{"id":29092,"date":"2026-05-22T04:14:15","date_gmt":"2026-05-22T04:14:15","guid":{"rendered":"https:\/\/leweiprecision.com\/"},"modified":"2026-05-23T04:16:53","modified_gmt":"2026-05-23T04:16:53","slug":"cnc-machining-surface-finishes","status":"publish","type":"post","link":"https:\/\/leweiprecision.com\/es\/cnc-machining-surface-finishes\/","title":{"rendered":"CNC Machining Surface Finishes: Anodising, Powder Coating &#038; Engineer&#8217;s Guide"},"content":{"rendered":"<p>For design engineers specifying surface finish on a CNC machined aluminium part, writing &#8216;black anodise&#8217; on a drawing without specifying Type II or Type III is a tolerance-time bomb. Type II anodising grows 4\u201312 \u00b5m of oxide layer \u2014 manageable with the right pre-machining stock allowance. Type III hard anodise grows 50 \u00b5m, half inward and half outward: a 20 mm bore becomes 19.95 mm after Type III. On a bore tolerance of \u00b10.025 mm, that dimensional growth renders the part out of spec before a single test is run. The fix costs $200\u2013$800 in rework per part \u2014 entirely avoidable with a complete surface finish specification.<\/p>\n\n\n\n<p>Surface finish is also not a cost-neutral choice. Powder coating adds 15\u201325% to part cost but provides excellent corrosion protection on steel and aluminium. Electropolishing on stainless steel adds 20\u201340% but produces the biocompatible, low-surface-energy surface required for pharmaceutical and medical equipment. Specifying a finish that is more expensive or more complex than the application requires is common \u2014 understanding what each finish actually provides, and what dimensional changes it imposes, is how engineers control both cost and dimensional compliance.<br><\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Surface Finish Options for CNC Machined Parts: Full Comparison<\/strong><\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th><strong>Finish<\/strong><\/th><th><strong>Material<\/strong><\/th><th><strong>Thickness\/Depth<\/strong><\/th><th><strong>Dimensional Change<\/strong><\/th><th><strong>Corrosion Resist.<\/strong><\/th><th><strong>Cost Add<\/strong><\/th><th><strong>Lo mejor para<\/strong><\/th><\/tr><\/thead><tbody><tr><td>Como mecanizado<\/td><td>Todos<\/td><td>Ra 0.8\u20133.2 \u00b5m tool marks<\/td><td>Zero<\/td><td>Bajo<\/td><td>$0<\/td><td>Engineering validation, internal parts, structural<\/td><\/tr><tr><td>Bead blast<\/td><td>All metals<\/td><td>Surface texture only<\/td><td>Negligible (&lt;0.01 mm)<\/td><td>Low (cosmetic only)<\/td><td>$5\u2013$20\/part<\/td><td>Uniform matte appearance; cosmetic pre-treatment<\/td><\/tr><tr><td>Type II anodise<\/td><td>Al only<\/td><td>4\u201312 \u00b5m oxide layer<\/td><td>+4\u201312 \u00b5m outward (\u224850\/50 in\/out)<\/td><td>Bien<\/td><td>$8\u2013$25\/part<\/td><td>Consumer electronics, general Al parts; colour options<\/td><\/tr><tr><td>Type III hard anodise<\/td><td>Al only<\/td><td>25\u2013125 \u00b5m (std 50 \u00b5m)<\/td><td>+25 \u00b5m outward (50 \u00b5m total)<\/td><td>Excellent; wear-resistant<\/td><td>$20\u2013$60\/part<\/td><td>Aerospace, wear surfaces, hydraulic cylinders<\/td><\/tr><tr><td>Recubrimiento en polvo<\/td><td>Steel, Al<\/td><td>60\u2013150 \u00b5m polymer layer<\/td><td>+60\u2013150 \u00b5m<\/td><td>Excelente<\/td><td>$12\u2013$40\/part<\/td><td>Industrial, outdoor, coloured structural parts<\/td><\/tr><tr><td>Passivation (ASTM A967)<\/td><td>Stainless<\/td><td>Chemical only; no thickness<\/td><td>Zero<\/td><td>Excellent (restores Cr2O3)<\/td><td>$5\u2013$15\/part<\/td><td>Required after all SS machining; food\/medical<\/td><\/tr><tr><td>Electropulido<\/td><td>Stainless, Al<\/td><td>Removes 5\u201340 \u00b5m<\/td><td>\u22125\u201340 \u00b5m (removing material)<\/td><td>Excellent; low surface energy<\/td><td>$15\u2013$60\/part<\/td><td>Pharmaceutical, medical, sanitary applications<\/td><\/tr><tr><td>Zinc plate (electroplate)<\/td><td>Acero<\/td><td>5\u201325 \u00b5m zinc<\/td><td>+5\u201325 \u00b5m<\/td><td>Good (sacrificial)<\/td><td>$8\u2013$20\/part<\/td><td>Indoor steel, fasteners, general corrosion