Small-bend-diameter-ratio pipeline inner arc cavity bottom cutting and reverse direction combined numerical control machining method

Abstract

The invention relates to an undercutting and reverse combined numerical control machining method for an inner arc cavity of a small-bend-diameter-ratio pipeline, and belongs to the technical field of numerical control milling machining. The problems that in the prior art, in the machining process of small-bend-diameter-ratio pipelines (R / D is smaller than or equal to 1.5), deformation is uneven, the wall thickness is difficult to control, a tool is prone to interference, loss is serious, cavity precision is reduced due to machining vibration, and production efficiency is low due to parameter adjustment depending on manual experience are solved. The mode of combining bottom cutting and reverse cutting is innovatively provided, the reverse path from the interior of the arc section to the end face of the three-face cutting edge is made to conduct cutting by optimizing the inverted cavity area cutter path, and in the cutting process, only the side cutting edge and the rear cutting edge conduct cutting, and the front cutting edge does not conduct cutting. In the programming process, the whole numerical control programming process becomes simple and convenient by editing the forward tool path, and the high-precision, short-period and intelligent machining target is successfully achieved.

Description

Technical Field

[0001] This invention relates to the field of CNC milling technology, and in particular to a CNC machining method that combines bottom cutting and reverse cutting of the inner arc cavity of a pipe with a small bending ratio. Background Technology

[0002] Integrated piping for nuclear power pressure vessels is a key component connecting the reactor pressure vessel and the steam generator. As a large, thick-walled pressure-bearing pipeline that transmits core heat energy in the nuclear steam supply system, its manufacturing quality directly affects the safety and service life of nuclear power equipment. With the development of efficient and clean energy, the manufacturing process of nuclear power main pipelines urgently needs to evolve towards higher precision, shorter cycle times, and greater intelligence.

[0003] Currently, the manufacturing of pipes with small bend ratios (R / D≤1.5) faces numerous challenges: traditional forging, extrusion, or welding processes are prone to uneven deformation, structural defects, high weld risks, and difficulties in wall thickness control, making it difficult to meet the stringent requirements of nuclear-grade pipes. While integral forging combined with machining can solve these problems, bottlenecks still exist. For example, cutting tools are prone to interference during machining, especially in complex curved surfaces where blind spots are formed; tool wear is severe, with three-sided milling cutters, for instance, exhibiting high cutting resistance, rapid wear, and even potential chipping; furthermore, vibrations during machining reduce cavity accuracy. In addition, traditional processes rely on manual experience to adjust parameters, requiring frequent machine stops for inspection, severely impacting production efficiency and hindering mass production.

[0004] Therefore, developing a high-precision, high-efficiency, and intelligent processing technology is of great significance for improving the manufacturing level of nuclear power pipelines. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a CNC machining method combining bottom cutting and reverse cutting of the inner arc cavity of a pipe with a small bend ratio, in order to solve at least one of the following problems in the prior art of machining pipes with a small bend ratio (R / D≤1.5): uneven deformation, structural defects, high weld risk, difficulty in controlling wall thickness, easy tool interference, severe wear, reduced cavity accuracy due to machining vibration, and low production efficiency due to reliance on manual experience to adjust parameters.

[0006] On one hand, embodiments of the present invention provide a CNC machining method combining bottom cutting of the inner arc cavity of a pipe with a small bend ratio and reverse machining, including the following steps:

[0007] S1. Establish a three-dimensional model of the target pipe component and solid blank;

[0008] S2. The pipe parts are machined from the end direction to the tangent point with the adjacent arc segment;

[0009] S3. Perform stepped hole machining on the inner cavity of the arc segment along at least two different directions;

[0010] S4. Create a positive cavity milling program to generate toolpaths for the positive cavity region of the machining arc segment;

[0011] S5. Create a reverse cavity milling program to generate toolpaths for machining the reverse cavity area of ​​the inner arc cavity;

[0012] S6. Obtain the corresponding tool path data based on the tool paths in S4 and S5, and convert the tool path data into a CNC machining program to drive the machine tool to perform machining.

[0013] Specifically, in step S1, the model includes a long straight segment, a short straight segment, and an intermediate arc segment.

[0014] Specifically, in step S2, after machining to the tangent point position with the adjacent arc segment, only the solid blank of the arc segment area is retained.

[0015] Furthermore, in step S3, the machining of the stepped hole satisfies the following conditions:

[0016] Holes are drilled axially from one end of the long straight section and the short straight section near the middle arc section to form stepped holes;

[0017] The diameter of the stepped hole gradually decreases from the distance from the middle arc segment to the distance from the middle arc segment;

[0018] The stepped holes are eccentrically machined to ensure that the stepped holes at both ends of the long straight section and the short straight section are connected.

