Understanding Surface Roughness in Engineering Drawings
https://youtu.be/lyakRLe4r0g?si=tKo0_UCW6b9lvNs3
Surface roughness defines the microscopic texture of a component’s surface and is a critical requirement in engineering drawings. It directly affects functional performance, manufacturability, cost, and product lifetime. A part can be dimensionally correct and still fail if the surface finish is wrong.
In engineering drawings, surface roughness is specified using standardized symbols defined in ISO 1302 or ASME standards. These symbols communicate how a surface must be produced and how smooth or rough it is allowed to be. The most common parameter is Ra (arithmetical mean roughness), typically expressed in micrometers (µm). Lower Ra values indicate smoother surfaces, while higher values allow rougher finishes.
Surface roughness requirements are closely linked to function. Sealing surfaces, sliding interfaces, bearing seats, and optical components usually require very fine finishes to reduce friction, wear, leakage, or noise. Structural or non-functional surfaces can often tolerate much rougher finishes, reducing machining time and cost. Over-specifying surface roughness is a common design mistake and leads directly to unnecessary manufacturing expense.
Engineering drawings may also indicate manufacturing constraints, such as whether material removal is allowed, whether a surface must be machined, or whether a specific process is prohibited. Additional parameters like Rz, lay direction, or sampling length may be specified when functional behavior depends on more than average roughness alone.
Correctly defining surface roughness ensures clear communication between design, manufacturing, and quality. It prevents assumptions, avoids rework, and ensures parts perform as intended. The following video explains surface roughness symbols, parameters, and real-world examples, showing how to specify surface finish correctly and when it truly matters.
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Use Excedify's 2D CAD tool to add surface roughness to drawings:
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