Machining 6061 for Anodize: Pre-Anodize Dimensions, Surface Finish, and Masking Decisions (2026)

Pre-anodize machining allowance is the material you leave on a 6061 feature so the finished dimension lands in tolerance after the oxide film grows. MIL-A-8625F sets that film at 0.00007 to 0.0010 in for Type II and 0.0005 to 0.0045 in for Type III, with 0.002 in as the Type III default. Half the film grows into the base metal and half outward, so a bore closes by the full film thickness and a shaft opens by the same amount.

A 6 mm H7 bore carries a 12 μm tolerance band. A 5 μm Type II film consumes 42 percent of it. A 50 μm Type III film closes that bore by more than four times the band, and the pin will not enter.

This guide covers the three checks that decide whether a 6061-T6 part survives the move from cutter to tank. Feature type, geometry, process route, setup, surface finish, quantity, and inspection method all shift the number you apply. Alloy and temper sit upstream, in the 6061 aluminum properties reference.

1. What the Three-S Handoff covers

The Three-S Handoff is three checks you run before a 6061 part leaves the machine for the anodize tank. Size sets the dimensional allowance. Surface sets the Ra you leave behind. Shield sets which features get no film at all. Each check has one drawing note that enforces it and one cost that lands on you when it is missing.

Size runs first because the allowance has to sit in the CAM program before the first roughing pass. Surface runs second because the prep you pick eats into Size. Shield runs last because masking and rack points are drawing instructions you can add without touching a toolpath, though plug masking a threaded hole can push a job out of EPOC CRAFTER’s 3 to 7 business day single-step anodizing window into the 7 to 14 day multi-step window.

Engineering note. When a feature carries a total tolerance band under 30 μm and the drawing calls Type III, mask it instead of machining an allowance, because film thickness varies across a rack and that variation eats the band before nominal growth is counted. The exception is a through bore you can hone afterward. Blind bores and features carrying a concentricity tolerance have no such recovery.

Table 1. The Three-S Handoff

Check	What it controls	Failure you see on the part	Drawing note that enforces it	What skipping it costs
1. Size	Allowance per surface. 2.5 to 12.5 μm for Type II, 12.5 to 37.5 μm for Type III	Bores undersize, shafts oversize, threads bind on the go gauge	Statement of whether dimensions apply before or after anodize	Whole-lot rework. Type III correction needs honing or lapping, not milling
2. Surface	Ra left by the last cutting pass, plus the prep that follows	Tool marks, concentric shade rings, and batch to batch color drift under a clear or dyed film	Ra value on cosmetic faces, plus the named prep method	A second operation. Blast or tumble adds a step and shifts Size again
3. Shield	Features that must receive no film, and where the rack touches	Threads the fastener will not enter, bare witness marks on an A surface	Mask list, plus an acceptable rack contact zone	Late per-part masking charge, or scrapped cosmetic parts

2. Size, how far each feature moves in the tank

Anodizing changes every unmasked dimension on a 6061 part because the oxide film consumes base metal and builds outward at once. A bore loses the full film thickness on diameter, a shaft gains it, a flat face moves outward by half. Type II moves parts 5 to 25 μm, Type III 25 to 75 μm.

2.1 Film thickness and the half in rule

About half the film thickness grows into the aluminum and half stands above the original surface. The half that stands above is what moves your dimension.

Direction matters more than magnitude. On an external surface the film adds outward, so a shaft grows by the full film on diameter. On an internal surface it adds toward the axis, so a bore shrinks by the full film. Two features cut to the same nominal move opposite ways in the same tank.

Threads move worst. Radial buildup on a 60 degree flank changes pitch diameter by four times the radial thickness, so a 50 μm Type III film pulls 100 μm off an internal pitch diameter. A 2B class thread has nowhere near that room, which is why threads get masked, not sized.

Enclosed geometry runs thicker than open faces and sharp corners run thinner, which is why a 0.3 to 0.5 mm edge break belongs on any part going to Type III. Check the values below against your own machining tolerance standards.

