Mechanical parts inner diameter precision measuring instrument

Abstract

This invention discloses a precision measuring instrument for the inner diameter of mechanical parts, relating to the field of laser measurement technology. The technical solution specifically includes a guide rail, a fixed base, a sliding base, a laser rangefinder, a reflector, an electromagnet, a negative pressure generating component, an adaptive centering component, and a hydraulic locking mechanism integrated within the sliding base. The negative pressure generating component is used to stress-free adsorb the fixed contact onto the hole wall; the electromagnet is used to drive the sliding base to move; the adaptive centering component is used to automatically correct the measurement posture to eliminate cosine error; the hydraulic locking mechanism uses the measurement contact force itself as the power to lock the sliding base. This invention effectively eliminates cosine error caused by manual placement deviations, prevents positional shifts caused by lateral torque generated during locking, ensures stable measurement reference and achieves constant measuring force, and significantly improves measurement repeatability and accuracy.

Description

Technical Field

[0001] This invention relates to the field of laser measurement technology, and more specifically to a precision measuring instrument for the inner diameter of mechanical parts. Background Technology

[0002] In the field of precision machining and inspection, internal diameter measurement mainly relies on contact measuring tools such as inside micrometers and inside dial indicators. Especially for small parts, when measuring with measuring tools, the operator needs to manually swing the instrument within the hole to find the "turning point" (i.e., the maximum distance through the diameter line), which requires a high level of skill and experience. Existing internal diameter measuring equipment has the following main drawbacks: First, if the instrument's orientation does not strictly pass through the diameter line of the hole, or if the probe axis is tilted relative to the hole wall, the measured value will be the chord length rather than the true diameter, resulting in a cosine error that is difficult to quantify and compensate for. Second, the contact force measurement relies entirely on feel; excessive force can cause elastic deformation of thin-walled parts, while insufficient force leads to unreliable contact, both of which introduce random errors and severely affect measurement repeatability. Third, regarding the force measurement problem, some improved instruments have introduced spring mechanisms to achieve constant measuring force, but manual judgment is still required for centering and posture correction. Finally, after the measuring head reaches the measuring position, the movable parts usually need to be fixed by means of threaded locking or cam clamping. These mechanical locking actions inevitably apply lateral forces or torques, causing minute, imperceptible shifts in the already aligned position. This results in the final locked state deviating from the actual contact measurement state, introducing new sources of error. Furthermore, the fixed end, relying solely on simple support, is prone to stick-slip or displacement during repeated measurements, compromising the measurement reference. Additionally, some components have bosses machined in the middle of ring-shaped workpieces during manufacturing, making it impossible for the dial indicator to avoid them during measurement, preventing the measuring tool from reaching the inner diameter of the component. Summary of the Invention

[0003] The main technical problem addressed by this invention is how to eliminate the cosine error caused by manual alignment deviation in existing internal diameter measuring instruments, as well as the measurement position offset caused by the lateral force applied by the locking mechanism, when measuring the internal diameter of small-sized parts. Simultaneously, this invention also addresses the problem of poor measurement repeatability caused by inconsistent contact force and easy displacement of the fixed end during measurement, thereby providing a precision measuring instrument for the internal diameter of mechanical parts that is easy to operate, self-aligning, stress-free, and highly accurate.

[0004] To address the aforementioned technical problems, the present invention adopts the following technical solution: a precision measuring instrument for the inner diameter of mechanical parts, comprising a guide rail, a fixed base, a sliding base, a laser rangefinder, a reflector, an electromagnet, a negative pressure generating component, an adaptive centering component, and a hydraulic locking mechanism integrated within the sliding base; one end of the guide rail is fixedly connected to the fixed base, and the other end is fitted with a sliding base capable of sliding along its axial direction, forming a measuring stroke between the sliding base and the fixed base; a fixed contact is installed at the end of the fixed base near the inner hole to be measured, and the negative pressure generating component is connected to the fixed contact for stress-free adsorption of the fixed contact onto the hole wall; an electromagnet is fixedly disposed on the side of the fixed base facing the sliding base. The electromagnet is used to generate magnetic repulsion with the permanent magnet embedded inside the sliding seat to drive the sliding seat to move away from the fixed seat; the adaptive centering component is installed on the sliding seat at the end away from the fixed seat to automatically correct the measurement posture to eliminate cosine error caused by manual placement deviation; a sliding contact is provided on the sliding seat in the central area of ​​the adaptive centering component, and the hydraulic locking mechanism is integrated in the sliding contact to lock the sliding seat using the measurement contact force itself as the locking power; the laser rangefinder is fixedly installed on the fixed seat, and the reflector is fixedly installed on the sliding seat and is positioned directly opposite the emitting end of the laser rangefinder to measure relative displacement.

