Anodizing is the finish most CNC machined aluminum parts receive, and it is a geometry decision, not a cosmetic step. The process grows a porous aluminum oxide layer from the surface, which improves wear and corrosion protection and accepts dyes, but the coating changes dimensions, follows the machined surface, and varies with the alloy. The drawing decisions that prevent rework are the coating thickness callout, the tolerance state relative to the coating, the mask plan, and the color acceptance process. This guide covers those decisions with a worked thickness example, the alloy-to-color variation that surprises teams, and the masking map that keeps critical features functional.
What the Coating Does to the Dimension
Anodizing converts the aluminum surface into oxide, and the coating grows from the surface: roughly half the thickness builds into the material and half above the original surface. Every coated surface therefore changes dimension, and the change is predictable only if the coating thickness is specified and the machining accounts for it.
A worked example shows the arithmetic. A bore that must fit a bearing at 20.000 mm after Type II anodizing with a 10 µm coating grows inward by about half the coating on each side. If the coating is 10 µm total and the growth splits roughly evenly, the bore shrinks by about 5 µm per side, or 10 µm on the diameter, so the bore must be machined to about 20.010 mm before coating. The numbers vary with the alloy and the process, but the structure of the calculation is the same for every coated feature: define the final size, define the coating, and machine the pre-coat size to land the final fit.
The drawing must say which state the tolerances apply to. "Anodize after machining" without a direction leaves the fit to chance; "bore finished after anodize" tells the machinist to leave stock and final-machine after coating, which adds a step. The practical rule is to decide per feature: threads and press-fit bores are usually masked or accounted for, while cosmetic surfaces take the coating growth without issue.
Type II vs Type III: The Coating Selection
| Type | Typical thickness | What it gives | Where it fits |
|---|---|---|---|
| Type II (sulfuric) | 5–25 µm | Corrosion protection, dyeability, moderate wear | Cosmetic and general parts |
| Type III (hard coat) | 25–75 µm or more | High wear resistance and hardness | Wear surfaces, tooling, high-use parts |
Type II is the default for appearance and general protection, and it accepts the full dye range. Type III is thicker, denser, and harder, used where the surface must slide or wear, with a thicker dimensional change, a rougher surface, and a limited color range, black and gray being common. The selection starts from the surface function: a part that slides against another component is a hard-coat candidate, and a part that just needs to look right is Type II. Specifying hard coat for cosmetics adds cost without value.
Masking: The Map That Keeps Features Functional
The mask plan is the map of what not to coat. Threads, press-fit bores, datum surfaces, and grounding points are the usual mask targets, and the drawing should define them before the finish order.
Threads deserve the first decision. Anodizing a thread changes its pitch diameter, and the coating can make a Class 2A thread bind. The three approaches are masking the thread, accounting for the coating in the machining, or chasing the thread after anodizing, and the choice should be on the drawing. For Type III, the thicker coating makes the mask decision more critical: a hard-coated thread with no allowance is a rework candidate.
Press-fit and bearing bores follow the same logic. If the bore must land a press-fit pin, either mask it or machine with the coating allowance, and the drawing should label the condition, because the supplier cannot know whether to allow for oxide growth or mask the bore. Datum surfaces that the next operation references should also be protected, because a coated datum changes the measurement baseline.
Alloy-to-Color: Why Two Batches of 6061 Differ
The alloy changes the anodizing result, and color is where the difference shows. 6061 and 6063 anodize consistently and are the standard for cosmetic work; 7075 contains more alloying elements and anodizes with a different, often darker or streaked, appearance; and cast or high-silicon alloys anodize poorly and are usually painted instead. A color approved on 6061 may not repeat on 7075, so the alloy should be confirmed against the finish requirement before a color is committed.
Even within one alloy, batches differ. The final shade depends on the alloy temper, the surface preparation, the coating thickness, and the dye and seal process, so two 6061 batches can produce different black shades if any of those variables shift. The controls are a signed color standard, the same process parameters, and an acceptance tolerance against the standard. The batch record should capture the alloy, temper, coating thickness, and finish steps so repeat orders reproduce the result.
Rack Marks and Surface Preparation
Rack marks are a fact of anodizing: the part contacts a rack to carry current, and the contact point does not coat the same as the rest of the surface. The drawing can control where the marks land by specifying rack points on non-cosmetic surfaces, and the supplier should place contact where the marks will not be visible.
Rack placement is worth agreeing in the DFM, because moving a contact point after the finish order can require a re-run or leave a visible mark on a customer-facing surface. The mask map and the rack plan together define what the coated part looks like and where the evidence of the process is allowed to show.
Surface preparation decides the cosmetic result. The oxide follows the machined surface and can exaggerate roughness, so tool marks and scratches are not hidden by the coating. A part that needs a uniform finish should be machined with the finish in mind, and any conditioning, polishing or bead blasting, should be specified before the anodize step. The finish callout should include the measurement method, because anodized roughness is measured on the coated surface, not the machined surface.
Design and Specification Checklist
- Final coated size defined for press-fit and bearing features
- Coating thickness specified with an acceptance method
- Tolerance state stated: before anodize, after anodize, or masked
- Threads assigned an approach: mask, allowance, or chase
- Mask map drawn for datum surfaces and grounding points
- Alloy confirmed against the color and appearance requirement
- Color standard signed against the actual process and alloy
- Rack points placed on non-cosmetic surfaces
- Surface preparation specified before coating
Conclusion
Anodizing is a finish that participates in the geometry: the coating changes dimensions, follows the surface, and varies with the alloy. Define the final size and the coating thickness together, mask the functional features, and control color with a signed standard against the actual process. A surface finishing step designed in costs little; the same step added after production costs rework, re-machining, and schedule.
FAQs
Should tolerances be specified before or after anodizing?
The drawing must say which state the tolerances apply to. If the final fit matters, specify the after-anodize size and machine the pre-coat size with the coating allowance; if the tolerance is intended before coating, label that condition explicitly. Threads and press-fit bores need a defined approach because the coating changes their size.
Which features should be masked during Type III hard anodizing?
Threads, press-fit bores, datum surfaces, and grounding points are the usual mask targets, and the thicker hard-coat layer makes the decision more critical. The mask map should be on the drawing, because a hard-coated thread with no allowance or a coated datum is a rework candidate.
Can an anodized threaded hole pass a GO/NO-GO gauge after coating?
Only if the thread was machined with a coating allowance or masked before anodizing. The coating adds thickness and changes the pitch diameter, so a thread machined to nominal size before coating can bind the GO gauge afterward. The drawing should state the approach and the inspection point relative to coating.
Why can two 6061 batches produce different black-anodize shades?
The final shade depends on the alloy temper, surface preparation, coating thickness, and the dye and seal process, and any of these variables can shift between batches. The controls are a signed color standard, consistent process parameters, and an acceptance tolerance, with the batch record capturing the variables so repeat orders reproduce the result.
Sources
- 6CProto Surface Finishing
- 6CProto CNC Machining Services
- 6CProto Standards and Tolerances
- ISO 1302:2002 – Geometrical product specifications, surface texture
- ISO 9001:2015 – Quality management systems