ISO 3669:2020

Overview

ISO 3669:2020 - Vacuum technology - Dimensions of knife-edge flanges specifies the dimensional requirements for fixed or rotatable bolted knife-edge flanges (often known as ConFlat® or CF-style flanges) used in vacuum systems from atmospheric pressure down to ultra-high vacuum levels (as low as 10−11 Pa). The third edition (2020) consolidates a single series of standard flange dimensions, includes detailed knife-edge sealing profiles, and provides tables and figures for flange and gasket sizes (nominal bores from common CF sizes through larger CF sizes).

Key topics and technical requirements

• Dimensional specifications: Precise outside diameters, bolt circle, bolt-hole positions, flange thickness, knife-edge geometry and seal recess dimensions are defined in figures and tables (see Figures 1–4, Table 1 and Table 2).
• Knife-edge sealing profile: Triangular knife-edge detail for metal-to-metal seals is specified; a correction in this edition updates the vertical distance from the knife-edge to the flange surface to 0.65 ± 0.05 mm.
• Gasket dimensions: Recommended metal gasket outside diameters and tolerances are given (Table 2). The standard notes that gaskets should be softer than the flange to avoid dulling the knife-edge.
• Materials guidance: Material selection should suit service temperature, sealing needs, corrosion resistance and magnetic properties. Austenitic stainless steel and OFHC copper gaskets are commonly used, but the standard does not limit material choice.
• Bolt hole options and threads: Flanges may use clearance or tapped holes; both metric and English (imperial) thread forms commonly in use are addressed.
• Leak check grooves: Recommended geometry and equidistant positioning between bolt holes to facilitate leak checking.

Applications and users

ISO 3669:2020 is essential for:

• Vacuum engineers and system integrators designing UHV and high-vacuum systems
• Manufacturers and machinists producing vacuum flanges and vacuum chambers
• Research labs (physics, accelerator facilities, fusion research), semiconductor fabs, analytical instrument OEMs and maintenance teams Practical benefits include ensuring flange interchangeability, reliable metal seals for bakeable UHV joints, and consistent specifications for procurement and quality control.

Related standards

• ISO 1609 (dimensions of non-knife edge flanges)
• ISO 965-1 (metric screw thread tolerances)
• ISO 286 (tolerances on linear sizes) Consult these referenced documents for complementary thread, tolerance and flange-type requirements.

Keywords: ISO 3669:2020, knife-edge flanges, ConFlat, vacuum technology, UHV flanges, metal gasket, flange dimensions, leak check groove, flange materials.