An over-positioning grinding method for hard and brittle pipe fittings with a large length-diameter ratio

Abstract

An over-positioning grinding method for hard and brittle pipe fittings with a large length-diameter ratio belongs to the field of grinding technology. Although the existing grinding methods can improve the grinding accuracy of the outer circle of hard and brittle pipe fittings, they cannot ensure the coaxiality of the outer circle and the inner circle of the hard and brittle pipe fittings after grinding at the same time, which results in inconsistent wall thickness of the hard and brittle pipe fittings after grinding. The present invention uses a rigid mandrel, a supporting plate and a guide wheel to perform over-positioning support on the hard and brittle pipe fittings during the processing, drives the guide wheel to rotate forward, feeds the hard and brittle pipe fittings with a large length-diameter ratio into the grinding area, the guide wheel contacts and drives the hard and brittle pipe fittings to move forward while rotating reversely, the grinding wheel rotates reversely and feeds towards the hard and brittle pipe fittings side, as the grinding and feeding of the grinding wheel, the guide wheel and the supporting plate feed towards the hard and brittle pipe fittings side to always maintain the over-positioning support of the guide wheel, the supporting plate and the rigid mandrel on the hard and brittle pipe fittings, and as the hard and brittle pipe fittings move, the grinding processing of the hard and brittle pipe fittings with a large length-diameter ratio is realized. The present invention is used for the grinding processing of hard and brittle pipe fittings.

Description

Technical Field

[0001] The present invention belongs to the technical field of grinding, relates to the field of external circle grinding of hard and brittle materials, and particularly relates to an over-positioning grinding method for hard and brittle pipe fittings with a large length-diameter ratio. Background Art

[0002] At present, hard and brittle pipe fittings with a large length-diameter ratio are widely used in fields such as semiconductors, nuclear energy, and chemical engineering. Hard and brittle pipe fittings applied under special working conditions need to have high dimensional and shape accuracy. Due to the preparation process, there are large dimensional and shape accuracy errors in the blank pipes of hard and brittle pipe fittings with a large length-diameter ratio, which cannot directly meet the use requirements. This means that it is necessary to process the hard and brittle pipe fittings to ensure their accuracy. However, due to the structural characteristics of a large length-diameter ratio and the material characteristics of high hardness and high brittleness of hard and brittle pipe fittings with a large length-diameter ratio, it is very difficult to perform high-precision and high-efficiency precision forming on such parts, which greatly limits the application of such parts as hard and brittle pipe fittings with a large length-diameter ratio in various industries.

[0003] Currently, the external circle of existing hard and brittle pipe fittings is generally processed by external circle grinding. However, the current processing method cannot improve the grinding accuracy of the external circle of hard and brittle pipe fittings and ensure the coaxiality of the external circle and the internal circle of the hard and brittle pipe fittings after grinding, which will result in inconsistent wall thicknesses of the hard and brittle pipe fittings after grinding. Summary of the Invention

[0004] In view of this, the present invention provides an over-positioning grinding method for hard and brittle pipe fittings with a large length-diameter ratio, which can perform over-positioning support on the hard and brittle pipe fittings during the processing, so as to improve the grinding accuracy of the external circle of the hard and brittle pipe fittings, and at the same time, can ensure the coaxiality of the external circle and the internal circle of the hard and brittle pipe fittings after grinding, realizing high-precision processing of hard and brittle pipe fittings with a large length-diameter ratio.

[0005] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0006] An over-positioning grinding method for hard and brittle pipe fittings with a large length-diameter ratio, and the specific grinding process is as follows:

[0007] S1, assembly and positioning of the hard and brittle pipe fitting: A rigid mandrel passes through the inner hole of the hard and brittle pipe fitting with a large length-diameter ratio and has a clearance fit with the hard and brittle pipe fitting. The rigid mandrel is tensioned and the hard and brittle pipe fitting is kept horizontal. Both ends of the rigid mandrel and near the hard and brittle pipe fitting are each supported by a vibration damping device to reduce the vibration amplitude and frequency of the rigid mandrel during the processing of the hard and brittle pipe fitting; The support plate contacts the outer pipe surface of the hard and brittle pipe fitting and supports the hard and brittle pipe fitting;

[0008] S2. Determine the grinding area: Arrange the grinding wheel and the guide wheel on both sides of the hard and brittle pipe fitting respectively, and keep the axes of the grinding wheel and the guide wheel at the same height. Adjust their heights so that there is a height difference between the axes of the guide wheel and the grinding wheel and the axis of the hard and brittle pipe fitting. The grinding surface of the grinding wheel faces the guiding and supporting surface of the guide wheel directly. The grinding wheel, the guide wheel and the supporting plate enclose the grinding area of the hard and brittle pipe fitting, and cooperate with the rigid mandrel to perform over-positioning support on the hard and brittle pipe fitting;

[0009] S3. Guide wheel positioning: Move the hard and brittle pipe fitting to the grinding area, and the guide wheel moves towards the hard and brittle pipe fitting side until the guiding and supporting surface of the guide wheel contacts and presses the outer pipe surface of the hard and brittle pipe fitting, so that there is a pre-pressure between the guide wheel and the hard and brittle pipe fitting. The hard and brittle pipe fitting is removed from the grinding area, and the position of the guide wheel remains unchanged;