protection<\/td><\/tr><tr><td>Hard chrome (decorative)<\/td><td>Steel, Al<\/td><td>5\u201330 \u00b5m Cr<\/td><td>+5\u201330 \u00b5m<\/td><td>Excelente<\/td><td>$25\u2013$80\/part<\/td><td>Wear-resistant shafts, sliding components<\/td><\/tr><tr><td>Black oxide<\/td><td>Acero<\/td><td>1\u20132 \u00b5m<\/td><td>Negligible<\/td><td>Low (oil-impregnated)<\/td><td>$5\u2013$12\/part<\/td><td>Internal steel parts, reduced light reflection<\/td><\/tr><tr><td>Chemical conversion (Alodine)<\/td><td>Al<\/td><td>&lt;1 \u00b5m conductive oxide<\/td><td>Negligible<\/td><td>Good (conductive)<\/td><td>$5\u2013$12\/part<\/td><td>EMI enclosures; pre-treatment before paint\/primer<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Lewei Precision provides all standard CNC surface finishes through our in-house and partner finishing network. Our <a href=\"https:\/\/leweiprecision.com\/es\/servicios\/mecanizado-de-precision\/\">mecanizado de precisi\u00f3n<\/a> process accounts for finish-specific dimensional stock in the machining programme \u2014 we machine to the pre-finish dimension, not the drawing nominal, to ensure post-finish dimensions are within tolerance.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Type II vs Type III Anodising: The Dimensional Impact Engineers Miss<\/strong><\/h2>\n\n\n\n<p>The anodising process converts aluminium surface material into aluminium oxide (Al2O3). Approximately 50% of the oxide layer grows outward from the original surface; 50% grows inward by converting existing aluminium. This &#8217;50\/50 rule&#8217; (MIL-A-8625F) has critical dimensional consequences:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th><strong>Especificaci\u00f3n<\/strong><\/th><th><strong>Oxide Thickness<\/strong><\/th><th><strong>Outward Growth<\/strong><\/th><th><strong>Dimensional Impact on \u00b10.025 mm Bore<\/strong><\/th><th><strong>Required Pre-Anodise Stock Allowance<\/strong><\/th><\/tr><\/thead><tbody><tr><td>Type II Clear (undyed)<\/td><td>4\u20138 \u00b5m<\/td><td>2\u20134 \u00b5m outward<\/td><td>Bore becomes ~4 \u00b5m smaller \u2014 within \u00b10.025 mm<\/td><td>Leave 0.003\u20130.005 mm oversized on precision bores<\/td><\/tr><tr><td>Type II Black (dyed)<\/td><td>8\u201312 \u00b5m<\/td><td>4\u20136 \u00b5m outward<\/td><td>Bore ~6 \u00b5m smaller \u2014 within \u00b10.025 mm with planning<\/td><td>Leave 0.005\u20130.008 mm oversized<\/td><\/tr><tr><td>Type III 25 \u00b5m spec<\/td><td>25 \u00b5m<\/td><td>12.5 \u00b5m outward<\/td><td>Bore ~13 \u00b5m smaller \u2014 may violate \u00b10.025 mm spec<\/td><td>Leave 0.012\u20130.015 mm oversized<\/td><\/tr><tr><td>Type III 50 \u00b5m std<\/td><td>50 \u00b5m<\/td><td>25 \u00b5m outward<\/td><td>Bore ~25 \u00b5m smaller \u2014 VIOLATES \u00b10.025 mm spec<\/td><td>Leave 0.025\u20130.030 mm oversized \u2014 or post-finish ream<\/td><\/tr><tr><td>Type III 75 \u00b5m heavy<\/td><td>75 \u00b5m<\/td><td>37.5 \u00b5m outward<\/td><td>Bore ~38 \u00b5m smaller \u2014 violates \u00b10.05 mm spec<\/td><td>Leave 0.038\u20130.045 mm oversized \u2014 post-finish bore required<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>The engineering action: for any critical bore, shaft, thread, or press-fit feature that must hold tolerance after anodising \u2014 specify the post-anodise dimension on the drawing AND the anodise specification. The machinist machines to a pre-anodise stock allowance; the finish brings the part to drawing nominal.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Powder Coating: Thickness and Tolerance Impact<\/strong><\/h2>\n\n\n\n<p>Powder coating applies 60\u2013150 \u00b5m of polymer to the part surface. Unlike anodising (which converts material), powder coating adds material on top. For parts with critical dimensions on coated surfaces \u2014 enclosure lid interfaces, thread clearances, or mating surfaces \u2014 the added thickness must be masked before coating or accounted for in pre-coating machining stock.