying on the fit between the workpiece's center hole and the tailstock center. Therefore, it is still limited by the accuracy loss caused by center hole deformation and the bending deformation of slender shafts caused by the tip clamping force. Moreover, its processing object is a pre-cut single screw, and the thread bottom shape still needs to be pre-machined. The root cause of the above problems lies in the reliance on the center hole reference.

[0017] 2. GRINDSTAR series grinding machines from JUNKER (Germany)

[0018] The GRINDSTAR series grinding machines employ plunge profile grinding technology, using a single grinding wheel spindle with two grinding wheels running axially to achieve "grinding instead of turning." However, this technology uses plunge profile grinding without axial movement, forming the entire contour in one step. The outer diameter cannot be used as an independent reference, and there is no distinction between rough and fine thread grinding. Furthermore, it does not break free from the positioning logic dependent on the center hole. Additionally, this equipment can only be equipped with a maximum of two grinding wheel spindles, making it impossible to integrate the entire sequence of machining—external cylindrical grinding, rough thread grinding, fine thread grinding, and precision cutting—on a single machine tool.

[0019] 3. Japanese Patent JPH10118843A

[0020] Japanese Patent JPH10118843A discloses a thread-machining device using a centerless grinding method. This device supports the workpiece with a support plate and an adjusting grinding wheel. Multiple annular raised stripes are formed on the grinding wheel, allowing the round bar to gradually grind into shape by sequentially contacting each stripe during rotation. While this method eliminates the need for clamping and center positioning, it is essentially an extension of centerless grinding. It lacks an independent axial feed axis (Z-axis), precision C-axis indexing function, and a multi-wheel spindle switching structure. Therefore, it cannot achieve precision thread generating grinding, and its machining accuracy falls far short of the P3 level precision requirements for micro-lead screws.

[0021] To address the shortcomings of the prior art, this invention provides a micro lead screw machining method and a composite grinding center based on a center hole-less reference. Summary of the Invention

[0022] The main objective of this invention is to provide a micro-lead screw machining method and a composite grinding center based on a center hole-less datum. This aims to completely abandon the traditional process that uses a center hole as a datum and a center-mounted clamping mechanism, thus solving problems such as accuracy loss, bending deformation of slender lead screws, and material waste. Specifically, this invention reverses the long-standing industry prejudice that "grinding instead of turning is unsuitable for initial thread forming," using grinding as the sole means of forming threads on hardened bar stock. It combines CBN grinding wheel technology and a multi-line / single-line composite grinding strategy to achieve high-efficiency machining while ensuring accuracy. Furthermore, this invention overcomes the limitation of existing composite grinding equipment (such as plunge-type form grinders) that cannot complete the entire sequence of outer diameter, thread, and cut-off machining with a unified outer diameter datum in a single clamping operation.

[0023] A method for machining miniature lead screws based on a centerless datum, using the outer circle established by centerless grinding as the sole positioning datum throughout the entire process, without relying on a center hole for positioning in any operation; the method includes the following steps:

[0024] S1: Provides round bars that have undergone overall hardening treatment;

[0025] S2: Perform centerless through-grinding on the round bar obtained in S1 to establish a uniform outer circle datum along the entire length of the bar;

[0026] S3: The bar after S2 is clamped to the composite grinding center. The ground outer circle of the bar is used as the reference. The bar is positioned and clamped by a fixture that applies only radial clamping force. No axial clamping force is applied to the two ends of the bar.

[0027] S4: On the compound grinding center, with the outer circle datum as the sole reference for clamping and positioning and all subsequent grinding operations, under the same clamping state and without releasing the workpiece, the grinding wheel head is driven to generate relative motion between itself and the workpiece, moving in multiple coordinate axis directions, including at least feed motion along the workpiece axis and cut-in motion perpendicular to the workpiece axis, sequentially completing the machining of the outer circle surface of the lead screw, the grinding of the thread, and the cutting off; wherein, the grinding of the thread is to directly form the initial tooth profile of the thread entirely by grinding, without any pre-forming processing using turning tools during the entire thread forming process;

[0028] S5: Feeding.