[0010] S4. Tool setting: Drive the guide wheel to rotate forward, and send the hard and brittle pipe fitting with a large length-diameter ratio into the grinding area. While the guide wheel contacts and drives the hard and brittle pipe fitting to move forward, it rotates in the reverse direction. The grinding wheel rotates in the reverse direction and feeds slightly towards the hard and brittle pipe fitting side to achieve the tool setting operation;

[0011] S5. Grinding of the hard and brittle pipe fitting: Set the rotational speed and feed speed of the guide wheel and the rotational speed and feed speed of the grinding wheel, and set the grinding amount of the hard and brittle pipe fitting. Drive the guide wheel to rotate forward, and send the hard and brittle pipe fitting with a large length-diameter ratio into the grinding area. While the guide wheel contacts and drives the hard and brittle pipe fitting to move forward, it rotates in the reverse direction. The grinding wheel rotates in the reverse direction and feeds towards the hard and brittle pipe fitting side. As the grinding and feeding of the grinding wheel progress, the guide wheel and the supporting plate feed towards the hard and brittle pipe fitting side to always maintain the over-positioning support of the guide wheel, the supporting plate and the rigid mandrel on the hard and brittle pipe fitting. As the hard and brittle pipe fitting moves, the grinding process of the hard and brittle pipe fitting with a large length-diameter ratio is realized.

[0012] Further, in the above S5, the linear speed of the grinding wheel is 30 m / s.

[0013] Further, in the above S5, when the outer diameter of the grinding wheel is 400 mm, the rotational speed of the grinding wheel is 1400 rpm / min - 1500 rpm / min.

[0014] Further, in the above S5, the working rotational speed of the guide wheel is 15 rpm / min - 20 rpm / min.

[0015] Further, the value range of the clearance between the rigid mandrel and the hard and brittle pipe fitting is 0.2 - 0.5 mm.

[0016] Further, the supporting plate is arranged on the side close to the grinding wheel and supports the hard and brittle pipe fitting obliquely. The supporting surface of the supporting plate is an inclined surface, and the inclination angle of the supporting surface is 30°.

[0017] Further, the grinding wheel is a parallel grinding wheel.

[0018] Further, the grinding wheel is a cup-shaped grinding wheel, and there is an offset angle θ between the axis of the cup-shaped grinding wheel and the axis of the hard and brittle pipe fitting, where θ > 0.

[0019] Further, in the step S2, the height difference between the axes of the guide wheel and the grinding wheel and the axis of the hard and brittle pipe fitting is obtained by the following method:

[0020] Let the outer radius of the hard and brittle pipe fitting with a large length-to-diameter ratio be R w , the radius of the guide wheel be R c , the radius of the grinding wheel be R g . Let the height difference between the axis of the hard and brittle pipe fitting and the axis of the grinding wheel be h; when the grinding wheel contacts the hard and brittle pipe fitting, let the initial contact point of the grinding wheel and the hard and brittle pipe fitting be A, the tangent at point A be l, when the guide wheel contacts the hard and brittle pipe fitting, let the initial contact point of the guide wheel and the hard and brittle pipe fitting be B, and when the support pallet contacts the hard and brittle pipe fitting, let the initial contact point of the support pallet and the hard and brittle pipe fitting be C;

[0021] Let the axis of the hard and brittle pipe fitting be O, the center of the guide wheel be O c , the center of the grinding wheel be O g , ∠AOC = α, ∠BOC = φ, ∠AO g O c = β g , ∠O g O c B = β c , β is the grinding angle of the grinding wheel, δ is the angle between OA and the horizontal line, the inclination angle of the supporting surface of the support pallet is γ, and a rectangular coordinate system is established with O g as the origin. The coordinates of the tangent point A in the coordinate system xO g y are (x, y); when there is an offset angle θ between the axis of the grinding wheel and the axis of the hard and brittle pipe fitting, the grinding wheel grinds at the offset angle θ, and the profile projection of the grinding wheel perpendicular to the feed speed direction section. Then, the elliptic curve equation in xO g y can be expressed as:

[0022]

[0023] where y is the height difference between the tangent point A and the axis center of the grinding wheel, and x is the horizontal distance from the tangent point A to the vertical axis y;

[0024] From formula (1), the following relational expressions are obtained:

[0025] x = R g sinθcosβ g (2)

[0026] y = R g sinβ g(3)

[0027] The slope of the tangent line l passing through the tangent point A on the elliptic curve can be expressed as:

[0028]

[0029] Since the tangent value of the slope of the tangent line l is the angle between the tangent line l and the X-axis, therefore, from formula (4), the angle δ between the tangent line l and the x-axis is:

[0030]

[0031] According to the geometric relationship, the following formula is obtained:

[0032] β c ≈h / (R w +R c )(6)

[0033] h = y + R w sinδ (7)

[0034] β = β c +δ = π - φ - α (8)

