Surface Finish
What parameter is most commonly used to specify surface finish on drawings?
Ra is the most common specification because it gives a consistent measure of average roughness across many CNC processes.
What does surface finish actually measure?
It measures the micro-scale irregularities of the surface—its peaks and valleys—not the part’s overall size, flatness, or positional tolerance.
Why can two parts with the same Ra look different?
Because surface texture also includes lay and waviness; a part can meet Ra and still show visible tool patterning or chatter marks.
Surface finish in CNC machining and millwork refers to the micro-level texture left on a surface after cutting, grinding, sanding, or post-processing. It is commonly specified with roughness metrics such as Ra, Rz, and Rq. In manufacturing, it describes both the resulting surface condition and the processes used to improve it, such as finishing passes or secondary treatments.
On the shop floor, surface finish is controlled by toolpath strategy, cutting parameters, tool selection, material behavior, and any post-processing. A typical as-machined finish for CNC milling is around Ra 1.6–3.2 µm, while turning often achieves Ra 0.8–1.6 µm without special optimization. The finish callout determines whether a part can be left as-machined or requires bead blasting, brushing, polishing, anodizing, or powder coating to meet functional and appearance requirements. Functionally, smoother finishes matter where parts must seal, slide, mate, or resist wear, as micro-peaks and valleys influence friction, contact, and fluid leakage. In millwork and edgebanding, surface finish must eliminate tool marks, telegraphing, fuzzing, or adhesive witness lines while preserving dimensional consistency at joints and edges, affecting both tactile quality and assembly fit.
Chatter-mark finish failure: A part can achieve the target Ra on paper yet appear visually poor if cutter chatter leaves directional ripples or waviness, causing a mismatch between roughness and actual quality.
Wrong finish for function: Using a general-purpose as-machined finish on sealing lands or sliding interfaces can cause leakage, stick-slip, or premature wear because surface asperities are too high.
Post-process dimensional drift: Secondary finishes like bead blasting, coating, or anodizing can alter part dimensions; if the tolerance stack ignores the added layer, threaded holes or press fits may fail in assembly.