[0029] According to the first embodiment of the present invention, in the micro screw machining method based on a center hole-less reference, when the nominal diameter of the micro screw is ≤6mm and the length-to-diameter ratio is ≥5, during the thread grinding process, a movable center frame set at the composite grinding center is supported on the ground outer circle of the bar, and the support surface of the center frame is in contact with the outer circle reference; when the nominal diameter of the micro screw is >6mm or the length-to-diameter ratio is <5, the center frame retracts its support for the bar.

[0030] According to the first embodiment of the present invention, the micro lead screw machining method based on a centerless datum includes rough grinding with a multi-line grinding wheel having multiple annular grinding edges to quickly remove excess material and initially grind out the thread profile, and then fine grinding with a single-line forming grinding wheel to ensure final accuracy.

[0031] According to the first embodiment of the present invention, the machining of the outer cylindrical surface of the lead screw based on a centerless datum includes:

[0032] When the outer diameter after centerless grinding does not reach the finished size or the workpiece has a stepped structure, the outer diameter grinding tool of the compound grinding center is used to grind it.

[0033] When the outer diameter has reached the finished size after centerless grinding, the outer diameter grinding step is omitted, and thread grinding is performed directly.

[0034] According to the first embodiment of the present invention, the micro lead screw machining method based on a centerless datum is wherein the cutting tool is a thin-plate CBN grinding wheel with a thickness ≤1.0mm.

[0035] According to a second aspect of the present invention, a micro-lead screw composite grinding center based on a centerless datum for implementing the above method is provided, comprising a bed, wherein the composite grinding center has no center structure that mates with the center hole of the workpiece throughout the entire grinding process, and the positioning and clamping of the workpiece are based solely on its outer circle:

[0036] The workpiece spindle is provided, which has a rotary indexing function and forms a C-axis; the workpiece spindle does not have a center structure for mating with the center hole of the workpiece; the workpiece spindle is provided with a clamp for clamping the outer circle of the bar only, and the clamping force of the clamp is radial.

[0037] A B-axis rotary table is provided, the rotation axis of which is perpendicular to the axis of the workpiece spindle; an A-axis drive device is provided on the B-axis rotary table, and a first grinding wheel spindle is provided on the A-axis drive device. A thread grinding wheel is installed on the first grinding wheel spindle, and the A-axis drive device drives the first grinding wheel spindle to swing around an axis parallel to the X-axis for adjusting the thread grinding angle.

[0038] The B-axis rotary table is also directly equipped with a second grinding wheel spindle and a third grinding wheel spindle, and grinding tools are respectively mounted on the second grinding wheel spindle and the third grinding wheel spindle; the composite grinding center is also equipped with a fourth grinding wheel spindle for cutting off, and a cutting grinding wheel is mounted on the fourth grinding wheel spindle; the first grinding wheel spindle, the second grinding wheel spindle, the third grinding wheel spindle and the fourth grinding wheel spindle are all independently driven;

[0039] The workpiece spindle and the B-axis rotary table have relative motion degrees of freedom in the Z and X directions, with the Z direction parallel to the workpiece spindle axis and the X direction perpendicular to the workpiece spindle axis.

[0040] The composite grinding center is not equipped with any device for applying axial clamping force at both ends of the workpiece.

[0041] According to the second aspect of the present invention, in the micro-lead screw composite grinding center based on a center hole-less reference, the A-axis drive device is disposed on the B-axis rotary table, and the grinding wheel spindle with a thread grinding wheel is mounted on the output end of the A-axis drive device. The A-axis drive device drives the grinding wheel spindle to swing around an axis parallel to the X-axis to adjust the thread grinding angle; other grinding wheel spindles directly fixed on the B-axis rotary table do not swing with the A-axis.

[0042] According to the second aspect of the present invention, the micro screw composite grinding center based on a centerless reference is provided on the B-axis rotary table, which is further provided with a grinding wheel spindle with a multi-line roughing thread grinding wheel and / or a grinding wheel spindle with an external cylindrical grinding wheel.

[0043] According to a second aspect embodiment of the present invention, in a micro-screw composite grinding center based on a centerless reference, the workpiece spindle is mounted on a slide plate movable along the Z-axis, and the B-axis rotary table is mounted on a slide plate movable along the X-axis.

