Method of machining a semi-spherical head having a multi-turn array of axial holes

Abstract

The application relates to processing of inclined holes on curved surfaces, and discloses a processing method of a hemispherical head with multiple-circle array axial holes, the hemispherical head comprising a connecting end surface at one end of the hemispherical head and perpendicular to the head axis, and multiple-circle array axial holes distributed on the hemispherical head with the head axis as the center, the processing method comprising the following steps: preparing a flat bottom forging as a blank, reserving a clamping surface on the blank, heat treating the blank, forming a positioning reference surface corresponding to the connecting end surface through rough machining and semi-finishing, reserving small hole bosses and clamping bosses on the outer surface of the head, the top surfaces of the small hole bosses and the clamping bosses being parallel to the positioning reference surface, then drilling the axial holes, heat treating again, finishing the inner surface, removing the small hole bosses and the clamping bosses, and finishing the axial holes. The application can be used for high-precision processing of the hemispherical head with multiple-circle array axial holes.

Description

Technical Field

[0001] This invention relates to a process for precision machining axial holes on thin-walled heads. Background Technology

[0002] Machining normal holes on end caps is relatively easy because it involves vertical drilling. For example, the three-degree-of-freedom machine tool disclosed in application CN112658706A can achieve this.

[0003] A certain piece of equipment requires a hemispherical thin-walled head (with a wall thickness t to inner diameter Di ratio of t / Di ≤ 0.01). The head has a flange at its end and multiple rings of annular arrayed small holes, all of which are axially arranged. The head has very small dimensional tolerances, and the machining accuracy of the small holes is extremely high. The machining challenge lies in achieving high-precision machining of the oblique holes on a curved surface. There are no precedents for this machining method, necessitating an innovative design of the machining process.

[0004] Chinese invention patent application CN101920452A, entitled "Manufacturing Method of Water Chamber Head for Nuclear Power Equipment", discloses a method for machining a cylindrical blank workpiece into a water chamber head, but all of its openings are normal holes and the machining allowance is large.

[0005] Chinese invention patent application CN119635206A, entitled "A method for processing a spherical head of a spacecraft fuel tank," also discloses a method for processing a spherical head. It completes the processing of inclined openings on curved surfaces and flanges at the openings by setting oblique and annular tooling. The process steps have multiple axial holes, but the axial holes to be processed are actually located on a plane rather than a curved surface. It also does not disclose a method for processing hemispherical heads with axial holes. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a processing method that can form a multi-ring array of axial holes on a hemispherical end cap while ensuring processing accuracy.

[0007] The technical solution adopted by this invention to solve its technical problem is: a method for processing a hemispherical end cap with multiple arrayed axial holes, wherein the end cap includes a connecting end face located at one end and perpendicular to the end cap axis, and multiple arrayed axial holes are distributed on the end cap with the end cap axis as the center. The processing method includes the following steps. A. Prepare a flat-bottomed forging as a blank, wherein the flat-bottomed forging includes a bottom surface parallel to the connecting end face, and perform heat treatment on the forging to eliminate stress; B. Roughly machine the inner and outer surfaces and connecting end faces of the head blank, and remove the forging surface; C. Semi-finished connection end face, using this face as the positioning reference surface, and leaving a finishing allowance; D. Semi-finish the inner surface and leave a finishing allowance; E. Turn over and reserve a clamping boss for clamping near the axis of the end cap. The top surface of the clamping boss is parallel to the positioning reference surface. In addition to the position of the clamping boss (3), a small hole boss is reserved at the position corresponding to each of the other axial holes on the outside. The top surface of the small hole boss is parallel to the positioning reference surface. The orthographic projection of the top surface in the direction of the end cap axis envelops the orthographic projection of the corresponding axial hole and has a machining allowance. Semi-finish the outer surface. F. Pre-drill holes on the reserved small hole boss, leaving a finishing allowance when drilling; G. Heat treatment of the workpiece; H. Fix the end cap using the clamping boss, and finish the inner surface and the connecting end face; I. Turn over, finish the outer surface, remove the remaining small hole boss, drill holes at the position of the clamping boss, leave a finishing allowance when drilling, and remove the remaining clamping boss. J. Finish all axial holes.