Table 2. Where each 6061 feature goes after anodize

Feature	Direction	Dimensional change	Type II (5 to 25 μm)	Type III (25 to 75 μm)	What to do
Shaft or boss OD	Grows	Full film on diameter	+5 to +25 μm	+25 to +75 μm	Machine undersize by the film value
Bore or pocket ID	Shrinks	Full film on diameter	−5 to −25 μm	−25 to −75 μm	Machine oversize, or mask
Single flat face	Grows outward	Half film per surface	+2.5 to +12.5 μm	+12.5 to +37.5 μm	Adjust only if the face is a datum
Slot or keyway width	Shrinks	Full film across the width	−5 to −25 μm	−25 to −75 μm	Machine wide by the film value
Internal thread PD	Shrinks	4x radial buildup	−10 to −50 μm	−50 to −150 μm	Mask, or tap after anodize
Blind bore or deep pocket	Shrinks more	Film runs 10 to 50 μm on Type II	−10 to −50 μm	Confirm with the finisher	Mask if toleranced
Sharp external corner	Grows less	About 80 percent of flat buildup	+2 to +10 μm	+10 to +30 μm	Break the edge 0.3 to 0.5 mm

2.2 Why bores fail first

Bores fail before any other feature because a fit-class bore carries the tightest band on the drawing and the film closes it from every side at once. A 20 μm Type II film consumes the entire 12 μm band on a 6 mm H7 bore and leaves the pin outside the hole.

Film variation compounds the nominal number. Hard anodize build measured across one rack moves 12 to 20 μm on a job set to 50 μm, so two parts from one batch land at different sizes. That spread turns a calculated allowance into a sorting operation.

Warp adds a second source of drift on thin sections, and it arrives before the tank. A part that moved during finish machining is already off nominal when you apply the allowance, which is why temper choice belongs upstream and is covered in T651 temper for low-distortion machining.

2.3 The pre-anodize sizing calculation

Pre-anodize target size equals final size plus the full film thickness for a bore, and final size minus it for a shaft. A Ø20.000 mm H8 bore with a +0.033/0 band taking a 20 μm Type II film becomes a pre-anodize target of Ø20.020 mm, cut at 20.020 to 20.033 mm so that film variation on the high side still lands inside tolerance after the bore closes.

Run the same bore against Type III at 50 μm and the method breaks. Pre-anodize target moves to Ø20.050 mm, and a rack spread of 15 μm against a 33 μm band leaves nothing for machining variation. Mask that bore instead.

According to Dewey Wu, General Manager and senior mechanical engineer at EPOC CRAFTER, “We hold Type II to ±2 μm and a 50 μm Type III run to ±5 μm, verified by eddy current to ASTM B244 at five points spread across the rack. We still mask any feature with a total band under 30 μm, because controlling thickness is not the same as controlling where it lands. A part racked at the top of the fixture sees a different current density than one at the bottom.”

Engineering note. When the film is Type II and the feature band is wider than 30 μm, machining an allowance holds the size for nothing beyond a CAM offset. Under 30 μm, or on Type III at any band, masking is the move. The exception is a through bore with no concentricity callout, where honing 12 to 25 μm of oxide back off recovers the size on parts already held to ±0.01 mm on toleranced features. Honing does not recover a blind bore or a bore carrying position or runout.

3. Surface, the Ra you must leave before the tank

A Type II film is transparent, so it magnifies whatever the last cutting pass left instead of hiding it. An as-machined 6061 face runs Ra 1.6 to 3.2 μm, which reads as visible tool marks under a clear or dyed film. Cosmetic faces need a named prep step before the tank, and that step moves the allowance you set in Size.

3.1 What Type II does to an as-machined finish

Anodizing converts the substrate, so it cannot cover a defect the way paint does. Scratches, burrs, weld discoloration, and oil residue on the bare part all show after the tank.

Directional tool marks turn into shade bands, because dye loads deeper into the valleys of the machined profile than into the peaks. A face-milled cover panel comes back striped and a bored sensor housing comes back with concentric rings, both inside tolerance and both rejected on appearance.

Alloy chemistry sets the ceiling. 6061 holds copper at 0.15 to 0.40 percent per ASTM B209/B209M-21a, low enough to take dye evenly, which is why it is the grade to specify when an A surface has to match. Higher zinc grades show grain through the film, covered in 6061 vs 7075 machinability and the aluminum grade selection logic.

