An In-Position Measurement Method of Point Replacing Surface for Triple Eccentric Butter�y Valve Precision Machining

: The machining quality of sealing surface of triple eccentric butterfly valve is crucial to its the sealing performance. Especially, the measuring of sealing surface size is the key to assess the machining quality. Due to the special structure of triple eccentric butterfly valve, the sealing surface size of triple eccentric butterfly valve is difficult and inaccurate to measure in position, which makes it difficult to ensure the machining quality of triple eccentric butterfly valve sealing surface. This will affect the sealing performance of triple eccentric butterfly valve, and result in poor bidirectional sealing, non-interchangeability, and lower machining productivity of the bodies and discs. Based on the above problems, this paper proposes a new measuring method for sealing surface size, i.e., in-position measuring method of point replacing surface. The reliability of method is analyzed and proved from the level of design principle of triple eccentric butterfly valve. Further, the tooling fixture and in-position measuring device of sealing surface size are designed and manufactured based on the measuring method of the point replacing surface. Different from the existing assembling boring process, this paper designs and manufactures a new boring tooling fixtures and develops an integrated process of measuring and machining. After the preliminary measuring and machining test, the reproducibility of the results was good, parts interchangeability and bidirectional sealing could be realized. And the application of this machining process significantly shortened the machining time and increases the machining efficiency by 25%.


Introduction
Butterfly valve refers to a kind of regulating valve whose closing part is a disc-shaped butterfly plate, rotating around the valve stem to achieve opening and closing.And butterfly valve mainly plays a role in cutting off and throttling [1] , is a typical valve representative of mechanical equipment through flow regulation.Butterfly valve structure has developed a variety of forms, experiencing the evolution from the concentric butterfly valve to the single eccentric butterfly valve, to the double eccentric butterfly valve, and then to the triple eccentric butterfly valve.The triple eccentric butterfly valve adds a sealing surface eccentric angle β on the basis of the double eccentric butterfly valve as shown in Fig. 1.The sealing surface is separated from the valve body during the opening process [2] .The concentric butterfly valve, single eccentric butterfly valve, double eccentric butterfly valve use rubber ring for sealing, so they can't be used in high temperature, corrosion and other working conditions.However, the triple eccentric butterfly valve can be sealed with metal hard sealing pairs, so they take the advantages of being used in high temperature, low temperature, high pressure and corrosion resistance working conditions.They are widely used in the oil and gas, metallurgy, chemical industry, electric power and other control pipelines that require high valve performance [3] .The sealing surface of triple eccentric butterfly valve is a metal inclined cone surface, and the sealing surface of valve body and valve disc are continuous and complete closed space surface [4] .Inclined cone surface seal is adopted, which requires high processing accuracy.And machining quality of the sealing surface depends largely on the precision of the machining process, which will directly affect the sealing performance of the triple eccentric butterfly valves.However, the precise measuring of the sealing surface size is the evaluation and guarantee of machining quality, so it is vital to accurately measure the sealing surface size of triple eccentric butterfly valves [5] .When machining inclined cone sealing surface, conventional means can't measure the sealing surface size, and traditional measuring tools lack of measuring reference and the result of measuring is inaccurate such as vernier calipers and micrometers; although the Coordinate Measuring Machine itself has high accuracy, it needs to disassemble the processed parts repeatedly during measuring, which introduces secondary measuring errors.And it relies heavily on the workers to obtain the measuring points, resulting in poor repeat accuracy [6][7] ; the point cloud data processing method based on laser measuring is inefficient and takes a long time, which is not suitable for the assembly line production of triple eccentric butterfly valves; it's insufficient to realize the measuring of sealing surface sizes with high quality and efficiency for the research and application of On-Machine Measurement (OMM) of triple eccentric butterfly valve sealing surface size.
Sun [8] conducted a numerical study on the effect of flow coefficient on triple eccentric butterfly valves with the different openings.In this study, a numerical simulation was employed to make friction included, and the results that the flow coefficients were in good agreement with experimentally determined flow coefficients were obtained.The differences of the flow coefficients obtained from numerical simulations of considering and neglecting friction were discussed.Ultimately, flow coefficients and pressure drops of the measuring were determined accurately.
Kan [9] et al. proposed a mathematical model of the sealing interface and derived a formula for evaluating the triple eccentric butterfly valves sealing pair interference.The five parameters of disc diameter, effective seal thickness, axial eccentricity, radial eccentricity and angular eccentricity were included in this model.The effects of axial eccentricity, radial eccentricity and angular eccentricity on the interference area and position were investigated.The results showed that the interference area decreases and then increases when the axial eccentricity or angular eccentricity increases, while the interference area monotonically decreases when the radial eccentricity increases.
Chen [10] et al. utilized the exceptional molding capabilities of CAD/CAM to address the design complexities of the triple eccentric butterfly valves' rotation center and enhance the sealing efficiency.Solid Edge software was used to analyze the process of 3D modeling and motion simulation as well as to check the static and dynamic interference.Reasonable eccentricity angle β, cone apex angle 2θ, radial eccentricity e and axial eccentricity a of the triple eccentric butterfly valve were obtained, which effectively shorten the design cycle of the triple eccentric butterfly valve and improved the design efficiency and quality.
Kim [11] et al. conducted an experiment to analyze flow characteristics of single eccentric, double eccentric, and concentric inclined plate butterfly valves.They conducted a detailed comparison of the valve flow coefficient, relative valve flow coefficient, and valve loss coefficient among various types of butterfly valves.The results showed that these coefficient curves were very different from the traditional butterfly valve coefficient curves.And triple eccentric butterfly valves were superior in flow or pressure control, and single or double eccentric butterfly valves were more suitable for on/off regulation of large flows.
Peng [5] et al. proposed the precision in-position measurement technique of sealing surface and the method of original point cloud data processing.The laser scanning method was used for inposition measurement of the sealing surface as shown in Fig. 2. The trough-clustering algorithm and the normal vector-surface fitting algorithm considering constraints were proposed to classify and simplify the point cloud data, and the key parameters and processing errors of the sealing surface were derived.Effectively experimental verification was carried out by comparing it with the Coordinate Measuring Machine.

