---
title: CNC Machining of Flange Sealing Surfaces (FF, RF, RTJ)
date: 2025-12-15T02:22:23Z
modified: 2025-12-17T02:57:03Z
permalink: "https://www.longanflange.com/blog/cnc-flange-sealing-surface/"
type: post
status: publish
excerpt: ""
wpid: 3233
categories:
  - Blog
timestamp: 2025-12-17T02:57:03Z
tags:
  - Blog
---

The flange sealing surface is a precise process that determines just how well your pipe joints can hold up under pressure. Below is a breakdown of what it is, the common sealing surfaces, and how CNC machining handles allowances, surface finishes, and tool path choices for these types.

## What is a Flange Sealing Surface?

A flange sealing surface is the specific part of the flange that makes contact with a gasket or another flange to create a leak-tight joint. The surface type can change depending on the pressure, temperature, and medium your system handles.

![](https://www.longanflange.com/wp-content/uploads/2025/12/image-1765937745129-595-2.webp)

Here’s a breakdown of the three main types:

- **Flat Face (FF)** – The entire face is in contact with the gasket, ensuring the load is evenly distributed. This is commonly used in low-pressure systems.
- **[Raised Face (RF)](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/asme-flange.md)** – Features a small raised ring in the center of the flange where the gasket sits. It’s commonly used in process piping or process plant applications as it allows a tighter seal under moderate to high pressure.
- **Ring Type Joint (RTJ)** – Has a precision-machined groove with a metal ring gasket. It’s designed to handle **[high-pressure, high-temperature systems](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/pressure-vessel-flanges.md)**.

## CNC Machining for Flange Sealing Surfaces

**[CNC (Computer Numerical Control) machining](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/manufacturing.md)** uses computers to shape flanges accurately, ensuring they are the right size and have a smooth finish for a perfect fit. Here are essential factors to consider when manufacturing flange sealing surfaces.

### Machining Allowance

Machining allowance refers to the small extra metal left over for delicate finishing, serving as a safety buffer. Below are suggestions for each:

- **FF:** Small allowance, as the broad face makes cutting predictable.
- **RF:** More careful removal is needed to preserve ring height.
- **RTJ:** Minimum allowance since groove depth can affect gasket compression.

### Surface Finish

The surface finish texture or roughness can affect the flange’s sealing reliability. Below are general recommendations for each type:

- **FF:** Fine or stock finish for even gasket contact.
- **RF:** Stock finish or spiral marks designed to grip the gasket.
- **RTJ:** Smooth grooves allow for a precise metal-to-metal seal.

### Tool Path Differences

Each sealing face type needs its own cutting path:

- **FF:** Simple circular or spiral cuts.
- **RF:** Layered steps to form the raised ridge.
- **RTJ:** Precision contouring with multiple passes for more consistent grooves.

These details will help give you perfect alignment and lead to leak-free, smoother operations.

## CNC Machining Tips for Better Sealing

Here are some recommendations:

- Always verify tool sharpness before facing. Dull tools can create uneven textures, weakening gasket grip.
- Check fixture stability as vibrations during machining can distort the sealing surface.
- Maintain consistent feed rates to achieve uniform spiral patterns.

## FAQs

**1.Can manual machining achieve the same finish?**

Not consistently. Unlike manual production, CNC machining guarantees repeatable accuracy, especially for RTJ grooves.

**2.What happens if the allowance is too large?**

It wastes time and labor costs, dulls tools, and even risks altering flange thickness.

**3.surface finishes affect gasket lifespan?**

Yes. Too rough can cause wear, while too smooth may reduce grip.

## Good Flow Control Through Flange Sealing Surfaces

Every flange sealing surface relies on innovative CNC machining to deliver lasting, more effective seals for your systems. For products built with the same attention to effectiveness and precision, check out LONG’AN’s **[complete flange line](https://www.longanflange.com/wp-content/uploads/wp-mfa-exports/page/stainless-steel-flange.md)**.

Talk to us today for a custom quote or more details on what we can offer you.