[0005] Furthermore, the fixed contact has an arc-shaped contact surface adapted to the inner wall of the component to be tested, and a sealed cavity is opened inside the fixed contact. A negative pressure adsorption hole communicating with the cavity is opened on the arc-shaped contact surface. The negative pressure generating component includes a vacuum pump installed on the fixed base. The vacuum pump is connected to the cavity inside the fixed contact through a gas pipeline. A control valve for controlling the gas flow is installed on the gas pipeline.

[0006] Furthermore, a sealing gasket is provided on the negative pressure adsorption hole, which is used to adapt to the shape of the hole wall so that the fixing contact is tightly attached to the hole wall.

[0007] Furthermore, the adaptive centering assembly includes a central sleeve fixedly disposed at the bottom of the sliding seat and a movable column slidably disposed within the cavity of the central sleeve; the front section of the movable column is fixedly connected to a U-shaped push rod, and the two ends of the U-shaped push rod are respectively equipped with movable balls that can roll freely; a second return spring is disposed within the cavity of the central sleeve, with its two ends respectively abutting against the inner wall of the cavity and the movable column, for providing elastic force to push the movable column outward.

[0008] Furthermore, a limiting slider is provided on the moving column, and a stop block that cooperates with the limiting slider is provided at the entrance of the central sleeve cavity to limit the extension distance of the moving column; the torque formed by the reaction force of the hole wall on one side of the movable ball drives the U-shaped push rod to swing until the movable balls on both sides simultaneously abut against the hole wall and achieve force balance, so as to forcibly correct the guide rail to the direction perpendicular to the line connecting the contact points of the movable balls on both sides.

[0009] Furthermore, the hydraulic locking mechanism has the following specific structure: a hydraulic oil chamber is provided inside the sliding contact and a first piston is disposed in the hydraulic oil chamber; the first piston is connected to a piston rod with one end extending out of the hydraulic oil chamber and connected to an abutting ball head; a first return spring is sleeved on the piston rod between the first piston and the abutting ball head, and a first limiting ring is used to limit the first return spring.

[0010] Furthermore, the hydraulic oil chamber is connected to a second hydraulic oil channel extending to the side wall of the sliding seat through a first hydraulic oil channel; two second pistons are symmetrically arranged in the second hydraulic oil channel, and a friction stop plate with the friction surface facing the side of the guide rail is fixedly connected to the second piston, and a second limiting ring is used to limit the stroke of the second piston.

[0011] The beneficial effects of this invention compared with the prior art are: (1) By setting an adaptive centering component, this invention utilizes the torque balance mechanism formed by the U-shaped push rod and the movable ball to automatically correct the measurement posture and force the guide rail to be corrected to the direction perpendicular to the line connecting the contact points of the movable balls on both sides, thereby effectively eliminating the cosine error caused by manual placement deviation and reducing the dependence on the operator's experience and skills; (2) By integrating a hydraulic locking mechanism, this invention uses the measurement contact force itself as a power source to drive the hydraulic oil to push the friction stop plate to lock, avoiding the lateral torque generated by traditional threaded locking or cam clamping, and preventing slight deviation of the already aligned position. (3) The present invention uses a negative pressure generating component to adsorb the fixed contact on the hole wall, thereby achieving stress-free fixing of the fixed end. This avoids the elastic deformation of thin-walled parts caused by mechanical clamping force and effectively prevents the fixed end from sticking or shifting during multiple measurements, thus greatly ensuring the stability of the measurement reference. (4) The present invention uses the magnetic repulsion force generated by the electromagnet and the permanent magnet embedded in the sliding seat to drive the sliding seat to move. Combined with the spring and hydraulic damping mechanism, it achieves constant control of the measurement contact force, solves the random error introduced by the difference in hand feel force, and significantly improves the repeatability and accuracy of the measurement. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the overall structure of the present invention.

[0013] Figure 2 This is a schematic diagram of one side of the fixing base of the present invention.

