A flange is a sealing part. Strength is rarely the problem — leaks are. What decides whether a flange works is the flatness and finish of the sealing face, the position of the bolt holes relative to the bore, and whether the gasket has a uniform load path. All three are machining decisions.
The same part can be produced several ways, and the choice is decided by volume and geometry rather than preference. We quote whichever is actually cheaper for your quantity.
| Approach | How it works | Best at | Watch out for |
|---|---|---|---|
| CNC turning | Turned from bar or tube: bore, sealing face and O-ring groove in one setup | Round flanges and fittings — concentricity is inherent | Bolt holes need a second operation (mill or drill) unless live tooling is used |
| Turn-mill (live tooling) | Turned and drilled/milled in one machine without re-fixturing | Flanges with bolt circles and cross-holes; concentricity and hole position both controlled | Machine availability; not every shop has live tooling |
| Milled from plate | Plate cut to outline, bore and sealing face machined, holes drilled | Non-round flanges, manifolds, and parts with complex outlines | Sealing face flatness depends on the plate; stress relief matters |
| Welded flange + machined face | Flange welded to pipe, then the sealing face machined after welding | Large or site-fabricated assemblies | Weld distortion — the face must be machined after welding, not before |
These are the decisions that account for most of the cost difference between two parts of the same overall size.
| Feature | Recommended | Why it matters |
|---|---|---|
| Sealing face flatness | 0.05mm typical; 0.02mm for gas or high pressure | The gasket cannot seal a face that is not flat |
| Sealing face finish | Ra 1.6 or better; Ra 0.8 for O-ring grooves | Too rough and the gasket cannot conform; too smooth can be worse for some gaskets |
| Bolt hole position | ±0.1mm to the bore (position tolerance preferred) | Misalignment loads the gasket unevenly |
| Bore-to-face perpendicularity | 0.05mm over the face diameter | Controls whether the gasket is compressed evenly |
| O-ring groove depth | Tolerance the depth, not the width | Depth sets compression; width only needs clearance |
| Wall thickness at the bore | ≥ 2mm for pressure; ≥ 3mm for high pressure | Thin walls distort under bolt load |
| Thread engagement | 1.5× diameter minimum in aluminum | Aluminum threads strip more easily than steel |
When a flange leaks, the instinct is to tighten it more. That usually makes it worse: uneven bolt load distorts the flange and opens the gap on the opposite side. The root cause is almost always the sealing face — either it is not flat enough, or the bolt holes are positioned so the clamp load is uneven.
A useful check before releasing a drawing: is the sealing face a single continuous surface with a clear load path from each bolt to the gasket? If the bolts sit far from the seal, or the face is interrupted by ports or slots, the gasket pressure will not be uniform regardless of how well it is machined.
| Cost element | Share of price | What to change if it is too high |
|---|---|---|
| Turning / milling time | 30–50% of part cost | Round parts turned in one setup are cheapest per feature |
| Setups | 20–35% | Bolt circles and cross-holes usually need a second operation |
| Material | 15–25% | Bar or plate size vs finished weight; tube can halve it for large bores |
| Sealing face finishing | 8–15% | Flatness and finish need a separate finishing pass |
| Pressure testing / inspection | 3–10% | If the part is pressure-rated, this is a real cost |
A flange with a large bore turned from solid bar wastes most of the material as chips. Switching to tube or a near-net forging can halve the material cost on large flanges — but only if the bore size is standard enough for tube to be available.
For small fittings the opposite is true: bar stock is cheap and fast, and the setup dominates anyway. Do not over-engineer the material strategy for a 50mm fitting; do look at it for a 300mm flange.
The sealing face should be masked during painting or coating, and protected in packing. A coated sealing face is usually worse than a bare one, and a scratched face leaks. Specify masking explicitly on the drawing.
For water and mild chemicals, 304 or brass is usually adequate. For chlorides, 316L. For aggressive chemicals or seawater, titanium or a plastic like PTFE or PVDF. The flange material and the gasket material have to be compatible with the same fluid.
Tapered threads (NPT) seal on the thread flank and need sealant; parallel threads (G/BSPP) seal on a face or an O-ring. Machining a tapered thread to the correct gauge depth is a skill — specify the standard and the gauge requirement rather than just the thread callout.
For a standard gasket at moderate pressure, 0.05mm across the face is normally adequate. For gas service, high pressure, or a metal-to-metal seal, 0.02mm or better. The governing factor is the gasket: a soft gasket conforms to a rougher face, a hard or metal gasket does not.
Position tolerance is better for bolt circles because it controls the pattern as a group relative to the bore, which is what determines whether the gasket is loaded evenly. Dimensioning each hole with ±X and ±Y creates a rectangular tolerance zone and no control over the pattern as a whole.
Yes. Tapered threads are cut with a tapered thread mill or a die, and the depth is checked with the appropriate gauge rather than by measurement. Specify the standard (NPT, BSPT) and whether a gauge check is required — a visually correct tapered thread can still leak if it is not cut to the right depth.
Brass or 304 stainless for clean water; 316L if chlorides are present; engineering plastics (PVC, PVDF, PTFE) for aggressive chemicals. The gasket must be compatible with the same fluid — a stainless flange with the wrong gasket still leaks.
Include the quantity, whether the finish is visible, and which fits or seals are critical. You get a line-item quote within 24 hours and DFM notes on the features driving your price.