[0019] Furthermore, in step S4, the positive cavity region includes a positive cavity region inside the short end cavity and a positive cavity region inside the long end cavity. The positive cavity region inside the short end cavity is the region formed by the axis of the short straight segment and the arc of the middle arc segment away from the center. The positive cavity region inside the long end cavity is the region formed by the axis of the long straight segment and the arc of the middle arc segment away from the center.

[0020] When machining the positive cavity area, the positive cavity areas in the short end cavity and the positive cavity areas in the long end cavity are machined independently. During machining, the tool axis direction is parallel to the axis of the short straight section or the long straight section, and interference with the inner hole of the short straight section or the long straight section is avoided by changing the tool head and tool holder of different sizes.

[0021] Furthermore, in step S5, the portion of the middle arc segment after removing the positive cavity region in the short end cavity and the positive cavity region in the long end cavity is the arc cavity inverted cavity region. The arc cavity inverted cavity region is divided equally into the short end cavity inverted cavity region and the long end cavity inverted cavity region by a diameter of the middle arc segment.

[0022] The toolpath for generating the inverted cavity region of the inner arc cavity is used to independently process the inverted cavity regions at the short end and the long end, including:

[0023] Set the cutter axis to be parallel to the axis of the long straight section or the short straight section;

[0024] Set the cutting layer to cover the furthest remaining end of the blank to the beginning of the arc segment;

[0025] Select conventional milling as the milling method, and allow bottom cutting;

[0026] Generate a reverse milling cutter path that processes from the end face of a long straight section or a short straight section towards the inside of the middle arc section;

[0027] Using the toolpath editor, set the conventional milling toolpath to reverse direction to obtain a climb milling toolpath that processes from the inside of the middle arc segment towards the end face of the long straight segment or the short straight segment.

[0028] Furthermore, both the conventional milling cutter path and the climb milling cutter path are machined using a three-sided milling cutter, and the undercut function is enabled; when machining the climb milling cutter path, the front edge of the three-sided milling cutter is out of cutting contact, and only the side edge and the rear edge participate in the cutting.

[0029] Specifically, the CNC program generated in step S6 can be dynamically adjusted based on the tool status and machining parameters fed back in real time during the machining process.

[0030] Specifically, the ratio of the bending radius R of the small bend ratio pipe to the outer diameter D of the pipe satisfies R / D≤1.5.

[0031] On the other hand, the present invention proposes a CNC machining system for implementing the above-described method, comprising:

[0032] The CAD module is used to create a 3D model of the target pipe component and the initial solid blank.

[0033] The CAM module is used to create the cavity milling cutter path for the positive and negative cavity regions;

[0034] The toolpath editing module is used to reverse the conventional milling toolpath in the cavity area into a climb milling toolpath;

[0035] The post-processing module is used to output CNC program code.

[0036] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0037] 1) When machining pipes with small bend ratios (R / D≤1.5) using traditional three-sided bottom cutting, the rapid change in the curvature of the inner arc cavity easily leads to simultaneous cutting by all three blades, resulting in high cutting resistance and rapid tool wear. This invention employs a combination of bottom cutting and reverse machining, combining... Figure 7As can be seen, in the machining of the short-end inner arc cavity, the first step is to generate a conventional milling path (light blue trajectory) from the end face to the inside of the arc segment. This path is then edited and reversed to a climb milling path from the arc segment to the straight segment. Only the side and back edges participate in the cutting, significantly reducing the cutting resistance. In practical applications, under the same cutting parameters, the insert change cycle is extended from 9 minutes to 43 minutes, allowing for higher linear speeds and reducing the machining time of the inner arc cavity from 40 hours to 16 hours. Figure 3 The eccentric stepped hole design, compared with traditional coaxial drilling, efficiently removes the arc-shaped allowance, optimizes the material removal distribution, reduces processing stress, and creates favorable conditions for subsequent reverse processing.

[0038] 2) Traditional methods using fixed cutting tools are difficult to adapt to the complex geometric changes within the inner cavity of small-diameter pipes. This invention innovatively proposes a tool adjustment strategy: in the positive cavity areas at both ends, smaller tool heads and tool holders are dynamically replaced based on the blank allowance distribution, avoiding tool interference (see...). Figure 4 This strategy not only improves machining accuracy (ellipticity error reduced from ±0.4mm to ±0.2mm), but also reduces tool wear, allowing for higher cutting speeds and shortening machining time in the positive cavity region (from 10 hours to 7 hours, see [link]). Figure 5 and Figure 6 ).

[0039] 3) In traditional reverse machining programming, the positive cavity area needs to be cut out in the 3D model and the tool axis adjusted. However, this invention addresses the characteristic of alternating positive and inverted cavity areas in the inner arc cavity of small-diameter pipes by proposing a toolpath editing technology that does not require model modification: by directly editing the positive toolpath to change the machining direction, the inverted cavity toolpath can be directly generated from the straight section of the pipe opening (see...). Figure 7 This method simplifies the programming process and avoids the problem of the simulated tool being unable to enter the cutting layer due to the inability to completely remove the positive cavity, thus significantly improving the ease of operation.