Machine tool holders
Laser displacement sensor Fig. 2 Sealing surface laser in-position inspection device [5] Many scholars researched on the triple eccentric butterfly valve mainly focused on the study of structure optimization and flow characteristics, but there were insufficient researches on the measuring of the sealing surface size.Restricted by the lack of measuring means, factories usually use with assembling boring process or gasket assembly to manufacture triple eccentric butterfly valves at this stage, as shown in Fig. 3. Assembling boring process is the method of boring the valve stem holes together after assembling the valve body and valve disc.First, the sealing surface of the valve body and the valve disc are machined respectively, and then the two are assembled, finally the valve stem hole of the assembly is boring.Gasket assembly method involves using measuring tools such as vernier calipers to roughly measure machining size, and the valve is debugged to achieve a better sealing by increasing the gaskets.Although the two, to a certain extent, can eliminate the measuring and machining error caused by measuring inaccuracy, they also bring new problems.Boring depth of the assembling boring process is large, the machining is difficult, the parts are not interchangeable, and it is difficult to realize the bidirectional sealing; gasket assembly method relies on the experience of the workers, assembly time is long, the parts are nointerchangeability, and the valves performance are not stable.These not only increase the difficulty of manufacturing triple eccentric butterfly valves, but also bring great difficulties to the maintenance of triple eccentric butterfly valves.To sum up, the precision measuring technology is the key to realize the high quality and efficiency machining of the triple eccentric butterfly valve.Therefore, the research on the precise measuring technology of the sealing surface size of the triple eccentric butterfly valve holds engineering value.In order to enhance the availability and efficiency of measuring, in-position measurement and on-machine measurement will become the main development trend of current measuring, which can provide timely and deterministic evaluation of machined parts and valuable feedback for ultraprecision machining [12] .For measuring of sealing surface size, due to the lack of accurate systems of the in-position measurement and 3D shape monitoring, a lot of efforts need to be spent to simulate and compensate these effects [13] .To solve the challenge of measuring the sealing surface size of the triple eccentric butterfly valve, a new method based on the design principle of the triple eccentric butterfly valve is proposed.The geometric analysis of the valve body and valve disc sealing surface is carried out, and the contour equation of the center cross-section of sealing surface is established to verify the feasibility of the measuring method.Analyze the structural characteristics of the triple eccentric butterfly valve, design the tooling fixtures for machining of the sealing surface and valve stem hole.Design the measuring device of sealing surface size based on the method of point replacing surface.Finally, the integrated process of measuring and machining is developed.