[0008] Furthermore, there are four pre-reserved clamping bosses for clamping, which are symmetrically distributed with respect to the axis of the end cap.

[0009] Furthermore, the surface of the forging is formed with a pair of clamping surfaces parallel to the axis of the end cap.

[0010] The beneficial effects of this invention are as follows: The design uses forgings as blanks, avoiding the springback problem that easily occurs when conventional thin-walled heads are formed by pressing steel plates, leading to dimensional deviations; after heat treatment of the blank, and before finishing after semi-finishing, the workpiece undergoes another heat treatment, which can fully release the stress during processing, further ensuring workpiece quality and machining accuracy; the reserved outer small hole boss facilitates the machining of axial holes, ensuring drilling position and accuracy; the reasonable selection of the positioning reference surface and clamping boss position during the machining process facilitates machining and clamping, especially for finishing the inner surface; considering all the above factors, a high-precision machining of a hemispherical head with multi-ring array axial holes is achieved. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the formed workpiece structure.

[0012] Figure 2 yes Figure 1 Top view.

[0013] Figure 3 This is a structural diagram of the blank part in step A.

[0014] Figure 4 This is a schematic diagram of removing the forged surface in step B.

[0015] Figure 5 This is a schematic diagram of the semi-finished connection end face in step C.

[0016] Figure 6 This is a schematic diagram of the semi-finished inner surface in step D.

[0017] Figure 7 This is a schematic diagram of the semi-finished outer surface in step E.

[0018] Figure 8 yes Figure 7 Top view.

[0019] Figure 9 This is a schematic diagram of pre-drilling holes on the small hole boss in step F.

[0020] Figure 10 This is a schematic diagram of the finishing of the inner surface and the connecting end face in step H.

[0021] Figure 11 This is a schematic diagram of the finishing of the outer surface and the small hole at the clamping boss position in step I.

[0022] Figure 12 This is a schematic diagram of a conventional direct drilling operation without employing this invention.

[0023] The markings in the figure are as follows: 1-head, 2-forging, 3-clamping boss, 4-small hole boss, 5-positioning reference surface, 6-clamping surface, 10-head axis, 20-bottom surface, 30-top surface of clamping boss, 40-top surface of small hole boss, 100-connecting end face, 101-inner surface, 102-outer surface, 103-axial hole. Detailed Implementation

[0024] The invention will be further described below with reference to the accompanying drawings.

[0025] like Figure 1 and Figure 2 As shown, in actual production, a hemispherical end cap 1 with multiple arrayed axial holes needs to be processed. The end cap 1 includes a connecting end face 100 located at one end and perpendicular to the end cap axis 10. Multiple arrayed axial holes 103 are distributed on the end cap 1 around the end cap axis 10, as shown in the figure. Figure 12 As shown, if conventional methods are used, such as pressing steel plates and drilling holes after pressing, it is difficult to guarantee the machining quality of both the workpiece body and the axial holes. In particular, drilling is difficult, especially the small axial holes near the straight edge of the end cap. The angle between the tool axis and the normal to the workpiece surface is about 54 degrees, which far exceeds the 5-degree limit.

[0026] During conventional drilling, the angle between the tool axis (drill centerline) and the workpiece surface normal (the direction perpendicular to the workpiece surface) should generally be controlled within 5°. This is based on a comprehensive optimization of cutting mechanics, machining accuracy, and tool life. The specific reasons and details are as follows: ①The core basis for the 5° limit: Cutting force balance: When the included angle is ≤5°, the force difference between the cutting edges on both sides of the drill bit is small, the radial eccentric force is controllable, and it can be offset by the rigidity of the drill bit itself or by appropriately reducing the feed rate, thus avoiding the hole axis from being skewed (the perpendicularity error can usually be controlled within 0.1mm / 100mm). Tool safety: When the angle is small (e.g., <3°), the impact on machining is negligible; when it is close to 5°, it is necessary to avoid excessive impact load by re-sharpening the drill bit (e.g., reducing the chisel edge and sharpening the cutting edge) or reducing the cutting speed (especially for hard materials); Material compatibility: For soft materials such as aluminum and copper, the included angle can be widened to 5-8° due to the low cutting resistance; however, for hard and brittle materials such as high-strength steel and cast iron, 5° is the critical value, and the drill bit is prone to chipping or breaking if it exceeds this value.