[0014] Figure 3 for Figure 2 Another perspective.

[0015] Figure 4 This is a schematic diagram of one side of the sliding seat of the present invention.

[0016] Figure 5 This is a schematic diagram of the adaptive centering component of the present invention.

[0017] Figure 6 This is a schematic diagram of the internal structure of the sliding contact of the present invention.

[0018] Figure 7 This is a schematic diagram of the structure of the second piston, friction stop plate, and second limiting ring of the present invention.

[0019] Reference numerals: 1. Guide rail; 2. Fixed seat; 20. Fixed contact; 201. Negative pressure adsorption hole; 3. Sliding seat; 30. Sliding contact; 301. First hydraulic oil passage; 302. First piston; 303. Piston rod; 304. Contact ball head; 305. First limiting ring; 306. First return spring; 31. Second hydraulic oil passage; 32. Second piston; 33. Friction stop plate; 34. Second limiting ring; 4. Laser rangefinder; 5. Reflector; 6. Electromagnet; 7. Negative pressure generating assembly; 71. Vacuum pump; 72. Gas pipeline; 73. Control valve; 8. Self-adaptive centering assembly; 81. Center sleeve; 82. Moving column; 83. U-shaped push rod; 84. Moving ball; 85. Second return spring; 86. Limiting slider. Detailed Implementation

[0020] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. The accompanying drawings are for illustrative purposes only, representing schematic diagrams only, not actual physical objects, and should not be construed as limiting the scope of this patent. To better illustrate the embodiments of the present invention, some components in the drawings may be omitted, enlarged, or reduced, and do not represent the actual dimensions of the product. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0021] like Figures 1 to 7 As shown, the present invention provides a precision measuring instrument for the inner diameter of mechanical parts, including a guide rail 1, a fixed base 2, a sliding base 3, a laser rangefinder 4, a reflector 5, an electromagnet 6, a negative pressure generating component 7, an adaptive centering component 8, and a hydraulic locking mechanism integrated in the sliding base 3.

[0022] The guide rail 1 serves as the main supporting frame of the measuring instrument and possesses high-precision straightness. A fixed base 2 is fixedly connected to one end of the guide rail 1, which is used for contact and positioning with one side wall of the inner hole to be measured. A sliding base 3 is sleeved on the other end of the guide rail 1, and the sliding base 3 can slide along the axial direction of the guide rail 1. The measuring stroke is formed between the sliding base 3 and the fixed base 2.

[0023] An electromagnet 6 is fixedly installed on the side of the fixed base 2 facing the sliding base 3. A permanent magnet (not shown in the figure) is embedded inside the sliding base 3 at the position corresponding to the electromagnet 6. When the electromagnet 6 is energized, the magnetic field polarity is the same as that of the permanent magnet, thereby generating a magnetic repulsive force to drive the sliding base 3 to move along the guide rail 1 away from the fixed base 2.

[0024] A fixed contact 20 is installed at one end of the fixed base 2 near the inner hole to be measured. The fixed contact 20 has an arc-shaped contact surface that adapts to the inner hole wall of the component to be measured. A sealed cavity is formed inside the fixed contact 20, and a negative pressure adsorption hole 201 communicating with the cavity is formed on the arc-shaped contact surface.

[0025] The negative pressure generating assembly 7 includes a vacuum pump 71, a gas pipeline 72, and a control valve 73. The vacuum pump 71 is mounted on the fixed base 2 and is connected to the cavity inside the fixed contact 20 via the gas pipeline 72. The control valve 73 is mounted on the gas pipeline 72 and is used to control the opening and closing of the gas pipeline 72.

[0026] The fixed contact 20 provides initial physical support. When the vacuum pump 71 operates, a negative pressure is generated at the negative pressure adsorption hole 201, firmly adsorbing the fixed contact 20 onto the hole wall. To further enhance the adsorption bond between the fixed contact 20 and the hole wall, a sealing gasket can be placed on the negative pressure adsorption hole 201 to better adapt to the shape of the hole wall, thereby achieving the effect of the fixed contact 20 tightly adhering to the hole wall. Compared with traditional mechanical tip support, this negative pressure adsorption method achieves stress-free fixation, avoiding deformation of thin-walled parts or slight displacement of the instrument itself caused by mechanical clamping force, greatly ensuring the stability of the measurement reference.

