How to Specify Surface Roughness in CNC Drawings

  • Date:
  • Views:6
  • Source:Axom



In the world of precision CNC machining, the clarity of your technical drawings is paramount. One of the most critical, yet often misunderstood, specifications is surface roughness. Correctly defining this parameter is not just a technical formality; it is a direct contributor to part functionality, performance, cost, and ultimately, customer satisfaction. For businesses seeking reliable, onestop CNC machining services, mastering this specification is key to ensuring their projects are manufactured correctly the first time.


cnc machining center
Understanding Surface Roughness

Surface roughness, often denoted as Ra (Arithmetic Average), refers to the fine, closely spaced, irregular deviations on a machined surface. It is a measure of the texture, quantified by the vertical deviations of a real surface from its ideal form. A lower Ra value indicates a smoother surface, while a higher Ra value signifies a rougher one. For example, a mirrorlike finish might have an Ra of 0.4 µm, while a rough machined part could be 3.2 µm or higher.

Why Proper Specification is Crucial

1. Functional Performance: Surface finish directly impacts how a part interacts with its environment. A low Ra is essential for sealing surfaces, reducing friction in bearing applications, improving fatigue strength, and ensuring proper adhesion for coatings or paints.

cnc machining online
2. Cost Control: Specifying an excessively smooth finish (unnecessarily low Ra) can exponentially increase machining costs. Achieving a finer finish requires more time, specialized tools, and additional processes like grinding or polishing. Conversely, a finish that is too rough may lead to part rejection or failure in the field.
3. Clear Communication: Using standardized ISO or ASME symbols on your drawings eliminates ambiguity between your design intent and our manufacturing team. This ensures you receive parts that perform exactly as required.

How to Specify Surface Roughness on Drawings



The most effective method is to use the international standard surface texture symbol (a checkmarklike symbol). The most common parameter to specify is the Ra value, which is placed to the right of the long leg of the symbol. For instance, specifying "Ra 1.6" within the symbol tells us that the average roughness must not exceed 1.6 micrometers.

General Note: A single roughness value in the title block can define the default finish for all surfaces.
Local Callouts: For critical features, apply the surface roughness symbol directly to the specific surface on the drawing. This overrides the general note and provides precise instructions.

Partner with a Pro for Flawless Results

As your onestop CNC machining partner, we don't just read your drawings—we help you optimize them. Our engineering expertise allows us to advise on the most costeffective and functional surface finishes for your specific application, material, and budget. By specifying surface roughness correctly, you empower us to deliver superiorquality components that enhance your product's reliability and performance, driving growth for your business through dependable manufacturing.

Let us handle the complexities of precision manufacturing. Provide us with your welldefined drawings, and we will deliver parts that meet your exact specifications, every time.