[0040] 4) This invention combines the cavity milling mode in CAD / CAM software and proposes a scientific zoning machining strategy: drilling eccentric stepped holes in both short and long straight sections, and visually dividing the machining areas of the outer and inner arc cavities (see...). Figure 4 Compared to traditional partitioning methods that rely on manual experience, this invention improves modeling efficiency and shortens programming time; it also increases actual machining efficiency and reduces roughing time, making it particularly suitable for machining solid bent tube forgings for nuclear power plants (see [link]). Figure 2 ).

[0041] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0042] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0043] Figure 1 : A schematic diagram of the process of a combined undercut and reverse processing method for the inner arc cavity of a pipe with a small bend ratio in this invention;

[0044] Figure 2 A three-dimensional schematic diagram of the blank with only the arc segment remaining after the straight sections at both ends have been machined.

[0045] Figure 3 : A schematic diagram of the stepped hole drilled from both the long and short ends;

[0046] Figure 4 Schematic diagram showing the division of the short-end cavity into positive cavity, the long-end cavity into positive cavity, and the inner arc cavity into inverted cavity region;

[0047] Figure 5 : Schematic diagram of the remaining blank after machining the positive cavity area inside the short end cavity;

[0048] Figure 6 : Schematic diagram of the blank in the inverted cavity area after machining the positive cavity area inside the long end cavity;

[0049] Figure 7 : Schematic diagram of the combination of short-end inner arc cavity bottom cutting and reverse setting and toolpath;

[0050] Figure 8 : Schematic diagram of the remaining blank during the machining process of the inner arc cavity at the short end;

[0051] Figure 9 : Schematic diagram of the remaining blank after machining the inner arc cavity at the short end;

[0052] Figure 10 : Schematic diagram of the remaining blank during the machining process of the inner arc cavity at the long end;

[0053] Figure 11 : Schematic diagram of blank residue after machining the inner arc cavity of a pipe with a small bend ratio.

[0054] Figure label:

[0055] 1- Positive cavity region inside the short end cavity; 2- Positive cavity region inside the long end cavity; 3- Inverted cavity region inside the inner arc cavity; 31- Inverted cavity region inside the long end cavity; 32- Inverted cavity region inside the short end cavity; 4- Machining residue inside the inner arc cavity. Detailed Implementation

[0056] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0057] This invention proposes an innovative process to address the high-precision and high-efficiency machining requirements of pipes with small bend ratios (R / D ≤ 1.5). Compared to the traditional integral forging + cutting process, this invention innovatively proposes a combination of undercutting and reverse cutting. By optimizing the toolpath, the three cutting edges perform cutting from the inside of the arc segment towards the end face in a reverse path. Furthermore, during the inner arc cavity cutting process, only the side and rear cutting edges cut, while the front cutting edge does not, significantly improving the cutting stress state. The innovatively designed dynamic tool adjustment mechanism can intelligently match tool specifications according to the geometric characteristics of the machining area, effectively solving the interference problem in machining small bend pipes. During programming, the use of forward toolpath editing simplifies the entire CNC programming process. Combined with a scientific intelligent machining area division method, the goal of high-precision, short-cycle, and intelligent machining is successfully achieved.

[0058] On the one hand, this invention proposes a CNC machining method that combines bottom cutting of the inner arc cavity of a pipe with a small bend ratio with reverse machining, such as... Figure 1 As shown, the machining datum is first established through 3D modeling. The material removal distribution is optimized using a multi-axial stepped hole layout. Then, with the help of a toolpath editor (such as the relevant function in UG software), the path in the inverted cavity area is intelligently switched to reverse cutting, and coordinated with the forward path in stages and regions, reconstructing traditional unidirectional machining into bidirectional composite machining. Combined with the eccentric stepped hole design, this fundamentally solves the problems of tool interference, high cutting resistance, and rapid tool wear in machining the inner cavity of pipes with small bending ratios, significantly improving machining efficiency and accuracy. Specifically, the following steps are included:

[0059] S1. Establish a three-dimensional model of the target pipe component and solid blank;

[0060] S2. The pipe parts are machined from the end direction to the tangent point with the adjacent arc segment;

[0061] S3. Perform stepped hole machining on the inner cavity of the arc segment along at least two different directions;

[0062] S4. Create a positive cavity milling program to generate toolpaths for the positive cavity region of the machining arc segment;

[0063] S5. Create a reverse cavity milling program to generate the toolpath for machining the reverse cavity region 3 of the inner arc cavity;

[0064] S6. Obtain tool path data based on the toolpaths in S4 and S5, and convert the tool path data into a CNC machining program to drive the machine tool to perform machining.