A measuring method of point replacing surface
As shown in Fig. 4-(1), the method of point replacing surface for measuring sealing surface size is proposed.The point replacing surface method acquires the predetermined size L by measuring a specific point that is perpendicular to the sealing surface at its central cross-section.And the size of the sealing surface at this point of positive cone radius R is derived by using geometric relations, which can reflect the actual machining condition of the entire sealing surface, and then, the measuring of the inclined cone can be converted into the positive cone.Fig. 4 The method of replacing the surface with a point During the process of machining and measuring, the inclined cone is normalized by an inclined plate with an inclination angle of β, which corresponds to the number of eccentric angles of the triple eccentric butterfly valve.At this time, the machine spindle rotation axis coincides with the cone rotation axis, and then the cone busbar is used as machining tool path for machining the sealing surface, as shown in Fig. 4-(2).When measuring the size of the sealing surface, the predetermined size L is measured based on the valve stem hole axis and the cone rotation axis perpendicular to the sealing surface.And then the positive cone radius R at this point is indirectly obtained.The two measuring references are positioned by designing the measuring device structure, which will be introduced in detail in the fourth chapter.

Geometric modeling of the sealing surface cross-section
Aiming at the inclined and central cross-sections of the triple eccentric butterfly valve, the geometric correlation model of the cross-sections with the positive cone rotary axis and valve stem axis is established, which is the theoretical basis of the point replacing surface.In order to further verify the feasibility of the point replacing surface measuring method, the geometric analysis is carried out at the design principle of the triple eccentric butterfly valve.

Contour line equation of the inclined cross-section
As shown in Fig. 5, O-CEDF takes the form of a cone.The cross-section CEDF is intercepted by a plane, the angle θ is the half-cone angle, the angle β is the eccentric angle.OA is the cone rotary axis, OB is perpendicular to the plane CEDF.And M is any point on the cross-section contour line, let its coordinates be (x, y).The coordinate system is established with the intersection point A of the midline of the cone and the plane CEDF as the origin, the major axis of CEDF as the x-axis, and the minor axis of CEDF as the y-axis.
In △OBM： ( ) ( ) In △OMA： ( ) ( ) By combining Equations ( 2) and (3), we can conclude that: By simplifying and organizing Equations (4), we can obtain: ( ) It can be seen from this Equation ( 5) that when θ+β< 90 º and θ< 45 º, the cross-section is an elliptical section, and the coordinates of the ellipse's center point, the long semi-axis and short semiaxis of the elliptical section are respectively:

