What Is Phonographic Finish?

Phonographic finish (also called spiral serrated finish) is a flange face machining pattern where a single continuous groove spirals outward from the bore to the RF outer edge—like the grooves on a vinyl record. This is the most common flange face finish for raised face (RF) flanges in industrial piping. ASME B16.5 specifies this finish with a surface roughness of 3.2 to 6.3 micrometers Ra for optimal gasket sealing.

Key Specifications

Parameter	Value
Pattern	Single continuous spiral groove (phonographic)
Standard	ASME B16.5, ASME SP-6 (for surface finish)
Surface roughness (Ra)	3.2 to 6.3 micrometers (125 to 250 AARH microinches)
Groove pitch	0.9 to 1.5 mm per revolution (approximately 45 grooves/inch)
Groove depth	Controlled by machining feed rate and tool radius
Application	Standard for all RF flanges under ASME B16.5
Gasket compatibility	Spiral wound, compressed fiber, graphite ring

Phonographic vs Concentric Serrated Finish

Feature	Phonographic (Spiral)	Concentric
Groove pattern	Single spiral (continuous)	Individual concentric circles
Machining	Single-pass lathe cut (standard)	Multiple passes or special tooling
Leak path	Spiral path from bore to OD (long, tortuous)	No radial leak path (each ring is closed)
Standard per ASME B16.5	Yes (default)	Specified on request
Cost	Standard (no premium)	Slight premium (more machining time)
Gasket compatibility	All standard gaskets	All standard gaskets
Use case	General process piping	Critical service (some operators prefer)

Common Mistake

Some engineers specify concentric serrations for all critical service believing it provides superior sealing because each groove forms a closed ring. In practice, ASME B16.5 does not mandate concentric over phonographic, and most spiral wound gaskets seal effectively on either pattern. Only specify concentric if the project specification or operator standard explicitly requires it—otherwise it adds cost without measurable benefit.

Why the Finish Matters

The serrated pattern serves two functions:

1. Gasket grip: Grooves bite into the gasket material, preventing blowout under pressure
2. Leak path length: The spiral creates a long, tortuous path that any leaking fluid must travel from bore to atmosphere

Surface Finish	Ra (micrometers)	Suitability
Too smooth (<1.6)	<1.6	Gasket may not grip; risk of blowout
Standard serrated	3.2-6.3	Optimal for spiral wound and sheet gaskets
Too rough (>6.3)	>6.3	Leak paths across groove peaks; gasket cannot fill valleys
Damaged/scored	Varies	Must re-machine or replace flange

Pro Tip

After any field machining or repair of a flange face, verify the Ra value with a surface roughness comparator or profilometer. A visual check alone is insufficient. Even light corrosion or handling damage can take the finish outside the 3.2-6.3 micrometer Ra range, compromising the seal when reassembled with a new gasket.

The phonographic finish is achieved in a single lathe pass during flange manufacturing, making it the most economical option. It is the industry default and does not need to be specified separately unless the project calls for a different finish (concentric, stock finish, or custom Ra).

Read the full guide to flanges