[0065] Further, in step S1, a three-dimensional model of the target pipe part and the initial solid blank is established using modeling software. The model includes a long straight section, a short straight section, and an intermediate arc section.

[0066] Preferably, a high-precision parametric 3D model of the target pipe part and its corresponding initial solid blank is constructed using NX software or Mastercam's CAD module. The specific steps are as follows:

[0067] (1) Based on the pipeline design specifications, a target model of a hollow pipeline containing a long straight section, a short straight section and an intermediate arc section is established, and its geometric features strictly follow the dimensional tolerance requirements of nuclear-grade pipelines.

[0068] (2) Simultaneously generate solid blank models with the same topological features, and accurately calculate the material removal area between the two through Boolean operations;

[0069] (3) Use the deviation analysis tool of NX software to perform three-dimensional visualization verification of the machining allowance to ensure that the allowance distribution in key areas (especially the arc transition area) meets the process control requirement of Δh≤0.5mm.

[0070] Specifically, the geometric features include the diameter (Φ) of the long straight segment and the short straight segment, the radius (R) of the intermediate arc segment, and the bending-to-diameter ratio (R / D).

[0071] It should be noted that the modeling method of the present invention not only provides accurate geometric references for subsequent CAM programming, but also can directly transfer machining feature information to the process simulation system through the NX PMI module, realizing digital closed-loop control of the entire nuclear-grade pipeline machining process.

[0072] Furthermore, in step S2, the end of the pipe part refers to the port position of its long end and short end straight section near the middle arc segment. After processing to the tangent position with the adjacent arc segment, only the solid blank of the arc segment area is retained.

[0073] Specifically, the initial blank undergoes rough machining of its straight ends, with the machining range precisely extended to the tangent point of the arc segment, such as... Figure 2As shown, the dark blue area represents the roughing process. Standard milling or turning techniques are used to remove excess material from the straight sections, while only retaining the complete solid blank in the curved areas. This process not only provides a reliable process benchmark for subsequent precision machining of the curved sections but also enables precise control of the machining allowance, thereby ensuring the machining accuracy and efficiency of subsequent processes.

[0074] Furthermore, in step S3 (see...) Figure 3 The machining of the stepped hole in the arc segment should simultaneously meet the following conditions: the cutting tool should be fed from the end face of the straight section near the middle arc segment of the long straight section and the short straight section along their respective axial directions to machine the stepped hole in the arc segment; the diameter of the stepped hole should gradually decrease from away from the middle arc segment to near the middle arc segment; the stepped hole should be machined eccentrically to ensure that the stepped holes at both ends are connected in the arc segment.

[0075] Compared with traditional coaxial drilling, the invention employs a unique eccentric stepped hole design, which has significant advantages: it maximizes the removal of internal allowances in the arc segment, reducing the subsequent milling load; it optimizes the material removal distribution, reducing machining stress; it ensures continuous tool path, avoiding machining vibration caused by blind holes; and it creates favorable initial conditions for subsequent reverse machining.

[0076] Specifically, the cutting process involves sequentially advancing the cutting tool axially from both ends of the straight section of the pipe, drilling stepped holes towards the center of the arc section to achieve bidirectional material removal. The closer to the interior of the arc section, the smaller the diameter of the stepped holes becomes, and eccentric machining is possible to ensure that the stepped holes at both ends are interconnected. The size and number of the stepped holes need to be designed based on the radius of rotation of the arc section to ensure continuity at both ends, without specific limitations. Figure 3 In this context, the two vertical sections refer to different diameter levels of the stepped hole. These levels are separated in the vertical direction, forming a stepped structure. This design helps to remove material more effectively while maintaining the stability and precision of the machining process.

[0077] Furthermore, to facilitate processing, the present invention divides the inner cavity of the arc segment into sections, such as... Figure 4 As shown, where:

[0078] The positive cavity (usually referring to the area adapted to conventional cavity machining methods and convenient for tool forward feed and other operations) is divided into a positive cavity region 1 in the short end cavity and a positive cavity region 2 in the long end cavity. The positive cavity region 1 in the short end cavity is the area formed by the axis of the short straight segment and the arc of the middle arc segment away from the center; the positive cavity region 2 in the long end cavity is the area formed by the axis of the long straight segment and the arc of the middle arc segment away from the center.

[0079] The inverted cavity (i.e., the area requiring special processing techniques or reverse tool feed during machining; in this invention, the positive cavity area is the opposite side of the inverted cavity area) is the inner arc cavity inverted cavity area 3. The portion of the intermediate arc segment excluding the positive cavity area 1 at the short end and the positive cavity area 2 at the long end constitutes the inner arc cavity inverted cavity area 3. This area is divided equally into the short end cavity inverted cavity area 32 and the long end cavity inverted cavity area 31 by a diameter of the intermediate arc segment.