[0044] According to the second aspect of the present invention, the micro screw composite grinding center based on a center hole-less reference is further provided with a center frame that can move along the Z direction for supporting the ground outer circle of the bar during the thread grinding process. The shape of the support surface of the center frame is adapted to the ground outer circle. When the nominal diameter of the micro screw is ≤6mm and the length-to-diameter ratio is ≥5, the center frame is set as an auxiliary support participating in the grinding process.

[0045] According to the second aspect of the present invention, the micro-lead screw composite grinding center based on a centerless reference is further provided with a grinding wheel dresser for on-machine dressing of grinding tools; the grinding wheel dresser is a diamond roller dresser and is disposed on the side of the workpiece spindle.

[0046] The micro-screw composite grinding center based on a centerless datum according to the second aspect of the present invention is further provided with an automatic loading and unloading mechanism for realizing continuous processing.

[0047] According to the second aspect of the present invention, the micro-lead screw composite grinding center based on a centerless reference is provided, wherein the fixture is a radial clamping fixture with adjustable clamping force.

[0048] One of the above-described technical solutions of the present invention has at least one of the following advantages or beneficial effects:

[0049] 1. Completely eliminates datum conversion errors and bending deformation: This invention uses the outer circle established by centerless grinding as the sole datum throughout the entire process—from clamping and positioning until the completion of all grinding operations, the outer circle remains the sole reference. Combined with a fixture that applies only radial force, the machining and finishing processes of the center hole are completely eliminated, avoiding the need for center clamping force. This combined technical approach simultaneously solves two long-standing independent problems in traditional processes—datum error and bending deformation—producing a synergistic technical effect. It is particularly suitable for the precision machining of miniature slender lead screws with a diameter ≤6mm and a length-to-diameter ratio ≥5.

[0050] 2. Significantly improve material utilization: The process chuck design is eliminated, and there is no need to reserve extra length for the blank, which increases the material utilization rate by 10%-20%; at the same time, the chuck cutting process is eliminated, simplifying the production process.

[0051] 3. Achieve integrated production of "bar stock in, finished product out": From hardened bar stock to finished lead screw, all processes are completed on the same composite grinding center in one clamping, which greatly reduces equipment investment and site occupation, and reduces the risk of precision loss caused by multiple loading and unloading and handling.

[0052] 4. Overcoming the technical prejudice that "replacing turning with grinding is uneconomical": This invention completes the initial thread forming by replacing turning with grinding. Combining the high grinding ratio characteristics of modern CBN grinding wheels with the efficient rough grinding strategy of grinding multiple tooth profiles simultaneously in a single feed using multi-wire grinding wheels, the overall grinding efficiency reaches or even exceeds that of the traditional "turning + grinding" process. Actual measurements (see Example 1) show that the total processing time for a single part using this invention is reduced by approximately 30% compared to the traditional process.

[0053] 5. The A-axis and B-axis are arranged in series, resulting in a compact structure and complete functionality: This invention places the A-axis drive device on the B-axis rotary table, with the first grinding wheel spindle mounted on the A-axis drive device, and the other grinding wheel spindles directly fixed to the B-axis rotary table. The B-axis is responsible for tool switching, while the A-axis is responsible for adjusting the thread grinding angle. The two work together without interference, achieving multi-tool composite machining functionality within a limited space. Compared to the solution of "each spindle having its own swing mechanism," the structure is significantly simplified, and the rigidity and precision are higher.

[0054] 6. Strong applicability: This method covers various miniature lead screws, including those with steps, those without steps, and those with large or small length-to-diameter ratios. It can be flexibly adapted to different workpiece characteristics by selecting whether to use a center rest, whether to perform additional external cylindrical grinding, and whether to configure multi-spindle grinding wheels or external cylindrical grinding wheels. Attached Figure Description

[0055] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0056] Figure 1 This is a schematic diagram of the structure of the micro-screw composite grinding center in some embodiments of the present invention;

[0057] Figure 2 This is a schematic diagram of the central frame structure in some embodiments of the present invention;

[0058] Figure 3 This is a cross-sectional view of the fixture and bar assembly in some embodiments of the present invention. Attached Figure Description

