The sealing face is where flange work goes wrong
I have scrapped more flanges on the sealing face than any other feature. A single chatter mark at 0.02 mm deep across a raised face is enough to create a leak path that shows up only after 150 bar of hydrotest pressure. The raised face or RTJ groove has to be flat, not looks-flat-on-the-bench flat, but measured flat across the full face with a dial indicator or CMM.
The rule of thumb I work with: on a raised-face flange, the sealing surface flatness should not exceed 0.05 mm across any 50 mm arc, and total face runout under 0.1 mm on flanges up to DN200. Go tighter on RTJ grooves. The groove surface finish needs to be Ra 1.6 or better, and the groove geometry (angle, depth, root radius) held to the standard. A sharp corner at the groove root is a stress riser; a rounded corner changes the seal contact pressure.
Bolt-circle patterns: position matters more than diameter
Everyone checks bolt-hole diameter. Not everyone checks true position relative to the bore and the sealing face. A hole drilled 0.3 mm off its nominal bolt-circle position still clears the bolt on assembly, but it loads the gasket unevenly. On a 4-bolt flange that is not catastrophic. On a 24-bolt heat-exchanger flange, one misplaced hole shifts the bolt preload pattern and you get a weep at 80 percent operating pressure.
- Bolt-hole position tolerance: +/-0.15 mm true position on standard RF flanges, +/-0.1 mm on RTJ and high-pressure flanges above ANSI 600
- Bolt-circle diameter: hold to +/-0.1 mm on flanges under DN150, +/-0.2 mm on larger sizes
- Spot-drill before drilling; a wandering drill on a curved face is the fastest way to blow a hole position
- Index holes from the bore center, not from the OD; the bore is the functional datum that aligns with the mating flange
Facing strategy for flatness and finish
Facing a flange to a good finish is not hard if you treat it like two operations: rough face then finish face with a separate tool and a spring pass. On 304/316 stainless, I use a 0.4 mm nose-radius insert for the finish pass at 0.1 mm/rev feed. That gives a predictable Ra 1.6 surface with no visible tool marks. Aluminum 6061 faces faster but is more prone to insert drag if you do not break the cut cleanly at the end.
The common mistake is clamping the flange on the back face and expecting the lathe to produce a perfectly perpendicular sealing face. If the chuck is gripping a rough casting or saw-cut blank, the face comes off with a slight dish. Bore the ID first, then face from the bore out while running on a mandrel or on bored soft jaws that reference the ID. That single sequence change eliminates most face-perpendicularity issues.
Material selection for flange applications
| Material | Common flange use | Pressure class | Machining notes |
|---|---|---|---|
| A105 / carbon steel | Pipeline, general industrial | ANSI 150-2500 | Good machinability, watch for mill scale |
| 304/304L stainless | Chemical, food, general corrosion | ANSI 150-600 | Work-hardens quickly, use sharp inserts |
| 316/316L stainless | Marine, chemical, offshore | ANSI 150-600 | Similar to 304 but tougher on tools |
| 6061-T6 aluminum | Pneumatic, lightweight hydraulic | Low pressure only | Easy to machine, seal grooves must be burr-free |
| Duplex / super duplex | Subsea, high-chloride | High pressure | Requires rigid setup, lower cutting speeds |
Groove work and weld-prep bevels
RTJ grooves require a form tool ground to the exact groove angle, typically 23 degrees for BX and R/RX rings depending on the standard. Groove depth is measured to the theoretical sharp corner, not to the bottom radius; that trips up a lot of shops. The groove walls need a consistent Ra 1.6 or better, and the root radius cannot have chatter.
Weld-neck flanges get a bevel on the hub end. The bevel angle (usually 37.5 degrees for standard V-groove weld prep) and land thickness are part of the flange specification, not an afterthought. I check the bevel angle with a gauge before the part leaves the machine.
Deburring is part of the job, not a bench operation
I do not send flanges to deburr with a belt sander. A belt sander can round the edge of a sealing face or open up a bolt-hole chamfer past specification. Break sharp edges with a hand deburr tool on the machine if possible, and use a dedicated chamfer tool for bolt holes where a 45-degree chamfer is specified. The sealing face gets zero contact with abrasive. Fingerprints are bad enough; grinding marks are a scrap ticket.
Frequently Asked Questions
What surface finish is required on a raised-face flange sealing surface?
Ra 1.6 to Ra 3.2 um (63-125 uinch) is standard for raised-face flanges with spiral-wound or sheet gaskets. RTJ grooves need Ra 1.6 or better, with controlled groove geometry including angle and root radius.
How flat does a flange face need to be?
Across any 50 mm arc, flatness should be within 0.05 mm for standard RF flanges. Total face runout should be under 0.1 mm on flanges up to DN200. High-pressure RTJ flanges require tighter control per ASME B16.5 or API specifications.
Can aluminum flanges handle hydraulic pressure?
6061-T6 aluminum flanges are suitable for low- to medium-pressure pneumatic and light hydraulic applications, typically below 100 bar. For higher pressures or critical hydraulic service, 316 stainless or carbon steel is standard.
What tolerance should bolt-hole position be held to?
+/-0.15 mm true position for standard ANSI RF flanges, +/-0.1 mm for RTJ and ANSI 600+ high-pressure flanges. Always dimension from the bore center as the functional datum.
Do I need a CMM to inspect a flange?
For standard production flanges, a bore gauge, dial indicator, and surface roughness sample are sufficient. Critical or high-pressure flanges, particularly RTJ, should get CMM inspection of groove geometry, bolt pattern, and face flatness.