[0035] Substituting δ, y, β c into formulas (7) and (8), we get:

[0036]

[0037] In trigonometric functions, when the angle is small, the following simplification occurs:

[0038]

[0039] cot(β g )≈1 / β g (12)

[0040] Therefore, substituting formulas (11) and (12) into formulas (9) and (10) can be changed to:

[0041]

[0042] h = R g β g +R w β g sinθ (14)

[0043] Combining formulas (13) and (14), we get:

[0044]

[0045] Furthermore, the process of obtaining the grinding angle β is as follows:

[0046] The following formula is obtained according to the geometric relationship:

[0047]

[0048] Stability growth coefficient A i The expression is:

[0049]

[0050] Combining formulas (4)-(17), formula (18) can be transformed into a formula with only variables of angles β and γ, and formula (18) becomes:

[0051]

[0052] Based on the open-source software python and the stability growth coefficient A i A stability diagram is obtained. When A i > 0, it is a stable region. According to the inclination angle γ of the supporting surface of the supporting plate, the range value of β is selected within the stable region.

[0053] The beneficial effects of the present invention compared with the prior art are as follows:

[0054] 1. The supporting plate obliquely supports the surface of the hard and brittle pipe fitting. The guide wheel and the grinding wheel also play a role in supporting and positioning the hard and brittle pipe fitting. The supporting of the supporting plate, the guide wheel and the grinding wheel to the hard and brittle pipe fitting is upward support, and a grinding area is formed. The rigid mandrel and the hard and brittle pipe fitting are in clearance fit, which can not only ensure the rotation and advancement of the hard and brittle pipe fitting, but also keep the hard and brittle pipe fitting horizontal all the time when the rigid mandrel is tensioned, that is, the hard and brittle pipe fitting is pressed in the grinding area and is supported by multiple points during the processing. It not only realizes the stable support of over-positioning of the hard and brittle pipe fitting, but also provides a positioning reference for the machining of the outer circle of the workpiece. Under this over-positioning support, the hard and brittle pipe fitting can improve the grinding accuracy of the outer circle of the hard and brittle pipe fitting, and at the same time ensure the coaxiality of the outer circle and the inner circle of the hard and brittle pipe fitting after grinding, realizing the high-precision machining of the hard and brittle pipe fitting with a large length-diameter ratio.

[0055] 2. The guide wheel adopts a hyperboloid of one sheet structure, which can not only drive the workpiece to rotate stably, realize the uniform machining of the outer circle surface of the workpiece, but also drive the workpiece to make a uniform forward movement, realizing the uniform machining of the workpiece in the length direction.

[0056] 3. The grinding wheel can be a parallel grinding wheel or a cup-shaped grinding wheel. When the grinding wheel is a cup-shaped grinding wheel, the rotation axis of the cup-shaped grinding wheel is perpendicular to the feed direction, so that the grinding in the feed direction becomes cylindrical grinding instead of surface grinding, which can reduce the grinding force. The outer cylindrical surface of the grinding wheel removes materials and has a smaller contact area. Therefore, more precise cutting control can be performed on the workpiece, especially suitable for high-precision cylindrical grinding, and higher surface quality and dimensional accuracy can be achieved. At the same time, the cutting area of the cup-shaped grinding wheel is concentrated, and the force during grinding is small, reducing the influence of the cutting force. The movement trajectory of the abrasive grains on the end face of the grinding wheel is parallel to the material forming surface, which can not only achieve grinding with large depth of cut, but also reduce the grinding force of the grinding wheel on the workpiece, avoid damaging the workpiece with brittle characteristics, and obtain a workpiece surface with less subsurface damage. By appropriately adjusting the angle or dressing of the cup-shaped grinding wheel, its service life can be extended and the replacement frequency of the grinding tool can be reduced. Description of the Drawings

[0057] The drawings, as part of this application, are used to provide a further understanding of the present invention.

[0058] Figure 1 It is a schematic diagram of the overall structure of the assembly of a brittle pipe fitting and an over-positioning grinding device.

[0059] Figure 2 It is a schematic diagram of the contact between the grinding wheel shape and the brittle pipe fitting during the processing of the brittle pipe fitting.

[0060] Figure 3 It is a schematic diagram of the over-positioning fit of the grinding wheel, rigid mandrel, support plate and guide wheel during the processing of the brittle pipe fitting.

[0061] Figure 4 It is a schematic diagram of the working principle of the over-positioning geometric layout during the grinding of the brittle pipe fitting.

[0062] Figure 5 It is to determine the grinding angle according to the stability diagram.

[0063] Figure 6 It is a physical diagram of the brittle pipe fitting after over-positioning grinding. Among them, a one-meter silicon carbide pipe was ground, and the wall thickness measurement was sampled and observed.

[0064] Figure 7 It is the roundness error of the brittle pipe fitting after processing. The physical object in the figure is the result after measuring a 100-mm-long silicon carbide pipe.