[0059] 1-Bed; 2-Z-axis transmission mechanism; 3-Z-axis slide; 4-X-axis transmission mechanism; 5-X-axis slide; 6-Workpiece spindle; 7-Grinding wheel dresser; 8-First grinding wheel spindle; 9-Grinding wheel tilting device; 10-External thread multi-line grinding wheel; 11-External thread grinding wheel; 12-Second grinding wheel spindle; 13-External cylindrical grinding wheel; 14-Third grinding wheel spindle (for external cylindrical grinding); 15-Hydraulic rotary table; 16-Center rest; 17-Clamping fixture for holding the outer diameter of the bar stock; 18-Grinding wheel head; 19-Cutting grinding wheel; 20-Fourth grinding wheel spindle (for cutting). Detailed Implementation

[0060] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0061] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0062] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0063] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" and "second" may explicitly or implicitly include one or more features.

[0064] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, it can be a fixed connection or a movable connection, a detachable connection or a non-detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection or a connection that allows communication between the two components; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components, an indirect connection, or an interaction between two components.

[0065] The following disclosure provides many different implementations or examples for different ways of implementing the present invention.

[0066] like Figures 1 to 3As shown, the machine tool bed 1 adopts a flat bed structure, and an X-axis transmission mechanism 4 and a Z-axis transmission mechanism 2 are arranged on the bed 1. The machine tool has five CNC coordinate axes: X, Z, A, B, and C. The first grinding wheel spindle 8, the grinding wheel spindle tilting device 9, the hydraulic rotary table 15, the external cylindrical grinding wheel 13, the second grinding wheel spindle 12, and the grinding wheel head 18 are placed on the X-axis slide plate 5, which moves back and forth. The workpiece spindle 6, the grinding wheel dresser 7, and the center rest 16 are placed on the Z-axis slide plate 3, which moves left and right. The clamp 17 for holding the outer diameter of the bar stock is installed on the workpiece spindle 6.

[0067] The bed 1 serves as the basic support component of the machine tool (flat bed structure), bearing all major components and ensuring the rigidity and precision stability of the entire machine. The Z-axis transmission mechanism 2 is a feed mechanism that drives the Z-axis slide plate to move along the length (left and right) of the bed, achieving axial positioning and feeding of the workpiece. The Z-axis slide plate 3 is a slide plate that moves left and right along the bed 1 under the drive of the Z-axis transmission mechanism 2, and is on which components such as the workpiece spindle 6 and the grinding wheel dresser 7 are mounted. The X-axis transmission mechanism 4 is a feed mechanism that drives the X-axis slide plate 5 to move along the width (back and forth) of the bed 1, achieving transverse feeding of the grinding wheel head. The X-axis slide plate 5 moves back and forth under the drive of the X-axis transmission mechanism 4. A movable slide plate is equipped with a grinding wheel holder, grinding wheel spindle, tilting device, hydraulic rotary table, etc.; the workpiece spindle 6 is the core component used to clamp and drive the workpiece rotation (C-axis), providing the rotational motion during workpiece grinding; the grinding wheel dresser 7 is used to shape and dress or sharpen the grinding wheel, ensuring the grinding wheel profile accuracy and grinding performance; the first grinding wheel spindle 8 is the spindle used to drive the rotation of the external thread grinding wheel (serial number 11) for grinding the external threads of the workpiece; the grinding wheel tilting device 9 is used to adjust the installation angle (A-axis) of the grinding wheel spindle (and grinding wheel) to adapt to the thread helix angle or grinding of a specific helical surface; the external thread multi-line grinding wheel 10... A grinding wheel for multi-start external threads, capable of simultaneously or sequentially machining multiple thread starts; an external thread grinding wheel 11 is a forming wheel for single-start external thread grinding, completing the finishing of the thread tooth surface; a second grinding wheel spindle 12 is a spindle used to drive the external thread grinding wheel (number 11) to rotate, used for grinding the external threads of the workpiece; an external cylindrical grinding wheel 13 is a grinding wheel used for grinding the external diameter; a third grinding wheel spindle 14 is a spindle used to drive the external cylindrical grinding wheel 13 to rotate, used for grinding the external diameter; a hydraulic rotary table 15 is a hydraulically driven rotary table that provides rotary motion (B-axis), used for indexing or adjusting the angular position of the workpiece / grinding wheel; a center rest 16... Mounted on the Z-axis slide plate 3, it assists in supporting slender workpieces during grinding, reducing bending deformation and improving machining rigidity; the clamp 17 for holding the outer circle of the bar is mounted on the workpiece spindle 6, used to clamp the outer circle of the bar, realizing workpiece clamping and rotation transfer; the grinding wheel frame 18 is mounted on the X-axis slide plate 5, and is an integral support component that supports multiple grinding wheel spindles (8, 12, 14) and the swing angle device 9, realizing the overall movement of the grinding wheel assembly; the cutting grinding wheel 19 is mounted on the fourth grinding wheel spindle 20, and is a grinding tool used to cut the finished lead screw from the bar; the fourth grinding wheel spindle 20 is the spindle used to drive the rotation of the cutting grinding wheel 19.

