In the field of modern precision manufacturing and machining, surface roughness is a critical indicator determining component performance, wear resistance, and fit tolerance. However, engineers and procurement professionals frequently encounter conversion challenges between Ra, Rz, micrometers (um), and micro-inches (uinch) due to varying international standards, industry sectors, and drawing notation habits.
To help clear up these concepts, this comprehensive Surface Finish Chart and Reference Guide has been compiled. With this guide, you can easily understand the surface finish levels achievable by various machining processes and perform quick conversions between different roughness units.
What is a Surface Finish Chart?
A Surface Finish Chart, also referred to as a Finish Surface Chart or Machining Surface Finish Chart in many technical documents, is a technical tool used to quantify, compare, and convert the microscopic geometric characteristics of a component surface.
In actual production, the most commonly used evaluation parameter is the Ra Surface Finish Chart. Ra stands for the Roughness Average (Arithmetic Average Roughness) and serves as the core global standard in the manufacturing sector.
Surface Finish Conversion Table: Units and Grades
Drawings from different countries and regions may utilize different roughness notations. For instance, North America typically uses micro-inches (uinch), while China and Europe generally use micrometers (um). Furthermore, roughness grades or triangle symbols (V) commonly found on legacy drawings are still frequently encountered.
Using the Surface Finish Conversion Chart below, you can visually perform necessary Surface Finish Conversion.
Surface Finish Conversion Table
| Roughness Grade (ISO) | Roughness Average Ra (um) | Roughness Average Ra (uinch) | Traditional Triangle Notation | Common Machining Process Reference |
| N1 | 0.025 | 1 | VVVV | Ultra-precision lapping, Superfinishing |
| N2 | 0.05 | 2 | VVVV | Mirror polishing, Fine lapping |
| N3 | 0.1 | 4 | VVVV | Lapping, Micro-bearing raceway finishing |
| N4 | 0.2 | 8 | VVV | Fine rolling, Ultra-fine turning/Fine grinding |
| N5 | 0.4 | 16 | VVV | Fine grinding, Fine reaming, Advanced cylindrical grinding |
| N6 | 0.8 | 32 | VVV | Standard grinding, Fine turning, Surface milling |
| N7 | 1.6 | 63 | VV | Fine turning, Standard milling, Reaming |
| N8 | 3.2 | 125 | VV | Rough turning, Medium milling, Broaching |
| N9 | 6.3 | 250 | VV | Rough milling, Boring, Sawing |
| N10 | 12.5 | 500 | V | Rough shaping, Rough boring, Heavy machining |
| N11 | 25 | 1000 | V | Flame cutting, Rough die casting, Forged surface |
| N12 | 50 | 2000 | V | Heavy forging, Sand casting |
Machining Processes vs. Surface Finish Scale
Different levels on the Surface Finish Scale correspond directly to manufacturing costs and production cycles. Pursuing a lower Ra value (a smoother surface) implies adding finishing steps such as lapping or polishing, which significantly increases manufacturing complexity.
When designing products, referring to a standard Surface Finish Reference is vital:
- Economical Machining (Ra 3.2 to 12.5 um): Typically achieved via rough turning, rough milling, or conventional drilling, suitable for non-mating surfaces.
- Commercial Precision Machining (Ra 0.8 to 1.6 um): The routine output of most CNC fine turning or fine milling, suitable for general mechanical mating surfaces.
- High-Precision Machining (Ra 0.1 to 0.4 um): Requires precision grinders, superfinishing, or polishing processes, typically used for seal seats, high-speed bearings, or high-pressure valve seats.
Conclusion and Resources
When aligning technical details with global clients or optimizing machining workflows, a precise chart eliminates communication gaps. Understanding the values within the Ra surface finish chart ensures that products fully meet design functionality while preventing over-machining to control production costs.
For convenient reference on the shop floor or in the design office, it is recommended to save this Surface Finish Chart PDF in your regular technical toolkit as a daily operational standard.