{ "slug": "surface-finish-ra", "title": "Surface Roughness (Ra) Quick Reference for Machined Parts", "description": "Surface roughness Ra chart in µm and µin: what each value looks like, what it costs versus as-machined, how to achieve it, and the material each finish removes.", "lang": "en", "tables": [ { "caption": "Surface roughness (Ra) values, appearance, cost and typical application", "columns": [ "Ra (µm)", "Ra (µin)", "Visual description", "How it feels", "Cost multiplier (vs as-machined)", "Typical application", "Achievable by" ], "rows": [ { "Ra (µm)": "0.05", "Ra (µin)": "2", "Visual description": "True mirror — sharp, undistorted reflection of objects", "How it feels": "Perfectly smooth, glass-like", "Cost multiplier (vs as-machined)": "4–6x", "Typical application": "Optical mirrors, precision optical components", "Achievable by": "Multi-stage polish and lap" }, { "Ra (µm)": "0.1", "Ra (µin)": "4", "Visual description": "Very smooth, hazy reflection — objects are visible but blurred", "How it feels": "Very smooth, barely detectable texture", "Cost multiplier (vs as-machined)": "3–4x", "Typical application": "Medical instruments, hydraulic components, mould faces", "Achievable by": "Precision polish" }, { "Ra (µm)": "0.2", "Ra (µin)": "8", "Visual description": "Semi-reflective, faint lines visible under close inspection", "How it feels": "Smooth, very slight texture detectable with a fingernail", "Cost multiplier (vs as-machined)": "2–3x", "Typical application": "Bearings, sealing faces, precision mating surfaces", "Achievable by": "Fine polish, tumble polish" }, { "Ra (µm)": "0.4", "Ra (µin)": "16", "Visual description": "Smooth, fine directional lines visible. Low reflectivity.", "How it feels": "Smooth, slight texture under a fingertip", "Cost multiplier (vs as-machined)": "1.5–2x", "Typical application": "General precision parts, cosmetic stainless", "Achievable by": "Hairline finish, fine polish" }, { "Ra (µm)": "0.8", "Ra (µin)": "32", "Visual description": "Smooth but machining marks visible on close inspection", "How it feels": "Even texture, slight roughness", "Cost multiplier (vs as-machined)": "1.2–1.5x", "Typical application": "CNC finishing pass, light-duty sealing surfaces", "Achievable by": "CNC finishing pass, light bead blast" }, { "Ra (µm)": "1.6", "Ra (µin)": "63", "Visual description": "Visible tool marks. Standard machined appearance.", "How it feels": "Clearly rough, machining marks felt", "Cost multiplier (vs as-machined)": "1x", "Typical application": "Non-critical machined faces, standard finish", "Achievable by": "As-machined" }, { "Ra (µm)": "3.2", "Ra (µin)": "125", "Visual description": "Rougher, distinct machining lines. Common on roughing passes.", "How it feels": "Rough, clearly felt", "Cost multiplier (vs as-machined)": "Included", "Typical application": "Non-critical surfaces, hidden features, pre-coating", "Achievable by": "As-machined (roughing)" }, { "Ra (µm)": "6.3", "Ra (µin)": "250", "Visual description": "Clearly rough, deep machining marks, saw-cut appearance", "How it feels": "Very rough, catches a fingernail", "Cost multiplier (vs as-machined)": "Included", "Typical application": "Hidden surfaces, pre-coating, weld preparation", "Achievable by": "Sawing, rough milling" } ] }, { "caption": "Which Finish Reaches Which Ra", "columns": [ "If your part needs…", "Finish", "Typical Ra target (µm)", "Cost factor" ], "rows": [ { "If your part needs…": "No cosmetic requirement, hidden or internal surface", "Finish": "As-machined", "Typical Ra target (µm)": "0.8–3.2 µm", "Cost factor": "Included" }, { "If your part needs…": "Uniform matte appearance, hide machining marks", "Finish": "Bead blast", "Typical Ra target (µm)": "1.6–6.3 µm", "Cost factor": "1x (baseline)" }, { "If your part needs…": "Uniform matte plus consistent colour before anodize or coating", "Finish": "Bead blast + anodize/coat", "Typical Ra target (µm)": "1.6–3.2 µm", "Cost factor": "1x + coating