[0068] Example 1

[0069] Taking the machining of a miniature lead screw (with a stepped design and a length-to-diameter ratio of 16.7) with a nominal diameter of 6mm, a length of 100mm, and a pitch of 1.0mm as an example:

[0070] S1: Select a 6.5mm diameter GCr15 bearing steel round bar and perform overall quenching heat treatment. In this embodiment, it is hardened to HRC60±2.

[0071] S2: Feed the hardened bar into a centerless grinder and use through grinding to obtain an outer diameter of 6.02mm ± 0.002mm, roundness ≤ 1μm, and roughness Ra ≤ 0.2μm.

[0072] S3: The bar stock is clamped to the composite grinding center. The ground outer diameter of the bar stock is held by a hydraulic elastic chuck. Only radial clamping force is applied. There is no center hole positioning and no center support.

[0073] S4: On the compound grinding center, using the outer diameter datum as the sole reference, and without releasing the workpiece under the same clamping condition, drive the grinding wheel head to generate relative movement between the workpiece in the X and Z directions, and sequentially complete the following: Grind each step of the outer diameter with the outer diameter grinding wheel to the finished size of 6.00mm; rotate the B-axis rotary table to cut the multi-thread grinding wheel into the working position for rough grinding of the thread section; rotate the B-axis rotary table to cut the thread finishing grinding wheel with A-axis oscillation into the working position for finishing grinding of the thread to the finished size; rotate the B-axis rotary table to cut the cutting grinding wheel into the working position for cutting off the finished product. During the machining process, the center rest 16 is supported on the ground outer diameter.

[0074] S5: Feeding.

[0075] Processing efficiency comparison: According to actual measurement, the total processing time (including loading and unloading) of a single piece from bar stock to finished product in this embodiment is 4 minutes and 20 seconds, while the total time of the traditional "soft turning → quenching → center hole repair → outer circle grinding → thread grinding" process is about 6 minutes and 30 seconds, which improves efficiency by about 33%.

[0076] Test results: coaxiality ≤2μm, cumulative pitch error ≤3μm / 100mm, achieving P3 level accuracy. Compared to traditional processes (usually P5 level), the accuracy is improved by two levels.

[0077] Example 2

[0078] Taking the machining of a miniature lead screw (without steps, a length-to-diameter ratio of 3.3, which is relatively small) with a nominal diameter of 6mm, a length of 20mm, and a pitch of 1.5mm as an example:

[0079] S1: Select a 6.5mm diameter GCr15 bearing steel round bar and perform overall quenching heat treatment.

[0080] S2: The hardened bar stock is fed into a centerless grinder and subjected to through-feed grinding to obtain an outer diameter of 6.02mm ± 0.002mm.

[0081] S3: Clamp the bar stock to the composite grinding center and use a hydraulic elastic chuck to hold the ground outer diameter of the bar stock.

[0082] S4: The drive wheel holder and the workpiece move relative to each other in the X and Z directions, completing the following steps sequentially: the centerless grinder has already achieved the finished diameter, so no additional outer diameter grinding is required; the B-axis rotary table rotates to bring the multi-thread grinding wheel into the working position for rough thread grinding; the B-axis rotary table rotates to bring the thread finishing grinding wheel with A-axis oscillation into the working position for finishing thread grinding to the finished product; the B-axis rotary table rotates to bring the cutting grinding wheel into the working position to cut off the finished product. In this embodiment, the workpiece has a small length-to-diameter ratio (3.3 < 5), and the center rest 16 is not used.