[0080] It should be noted that during actual machining, there may be overlap between the positive cavity region 1 in the short end cavity and the positive cavity region 2 in the long end cavity. These overlapping regions do not require special differentiation during machining and can be machined multiple times to ensure the integrity and accuracy of the machining process.

[0081] Furthermore, in step S4, when machining the positive cavity area, the positive cavity area 1 in the short end cavity and the positive cavity area 2 in the long end cavity are machined independently. During machining, the tool axis direction is parallel to the straight section axis, and the tool enters from both ends of the pipe to gradually remove the excess material in the positive cavity area. Interference with the inner hole of the straight section is avoided by changing the tool disc and tool holder of different sizes.

[0082] Specifically, the CAM function of NX software is used and the cavity milling mode is selected for machining. First, the tool axis is set to be parallel to the axis of the short end straight section. The excess material in the positive cavity area of ​​the short end is removed by cavity milling operation, thereby completing the machining of positive cavity area 1 in the short end cavity. Then, a new cavity milling program is created, and the tool axis is adjusted to be parallel to the axis of the long end straight section in order to machine positive cavity area 2 in the long end cavity.

[0083] Figure 5 and Figure 6 The images show the remaining blanks at the short and long ends after the positive cavity machining is completed, with the dark blue areas indicating the remaining blank portions. These dark blue areas represent the inverted cavity areas that require further undercutting after the positive cavity machining is finished.

[0084] This invention employs a tool adjustment strategy: for machining the blank in the positive cavity regions at both ends, the further away from the straight cutting point, the smaller the tool head and tool holder need to be to avoid interference. Compared with traditional fixed-tool machining, this strategy significantly improves machining accuracy and efficiency while reducing tool wear.

[0085] This tool adjustment strategy, combined with the regional machining method, provides a precise benchmark for the subsequent machining of the inner arc cavity inverted cavity region 3.

[0086] Further, in step S5, the portion of the middle arc segment after removing the positive cavity region 1 in the short end cavity and the positive cavity region 2 in the long end cavity is the inverted arc cavity region. The inverted arc cavity region is divided equally into the inverted short end cavity region 32 and the inverted long end cavity region 31 by a diameter of the middle arc segment.

[0087] For the inner arc cavity inverted cavity region 3 (this region is the only remaining part of the workpiece to be processed, refer to...) Figure 6 The remaining blank (marked in dark blue) is used to create a cavity milling program. This area is subdivided into short-end cavity inverted cavity area 32 and long-end cavity inverted cavity area 31, which will be machined independently to ensure accuracy and efficiency.

[0088] The specific processing steps are as follows:

[0089] 1. Set the tool axis to be parallel to the axis of the straight section at one end;

[0090] 2. Set the cutting layer to cover the furthest remaining end of the blank up to the start of the arc segment;

[0091] 3. Select conventional milling as the milling method and allow undercut operations;

[0092] 4. Generate a reverse milling cutter path that allows undercutting and machining from the end face to the inside of the arc segment.

[0093] Furthermore, using the toolpath editor, the generated conventional milling toolpath is set to reverse, thereby obtaining a climb milling toolpath that allows undercutting with three cutting edges, machining from the inside of the arc segment towards the end face. This adjustment optimizes the cutting process and improves machining quality and efficiency.

[0094] It should be noted that in traditional machining, pipe arc segments typically employ a unidirectional forward cutting strategy, i.e., cutting inward from the starting end of the arc segment. However, for the inverted cavity region 3 of the inner arc, especially for pipes with a small radius of curvature, the machining process faces significant challenges. Because the curvature of the inner hole in such pipe arc segments changes rapidly, during unilateral machining, the lateral allowance increases with the depth of cut, causing the rake, side, and flank edges of a three-sided end mill to participate in the cutting simultaneously. This leads to a sharp increase in cutting resistance and a deterioration in machining conditions. Simultaneously, this region is narrow and contains blind spots, resulting in a high risk of tool interference and uneven cutting force distribution. Therefore, special tools such as three-sided end mills must be used to ensure rigidity.

[0095] However, while the multi-flute cutting mechanism of three-sided end mills can handle the machining requirements of complex cavities, it also brings a series of process problems. Simultaneous multi-flute cutting increases the contact area between the tool and the workpiece, complicates the distribution of cutting forces, and easily causes localized stress concentration, increasing the risk of chipping. Furthermore, the limited space in the chip flute makes chips prone to entanglement and blockage, hindering coolant cooling, leading to heat buildup and accelerated tool wear, especially the prominent crater wear on the flank face. These problems further degrade the surface finish, manifesting as chatter marks, burrs, or dimensional deviations, while tool durability is significantly lower than in conventional machining. In deep cavity machining, these adverse factors are amplified, ultimately leading to reduced machining efficiency and difficulty in maintaining stable workpiece accuracy due to tool wear.