Geometric correlation model of the center cross-section
As show in Fig. 6, the thickness of the valve disk is E, cone OAB is intercepted by two planes at a specific angle β.The height of the cone is H0, the radius of the cone bottom surface is R, the θ angle is the half-cone angle, the β angle is the eccentric angle, OO2 is the connecting line at the middle of the ellipse.The ellipse long semi-axis of the small end surface is a0, and the short semiaxis is b0.The long semi-axis of the neutral surface is a1, and the short semi-axis is b1.And the long semi-axis of the big end surface is a2, and the short semi-axis is b2.The following relational equation is available based on its positional relationship: According to the position relationship, a0, a1, a2 can be calculated as follows: ( ) In the design process of triple eccentric butterfly valve, the half-cone angle θ, eccentric angle β, valve disc thickness E, and the length of the long semi-axes and short semi-axes are important design sizes, which are related to the structural strength and sealing performance of the triple eccentric butterfly valve.This measuring method examines the theory from the design principle of the triple eccentric butterfly valve, which provides theoretical support for the feasibility of the point replacing surface measuring method.
For the center cross-section of a diagonally-cutting valve disc, we only need to know one of the long semi-axis a0 or a2 to determine whether the sealing surface has been machined to the required size.The long semi-axis a2 is taken as the object of study, and it can be seen from Equation (11) that the size of a2 can be obtained as long as the radius R of the cone bottom surface is measured.Therefore, as long as we can measure the radius R of the positive cone at the center cross-section, we can use this radius R to reflect the actual size of the valve disc.

Measuring device Design based on point replacing surface method
Based on the measuring method of point replacing surface, a special device for measuring the sealing surface size is designed.The measuring references are positioned through the structure of measuring device, and the cone radius R of the sealing surface center cross-section at a certain point is measured, which provides a dimensional reference for the machining of the sealing surface.

Designing of tooling fixture
The sealing surface measuring device of triple eccentric butterfly valve lets the inclined cone sealing surface back to positive cone sealing surface through the inclined plate, positioning mould of valve body and valve disc, which makes the cone rotation axis and the machine spindle rotation axis coaxial.The machine press plate and bolts are used to press and fix, and the machining tool path is along the cone busbar, as shown in Fig. 7.As shown in Fig. 8-( 1), the inclined plate is a kind of disc-shaped tooling fixture with inclined angle β.The inclined angle β is the same as the eccentric angle β of the triple eccentric butterfly valve.There are two mounting holes of different sizes on the inclined end surface of the incline plate, which are respectively used for installing the positioning mould of the valve body and valve disc.And the bottom end surface is used for assembly with the machine chuck.As shown in Fig. 8-(2), the valve body positioning mould is a cylindrical tooling fixture.Tts bottom is assembled with the corresponding holes on the inclined end face of the inclined plate.The top is assembled with the seam allowance of the valve body, and the positioning blocks are used to complete the positioning of the valve body together.Finally, it is clamped through the reserved thread hole and pin hole in the middle.In order to ensure the precision of the two and the same angle of inclination, the bottom and top end surfaces of the positioning mould and inclined end surface of the inclined plate require higher accuracy.As shown in Fig. 8-(3), the positioning mould of valve body and the valve disc are positioned in the same way, the difference is that there is a rectangular groove and a hole in the middle.And the side surface of the rectangular groove is fitted with the side surface of the valve disc, the hole is coaxial with the valve stem hole of the triple eccentric butterfly valve, then the positioning of the valve disc is completed.Finally, bolts and the pressure plate are used to press and fix.The inclined plate is related to the eccentric angle β of the triple eccentric butterfly valve.When machining triple eccentric butterfly valves with different eccentric angles, it is necessary to replace the inclined plate with the same eccentric angle.The positioning mould is related to the caliber of the triple eccentric butterfly valve.When machining triple eccentric butterfly valves of different calibers, it is necessary to replace the valve body positioning mould with the same caliber.