[0083] S5: Feeding.

[0084] In this embodiment, the total processing time for a single piece is approximately 2 minutes and 50 seconds.

[0085] Test results: coaxiality ≤3μm, cumulative pitch error ≤3μm / 100mm, reaching P3 level accuracy.

[0086] Alternative implementation methods

[0087] It should be noted that in this invention:

[0088] When the centerless grinding has brought the outer diameter to the finished size, no additional outer diameter grinding is required in S4, and the thread grinding can proceed directly.

[0089] When the lead screw has no step, the grinding of the outer circle step can be omitted;

[0090] When using pre-cut bar stock, the cutting step in S4 can be omitted;

[0091] The number of threads on a multi-thread grinding wheel can be selected based on the number of thread starts and the thread pitch; it can be 2 threads, 3 threads, 4 threads, 5 threads, 6 threads, 8 threads, or more.

[0092] When the length-to-diameter ratio is small (e.g., less than 5), the center frame 16 can be omitted;

[0093] The grinding wheel spindle configuration on the B-axis rotary table can be flexibly selected according to processing needs. Multi-line roughing thread grinding wheels and external cylindrical grinding wheels are both optional.

[0094] The A-axis drive device can be selected to drive the first grinding wheel spindle to swing as needed, and the A-axis can be locked at a fixed angle when swinging is not required.

[0095] The cutting grinding wheel 19 is standard and is installed on the independent fourth grinding wheel spindle 20. It is used to cut the finished lead screw from the bar stock.

[0096] All the above-described alternative embodiments fall within the protection scope of this invention.

[0097] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions without departing from the concept of the present invention are all within the protection scope of the present invention.

Claims

1. A method for machining miniature lead screws based on a center hole-less datum, characterized in that, The entire process uses the outer circle established by centerless grinding as the sole positioning datum, completely eliminating the need for a center hole and performing any positioning operations without relying on a center hole; the method includes the following steps: S1: Provides round bars that have undergone overall hardening treatment; S2: Perform centerless through-grinding on the round bar obtained in S1 to establish a uniform outer circle datum along the entire length of the bar; S3: The bar after S2 is clamped to the composite grinding center. The ground outer circle of the bar is used as the reference. The bar is positioned and clamped by a fixture (17) that applies only radial clamping force. No axial clamping force is applied to the two ends of the bar. S4: On the compound grinding center, with the outer circle datum as the sole reference for clamping and positioning and all subsequent grinding processes, under the same clamping state and without releasing the workpiece, the grinding wheel head and the workpiece move relative to each other in at least the Z and X directions to sequentially complete the processing of the outer circle surface of the lead screw, the grinding of the thread, and the cutting off; wherein, the grinding of the thread is to directly form the initial tooth profile of the thread entirely by grinding, without any pre-forming processing using turning tools during the entire thread forming process; S5: Feeding.

2. The method for machining miniature lead screws based on a centerless datum according to claim 1, characterized in that, When the nominal diameter of the micro screw is ≤6mm and the length-to-diameter ratio is ≥5, during the thread grinding process, the movable center frame (16) set at the composite grinding center is supported on the ground outer circle of the bar, and the support surface of the center frame (16) is in contact with the outer circle reference; when the nominal diameter of the micro screw is >6mm or the length-to-diameter ratio is <5, the center frame (16) is disengaged from the ground outer circle of the bar.

3. The method for machining miniature lead screws based on a centerless datum according to claim 1, characterized in that, The grinding of the thread includes rough grinding with a multi-line grinding wheel with multiple annular grinding edges to quickly remove excess material and initially grind out the thread profile, and then fine grinding with a single-line forming grinding wheel to ensure the final accuracy.

4. The method for machining miniature lead screws based on a centerless datum according to claim 1, characterized in that, The machining of the outer cylindrical surface of the lead screw includes: When the outer diameter after centerless grinding does not reach the finished size or the workpiece has a stepped structure, the outer diameter grinding tool of the compound grinding center is used to grind it. When the outer diameter has reached the finished size after centerless grinding, the outer diameter grinding step is omitted, and thread grinding is performed directly.

5. The method for machining miniature lead screws based on a centerless datum according to claim 1, characterized in that, The cutting tool is a thin-bladed grinding wheel.