[0027] ② The dangers of exceeding the angle limit: Hole accuracy deterioration: When the included angle is greater than 5°, the cutting edge on one side of the drill bit contacts the workpiece first at the moment of entry, causing the hole axis to deviate to the side with less force, resulting in enlarged hole diameter (especially in deep hole machining, the cumulative deviation is more obvious), and out-of-tolerance perpendicularity. Increased tool damage: Increased skew angle causes a sharp increase in radial force and torque on the cutting edge. High-speed steel drill bits are prone to excessive edge wear, while carbide drill bits are prone to chipping or shank breakage due to their high brittleness. Deterioration in surface quality: Tears, burrs, or "step-like" defects will appear on the surface of the workpiece, especially on plastic materials, requiring an additional deburring process and affecting processing efficiency.

[0028] Therefore, conventional methods cannot be used to process the head. Figures 1 to 11 As shown, the processing method of this invention utilizes existing CNC machine tools to complete the machining of the end cap. The overall processing approach uses a flat-bottomed forging as the blank, and ensures machining accuracy through multiple machining processes including roughing, semi-finishing, and finishing. Simultaneously, a boss is formed during the semi-finishing process. This boss has a plane perpendicular to the axial hole axis to ensure drilling accuracy. Furthermore, stress issues caused by forging and machining are addressed through two heat treatments. The processing method includes the following steps: A. Reference Figure 3 A flat-bottomed forging 2 is prepared as a blank. The forging 2 includes a bottom surface 20 parallel to the connecting end face. The forging 2 is heat-treated to relieve stress. The flat-bottomed structure of the forging facilitates forging, placement, and transportation. Figure 3 As shown, the forging 2 is a polyhedral shape that approximates a hemisphere in order to reasonably reduce the machining allowance. The surface of the forging 2 is also formed with a pair of clamping surfaces 6 parallel to the head axis 10, which facilitates fixing the workpiece during rough machining. B. Reference Figure 4 Rough machining of the inner and outer surfaces and connecting end faces of the head blank, removing the forging surface, is a conventional step in obtaining a finished part from a forged blank. C. Reference Figure 5 The connecting end face 100 is semi-finished and used as the positioning reference surface 5, with a finishing allowance reserved. The connecting end face 100 is selected as the positioning reference surface for hemispherical workpieces to facilitate the subsequent CNC machining program. D. Reference Figure 6 Semi-finish the inner surface 101 and leave a finishing allowance; semi-finishing the inner surface first can reduce the cutting amount of drilling, which is beneficial to ensuring drilling accuracy. E. Reference Figure 7 and Figure 8 Turn the workpiece over and reserve a clamping boss 3 near the axis 10 of the end cap for clamping. The top surface 30 of the clamping boss is parallel to the positioning reference surface 5 so that the workpiece can be clamped during subsequent finishing of the inner surface. To ensure drilling accuracy, small hole bosses 4 are also reserved on the outer side at positions corresponding to each of the other axial holes 103, except for the position of the clamping boss 3. The top surface 40 of the small hole bosses 4 is parallel to the positioning reference surface 5, that is, perpendicular to the axis 10 of the end cap. The orthographic projection of the top surface in the direction of the end cap axis envelops the orthographic projection of the corresponding axial hole 103 and has a machining allowance. Semi-finish the outer surface 102. Semi-finishing the inner surface can reduce the cutting amount in drilling, which is beneficial to ensuring the pre-drilling accuracy in subsequent step F. From the perspective of minimizing subsequent machining allowance, it is best for the small hole boss 4 to correspond one-to-one with all axial holes 103. However, when the position of the small hole boss 4 coincides with the position of the reserved clamping boss 3, the clamping boss 3 can be used as the small hole boss 4. According to the clamping requirements, the size of the clamping boss 3 can be appropriately enlarged. For example, the position of a single clamping boss 3 can correspond to two or more axial holes 103. Since a small platform is reserved at the machining hole position, the angle between the tool axis and the normal of the workpiece surface is 0 degrees when drilling, which can effectively avoid various defects such as tool slippage, hole perpendicularity deviation, and burrs, thereby effectively ensuring the machining accuracy and tolerance of the axial holes on the end cap. F. Reference Figure 9 Pre-drill holes on the reserved small hole boss 4, and leave a finishing allowance when drilling, that is, at least two machining steps should be considered to complete the cutting at the axial hole. G. Heat treatment of the workpiece; This step is completed after the roughing and semi-finishing of the workpiece to eliminate most of the machining stress, which helps to ensure the dimensional accuracy of the final product. H, Reference Figure 10 The end cap 1 is fixed by the clamping boss 3, and the inner surface 101 and the connecting end face 100 are precision machined to achieve the final required dimensional accuracy of the inner surface 101 and the connecting end face 100. I. Reference Figure 11 Turn it over, finish the outer surface 102, remove the remaining small hole boss 4, drill a hole at the position of the clamping boss 3, leave a finishing allowance when drilling, remove the remaining clamping boss 3, so as to achieve the final required dimensional accuracy of the outer surface 102. J. Finish all axial holes 103 to achieve the final required dimensional accuracy of axial holes 103.