[0065] Description of the reference numerals: 1 - rigid mandrel; 2 - brittle pipe fitting; 3 - support plate; 4 - guide wheel; 5 - grinding wheel; 6 - vibration damping device. Detailed Description of the Embodiment

[0066] The following detailed description of the present invention is made in conjunction with specific embodiments.

[0067] See Figure 1 and Figure 2 , the over-positioning grinding method of a hard and brittle pipe fitting with a large length-diameter ratio in this embodiment is realized by using an over-positioning grinding device. As shown in Figure 1 , the figure shows a schematic diagram of the overall structure of the assembly of the hard and brittle pipe fitting and the over-positioning grinding device. The over-positioning grinding device includes a rigid mandrel 1, a vibration damping device 6, a support plate 3, a grinding wheel 5 and a guide wheel 4. The rigid mandrel 1 is horizontally arranged and used to support the hard and brittle pipe fitting 2 with a large length-diameter ratio. Since the hard and brittle pipe fitting 2 is a pipe fitting with a large length-diameter ratio and the rigid mandrel 1 passes through the inner hole of the rigid mandrel 1 and is used to support the hard and brittle pipe fitting 2, the rigid mandrel 1 is also a support member with a large length-diameter ratio. The rigid mandrel 1 is prone to bending downwards. Therefore, when the rigid mandrel 1 supports the hard and brittle pipe fitting 2, both ends of the rigid mandrel 1 are tensioned by a tensioning device to keep the rigid mandrel 1 and the hard and brittle pipe fitting 2 always horizontal. The support plate 3 is arranged below the rigid mandrel 1 and is also used to support the hard and brittle pipe fitting 2 with a large length-diameter ratio. The support of the rigid mandrel 1 and the support plate 3 for the hard and brittle pipe fitting 2 keeps the hard and brittle pipe fitting 2 in a horizontal state during the processing. There are two vibration damping devices 6, and the two vibration damping devices 6 are respectively arranged at both ends of the rigid mandrel 1 and are used to support the rigid mandrel 1. Since the hard and brittle pipe fitting 2 will generate vibrations during the processing, the rigid mandrel 1 will also generate vibrations under the action of inertia. The vibration damping device 6 can reduce the vibration amplitude and frequency of the rigid mandrel 1, and then reduce the vibration frequency and amplitude of the hard and brittle pipe fitting 2, improving the processing accuracy of the hard and brittle pipe fitting 2. The guide wheel 4 is installed on one side of the rigid mandrel 1 and is used to rotate the workpiece and push the workpiece forward to ensure the smooth processing of the workpiece. The grinding wheel 5 is installed on the other side of the rigid mandrel 1 and is used to grind the outer circle of the hard and brittle pipe fitting 2.

[0068] The over-positioning grinding process of the hard and brittle pipe fitting 2 is as follows:

[0069] S1, assembly and positioning of the hard and brittle pipe fitting: As shown in Figure 1 , the rigid mandrel 1 passes through the inner hole of the hard and brittle pipe fitting 2 with a large length-diameter ratio and has a clearance fit with the hard and brittle pipe fitting 2. The value range of the clearance between the rigid mandrel 1 and the hard and brittle pipe fitting 2 is 0.2 - 0.5 mm, that is, it ensures that the hard and brittle pipe fitting 2 can move along the length direction of the rigid mandrel 1 and can also realize the positioning and support of the hard and brittle pipe fitting 2. Tighten the rigid mandrel 1 and keep the hard and brittle pipe fitting 2 horizontal; each of the two ends of the rigid mandrel 1 and near the hard and brittle pipe fitting 2 is supported by a vibration damping device 6 to reduce the vibration amplitude and frequency of the rigid mandrel 1 during the processing of the hard and brittle pipe fitting 2, thereby reducing the vibration frequency and amplitude of the hard and brittle pipe fitting 2 and improving the processing accuracy of the workpiece; the support plate 3 slightly contacts the outer pipe surface of the hard and brittle pipe fitting 2 and obliquely supports the hard and brittle pipe fitting 2. The support plate 3 is arranged on the side close to the grinding wheel 5, and the supporting surface of the support plate 3 is an inclined surface of 30°.

[0070] S2. Determine the grinding area: Arrange the grinding wheel 5 and the guide wheel 4 on both sides of the hard and brittle pipe fitting 2 respectively, and keep the axes of the grinding wheel 5 and the guide wheel 4 at the same height. Adjust their heights so that the axes of the guide wheel 4 and the grinding wheel 5 are lower than the axis of the hard and brittle pipe fitting 2. The grinding surface of the grinding wheel 5 faces the guiding and supporting surface of the guide wheel 4 directly. The rigid mandrel 1, the grinding wheel 5, the guide wheel 4 and the supporting plate 3 enclose the grinding area of the hard and brittle pipe fitting 2 with over-positioning support;

[0071] S3. Guide wheel positioning: Move the hard and brittle pipe fitting 2 to the grinding area. The guide wheel 4 moves towards the hard and brittle pipe fitting 2 until the guiding and supporting surface of the guide wheel 4 contacts and presses the outer pipe surface of the hard and brittle pipe fitting 2, so that there is a pre-pressure between the guide wheel 4 and the hard and brittle pipe fitting 2. The hard and brittle pipe fitting 2 is removed from the grinding area, and the position of the guide wheel 4 is kept unchanged;