Designing of measuring device
As shown in Fig. 9-(1), the measuring device of valve body sealing surface is mainly composed of the measuring body, measuring ruler, center positioning shaft, valve stem hole positioning sleeve and positioning pin shaft.The top of the measuring body is equipped with a measuring ruler with a scale, the bottom is assembled with the center positioning shaft, the right side is connected to the valve stem hole positioning sleeve by a screw of short shaft, and a through-hole for installing the positioning pin shaft is left in the middle of the short shaft.The measuring ruler is a marked scale and the ruler's measuring contact is spherical.The measuring ruler is used together with the measuring body's scale during measuring.The bottom of the center positioning shaft is a conical structure, it is installed in the center hole of the positioning mould to ensure the coaxiality of the center positioning shaft and the inclined plate, and axis of the two is used as positioning reference 1 for measuring.As shown in Fig. 9-(2), the valve stem hole positioning sleeve is fitted with the valve stem hole to ensure the precision of the distance from the stem hole to the measuring ruler, and the distance between the two is the positioning reference 2 for measuring.Finally, the entire measuring device of valve body sealing surface is limited to the measuring position, and then measuring ruler is moved for size measuring.As shown in Fig. 10-( 1), the measuring device of valve disc sealing surface is mainly composed of a measuring body, measuring ruler, valve stem hole positioning shaft and positioning pin shaft.The measuring body is used to install the measuring ruler with a scale and the stem hole positioning shaft.The measuring contact of the measuring ruler is a linear contact, and it is used together with the measuring body's scale during measuring.The seam allowance of valve disc is assembled with the valve disc positioning mould to ensure the coaxiality of the two, which is used as a reference 1 for measuring.The valve stem hole positioning shaft is fitted with the valve stem hole of the triple eccentric butterfly valve to ensure the coaxiality of the two, which is used as the reference 2 for measuring.The measuring device of the valve disc sealing surface is limited to the measuring position by the center positioning shaft and valve stem hole positioning shaft, and then measuring ruler is moved to measure the size, as shown in Fig. 10-(2).The measuring method is similar to vernier caliper, the scale of the measuring body is the main ruler, the scale of the measuring ruler is the sub-ruler, its total length is 49mm, the length of each part is 0.98mm, and the minimum scale of the main ruler is 0.02mm, so the accuracy of this measuring ruler is 0.02mm.The number of scales read =The main ruler scale + the number of grids aligned with the sub-ruler * accuracy (0.02mm).
As shown in Fig. 11, the example is as follows: first, look at the position of the sub-ruler "0", which determines the first two digits.And 0 in the figure is after 2.2cm, which means that the measuring size is 2.2XXcm.Then observe the sub-ruler division (accuracy) of how many frames, 50 divisions in the figure, that is, the accuracy is 0.02mm.Look at the digits of the sub-ruler and the main ruler in the red box, the number of frames in the coincident part is 17, and each unit is 0.02, and the final number of digits is 0.34mm (0.034cm).Finally, the target inner diameter was 2.234cm.

Measuring method
Before measuring, it is first necessary to use a coordinate measuring machine to calibrate the size of the measuring device, such as the vertical distance h between the valve stem hole and the measuring ruler, the distance S between the central axis and the measuring body, etc. Moving the measuring ruler until the measuring ruler contact is in contact with the sealing surface.And the measuring ruler contact is perpendicular to the sealing surface of the center cross-section.At this the predetermined size can be measured, and then the converted size L2 can be obtained by geometric conversion.Finally, the size of the required positive cone radius R can be obtained.
As shown in Fig. 12-(1), the schematic diagram for measuring the valve body sealing surface of the triple eccentric butterfly valve is displayed.When measuring the size of valve body sealing surface, we can be known according to the geometric relationship: L1 is the actual measured size, θ is the half-cone angle of the cone, and S is the fixed size.At this time, size R can provide a dimensional reference for machining.The long semi-axis length of the ellipse large end surface can be obtained by bringing the size R into Equation ( 11): θ is the half-cone angle of the cone, β is the eccentric angle, and R is the cone radius of center cross-section at the point.
As shown in Fig. 12-(1), the schematic diagram for measuring the valve disc sealing surface of the triple eccentric butterfly valve is displayed.When measuring the size of valve disc sealing surface, we can be known according to the geometric relationship: L1´ is the actual measured size, the θ is the half-cone angle of the cone, S´is the fixed size.
At this time, size R can provide a dimensional reference for machining.The long semi-axis length of the ellipse large end surface can be obtained by bringing the size R into Equation ( 11): θ is the half-cone angle of the cone, β is the eccentric angle, and R is the cone radius of center cross-section at the point.
When the length of the long semi-axis a2 is obtained, other sizes of the entire elliptical section can be obtained according to the calculation formula of the triple eccentric butterfly valve mentioned in Chapter 2. The theoretical verification is carried out from the design level of the triple eccentric butterfly valve, and the schematic diagram of the geometric calculation is shown in Fig. 12.