cost" }, { "If your part needs…": "Directional grain appearance (stainless panels, decorative trim)", "Finish": "Hairline / brush finish", "Typical Ra target (µm)": "0.4–1.6 µm", "Cost factor": "1.2–1.5x" }, { "If your part needs…": "Deburring and slight smoothing on many small parts at once", "Finish": "Vibratory tumbling", "Typical Ra target (µm)": "0.8–3.2 µm", "Cost factor": "0.5–1x (batch)" } ] }, { "caption": "How Much Material Each Finish Removes", "columns": [ "Process", "Material removal per surface", "Removal on a diameter (both sides)", "Can you control it?", "Recommendation" ], "rows": [ { "Process": "As-machined", "Material removal per surface": "0 mm", "Removal on a diameter (both sides)": "0 mm", "Can you control it?": "N/A", "Recommendation": "No allowance needed." }, { "Process": "Bead blasting", "Material removal per surface": "0.002–0.005 mm", "Removal on a diameter (both sides)": "0.004–0.01 mm", "Can you control it?": "Limited — depends on media, pressure, time", "Recommendation": "Negligible for most tolerances. Mask threads and bores." }, { "Process": "Hairline / brush", "Material removal per surface": "0.005–0.02 mm", "Removal on a diameter (both sides)": "0.01–0.04 mm", "Can you control it?": "Moderate — controlled by grit and passes", "Recommendation": "Allow 0.02 mm on brushed faces." }, { "Process": "Vibratory tumbling", "Material removal per surface": "0.01–0.05 mm", "Removal on a diameter (both sides)": "0.02–0.1 mm", "Can you control it?": "Low — batch process, hard to control precisely", "Recommendation": "Allow 0.02–0.05 mm over the batch range." }, { "Process": "Sanding / grinding", "Material removal per surface": "0.01–0.1 mm", "Removal on a diameter (both sides)": "0.02–0.2 mm", "Can you control it?": "Good — controlled by grit progression", "Recommendation": "Allow 0.02–0.05 mm for fine grinding." }, { "Process": "Polishing", "Material removal per surface": "0.01–0.1 mm", "Removal on a diameter (both sides)": "0.02–0.2 mm", "Can you control it?": "Moderate — depends on starting surface", "Recommendation": "Allow 0.02–0.05 mm for polish from machined." }, { "Process": "Mirror polishing", "Material removal per surface": "0.02–0.1 mm", "Removal on a diameter (both sides)": "0.04–0.2 mm", "Can you control it?": "Moderate — multi-stage, cumulative", "Recommendation": "Allow 0.05 mm minimum on mirror-polished surfaces." }, { "Process": "Lapping", "Material removal per surface": "0.005–0.05 mm", "Removal on a diameter (both sides)": "0.01–0.1 mm", "Can you control it?": "Very good — controlled by time and abrasive", "Recommendation": "Lapping is often used to achieve final dimensional accuracy." } ] }, { "caption": "Features You Should Mask Before Finishing", "columns": [ "Feature type", "Risk level", "Recommendation" ], "rows": [ { "Feature type": "Press-fit bores (H7/p6)", "Risk level": "High", "Recommendation": "Mask or do not finish these surfaces. Even 0.01 mm removal changes the interference fit." }, { "Feature type": "Threaded holes", "Risk level": "Medium", "Recommendation": "Bead blasting is usually fine (minimal removal). Polishing removes thread peaks and may cause a loose fit. Mask or avoid." }, { "Feature type": "Shaft diameters in bearings", "Risk level": "High", "Recommendation": "Do not polish or blast bearing journals unless you account for the removal in the tolerance stack." }, { "Feature type": "Sealing surfaces (O-ring grooves)", "Risk level": "Medium", "Recommendation": "Sanding or lapping sealing faces is fine if controlled. Avoid blasting (media embedment)." }, { "Feature type": "Flatness-critical faces", "Risk level": "High", "Recommendation": "Hand polishing creates waviness on large flat surfaces. Use machine grinding or lapping for flatness requirements." }, { "Feature type": "External cosmetic surfaces", "Risk level": "Low", "Recommendation": "Dimensional removal is acceptable on appearance surfaces as long as it is within the profile tolerance." } ] } ] }