[0072] S4. Tool setting: Drive the guide wheel 4 to rotate forward. Feed the hard and brittle pipe fitting 2 with a large length-diameter ratio into the grinding area. While the guide wheel 4 contacts and drives the hard and brittle pipe fitting 2 to move forward, it rotates in the reverse direction. The grinding wheel 5 rotates in the reverse direction and feeds slightly towards the hard and brittle pipe fitting 2 to achieve the tool setting operation;

[0073] S5. Grinding of the hard and brittle pipe fitting: Set the rotation speed and feed speed of the guide wheel 4 and the rotation speed and feed speed of the grinding wheel 5, and set the grinding amount of the hard and brittle pipe fitting 2. Drive the guide wheel 4 to rotate forward. The working rotation speed of the guide wheel 4 is 15 rpm / min

[0074] ~20 rpm / min. Feed the hard and brittle pipe fitting 2 with a large length-diameter ratio into the grinding area. While the guide wheel 4 contacts and drives the hard and brittle pipe fitting 2 to move forward, it rotates in the reverse direction. The grinding wheel 5 rotates in the reverse direction and feeds towards the hard and brittle pipe fitting 2. The linear speed of the grinding wheel 5 is 30 m / s. When the outer diameter of the grinding wheel 5 is 400 mm, the rotation speed of the grinding wheel 5 is 1400 rpm / min~1500 rpm / min; As the grinding and feeding of the grinding wheel 5 proceed, the surface of the hard and brittle pipe fitting 2 is gradually removed. The pre-tightening force of the guide wheel 4 and the supporting plate 3 on the hard and brittle pipe fitting 2 gradually decreases, which will affect the machining accuracy of the hard and brittle pipe fitting 2. Therefore, during the machining of the hard and brittle pipe fitting 2, in addition to the gradual feeding of the grinding wheel 5, the guide wheel 4 and the supporting plate 3 also gradually feed, ensuring that the pre-tightening force on the hard and brittle pipe fitting 2 is relatively constant, and at the same time always maintaining the over-positioning support of the guide wheel 4, the supporting plate 3 and the rigid mandrel 1 on the hard and brittle pipe fitting 2. As the hard and brittle pipe fitting 2 moves, the grinding machining of the hard and brittle pipe fitting 2 with a large length-diameter ratio is realized.

[0075] Such as Figure 3 and Figure 4As shown, since the support pallet 3 is arranged on one side close to the grinding wheel 5 and obliquely supports the surface of the hard and brittle pipe fitting 2, the contact points of the guide wheel 4 and the grinding wheel 5 with the hard and brittle pipe fitting 2 are both lower than the axis of the hard and brittle pipe fitting 2. Therefore, when the hard and brittle pipe fitting 2 is being ground, the guide wheel 4 and the grinding wheel 5 also play a role in supporting and positioning the hard and brittle pipe fitting 2. However, the support of the support pallet 3, the guide wheel 4, and the grinding wheel 5 for the hard and brittle pipe fitting 2 is all upward support, forming a grinding area. The rigid mandrel 1 and the hard and brittle pipe fitting 2 are in clearance fit, which can not only ensure the rotation and advancement of the hard and brittle pipe fitting 2, but also keep the hard and brittle pipe fitting 2 horizontal all the time when the rigid mandrel 1 is tensioned, that is, pressing the hard and brittle pipe fitting 2 in the grinding area and being supported by multiple points during the processing. This not only realizes the stable support of over-positioning of the hard and brittle pipe fitting 2, but also provides a positioning reference for the machining of the outer circle of the workpiece. Under this over-positioning support, the hard and brittle pipe fitting 2 can improve the grinding accuracy of the outer circle of the hard and brittle pipe fitting 2, and at the same time ensure the coaxiality of the outer circle and the inner circle of the hard and brittle pipe fitting 2 after grinding, realizing the high-precision machining of the hard and brittle pipe fitting 2 with a large length-diameter ratio, such as Figure 3 in, the circular contour line formed by the dotted line is the contour line of the hard and brittle pipe fitting 2 at the initial position, and the solid circular contour line is the contour line of the hard and brittle pipe fitting 2 after grinding. By Figure 3 it can be seen that the two contour lines before and after grinding are coaxial and coaxial with the inner circle, so the coaxiality of the inner circle and the outer circle of the workpiece after grinding is ensured.