6. The method for machining miniature lead screws based on a centerless datum according to claim 1, characterized in that, The cutting tool is a thin-bladed CBN grinding wheel with a thickness ≤1.0mm.

7. A micro-screw composite grinding center based on a centerless datum for implementing the method of any one of claims 1-6, comprising a bed (1), characterized in that: This composite grinding center does not have a center structure that mates with the center hole of the workpiece, nor does it have any device for applying axial clamping force at both ends of the workpiece. The positioning and clamping of the workpiece are based solely on its outer circle. A workpiece spindle (6) is provided, which has a rotation indexing function and forms a C-axis; the workpiece spindle (6) does not have a center structure for cooperating with the center hole of the workpiece; the workpiece spindle (6) is provided with a clamp (17) for clamping the outer circle of the bar only, and the clamping force of the clamp (17) is radial. A B-axis rotary table (15) is provided, and the rotation axis of the B-axis rotary table (15) is perpendicular to the axis of the workpiece spindle (6). An A-axis drive device (9) is provided on the B-axis rotary table (15), and a first grinding wheel spindle (8) is provided on the A-axis drive device (9). A thread grinding wheel is installed on the first grinding wheel spindle (8). The A-axis drive device (9) drives the first grinding wheel spindle (8) to swing around an axis parallel to the X-axis to adjust the thread grinding angle. The B-axis rotary table (15) is also directly provided with a second grinding wheel spindle (12) and a third grinding wheel spindle (14), and grinding tools are respectively installed on the second grinding wheel spindle (12) and the third grinding wheel spindle (14); The composite grinding center is also provided with a fourth grinding wheel spindle (20) for cutting off, and a cutting grinding wheel (19) is installed on the fourth grinding wheel spindle (20); the first grinding wheel spindle (8), the second grinding wheel spindle (12), the third grinding wheel spindle (14) and the fourth grinding wheel spindle (20) are all independently driven; The workpiece spindle (6) and the B-axis turntable (15) have relative motion degrees of freedom in the Z and X directions, with the Z direction parallel to the workpiece spindle axis and the X direction perpendicular to the workpiece spindle axis.

8. The micro-lead screw composite grinding center based on a centerless datum according to claim 7, characterized in that, The second grinding wheel spindle (12) is equipped with a multi-line roughing thread grinding wheel (10), and the third grinding wheel spindle (14) is equipped with an external cylindrical grinding wheel (13).

9. The micro-screw composite grinding center based on a centerless datum according to claim 7, characterized in that, The A-axis drive device (9) is mounted on the B-axis turntable (15). The first grinding wheel spindle (8) with a threaded grinding wheel is mounted on the output end of the A-axis drive device (9). The second grinding wheel spindle (12) and the third grinding wheel spindle (14) are directly fixed on the B-axis turntable (15) and do not swing with the A-axis.

10. The micro-lead screw composite grinding center based on a centerless datum according to claim 7, characterized in that, The workpiece spindle (6) is mounted on a sliding plate (3) that can move along the Z direction, and the B-axis turntable (15) is mounted on a sliding plate (5) that can move along the X direction.

11. The micro-screw composite grinding center based on a centerless datum according to claim 7, characterized in that, A center frame (16) movable along the Z direction is also provided for supporting the ground outer circle of the bar during the thread grinding process. The support surface shape of the center frame (16) is adapted to the ground outer circle. When the nominal diameter of the micro screw is ≤6mm and the length-to-diameter ratio is ≥5, the center frame (16) participates in the auxiliary support of the grinding process. When the length-to-diameter ratio is <5, the center frame (16) withdraws from supporting the bar.

12. The micro-lead screw composite grinding center based on a centerless datum according to claim 7, characterized in that, A grinding wheel dresser (7) is also provided for on-machine dressing of grinding tools; the grinding wheel dresser (7) is a diamond roller dresser and is located on the side of the workpiece spindle (6).

13. The micro-screw composite grinding center based on a centerless datum according to claim 7, characterized in that, It is also equipped with an automatic loading and unloading mechanism for continuous processing.

14. The micro-screw composite grinding center based on a centerless datum according to claim 7, characterized in that, The clamp (17) is a radial clamping clamp with adjustable clamping force.

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