The connection method of vacuum fittings depends on the system vacuum level, thermodynamic state, and mechanical load characteristics, mainly divided into three categories: welding connection, flange connection, and compression connection. Welding connection is suitable for ultra-high vacuum (UHV) main pipelines, using fully automatic TIG welding. The uniformity deviation of weld penetration is ≤± 0.1 mm. After welding, it undergoes solid solution treatment and electrolytic polishing. The roughness Ra of the weld area is ≤ 0.15 μ m, and X-ray inspection meets the ASME Section V Level 2 standard. Flange connections are divided into three categories according to ISO standards: CF (ConFlat), ISO-KF (Quick Flange), and ISO-F. CF flanges use copper gaskets, which are fitted with octagonal grooves and conical surfaces to form plastic flow seals under the action of bolt pre tightening force. The gasket compression rate is controlled at 25-30%; ISO-KF flanges rely on clamps to apply radial clamping force, causing synchronous compression between the double cone flange and FKM sealing ring. The permanent deformation rate of the sealing ring compression must be ≤ 18% (175 ℃× 96 h); ISO-F flanges adopt a planar sealing structure, relying on high-precision grinding surfaces (flatness ≤ 1 μ m) and oxygen free copper gaskets for cold flow deformation to achieve sealing. The compression type connection is only used for medium vacuum working conditions. Axial compression force is generated by expanding the pipe body and locking the nut. The expansion angle is strictly controlled at 37 °± 0.5 °, and the depth tolerance of the expansion is ± 0.03 mm. All connection interfaces must be cleaned by acetone ethanol two-step ultrasonic cleaning before assembly, with residual carbon hydrogen compound content ≤ 5 ng/cm ². The assembly torque is executed according to the M value specified in ISO 15510, with an error margin of ± 3%.