Measuring device manufacturing
In order to further verify the feasibility of measuring device of sealing surface, the designed fixture and the various parts of the measuring device are CNC machined and manufactured, and the parts are assembled.
As illustrated in Fig. 13, in order to guarantee the precision of machining parts, the machining equipment used is five-axis CNC machining center.And the coordinate measuring machine is employed for dimensional calibration.The tooling fixtures and measuring gauges are made of 40Cr alloy structural steel and each part has an engineering drawing.The unnoted linear dimensional tolerance is GB/T1804-2000-m and the unannotated position tolerance is GB/T1184-1996-K, with acute angles are obtuse.The valve body and disc's technical requirements are analyzed from three dimensions: dimensional deviation, dimensional tolerance, geometric tolerance.After inspecting and verifying the drawings, the actual parts are machined and manufactured  For the problems of the assembling boring process and gasket assembly used in the traditional triple eccentric butterfly valve machining, a new triple eccentric butterfly valve machining technology is proposed, that is the integrated process of measuring, machining with the turning and boring.
As shown Fig. 14, in order to unify the reference of processing and positioning in the boring machining and ensure the accuracy of boring, a new boring tooling fixture has been designed.Its structure is the same as the positioning mould of the valve body and valve disc.The similar positioning structure not only improves the machining accuracy of boring and subsequent sealing surfaces, but also improves the sealing performance of the triple eccentric butterfly valve.Further, it is used alongside a specially-designed measuring gauge developed in this study, which helps in resolving the issues of low machining efficiency and interchangeability.The sealing surface machining process of the triple eccentric butterfly valve is shown in Fig. 17.When the sealing surface size measuring device is used for measuring, the measuring result is slightly deviated from the theoretical value due to the existence of manufacturing error.But after a few incorrect attempts, the results were repeatable enough to be able to interchange parts.Comparing with the traditional assembling boring process, the new integrated process proposed in this paper greatly shorts the processing time and significantly increases the processing efficiency by about 25%.

Fig. 1
Fig. 1 Structure of triple eccentric butterfly valve

Fig. 2
Fig. 2 Assembly boring process and gasket assembly

( 2 )
Measuring of the positive cone surface.

Fig. 5
Fig. 5 Schematic diagram of elliptic contour line solution

Fig. 6
Fig. 6 Schematic diagram of the valve disc center section

2 )
Fig. 9 Measuring device of valve body sealing surface

2 )
Fig. 10 Measuring device of valve disc sealing surface

( 1 )
Fig. 12 Schematic of the geometry calculation of the sealing surface measuring Inclined plate Valve body positioning mould Valve disc positioning mould Measuring ruler Measurement body of valve body Measurement body of valve disc Center positioning axis Valve stem hole positioning sleeve

Fig. 13
Fig.13 Tooling fixture and measuring device 6 Integrated processing of measuring and machining

( 1 ) 5 Fig. 15 Fig. 16
Fig. 14 Boring fixtureThe integrated process of measuring and machining removes traditional assembling boring process, machining valve stem hole first，and then machining sealing surface based on valve stem hole.During machining, measuring device of sealing surface size designed by this paper is used for measuring.The detailed process for machining is displayed in Tab. 1 and Tab. 2.