ISO 3669:2026

ISO 3669:2026 is an International Standard from ISO for knife-edge flanges used in vacuum technology. It specifies the dimensions of fixed or rotatable bolted knife-edge flanges for vacuum systems from atmospheric pressure down to 10−11 Pa, so manufacturers, designers and users can build compatible flange and gasket assemblies.

What does ISO 3669:2026 specify?

ISO 3669:2026 specifies the dimensions of knife-edge flanges and their metal gaskets, with the aim of keeping flange and gasket parts interchangeable. It is a dimensional standard for vacuum hardware, not a performance test standard.

The document is organized into Scope, Normative references, Terms and definitions, Symbols and abbreviated terms, and Requirements. Clause 5 is split into materials and dimensions, and the dimensional content is presented in Figures 1 to 4 and Tables 1 and 2.

What are the key requirements of ISO 3669:2026?

ISO 3669:2026 sets the main design and geometry rules that determine whether two knife-edge flanges will mate and seal properly. Clause 5 is the practical core of the standard.

Materials and bolt holes

• In 5.1.1, flange material has to suit the service conditions, including temperature, sealing capacity, corrosion resistance, magnetic permeability, gasket type and size. In practice, this means the material choice is part of seal performance, not just a procurement detail.
• A note says austenitic stainless steel is commonly used. That gives buyers and manufacturers a common baseline when they need a vacuum-compatible flange material.
• In 5.1.2, the flange shall have either clearance or tapped bolt holes. Table 1 shows both English and metric tapped hole options, and the metric thread form follows ISO 965-1.
• This matters because the bolt-hole style and thread system must match the mating hardware and the manufacturing route.

Grooves, gasket and seal behavior

• In 5.1.3, leak check grooves are recommended and should be arranged equidistantly between the bolt holes. This supports leak detection by giving trace gas a path to the seal zone.
• In 5.1.4, the gasket should generally be softer than the flange to avoid dulling the knife-edge. The note says OFHC copper is commonly used, but the standard does not limit gasket material to OFHC copper.
• In practice, the gasket choice affects sealing, repeatability and the life of the flange edge.

Dimensions and compatibility

• In 5.2, the flange and metal gasket dimensions are given in Figures 1 to 3 and Tables 1 and 2. These are the machining and inspection references for production and incoming checks.
• Table 1 covers the named flange sizes and their key dimensions, including bolt circle, bolt-hole size, position tolerance, seal recess, knife-edge, pipe-connection depth, inner ring setback and flange thickness.
• Figure 1 distinguishes rotatable and non-rotatable flanges, and the note says the inner and outer parts of a rotatable flange should be fabricated as a set. That matters when matching parts from production or stock.
• Figure 4 gives the recommended leak check groove dimensions and shows where the groove should sit relative to the bolt holes.

What terms does ISO 3669:2026 define?

• Knife-edge flange - a metal sealed flange with a knife edge for high and ultra-high vacuum service; the seal is made by bolting two knife-edge flanges together with a deformable metal gasket between them.
• Nominal outside diameter - the value used to identify the flange, following the historical convention in Table 1.
• Leak check groove - a groove machined into the seal side of the flange to help trace gas move from the outer perimeter toward the seal zone.
• OFHC - oxygen-free high conductivity copper, which the document notes as a commonly used gasket material.
• l - the symbol for nominal outside diameter of flange.
• φ - the symbol for position tolerance of the bolt hole centre.

Who uses ISO 3669:2026?

ISO 3669:2026 is used by vacuum equipment manufacturers, flange and gasket suppliers, and engineers who design or assemble vacuum systems. Quality teams and buyers use it to check that flange dimensions, bolt-hole patterns and gasket sizes match the required mating parts.

It is especially relevant for systems that use fixed or rotatable bolted knife-edge flanges, including applications that need compatibility across different suppliers.

What changed in ISO 3669:2026 from the previous edition?

ISO 3669:2026 is the fourth edition and replaces ISO 3669:2020. The document says the revision is technical.

The stated changes are:

• the title has been updated
• flange type 295CF has been added for use with turbo molecular pumps
• flange type 350ACF has been added and is compatible with ASTM E2734 flange type 350CF
• the knife-edge angle range has been expanded to 0°-40°

Which standards are used with ISO 3669:2026?

ISO 3669:2026 points to several related standards and references:

• ISO 286 (all parts) - supports the geometrical product specification system for linear size tolerances
• ISO 965-1 - provides the tolerance basis for the metric screw threads used for tapped holes in Table 1
• ISO 1609 - covers non-knife-edge flanges and is listed as a related vacuum flange standard
• ASTM E2734M - the introduction says ISO 3669:2026 was revised to harmonize with it so flanges and gaskets remain compatible
• ASTM E2734 - the introduction links flange type 350ACF to ASTM E2734 flange type 350CF

What does the ISO 3669:2026 document contain?

ISO 3669:2026 contains three technical drawings and two main dimensional tables. Figures 1 to 3 show the basic flange dimensions, the knife-edge detail and the metal gasket.

Figure 4 gives the recommended leak check groove layout. Table 1 gives the dimensional data for the flange types, and Table 2 gives the gasket outside diameters and tolerances.

The bibliography also traces the design background to earlier vacuum-flange work and related standards, which is useful for design comparison and compatibility checks.