[0027] The laser rangefinder 4 is fixedly mounted on the fixed base 2, and the reflector 5 is fixedly mounted on the sliding base 3, with the emitting end of the laser rangefinder 4 facing the reflector 5. The laser rangefinder 4 is used to measure the relative displacement between itself and the reflector 5. This displacement data, combined with the instrument's own fixed dimensional constants (specifically, the inherent dimensional distance between the contact point of the fixed contact 20 and the hole wall and the laser rangefinder 4 on the fixed base 2; and the inherent dimensional distance between the contact point of the sliding contact 30 and the hole wall and the reflector 5 on the sliding base 3), allows for the accurate calculation of the diameter of the inner hole to be measured.

[0028] The adaptive centering component 8 is installed at the front end of the sliding seat 3 (i.e., the end away from the fixed seat 2). The component includes a center sleeve 81, a moving column 82, a U-shaped push rod 83, a movable ball 84, a second return spring 85, and a limiting slider 86.

[0029] The central sleeve 81 is fixedly mounted on the bottom of the sliding seat 3. A cavity is provided inside the central sleeve 81. The rear section of the moving column 82 is slidably mounted within the cavity of the central sleeve 81. The U-shaped push rod 83 is U-shaped, and its middle section is fixedly connected to the front section of the moving column 82. Movable balls 84 are installed at the ends of the two arms of the U-shaped push rod 83, allowing them to roll freely. The second return spring 85 is located within the cavity of the central sleeve 81, with its two ends abutting against the inner wall of the cavity and the moving column 82, respectively, providing an outward pushing force against the moving column 82. The limiting slider 86 is mounted on the moving column 82, and a stop block cooperating with the limiting slider 86 is provided at the entrance of the cavity of the central sleeve 81 to limit its extension distance.

[0030] The adaptive centering component 8 is used to automatically correct the measurement posture. When the instrument is placed into the inner hole, the movable ball 84 on the U-shaped push rod 83 contacts the hole wall before the central sliding contact 30. If the guide rail 1 is not strictly aligned with the diameter line, the movable ball 84 on one side will generate a reaction force after contacting the hole wall. This reaction force forms a torque that drives the U-shaped push rod 83 to swing until the movable balls 84 on both sides contact the hole wall simultaneously. This dual-point positioning mechanism forces the guide rail 1 to be corrected to a direction perpendicular to the chord line (i.e., the diameter direction), fundamentally eliminating the cosine error caused by manual placement deviation. At the same time, the design of the movable ball 84 allows it to adapt to trace amounts of oil or impurities in the hole, providing good passability.

[0031] A sliding contact 30 is provided on the sliding seat 3 and located in the central area of ​​the adaptive centering component 8. The hydraulic locking mechanism has the following structure: a hydraulic oil chamber is provided inside the sliding contact 30, and the first piston 302 is disposed within this hydraulic oil chamber; one end of the piston rod 303 is connected to the first piston 302, and the other end extends out of the hydraulic oil chamber and is connected to an abutment ball head 304. The first return spring 306 is sleeved on the piston rod 303 and located between the first piston 302 and the abutment ball head 304; the first limiting ring 305 is used to limit the first return spring 306.

[0032] The hydraulic oil chamber is connected to the second hydraulic oil channel 31 via the first hydraulic oil channel 301. The second hydraulic oil channel 31 extends to the side wall of the sliding seat 3, and the second piston 32 is disposed within the second hydraulic oil channel 31. Two second pistons 32 are provided, symmetrically disposed within the second hydraulic oil channel 31. The friction stop plate 33 is fixedly connected to the second piston 32, and the friction surface of the friction stop plate 33 faces the side of the guide rail 1. The second limiting ring 34 is used to limit the stroke of the second piston 32, ensuring that the second piston 32 can accurately control its movement distance under hydraulic oil pressure, thereby achieving reliable locking of the sliding seat 3. When the ball head 304 contacts the contact hole wall, the reaction force first compresses the first return spring 306, providing a first-stage elastic buffer to reduce impact. Subsequently, the force is transmitted to the first piston 302, squeezing the silicon-based hydraulic oil in the hydraulic oil chamber. The hydraulic oil rapidly pushes the second piston 32 through the channel, causing the friction stop plate 33 to press against the side of the guide rail 1. Utilizing the contact force itself as the locking power eliminates the need for an external power source. Hydraulic damping provides a smooth second-level buffer, eliminating vibration and bouncing at the moment of contact. This purely mechanical yet hydraulically linked locking method offers extremely fast response and generates no additional lateral torque at the moment of locking, ensuring that the positional accuracy in the locked state is the same as the true position during contact measurement, thus avoiding the micro-motion errors associated with traditional threaded locking.

