How much thickness does anodizing add to aluminum parts?
How Much Thickness Does Anodizing Add to Aluminum Parts?
Anodizing thickness and actual dimensional growth
Anodizing usually adds about half of the specified coating thickness as outward dimensional growth on each exposed aluminum surface, while the rest grows into the base metal. Type II sulfuric anodizing is often specified around 8-25 µm total film thickness, and Type III hard anodizing is often specified around 25-100 µm total film thickness, depending on alloy, process, color, sealing, and standard. The exact dimensional change must be confirmed with the anodizer because alloy chemistry, racking contact, current density, masking, and feature geometry can change local build-up. Buyers should state whether drawing dimensions apply before anodizing or after anodizing, especially for bores, slots, threads, grooves, pins, and sliding fits. This note is more useful than a general “allow for anodize” comment because it tells machining, finishing, and inspection teams which condition controls acceptance.
Type II (Decorative anodizing) is commonly used for color, appearance, moderate corrosion resistance, and light wear protection. A 20 µm Type II film may create roughly 10 µm outward growth per exposed side as a screening estimate, but the buyer should not treat that ratio as a guaranteed tolerance. Clear, black, and dyed finishes can behave differently after sealing, and cosmetic requirements may restrict touch marks or racking locations. On tight CNC parts, call out critical masked faces, post-anodize dimensions, and whether plug gauges or thread gauges are required after finishing. Cosmetic surfaces should be separated from functional datums so a color decision does not accidentally change a bearing or seal fit.
Type III (Hard anodizing) is used when wear resistance, higher hardness, or more severe corrosion exposure matters. A 50 µm hardcoat may create roughly 25 µm outward growth per exposed side, which can reduce a bore diameter by about 50 µm if both sides of the bore are coated. That change can lock up bearings, pins, bushings, sealing glands, and threaded assemblies. Hardcoat also increases surface hardness while leaving the aluminum substrate unchanged, so the design should still check base-metal strength, fatigue risk, edge condition, and coating thickness limits. For sliding parts, the RFQ should define the mating material, lubrication, target clearance, and whether running clearance is verified after finishing.
For example, if a Type III anodized part has a 50 µm total coating thickness, a flat outside face may grow outward by about 25 µm, while an internal bore may lose about 50 µm in diameter when both bore walls are coated. This is why a drawing note must define the acceptance condition. A note such as “dimension applies after anodizing” tells the CNC supplier to machine allowance into the part. A note such as “mask bore before anodizing” tells the finishing supplier to protect that feature instead of compensating by machining. Without that decision, a part can be correct before finishing and fail assembly after finishing. The same logic applies to dowel holes, O-ring grooves, counterbores, and threads because coating can change fit even when the nominal coating thickness looks small.
Design controls for CNC machined aluminum parts
When designing tight-tolerance aluminum components, account for anodizing buildup before final machining dimensions are released. Critical features include mating surfaces, precision bores, dowel holes, bearing seats, threaded holes, sealing grooves, sliding rails, and datum faces. At Neway, the useful RFQ discussion should define coating type, target thickness range, masked areas, post-finish dimensions, and inspection tools rather than relying on a generic “black anodized” note. Aluminum 6061 is often easier to anodize consistently than some high-copper aluminum grades, while 7075 may need closer color and corrosion review because alloying elements affect anodized appearance and coating behavior. The buyer should send a drawing that separates cosmetic surfaces from functional surfaces, identifies threads that need plug control, and states whether thickness is measured by coating gauge, cross-section, or agreed process certification. If a feature is too important to coat, define the masking boundary and the acceptable transition line.
Connect machining allowance, masking, and anodizing inspection
Neway-related RFQs that include CNC machining and aluminum anodizing services should treat finishing as part of the manufacturing route, not a final decoration step. The order should identify the anodizing type, color, sealing requirement, target thickness, tolerance stack, masking map, racking restrictions, and post-finish inspection report. If the part has threaded holes, bearing fits, O-ring grooves, or precision dowel holes, ask whether those features should be machined undersize or oversized, masked, chased after anodizing, or inspected only after finishing. A first-article check should compare coating thickness, bore size, thread fit, racking marks, and appearance before the same allowance is released to production. The best answer to “how much thickness does anodizing add” is therefore conditional: use the specified film thickness for planning, estimate about half as outward growth per surface, and confirm the final allowance with the anodizer before production machining.