[0076] In this embodiment, the guide wheel 4 adopts a hyperboloid of one sheet structure. It can not only drive the workpiece to rotate stably to achieve uniform machining of the outer cylindrical surface of the workpiece, but also drive the workpiece to move forward at a constant speed to achieve uniform machining of the workpiece in the length direction. The grinding wheel 5 can be a parallel grinding wheel or a cup-shaped grinding wheel. When the grinding wheel 5 is a cup-shaped grinding wheel, the rotation axis of the cup-shaped grinding wheel is perpendicular to the feed direction, so that the grinding in the feed direction becomes the grinding of the outer cylindrical surface instead of the plane grinding, which can reduce the grinding force. The outer cylindrical surface of the grinding wheel removes materials and has a smaller contact area. Therefore, more precise cutting control can be performed on the workpiece, especially suitable for high-precision external cylindrical grinding, and higher surface quality and dimensional accuracy can be achieved. At the same time, the cutting area of the cup-shaped grinding wheel is concentrated, and the force during grinding is small, reducing the influence of the cutting force. The movement trajectory of the abrasive grains on the end face of the grinding wheel is parallel to the material forming surface, which can not only achieve large-depth-of-cut grinding, but also reduce the grinding force of the grinding wheel on the workpiece, avoid damaging the workpiece with brittle characteristics, and obtain a workpiece surface with less subsurface damage. By appropriately adjusting the angle or dressing of the cup-shaped grinding wheel, its service life can be extended and the replacement frequency of the grinding tool can be reduced. Since the cup-shaped grinding wheel uses the annular grinding surface at the cup mouth to remove materials, if the axis direction of the cup-shaped grinding wheel is perpendicular to the axis direction of the workpiece, there will be two grinding contact surfaces between the cup-shaped grinding wheel and the workpiece surface, resulting in repeated grinding of the workpiece and affecting the machining quality of the workpiece. Therefore, there is an included angle between the grinding surface of the cup-shaped grinding wheel and the workpiece surface, and the included angle θ > 0, so that the cup-shaped grinding wheel and the workpiece surface are in point contact grinding with a smaller area, avoiding low damage to the workpiece. The parallel grinding wheel has only one grinding end face, so when using the parallel grinding wheel for machining, the parallel grinding wheel only needs to be parallel to the brittle pipe fitting 2.

[0077] When the workpiece enters the grinding area for grinding, the supporting surface of the supporting plate 3 slightly contacts the workpiece surface to achieve the supporting purpose. It should be noted that before the guide wheel 4 contacts the workpiece surface, there is only a slight contact between the supporting plate 3 and the workpiece surface. When the guide wheel 4 contacts and drives the workpiece to rotate and move forward, the contact surface between the workpiece and the supporting surface of the supporting plate 3 changes from slight contact to contact, and the contact area between the two becomes larger. The guide wheel 4, the rigid mandrel 1 and the supporting plate 3 generate an appropriate pre-tightening force on the workpiece to ensure the machining accuracy of the workpiece surface. If there is a large-area contact support between the supporting plate 3 and the workpiece surface before the guide wheel 4 contacts the workpiece surface, when the guide wheel 4 contacts and drives the workpiece to rotate and move forward, the supporting force between the supporting plate 3 and the workpiece becomes larger, which easily leads to poor machining accuracy of the workpiece. In addition, the inclination angle of the supporting surface of the supporting plate 3 is preferably about 30°. The stability range is the largest within this range of the support plate angle, and the rounding effect is the best.

[0078] In the actual machining process, it is necessary to first determine the grinding angle of the grinding wheel for grinding. However, when adjusting the positions of the guide wheel, the grinding wheel and the hard and brittle pipe fitting, it is difficult to measure the grinding angle, which increases the difficulty of adjusting the positions of the guide wheel, the grinding wheel and the hard and brittle pipe fitting. Therefore, in this embodiment, the height difference h between the axis of the hard and brittle pipe fitting and the axis of the grinding wheel is determined based on the grinding angle β, so that it is easier to adjust the positions of the grinding wheel and the guide wheel.

[0079] As Figure 4 shown in the schematic diagram of the working principle of the over-positioning geometric layout during the grinding of the hard and brittle pipe fitting. The height difference h between the axes of the guide wheel and the grinding wheel and the axis of the hard and brittle pipe fitting is obtained by the following method:

[0080] Let the outer radius of the hard and brittle pipe fitting with a large length-diameter ratio be R w , the radius of the guide wheel be R c , and the radius of the grinding wheel be R g . Let the height difference between the axis of the hard and brittle pipe fitting and the axis of the grinding wheel be h; when the grinding wheel contacts the hard and brittle pipe fitting, let the initial contact point of the grinding wheel and the hard and brittle pipe fitting be A, and the tangent at point A be l. When the guide wheel contacts the hard and brittle pipe fitting, let the initial contact point of the guide wheel and the hard and brittle pipe fitting be B. When the support plate contacts the hard and brittle pipe fitting, let the initial contact point of the support plate and the hard and brittle pipe fitting be C;

[0081] Let the axis of the hard and brittle pipe fitting be O, the center of the guide wheel be O c , and the center of the grinding wheel be O g , ∠AOC = α, ∠BOC = φ, ∠AO g O c = β g , ∠O g O c B = β c , β is the grinding angle of the grinding wheel, δ is the angle between OA and the horizontal line, the inclination angle of the supporting surface of the support plate is γ, and a rectangular coordinate system is established with O g as the center. The coordinates of the tangent point A in the coordinate system xO g y are (x, y); when there is an offset angle θ between the axis of the grinding wheel and the axis of the hard and brittle pipe fitting, the grinding wheel grinds at the offset angle θ, and the contour projection of the grinding wheel perpendicular to the feed speed direction. Then, the elliptic curve equation in xO g y can be expressed as:

[0082]

[0083] where y is the height difference between the tangent point A and the axis center of the grinding wheel, and x is the horizontal distance from the tangent point A to the vertical axis y;

[0084] From formula (1), the following relational expression is obtained:

[0085] x = R g sinθcosβ g (2)

[0086] y = R g sinβ g (3)

[0087] The slope of the tangent line l passing through the tangent point A on the elliptic curve can be expressed as:

[0088]

[0089] Since the tangent value of the slope of the tangent line l is the angle between the tangent line l and the X-axis, therefore, from formula (4), the angle δ between the tangent line l and the x-axis is:

[0090]

[0091] According to the geometric relationship, the following formula is obtained:

[0092] β c ≈ h / (R w + R c )(6)

[0093] h = y + R w sinδ (7)

[0094] β = β c + δ = π - φ - α (8)

[0095] Substituting δ, y, β c into equations (7) and (8), we get:

[0096]

[0097] In trigonometric functions, when the angle is small, the following simplification occurs:

[0098]

[0099] cot(β g ) ≈ 1 / β g (12)

[0100] Therefore, substituting formula (11) and formula (12) into formula (9) and (10) can be changed to:

[0101]

[0102] h = R g β g + R w β g sinθ (14)

[0103] Combining formulas (13) and (14) gives:

[0104]

[0105] In this embodiment, the process of obtaining the grinding angle β is as follows:

[0106] According to the geometric relationship, the following formula is obtained:

[0107]

[0108] Stability growth coefficient A i has the following expression:

[0109]

[0110] Combining formulas (4)-(17), formula (18) can be transformed into an expression with only the variables of angles β and γ, and formula (18) becomes:

[0111]

[0112] Based on the open-source software python and the stability growth coefficient A i a stability diagram is obtained. As Figure 5 shown, when A i > 0, it is a stable region. According to the inclination angle γ of the supporting surface of the supporting plate, the range value of β is selected within the stable region.

[0113] In this embodiment, based on the grinding angle β of the grinding wheel, the height difference h between the axis of the grinding wheel (the axis of the guide wheel) and the axis of the hard and brittle pipe fitting is determined. After the position of the hard and brittle pipe fitting is determined, based on the height difference h, the heights of the grinding wheel and the guide wheel can be determined, thereby reducing the installation difficulty of the grinding wheel and the guide wheel.