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Thread clearance: metric threads in coated steel housings must be chased (re-tapped) after powder coating if the coating enters thread relief areas \u2014 powder in threads prevents fastener engagement<\/li>\n\n\n\n<li>Mating surface interfaces: if two powder-coated aluminium panels must mate with a precision seal, mask the sealing faces during coating or increase mating face clearance by 0.15\u20130.30 mm to accommodate the coating thickness<\/li>\n\n\n\n<li>Press-fit bores: mask press-fit bores before powder coating \u2014 coating inside a press-fit bore changes the interference fit by 60\u2013150 \u00b5m, which is catastrophic for designed press-fit interference of 0.01\u20130.05 mm<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Passivation vs Electropolishing for Stainless Steel<\/strong><\/h2>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><th><strong>Propiedad<\/strong><\/th><th><strong>Passivation (ASTM A967)<\/strong><\/th><th><strong>Electropolishing<\/strong><\/th><\/tr><\/thead><tbody><tr><td>Proceso<\/td><td>Citric or nitric acid removes free iron, restores Cr2O3 passive layer<\/td><td>Electrochemical removal of 5\u201340 \u00b5m of surface material<\/td><\/tr><tr><td>Dimensional change<\/td><td>Zero \u2014 purely surface chemistry, no material removal<\/td><td>Removes 5\u201340 \u00b5m (can affect tight tolerances \u2014 specify on drawing)<\/td><\/tr><tr><td>Surface finish improvement<\/td><td>None \u2014 does not improve Ra<\/td><td>Yes \u2014 removes microscopic peaks; Ra improves 30\u201350%<\/td><\/tr><tr><td>Resistencia a la corrosi\u00f3n<\/td><td>Restores standard SS passive layer<\/td><td>Superior \u2014 smoothed surface has fewer corrosion initiation sites<\/td><\/tr><tr><td>Biocompatibility<\/td><td>Adequate for instrument-grade surfaces<\/td><td>Superior \u2014 required for implantable and pharmaceutical wetted surfaces<\/td><\/tr><tr><td>Coste<\/td><td>$5\u2013$15\/part<\/td><td>$15\u2013$60\/part<\/td><\/tr><tr><td>Required for<\/td><td>All machined SS parts in food, medical, pharmaceutical<\/td><td>Medical implants, pharmaceutical process equipment, sanitary surfaces<\/td><\/tr><tr><td>Application note<\/td><td>Mandatory after all SS machining \u2014 not optional<\/td><td>Specify pre\/post-electropolish dimensions; dimensional change must be planned<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Surface Finish Selection Decision Tree<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Aluminium, cosmetic\/consumer, colour required \u2192 Type II anodise (specify pre-anodise dimension on precision features)<\/li>\n\n\n\n<li>Aluminium, wear\/hydraulic\/aerospace \u2192 Type III hard anodise (specify pre-anodise stock allowance; post-finish ream precision bores)<\/li>\n\n\n\n<li>Aluminium, <a href=\"https:\/\/www.reddit.com\/r\/woodworking\/comments\/qofahn\/emi_shielding\/\" target=\"_blank\" rel=\"noopener\">EMI shielding enclosure<\/a> \u2192 Alodine (chemical conversion coating \u2014 conductive, &lt;1 \u00b5m, no dimensional impact)<\/li>\n\n\n\n<li>Steel, indoor general engineering \u2192 Zinc plate or black oxide with oil<\/li>\n\n\n\n<li>Steel, outdoor\/industrial \u2192 Powder coat (mask threads, press fits, and mating precision surfaces)<\/li>\n\n\n\n<li>Stainless steel, any application \u2192 Passivation mandatory (ASTM A967); electropolish additionally for pharmaceutical or medical<\/li>\n\n\n\n<li>Any metal, engineering validation only \u2192 As-machined (save finish cost until design is frozen)<\/li>\n<\/ul>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Preguntas frecuentes<\/strong><\/h2>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>What is the difference between Type II and Type III anodising?<\/strong><\/h3>\n\n\n\n<p>Type II (decorative\/standard) anodising produces an aluminium oxide layer of 4\u201312 \u00b5m \u2014 2\u20136 \u00b5m of outward dimensional growth. Type III (hard anodise) produces 25\u2013125 \u00b5m of oxide, standard 50 \u00b5m \u2014 12.5\u201362.5 \u00b5m of outward dimensional growth. Type III provides 10\u00d7 more wear resistance than Type II but imposes dimensional changes that must be accounted for in pre-anodise machining stock. Type II is suitable for corrosion protection and cosmetic applications. Type III is required for wear contact surfaces, hydraulic cylinder bores, aerospace sliding interfaces, and applications requiring maximum surface hardness (HV 400\u2013600 vs HV 60\u201370 for untreated aluminium).