[0033] Working principle:

[0034] (a) Measurement preparation and preliminary positioning: The operator holds the guide rail 1 and places one end of the fixed base 2 into the inner hole of the part to be measured. By visual observation, the guide rail 1 is roughly positioned in the direction of the diameter line of the inner hole, and the arc-shaped surface of the fixed contact 20 is placed against the side wall of the inner hole of the part to achieve preliminary single-point positioning.

[0035] (II) Adaptive Alignment: When electromagnet 6 is activated, the magnetic field generated by electromagnet 6 repels the permanent magnet inside the sliding seat 3, pushing the sliding seat 3 along the guide rail 1 towards the opposite hole wall. During the movement of the sliding seat 3, the U-shaped push rod 83 of the adaptive alignment component 8 first contacts the hole wall. If the guide rail 1 is strictly aligned with the diameter line, the movable balls 84 on both sides of the U-shaped push rod 83 simultaneously contact the hole wall, without generating a lateral force. If there is an angular deviation in the guide rail 1, the movable ball 84 on one side of the U-shaped push rod 83 contacts the hole wall first and receives a reaction force from the hole wall. This reaction force forms a torque around the hinge point, pushing the U-shaped push rod 83 to swing, causing its other side to move closer to the hole wall. Since the movable balls 84 can roll, the U-shaped push rod 83 will automatically adjust its posture until the movable balls 84 on both arms simultaneously contact the hole wall and achieve force balance. At this time, the guide rail 1 is forcibly corrected to a direction perpendicular to the line connecting the contact points of the movable balls 84 on both sides, thus completing the adaptive alignment.

[0036] (III) Hydraulic Locking: As the electromagnet 6 continues to push the sliding seat 3, the contact ball 304 of the central sliding contact 30 finally abuts against the inner wall of the hole. At the moment of contact, the reaction force first compresses the first return spring 306, absorbing the impact energy. Subsequently, the piston rod 303 pushes the first piston 302 to squeeze the silicon-based hydraulic oil in the hydraulic chamber. After being pressurized, the hydraulic oil transmits pressure through the first hydraulic oil flow channel 301 and the second hydraulic oil flow channel 31, pushing the second piston 32 to move outward. The second piston 32 drives the friction stop plate 33 to quickly press against the side of the guide rail 1, generating a strong frictional force, locking the sliding seat 3 onto the guide rail 1 without gaps. At this time, the measuring device is in a fully locked state and maintains a constant measuring force.

[0037] (iv) Vacuum adsorption and high-precision measurement: After hydraulic locking is completed (or the preset delay is reached), vacuum pump 71 is started. Vacuum pump 71 evacuates air from the inside of fixed contact 20 through gas pipe 72, creating negative pressure at negative pressure adsorption hole 201, which firmly adsorbs fixed contact 20 onto the inner wall of the part. At this point, both ends of the measuring instrument are reliably fixed. Laser rangefinder 4 emits laser light to reflector 5 to measure accurate distance data, and calculates the high-precision inner diameter value based on this data.

[0038] (V) Completion of Reset: After the measurement is completed, the electromagnet 6 is de-energized, and the magnetic repulsion disappears; the vacuum pump 71 stops working, and the negative pressure is released. The operator manually pulls the sliding seat 3 backward, and the pressure in the hydraulic chamber is released. Under the restoring force of the first reset spring 306 and the second reset spring 85, the sliding contact 30, the self-adaptive centering assembly 8, and each piston are reset, and the instrument can be removed from the hole.