[0096] To address the aforementioned problems, this invention proposes an innovative integrated toolpath strategy combining inner arc cavity bottom cutting and reverse machining. Traditional reverse machining requires removing the positive cavity area to open the inverted cavity and changing the tool axis direction. However, in small-diameter hollow pipes, the positive and inverted cavities are interspersed, making it impossible to directly remove the entire positive cavity area. This solution changes the machining direction by editing the forward toolpath, allowing the CNC toolpath for the inverted cavity area to be directly generated from the straight sections at both ends of the pipe opening, without modifying the 3D model or recalculating the toolpath.

[0097] The specific implementation method is as follows: First, a conventional milling toolpath is generated as the basic path, and then the direction is reversed through the toolpath editing function of NX software to optimize it into a climb milling machining path. The optimized toolpath allows the tool to participate in cutting only with the side and rear edges, while the front edge is out of contact, which significantly reduces cutting resistance. At the same time, the machining direction is from the inside of the arc segment to the outside, making the allowance distribution more uniform and effectively suppressing vibration, thereby greatly improving tool life and machining stability.

[0098] The specific steps are as follows:

[0099] 1. Create a cavity milling program: First, create a new cavity milling program. In this program, set the tool axis direction to be parallel to the axis of the short end straight section (see...). Figure 7 The toolpath is shown in light blue, with the starting layer of the toolpath located near the arc segment and the ending layer near the short end of the pipe.

[0100] 2. Set the cutting layer: Next, configure the cutting layer to ensure that its coverage extends from the farthest end of the remaining blank to the beginning of the arc segment.

[0101] 3. Select Milling Mode: Select conventional milling as the milling mode, meaning the milling cutter's rotation direction is opposite to the workpiece's feed direction. It also allows undercutting, thus generating a three-sided conventional milling toolpath that allows undercutting, machining from the end face towards the inside of the arc segment.

[0102] 4. Edit the toolpath: Finally, use the toolpath editor to set the generated conventional milling toolpath to reverse. This operation causes the toolpath to machine from the inside of the arc towards the end face, thus obtaining a climb milling toolpath that allows undercutting on three sides.

[0103] The same machining logic also applies to the toolpaths used to generate the inverted cavity region 31 within the long-end cavity. Figure 8 and Figure 9 The machining process of the short-end inner arc cavity region is shown, and Figure 10 This illustrates the machining process of the inner arc cavity region at the long end. In these figures, the pink area indicates the remaining blank portion after the inner arc cavity is machined.

[0104] For the different machining areas at the long and short ends, we adopted a unified toolpath optimization strategy. This strategy ensures consistency throughout the entire cavity machining process by maintaining a consistent tool axis projection algorithm and adaptive feed control. This intelligent toolpath editing technology provides an efficient and reliable solution for machining complex internal cavities.

[0105] Of particular note is that both the initial conventional milling path and the subsequent climb milling path employed a three-sided milling cutter with the undercut function enabled. The undercut function, when generating the toolpath using CAM software, allows the tool's rear cutting edge to pass through a small obstruction space to machine the geometry behind it. This prevents friction between the tool holder and the part geometry. Typically, this undercut function is used in conjunction with a three-sided milling cutter to machine chamfered areas, such as T-slots, ensuring that the chamfered area is completely cut.

[0106] During the machining process of a climb milling cutter, the cutting proceeds from the inside of the arc segment towards the end face. At this time, only the side cutting edge and the rear cutting edge of the tool participate in the cutting, while the front cutting edge does not participate in the cutting contact.

[0107] The optimized climb milling cutter path effectively reduces cutting force fluctuations, thereby reducing tool vibration. Simultaneously, this cutter path design can stably control the ellipticity error of the arc-shaped cavity to within 3%.

[0108] The results show that the optimized machining scheme significantly improves process performance. Cutting force fluctuations are greatly reduced, and the machining process is more stable; surface quality is significantly improved, and contour accuracy is steadily increased; at the same time, tool wear is effectively controlled, and tool life is significantly extended. This method is particularly suitable for machining complex internal cavities with alternating upright and inverted cavities, ensuring machining accuracy while avoiding cumbersome model modification processes, and has significant engineering application value.

[0109] Further, in step S6, corresponding toolpath data is generated according to the machining steps, and the toolpath data is post-processed to convert it into a CNC machining program. The generated CNC program can be dynamically adjusted according to the tool status and machining parameters fed back in real time during the machining process. The bent pipe blank is then clamped on a floor-type boring machine or a multi-axis CNC milling machine, and milling is performed according to the CNC program.

[0110] like Figure 11 As shown, after machining the inner arc cavity of a pipe with a small bend ratio, a small amount of machining residue may remain that has not been completely removed. If the residue is small, it can be smoothed by simple grinding; if the residue is more obvious, the tool shank diameter can be reduced, the effective cutting radius of the three-sided cutting edge can be increased, and the above machining steps can be repeated to ensure that the blank is completely removed.