[0114] In addition, the height difference h determined based on the grinding angle β of the grinding wheel can ensure the grinding accuracy of the outer circle of the hard and brittle pipe fitting, avoid the situation that the guide wheel cannot stably drive the workpiece to rotate and cause the workpiece to jump due to too high height difference h, and avoid the generation of roundness on the surface of the hard and brittle pipe fitting due to too low height difference h. The height difference h determined based on the grinding angle β can effectively ensure the stable circularity of the outer diameter of the hard and brittle pipe with a large length-diameter ratio.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for over-positioning grinding of hard and brittle pipe fittings with a large aspect ratio, characterized in that: The specific grinding process is as follows: S1, assembly and positioning of hard and brittle pipe fittings: the rigid mandrel passes through the inner hole of the hard and brittle pipe fittings with a large aspect ratio and is clearance-matched with the hard and brittle pipe fittings, the rigid mandrel is tensioned and the hard and brittle pipe fittings are kept level, and the two ends of the rigid mandrel and the positions close to the hard and brittle pipe fittings are each supported by a vibration reduction device to reduce the vibration amplitude and frequency of the rigid mandrel during the processing of the hard and brittle pipe fittings; the support plate contacts the outer pipe surface of the hard and brittle pipe fittings and supports the hard and brittle pipe fittings; S2, determine the grinding area: arrange the grinding wheel and the guide wheel on both sides of the hard and brittle pipe respectively, and keep the axes of the grinding wheel and the guide wheel at the same height, adjust the height of the two so that there is a height difference between the axes of the guide wheel and the grinding wheel and the axis of the hard and brittle pipe, and the grinding surface of the grinding wheel is positively opposite to the guide support surface of the guide wheel. The grinding wheel, the guide wheel and the support plate enclose the grinding area of ​​the hard and brittle pipe, and the rigid mandrel is used to position and support the hard and brittle pipe; The height difference between the axis of the guide wheel and the grinding wheel and the axis of the hard and brittle pipe is obtained in the following way: Assume that the outer radius of the hard and brittle pipe with a large aspect ratio is R w , the radius of the guide wheel is R c , the radius of the grinding wheel is R g , let the height difference between the axis of the hard and brittle pipe fitting and the axis of the grinding wheel be h; when the grinding wheel contacts the hard and brittle pipe fitting, let the initial contact point between the grinding wheel and the hard and brittle pipe fitting be A, and the tangent at point A be l; when the guide wheel contacts the hard and brittle pipe fitting, let the initial contact point between the guide wheel and the hard and brittle pipe fitting be B; when the support plate contacts the hard and brittle pipe fitting, let the initial contact point between the support plate and the hard and brittle pipe fitting be C; Assume the axis of the hard and brittle pipe is O and the center of the guide wheel is O c , the grinding wheel center is O g , ∠AOC=α, ∠BOC=φ, ∠AO g O c =β g , ∠O g O c B=β c , β is the grinding angle of the grinding wheel, δ is the angle between OA and the horizontal line, the inclination angle of the supporting surface of the support plate is γ, and O g Establish a rectangular coordinate system for the center of the circle, and the tangent point A is in the coordinate system xO g The coordinates in y are (x, y); when there is an offset angle θ between the axis of the grinding wheel and the axis of the hard and brittle pipe, the grinding wheel grinds at the offset angle θ, and the contour projection of the grinding wheel in the cross section perpendicular to the feed speed direction is at xO g The elliptic curve equation in y can be expressed as: Where y is the height difference between the tangent point A and the axis of the grinding wheel, and x is the horizontal distance from the tangent point A to the vertical axis y; The following relationship is obtained from formula (1): x=R g sinθcosβ g (2) y=R g sinβ g (3) The slope of the tangent line l passing through the tangent point A on the elliptic curve can be expressed as: Since the tangent value of the slope of the tangent line l is the angle between the tangent line l and the x-axis, the angle δ between the tangent line l and the x-axis is obtained from formula (4): According to the geometric relationship, the following formula is obtained: b c ≈h / (R w +R c )(6) h=y+R w sinδ (7) β=β c +δ=π-φ-α (8) δ, y, β c Substituting into equations (7) and (8), we obtain: In trigonometry, when the angles are small, the following simplifications occur: cot(b g )≈1 / β g (12) Therefore, substituting formula (11) and formula (12) into formula (9) and (10) can be transformed into: h=R g b g +R w b g sinth (14) Combining formulas (13) and (14) we get: S3, guide wheel positioning: the hard and brittle pipe is moved to the grinding area, and the guide wheel is moved toward the side of the hard and brittle pipe until the guide bearing surface of the guide wheel contacts and squeezes the outer surface of the hard and brittle pipe, so that there is a pre-pressure between the guide wheel and the hard and brittle pipe, and the hard and brittle pipe is moved out of the grinding area, and the guide wheel is kept in place; S4, tool setting: drive the guide wheel to rotate forward, and send the hard and brittle pipe with a large aspect ratio into the grinding area. The guide wheel contacts and drives the hard and brittle pipe forward while rotating in the opposite direction. The grinding wheel rotates in the opposite direction and feeds a small amount to the side of the hard and brittle pipe to achieve tool setting operation. S5, grinding of hard and brittle pipe fittings: set the rotation speed and feed speed of the guide wheel and the rotation speed and feed speed of the grinding wheel, and set the grinding amount of hard and brittle pipe fittings; drive the guide wheel to rotate forward to feed the hard and brittle pipe fittings with a large aspect ratio into the grinding area. The guide wheel contacts and drives the hard and brittle pipe fittings forward while rotating in the opposite direction. The grinding wheel rotates in the opposite direction and feeds toward the side of the hard and brittle pipe fittings. As the grinding wheel grinds and feeds, the guide wheel and the support plate feed toward the side of the hard and brittle pipe fittings to always maintain the over-positioning support of the guide wheel, the support plate and the rigid mandrel on the hard and brittle pipe fittings. As the hard and brittle pipe fittings move, the grinding of the hard and brittle pipe fittings with a large aspect ratio is achieved.

2. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: In the above-mentioned S5, the linear speed of the grinding wheel is 30 m / s.

3. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: In the above-mentioned S5, when the outer diameter of the grinding wheel is 400 mm, the rotation speed of the grinding wheel is 1400 rpm / min to 1500 rpm / min.

4. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: In the above-mentioned S5, the operating speed of the guide wheel is 15 rpm / min to 20 rpm / min.

5. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: The fitting clearance between the rigid mandrel and the hard and brittle pipe fittings ranges from 0.2 to 0.5 mm.

6. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: The support plate is arranged close to the grinding wheel side and obliquely supports the hard and brittle pipe fittings. The supporting surface of the support plate is an inclined surface, and the inclination angle of the supporting surface is 30 degrees.

7. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: The grinding wheel is a parallel grinding wheel.

8. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: The grinding wheel is a cup-shaped grinding wheel, and there is an offset angle θ between the axis of the cup-shaped grinding wheel and the axis of the hard and brittle pipe, θ>0.

9. The over-positioning grinding method for hard and brittle pipes with a large aspect ratio according to claim 1 is characterized in that: The process of obtaining the grinding angle β is as follows: According to the geometric relationship, the following formula is obtained: Stability growth factor A i The expression is: Combining formulas (4)-(17), formula (18) can be converted into variables with only angles β and γ, and formula (18) becomes: Based on open source software python and stability growth coefficient A i Get the stability diagram, when A i When >0, it is a stable region, and the range value of β is selected in the stable region according to the inclination angle γ of the supporting surface of the support plate.

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