Claims

1. A precision measuring instrument for the inner diameter of mechanical parts, characterized in that, The system includes a guide rail (1), a fixed base (2), a sliding base (3), a laser rangefinder (4), a reflector (5), an electromagnet (6), a negative pressure generating component (7), an adaptive centering component (8), and a hydraulic locking mechanism integrated within the sliding base (3). One end of the guide rail (1) is fixedly connected to the fixed base (2), and the other end is fitted with a sliding base (3) that can slide along its axial direction. The sliding base (3) and the fixed base (2) form a measurement stroke. A fixed contact (20) is installed at the end of the fixed base (2) near the inner hole to be measured. The negative pressure generating component (7) is connected to the fixed contact (20) to stress-free adsorb the fixed contact (20) onto the hole wall. An electromagnet (6) is fixedly installed on the side of the fixed base (2) facing the sliding base (3). The sliding seat (3) is used to generate magnetic repulsion with the permanent magnet embedded inside the sliding seat (3) to drive the sliding seat (3) to move away from the fixed seat (2); the adaptive centering component (8) is installed on the sliding seat (3) at the end away from the fixed seat (2) to automatically correct the measurement posture to eliminate cosine error caused by manual placement deviation; a sliding contact (30) is provided on the sliding seat (3) in the central area of ​​the adaptive centering component (8), and the hydraulic locking mechanism is integrated in the sliding contact (30) to lock the sliding seat (3) by using the measurement contact force itself as the locking power; the laser rangefinder (4) is fixedly installed on the fixed seat (2), and the reflector (5) is fixedly installed on the sliding seat (3) and is set directly opposite to the emitting end of the laser rangefinder (4) to measure relative displacement.

2. The precision measuring instrument for the inner diameter of mechanical parts according to claim 1, characterized in that, The fixed contact (20) has an arc-shaped contact surface adapted to the inner wall of the component to be tested. The fixed contact (20) has a sealed cavity inside. The arc-shaped contact surface has a negative pressure adsorption hole (201) communicating with the cavity. The negative pressure generating component (7) includes a vacuum pump (71) installed on the fixed base (2). The vacuum pump (71) is connected to the cavity inside the fixed contact (20) through a gas pipe (72). The gas pipe (72) is equipped with a control valve (73) for controlling the gas flow.

3. The precision measuring instrument for the inner diameter of mechanical parts according to claim 2, characterized in that, A sealing gasket is provided on the negative pressure adsorption hole (201). The sealing gasket is used to adapt to the shape of the hole wall so that the fixing contact (20) is tightly attached to the hole wall.

4. The precision measuring instrument for the inner diameter of mechanical parts according to claim 1, characterized in that, The adaptive centering component (8) includes a central sleeve (81) fixedly disposed at the bottom of the sliding seat (3) and a movable column (82) slidably disposed in the cavity of the central sleeve (81); the front end of the movable column (82) is fixedly connected to a U-shaped push rod (83), and the two arms of the U-shaped push rod (83) are respectively equipped with movable balls (84) that can roll freely; a second return spring (85) is provided in the cavity of the central sleeve (81), with its two ends respectively abutting against the inner wall of the cavity and the movable column (82), for providing elastic force to push the movable column (82) outward.

5. A precision measuring instrument for the inner diameter of mechanical parts according to claim 4, characterized in that, The movable column (82) is provided with a limiting slider (86), and the cavity entrance of the central sleeve (81) is provided with a stop block that cooperates with the limiting slider (86) to limit the extension distance of the movable column (82); the torque formed by the reaction force of the hole wall on one side of the movable ball (84) drives the U-shaped top rod (83) to swing until the movable balls (84) on both sides simultaneously abut against the hole wall and reach force balance, so as to forcibly correct the guide rail (1) to the direction perpendicular to the line connecting the contact points of the movable balls (84) on both sides.

6. A precision measuring instrument for the inner diameter of mechanical parts according to claim 1, characterized in that, The hydraulic locking mechanism has the following structure: a hydraulic oil chamber is provided inside the sliding contact (30) and a first piston (302) is provided inside the hydraulic oil chamber; the first piston (302) is connected to a piston rod (303) with one end extending out of the hydraulic oil chamber and connected to a contact ball (304); a first return spring (306) is sleeved on the piston rod (303) between the first piston (302) and the contact ball (304), and a first limiting ring (305) is used to limit the first return spring (306).

7. A precision measuring instrument for the inner diameter of mechanical parts according to claim 6, characterized in that, The hydraulic oil chamber is connected to the second hydraulic oil channel (31) extending to the side wall of the sliding seat (3) through the first hydraulic oil channel (301); two second pistons (32) are symmetrically arranged in the second hydraulic oil channel (31), and a friction stop plate (33) with the friction surface facing the side of the guide rail (1) is fixedly connected to the second piston (32). The second limiting ring (34) is used to limit the stroke of the second piston (32).

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