[0111] By organically combining reverse programming technology with undercutting machining processes, this system achieves optimized control of the machining process: significantly improving efficiency while ensuring machining accuracy, and reducing operational complexity through standardized toolpath generation and editing workflows. This innovative machining method reduces the heavy reliance on manual experience in traditional processes, making the entire machining process more stable and reliable, and providing an effective solution for high-quality machining of pipes with small bend ratios.

[0112] This invention also provides a CNC machining system for implementing the above-mentioned method for machining the inner arc cavity of a pipe with a small bending ratio. The system includes: a CAD module for creating a three-dimensional model of the pipe and the blank; a CAM module for generating the cavity milling cutter paths for the positive and negative cavity regions; a toolpath editing module for reversing the conventional milling cutter path for the negative cavity region into a climb milling cutter path; and a post-processing module for outputting CNC program code.

[0113] The CNC machining system of the present invention first uses the CAM module to generate an initial conventional milling path, and then uses the direction reversal function of the toolpath editor to quickly convert it into an optimized climb milling path. This not only simplifies and simplifies the programming process, avoiding the tediousness of rebuilding the model and setting complex parameters, but more importantly, this unique machining method allows the three-sided milling cutter to participate in cutting only with the side and back edges, thereby greatly improving machining accuracy.

[0114] It should be noted that the software, modules, and codes (such as UG, CAD, CAM, etc.) involved in this invention are all well known to those skilled in the art.

[0115] The present invention will be described in more detail below through specific embodiments. These embodiments are merely descriptions of the best implementation of the invention and do not limit the scope of the invention in any way.

[0116] Example 1

[0117] A CNC machining method combining bottom cutting of the inner arc cavity of a pipe with a small bend ratio and reverse machining includes the following steps:

[0118] S1. Enable the software's CAD function to create a 3D model of the pipe parts and their blanks: long straight section (Φ300mm), short straight section (Φ300mm), and arc section (R=360mm, R / D=1.2).

[0119] S2. Process the straight sections of the long and short ends separately to the tangent point of the arc segment;

[0120] S3. Drill stepped arc holes from both the short straight section and the long straight section: Long straight section end: machine from Φ280mm to Φ200mm in a stepped manner; Short straight section end: machine from Φ280mm to Φ180mm in a stepped manner.

[0121] S4. Enable the CAM function in the software, select the cavity milling mode, with the cutter axis parallel to the axis of the short end straight section, and machine the positive cavity area 1 inside the short end cavity; create a cavity milling program, with the cutter axis parallel to the axis of the long end straight section, and machine the positive cavity area 2 inside the long end cavity;

[0122] S5. Create a reverse cavity milling program, with the tool axis parallel to the axis of the short straight section. Select the "Allow undercut" option in the cutting parameters and generate a toolpath for machining the reverse cavity region 32 inside the short end cavity. Edit the generated reverse cavity toolpath to reverse it. Repeat the above steps to generate a toolpath for machining the reverse cavity region inside the long straight section cavity.

[0123] S6. Generate the toolpath code for each step of the machining process, and post-process the toolpath code to generate a CNC program; clamp the bent pipe blank on a floor-type boring machine and perform milling machining according to the CNC program.

[0124] Example 2

[0125] The only difference from Example 1 is the processing of a bend with R / D = 1.5 (arc segment R = 200mm).

[0126] Example 3

[0127] The only difference from Example 1 is that the toolpath is generated using Mastercam software.

[0128] Comparative Example 1

[0129] The difference from Example 1 is that a three-sided milling cutter is used for forward machining (straight section → arc section direction), with the front / side / rear edges cutting simultaneously.

[0130] Comparative Example 2

[0131] The difference from Example 1 is that the undercut function is not used: that is, in the CAD function, most of the positive cavity areas in the model are removed (the inner cavity shape of the workpiece is special and cannot be completely removed). In the CAM function, the milling cutter axis direction is changed, and then the CNC program is generated.

[0132] Comparison table of characterization results

[0133] Table 1

[0134]

[0135] As shown in Table 1, under the same cutting parameters, the tool change cycle of Example 1 was significantly extended, increasing from 9 minutes in Comparative Example 1 to 43 minutes. This improvement not only increased tool life but also allowed for higher linear speeds (from 20 m / min in Comparative Examples 1 and 2 to 31 m / min and 33 m / min in Examples 1 and 2, respectively), reducing the machining time for the inner arc cavity from 40 hours to 16 hours. The machining time for the positive cavity region was reduced from 10 hours to 7 hours.

[0136] Furthermore, the embodiments outperform the comparative examples in terms of ellipticity and surface roughness. The ellipticity errors of the embodiments are all controlled within 3%, while the ellipticity errors of the comparative examples range from 6.2%, with some exhibiting significant residuals. Similarly, the surface roughness of the embodiments is also significantly better than that of the comparative examples, with roughness values ​​all below 3.2, while the roughness values ​​of the comparative examples are as high as 8.0, with some exhibiting significant residuals. These data demonstrate that the method of the embodiments has a clear advantage in terms of processing accuracy and surface quality.