[0029] If necessary, the workpiece can also be cleaned and its dimensions re-inspected after the above steps are completed.

[0030] In the above machining process, the workpiece is fixed by means of the bottom surface, clamping surface, positioning reference surface, and clamping boss, eliminating the need for additional tooling design and preparation, thus saving costs. Through the cooperation of the above steps, a high-precision machining of a hemispherical head with multi-ring arrayed axial holes is completed, and the dimensions fully meet the design requirements after re-inspection.

Claims

1. A method for processing a hemispherical end cap with a multi-ring array of axial holes, the end cap (1) comprising a connecting end face (100) located at one end and perpendicular to the end cap axis, characterized in that: The end cap (1) has multiple rings of arrayed axial holes (103) centered on the end cap axis (10). The processing method includes the following steps. A. Prepare a flat-bottomed forging (2) as a blank. The forging (2) includes a bottom surface (20) parallel to the connecting end face. Heat treat the forging (2) to eliminate stress. B. Roughly machine the inner and outer surfaces and connecting end faces of the head blank, and remove the forging surface; C. Semi-finished connection end face (100), using this face as the positioning reference surface (5), and reserving finishing allowance; D. Semi-finish the inner surface (101) and leave a finishing allowance; E. Turn over and reserve a clamping boss (3) for clamping near the head axis (10). The top surface (30) of the clamping boss is parallel to the positioning reference surface (5). In addition to the position of the clamping boss (3), a small hole boss (4) is reserved at the position corresponding to each of the other axial holes (103). The top surface (40) of the small hole boss (4) is parallel to the positioning reference surface (5). The orthographic projection of the top surface in the direction of the head axis envelops the orthographic projection of the corresponding axial hole (103) and has a machining allowance. Semi-finish the outer surface (102). F. Drill holes in the reserved small hole boss (4), and leave a finishing allowance when drilling; G. Heat treatment of the workpiece; H. Fix the end cap (1) using the clamping boss (3), and finish the inner surface (101) and the connecting end face (100). I. Turn over, finish the outer surface (102), remove the remaining small hole boss (4), drill a hole at the position of the clamping boss (3), leave a finishing allowance when drilling, and remove the remaining clamping boss (3). J. Finish all axial holes (103).

2. The processing method of a hemispherical head with a multi-ring array of axial holes as described in claim 1, characterized in that: The reserved clamping bosses (3) are four in number and are centrally symmetrically distributed with respect to the head axis (10).

3. A method for processing a hemispherical head with a multi-ring array of axial holes as described in claim 1 or 2, characterized in that: The surface of the forging (2) is formed with a pair of clamping surfaces (6) parallel to the head axis (10).

Images