To ensure that a flange mates with the gasket and the companion flange perfectly, some roughness is required on the flange surface area (RF and FF flange finish only). The type of roughness on the flange face surface defines the type of “flange face finish”.
Common types are stock, concentric serrated, spiral serrated and smooth flange finish.
Steel flanges are available with four basic face finish, however, the common objective of any type of flange face finish is to create the desired roughness on the face of a flange to ensure a strong match between the flange, the gasket, and the mating flange and thus provide a high-quality seal.
Source: Officine Orsi, Italy
Let’s now dive into the most common flange face finish types:
The stock finish is the most widespread type of finish as it suits the large majority of applications. The pressure embeds the soft face of the gasket into the flange finish and results in the formation of a good seal due to the friction existing between the contacting parts.
As the mating flanges are bolted together, gaskets get “squeezed” into the flange face surface and create a very tight seal.
A stock finish face is manufactured using a phonographic spiral groove featuring a 1.6mm radius round-nose tool with a depth of 0.15mm and a feed-rate of 0.8mm per revolution. The resulting “Ra” value (AARH) for the surface ranges from 125µinch to 500 µinch (125 µm to 12.5 µm).
Spiral serrated finish is a phonographic spiral groove type that differs from the stock finish as the groove is crafted by a 90 degrees tool (instead of a round nosed one) that creates a “V” geometry with a 45-degree serration angle.
A serrated finish, concentric or spiral, has from 30 to 55 grooves per inch and roughness between 125 to 250 µinch.
The concentric serrated flange finish features concentric grooves instead of spirals.
The grooves are crafted by the same 90-degree tool used for the spiral serrated finish, but the serrations have an even design on the face of the flange. To have concentric grooves, the tool has a feed rate of 0.039mm per revolution and a depth of 0.079mm.
SMOOTH FINISH OR FACE
Flanges with a smooth finish do not show visible tool markings at naked eye.
This type of flange finish is used with metal-facing gaskets such as the jacketed type.
As per the stock finish, this is achieved by having the contact surface machined with a continuous spiral groove generated by a 0.8mm radius round-nosed tool at a feed rate of 0.3mm per revolution with a depth of 0.05mm (that creates a roughness between Ra 3.2 and 6.3 micrometers, i.e. 125 – 250 microinches).
The cold water finishes appear shiny to the naked eye and very smooth. The AARH value for these surfaces ranges between 85 µinch to 100 µinch. They are used with metal to metal seals (no gasket).