[0137] This invention significantly improves the accuracy, efficiency, and tool economy of pipe bending through a combination of pre-machining of stepped holes, step-by-step milling of forward / inverted cavities, and bottom-cutting reverse toolpaths. It is especially suitable for fields with high precision requirements, such as nuclear power.

[0138] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A CNC machining method combining bottom cutting of the inner arc cavity of a pipe with a small bend ratio and reverse machining, characterized in that, Includes the following steps: S1. Establish a three-dimensional model of the target pipe component and solid blank; S2. The pipe parts are machined from the end direction to the tangent point with the adjacent arc segment; S3. Perform stepped hole machining on the inner cavity of the arc segment along at least two different directions; S4. Create a positive cavity milling program to generate toolpaths for the positive cavity region of the machining arc segment; S5. Establish the inverted cavity milling program and generate the toolpath for machining the inverted cavity area (3) of the inner arc cavity; S6. Obtain tool path data based on the toolpaths in S4 and S5, and convert the tool path data into a CNC machining program to drive the machine tool to perform machining.

2. The method according to claim 1, characterized in that, In step S1, the model includes a long straight segment, a short straight segment, and an intermediate arc segment.

3. The method according to claim 1, characterized in that, In step S2, after machining to the tangent point with the adjacent arc segment, only the solid blank of the arc segment area is retained.

4. The method according to claim 2, characterized in that, In step S3, the machining of the stepped hole satisfies the following conditions: Holes are drilled axially from one end of the long straight section and the short straight section near the middle arc section to form stepped holes; The diameter of the stepped hole gradually decreases from the distance from the middle arc segment to the distance from the middle arc segment; The stepped holes are eccentrically machined to ensure that the stepped holes at both ends of the long straight section and the short straight section are connected.

5. The method according to claim 4, characterized in that, In step S4, the positive cavity region includes a positive cavity region (1) in the short end cavity and a positive cavity region (2) in the long end cavity. The positive cavity region (1) in the short end cavity is the region formed by the axis of the short straight segment and the arc of the middle arc segment away from the center. The positive cavity region (2) in the long end cavity is the region formed by the axis of the long straight segment and the arc of the middle arc segment away from the center. When machining the positive cavity area, the positive cavity area (1) in the short end cavity and the positive cavity area (2) in the long end cavity are machined independently. During machining, the direction of the tool axis is parallel to the axis of the short straight section or the long straight section. By changing the tool disc and tool holder of different sizes, interference with the inner hole of the short straight section or the long straight section is avoided.

6. The method according to claim 5, characterized in that, In step S5, the portion of the middle arc segment from the short end cavity positive cavity region (1) and the long end cavity positive cavity region (2) is the arc cavity inverted cavity region. The arc cavity inverted cavity region is divided equally into the short end cavity inverted cavity region (32) and the long end cavity inverted cavity region (31) by a diameter of the middle arc segment. The toolpath that generates the inverted cavity region (3) of the inner arc cavity is machined independently for the inverted cavity region (32) at the short end and the inverted cavity region (31) at the long end, including: Set the cutter axis to be parallel to the axis of the long straight section or the short straight section; Set the cutting layer to cover the furthest remaining end of the blank to the beginning of the arc segment; Select conventional milling as the milling method, and allow bottom cutting; Generate a reverse milling cutter path that processes from the end face of a long straight section or a short straight section towards the inside of the middle arc section; Using the toolpath editor, set the conventional milling toolpath to reverse direction to obtain a climb milling toolpath that processes from the inside of the middle arc segment towards the end face of the long straight segment or the short straight segment.

7. The method according to claim 6, characterized in that, Both the conventional milling cutter path and the climb milling cutter path are machined using a three-sided milling cutter, and the undercut function is enabled; when machining the climb milling cutter path, the front edge of the three-sided milling cutter is out of cutting contact, and only the side edge and the rear edge participate in the cutting.

8. The method according to claim 1, characterized in that, The CNC program generated in step S6 can be dynamically adjusted based on the tool status and machining parameters fed back in real time during the machining process.

9. The method according to any one of claims 1-8, characterized in that, The ratio of the bending radius R to the outer diameter D of the pipe with a small bending radius ratio satisfies R / D≤1.

5.

10. A CNC machining system for implementing the method according to any one of claims 1-9, characterized in that, include: The CAD module is used to create a 3D model of the target pipe component and the initial solid blank. The CAM module is used to create the cavity milling cutter path for the positive and negative cavity regions; The toolpath editing module is used to reverse the conventional milling toolpath in the cavity area into a climb milling toolpath; The post-processing module is used to output CNC program code.

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