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Technical Notes

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CF Flanges

The CF flange (originally called ConFlat) is a sexless design—both flanges are identical. Typical flange materials are austenitic stainless steel types 304L, 316L, 316LN, and surfacedhardened aluminum (made from a weldable Al alloy). The seal mechanism is a knife-edge that is machined below the flange’s flat surface. As the bolts of a flange-pair are tightened, the knifeedges make annular grooves on each side of a soft metal gasket. The extruded metal fills all the machining marks and surface defects in the flange, yielding a leak-tight seal (Figure 1). The CF seal operates from 760 Torr (1013 mbar) to < 1 x 10-13 Torr (<1.3 x 10^-13 mbar), and within the temperature range -196° C to 450° C. Flange sizes in North America (NA) are determined by outside diameter (O.D.). In Europe and much of Asia (E/A), the internal diameter (I.D.)—actually Figure 1: ConFlat Flange Assembly 1-2 the nominal I.D. of the largest tube that can be welded to a bored flange—is used.

The NA sizes are: 11/3" ("mini"), 23/4", 41/2", 6", 8", 10", 12", 131/4", 14" and 161/2" O.D.. The E/A sizes are digits (representing the nominal bore in mm) prefixed with either DN or NW (depending on manufacturer): DN16, DN40, DN63, DN100, DN160, DN200, and DN250. In addition, the NA system has intermediate O.D. sizes to enable the use of "standard" inch dimension tubes. They are 21/8" (1" diameter tube), 33/8" (2"), 45/8" (3"), 63/4" (5"). For other dimensions, see the ordering tables in this catalog.

CF flanges (with identical O.D.s up to 10" inclusive) from different (reputable) manufacturers will almost certainly seal.

The vacuum equipment industry adheres to the de facto ConFlat standard.

CF flanges are offered in four distinct versions or combinations thereof:

• Fixed flange (Figure 2): once welded to a fitting, it has its bolt-hole orientation fixed with respect to that fitting.
• Rotatable flange (Figure 3): when welded to a fitting, it enables the bolt-hole ring to rotate.
• Through-hole flange: indicates a mating flangepair accepts bolts that go through both flanges and is secured by nuts or plate nuts.
• Tapped flange: connected to a through-hole flange using only bolts. Vacuum components with a rough axis of symmetry—e.g., a gate valve—can have fixed CF flanges welded with the bolt holes either split by the axis or in-line with the axis. Regrettably, NA has adopted the former and E/A the latter as standards.

Positive Pressures

When discussing system pressures, a common question is: What is the maximum internal pressure?

Positive pressures are inherently dangerous and failure comes without warning. In a vacuum system, failure depends on various strength properties of bolts, clamps, chamber walls, welds, feedthroughs, valves, viewports, etc. The only safe answer is that the internal absolute pressure cannot exceed the external absolute pressure.

CF Flange Gaskets

The gasket material sealing two flanges determines the joint’s maximum temperature and, to an extent, the base pressure of the vacuum volume.

Metal gaskets are impermeable to gases and withstand moderately high temperatures indefinitely. The normal gaskets for stainless steel CF flanges are punched from 1/4-hard, high-purity, oxygen-free (OFHC) copper stock, chemically cleaned and finished to be scratch- and burr-free.

Bolting the flanges together causes the CF’s knife-edges to deform the gasket. Under plastic flow, the copper fills in the knife-edges’ imperfections and, in doing so, work-hardens—inducing the "springy" characteristics that keep the seal, even as the knife-edges are cycled from - 196° C to 450° C.

Given a correctly cleaned, baked, and pumped CF-flanged chamber, metal gaskets enable base pressures below 1 x 10^-13 Torr (1.3 x 10^-13 mbar). The work-hardening aspect means metal gaskets are one-time use only. There is strong anecdotal evidence that baking to 550° C for prolonged periods partially anneals the copper from its workhardened state, since the joints will leak on cooling. When mounting fragile flanged components—e.g., viewports—fully annealed copper gaskets are recommended.

For systems requiring frequent high-temperature bakeouts, silver-plated copper gaskets are recommended. Without the plating, copper oxide forms on parts of the gasket exposed to air. It may flake into the chamber during flange disassembly.

Aluminum CF flanges must use aluminum gaskets — never copper. However, aluminum gaskets can be used on stainless steel CF flanges if baking is restricted to 200° C.

CF Flange Hardware

Regular nuts and bolts are not suitable for CF flange applications. Various high-tensile strength nut/bolt combinations made from low magnetic permeability, 18-8 stainless steel are used.

It is strongly recommended that all bolt, nut, platenut combinations are lubricated, either by one component being silver-plated or the application of a suitable thread lubricant.

Hex Head

Hex-head bolts (socket hex head for the mini flange) are long enough to penetrate two (through-hole) flanges and leave sufficient length for washers, nuts, or plate-nuts. For tapped and blind-tapped flanges, a shorter bolt length is used that does not penetrate the tapped flange or bottom out in a blind-tapped flange.

12-Point

Where high temperature bakeouts and frequent reassemblies are needed, (dry lubricated) 12-point cap screws are the best choice. Note that cap screws do not require special wrenches—the appropriate sized 12-point socket or flat wrench suffices.

Metric

Tapped metric flanges must use metric bolts. While through-hole flanges (built to metric specifications) will accept inch or metric nuts/bolts, it is strongly recommended that, for vacuum systems containing some tapped metric fittings, all nuts and bolts have metric threads.

Studs and Threaded Rods

This hardware is used to:

• Connect double-sided flanges
• Make bolts "emerge" from blind-tapped flanges
• Mount protective plastic covers over glass viewports

Both inch and metric threads are available for these and most other components.

Plate Nuts

These are metal plates with two threaded holes, shaped to match the corresponding CF flange’s bolt hole patterns and replace the individual nuts (Figure 4). The plate nut’s major advantage is, once two bolts are manually started, the plate nut acts as its own backing wrench and washer—the bolts are tightened using one wrench.

Flanges, Cross Reference

Flange O.D. (in.)	Flange O.D. (mm)	Bolt Torque (ft.-lbs.)	Bore (mm)	Tube I.D. (mm)	VAT & Balzers	Leybold	V.G.	KJLC
1-1/3	34	7	Blank	—	DN16CF	NW16	FC90	F0133X000
			19	16			FC919	F0133X075
2-3/4	70	12	Blank	—	DN40CF	NW35	FC2	F0275X000
			38	35			FC38	F0275X150
			41	38			FC41	F0275X162
4-1/2	114	15	Blank	—	DN63CF	NW63	FC4	F0450X000
			63	60			FC64	F0450X250
6	152	15	Blank	—	DN100CF	NW100	FC6	F0600X000
			102	100			FC100	F0600X400
8	203	15	Blank	-	DN150CF	NW150	FC8	F0800X000
			152	150			FC150	F0600X600
10	254	15	Blank	-	DN200CF	NW200	FC10	F1000X000
			203	200			FC200	F1000X800

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