<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Does powder coating affect part tolerances?<\/strong><\/h3>\n\n\n\n<p>Yes. Powder coating adds 60\u2013150 \u00b5m to every coated surface. On precision mating surfaces, thread bores, press-fit interfaces, and seal faces \u2014 the coating thickness directly affects fit. Solutions: mask precision features before coating (adds $5\u2013$25 per masking step); machine to a pre-coating stock allowance on critical features (coordinate with the coating supplier for expected thickness on your part geometry); or re-machine \/ re-tap after coating (adds one operation per feature). Always specify masking requirements on the drawing or purchase order \u2014 not verbally.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\"><strong>Is passivation required after CNC machining stainless steel?<\/strong><\/h3>\n\n\n\n<p>Yes \u2014 passivation per ASTM A967 (citric acid or nitric acid) is required after all <a href=\"https:\/\/leweiprecision.com\/es\/servicios\/mecanizado-cnc\/\">Mecanizado CNC<\/a> operations on stainless steel for food-contact, medical, and pharmaceutical applications. CNC machining disrupts the natural chromium oxide passive layer on stainless steel by embedding iron from tooling, coolant, and fixture contact. These iron deposits cause surface rust and galvanic corrosion in service. Passivation dissolves the free iron and restores the full Cr2O3 passive layer, providing the corrosion resistance the alloy is specified for. Passivation adds $5\u2013$15 per part and 1\u20132 days of lead time \u2014 always include it in the manufacturing process for SS food and medical parts.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\"><strong>Conclusion: Specify the Complete Finish \u2014 Including the Dimensional Impact<\/strong><\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>&#8216;Black anodise&#8217; is not a complete finish specification \u2014 specify Type II or Type III, thickness (\u00b5m), and pre-anodise stock allowance on all precision features<\/li>\n\n\n\n<li>Type III hard anodise grows 25 \u00b5m outward at 50 \u00b5m spec \u2014 a \u00b10.025 mm bore will be out of tolerance after Type III without pre-anodise stock allowance<\/li>\n\n\n\n<li>Passivation is mandatory after CNC machining stainless steel for food\/medical\/pharmaceutical \u2014 not optional<\/li>\n<\/ul>","protected":false},"excerpt":{"rendered":"<p>For design engineers specifying surface finish on a CNC machined aluminium part, writing &#8216;black anodise&#8217; on a drawing without specifying Type II or Type III is a tolerance-time bomb. Type II anodising grows 4\u201312 \u00b5m of oxide layer \u2014 manageable with the right pre-machining stock allowance. Type III hard anodise grows 50 \u00b5m, half inward [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_seopress_robots_primary_cat":"","_seopress_titles_title":"","_seopress_titles_desc":"","_seopress_robots_index":"","_seopress_analysis_target_kw":"","footnotes":""},"categories":[13],"tags":[],"class_list":["post-29092","post","type-post","status-publish","format-standard","hentry","category-cnc-machining"],"acf":[],"_links":{"self":[{"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/posts\/29092","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/comments?post=29092"}],"version-history":[{"count":2,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/posts\/29092\/revisions"}],"predecessor-version":[{"id":29094,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/posts\/29092\/revisions\/29094"}],"wp:attachment":[{"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/media?parent=29092"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/categories?post=29092"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/leweiprecision.com\/es\/wp-json\/wp\/v2\/tags?post=29092"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}