The KF and ISO families are the quick-release side of vacuum engineering. They trade bake-out temperature for access speed, which is the right trade for forelines and service ports and the wrong one for a process chamber.
The three families and when each is used
KF (also called NW or QF) is the small quick-release family, KF10 to KF50, sealed by a centring ring and an O-ring and closed with a claw clamp — it is used for roughing lines, gauge ports and small service connections. ISO-K is the larger clamp family and ISO-F the bolted version of the same nominal bore, both from about ISO 63 upward, used on larger HV ports and pump inlets. They share sealing dimensions above a certain size, which is why an ISO-K and ISO-F of the same nominal bore mate to the same O-ring.
What has to be right on the machined part
The O-ring groove depth and width, the sealing face flatness, and the centring or claw geometry. A KF flange that is slightly out of flat will pull the centring ring to one side and pinch the O-ring, producing a leak that reappears whenever the joint is disturbed. On ISO sizes the bolt circle and clamp recess have to be concentric with the bore or the flange will not seat evenly.
Materials
304 and 316L stainless steel for most service, with aluminium used where weight matters. Stainless is preferred on any joint that will be repeatedly opened, because aluminium sealing faces gall and lose flatness after a few assembly cycles.
Where KF/ISO stops and CF begins
The O-ring is the limit. Viton is good to about 150 °C and a pressure around 10⁻⁸ mbar; above that it permeates and outgasses. Once a joint has to be baked or to hold UHV, the flange family has to change to ConFlat, and the port on the chamber has to be designed for it from the start.
Flange family selection
| Family | Nominal sizes | Sealing | Bake-out limit | Best for |
|---|---|---|---|---|
| KF / NW / QF | KF10 to KF50 | Centring ring + O-ring, claw clamp | ~150 °C | Roughing lines, gauge ports, frequent access |
| ISO-K | ISO 63 to ISO 630 | O-ring + clamp | ~150–200 °C | Larger HV ports, pump inlets |
| ISO-F | ISO 63 to ISO 630 | O-ring + bolted | ~150–200 °C | Same as ISO-K where vibration rules out clamps |
| CF (ConFlat) | CF16 to CF250+ | Copper gasket + knife edge | Up to 450 °C | UHV, bakeable joints, semiconductor process |
What we control in-house
- 304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
- Clean-environment TIG welding with full penetration and no micro-porosity
- Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
- In-house electropolishing and passivation — not subcontracted
- Chamber bake-out to 150–400 °C for UHV outgassing control
- Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
- Zeiss CMM verification to ±0.02 mm across large work envelopes
- 12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
- SEMI-referenced documentation and inspection protocols
Frequently asked questions
What does KF stand for?
KF comes from the German "Kleinflansch", meaning small flange. You will also see it written NW (Nennweite, nominal width) and QF (quick flange) — all three describe the same clamp-and-O-ring family.
What is the difference between ISO-K and ISO-F?
They share the same sealing dimensions and the same O-ring for a given nominal bore. ISO-K is closed with clamps around the outside; ISO-F is bolted through a bolt circle. ISO-F is chosen where vibration or thermal cycling would rattle a clamp loose.
Can KF flanges be used for UHV?
Not for the UHV part of the system. The elastomer O-ring both permeates and outgasses, and it cannot be baked. KF is fine on the roughing side of a UHV system, but every joint on the high-vacuum side should be ConFlat.
What temperature can a KF flange take?
About 150 °C with a Viton O-ring, and lower with nitrile. Above that the O-ring degrades and the outgassing rate rises sharply.
Do you supply flanges or only machine them?
We machine vacuum flanges to standard dimensions and to customer drawings, in standard sizes and custom geometries. We also fabricate the chamber that carries them, so the port and the flange are machined as one operation rather than fitted afterwards.
Related capabilities
Need this made to spec?
Send the drawing or the specification and we will come back with a DDP price, lead time and inspection plan.