Color holds to ΔE 2 or better inside one controlled run, and two runs separated by weeks can still drift past that. Assemblies that have to match visually go into one batch, and the drawing states it.

Table 3. Pre-anodize prep options on 6061

Prep	Ra after prep	Effect on size	Look under Type II	Cost and lead time	Choose when
As-machined, no prep	1.6 to 3.2 μm	None	Tool marks and shade bands visible	Single-step finish, 3 to 7 business days	Internal or hidden faces
Caustic etch	Matte, not controlled	Pulls about 12 μm per face	Uniform matte, grain shows if over-etched	Included in the anodize line	Non-toleranced faces needing a blended look
Bead blast	1.6 to 3.2 μm	Not a controlled removal	Even matte, standard pre-anodize prep	Adds a step, still inside 3 to 7 days	Most cosmetic housings and brackets
Vibratory tumble	Finer than blast	Rounds edges	Finest matte of the three, tool marks gone	Hours per load, adds a day	Small parts in batches
Brushed	0.4 to 0.8 μm	Light	Linear grain, directional	Multi-step, 7 to 14 days	Trim and panel faces
Mirror polish	0.05 to 0.1 μm	Labor scales with area	Specular, shows every remaining defect	Multi-step, 7 to 14 days	Consumer-facing show surfaces only

3.2 Getting there without a second operation

A finish pass reaches Ra 0.8 to 1.6 μm on 6061-T6 with a sharp cutter, a light stepover, and one cutting direction across the face, which is close enough that a clear film reads as uniform without blasting. The cost is cycle time on the machine instead of a line charge at the finisher.

Prep and allowance are one calculation, not two. A caustic etch takes about 12 μm off every exposed face, so a bore already sized 20 μm oversize for Type II ends up 32 μm oversize before the film starts growing. Pick the prep first, then set the offset.

Engineering note. When the cosmetic face carries no dimensional tolerance, bead blast before anodizing removes tool marks at a fixed line cost and keeps the finish inside the 3 to 7 business day window. When that face is a datum pad or a sealing surface, a finish pass to Ra 0.8 to 1.6 μm holds both the look and the size. The exception is one face carrying both requirements, where you mask the toleranced zone and blast the rest. Dry blasting embeds media in the aluminum and interferes with film formation, so show finishes get wet blast or tumble.

4. Shield, masking and racking decisions

Masking before anodizing removes the film from a feature entirely, so the machined size is the final size and no allowance applies. Racking is the other half of the same decision, since every part touches a fixture somewhere and that contact point comes out bare. Both are drawing instructions, not machining operations.

4.1 Mask or machine after anodize

Mask any feature whose tolerance band is narrower than the film variation you would have to absorb. Plug masking a threaded hole adds a per-part charge and pushes the job toward the 7 to 14 day multi-step window, still cheaper than sorting bound threads. Features not called out on the drawing receive full coverage.

Machining after anodize is the fallback, not the plan. Type III oxide is Al₂O₃ and does not mill, so recovery runs through honing or lapping with silicon carbide or diamond. That is why masking gets decided at drawing release, not after first article, a pattern covered in DFM mistakes that delay CNC prototypes.

Table 4. Mask, size, or recover

Feature	Type II route	Type III route	Impact on cost and lead time
Internal thread	Mask, or tap after anodize	Mask	Per-part plug charge, adds a masking setup
Bearing bore, band under 30 μm	Mask	Mask	Plug or tape, plus an inspection point
Slip-fit pin bore, band over 30 μm	Machine oversize by film value	Mask	CAM offset only on Type II
Datum pad or ground point	Mask	Mask	Electrical grounding needs bare metal anyway
Through bore, no runout callout	Machine oversize	Hone 12 to 25 μm after anodize	Adds a post-finish operation and a second inspection
Blind bore, toleranced	Mask	Mask	No recovery path exists after the tank
Cosmetic outer surface	Full coverage	Full coverage	None

Engineering note. When a bore carries a fit class and the part goes to Type III, mask it and machine to final size, because the 12 to 20 μm rack spread on a hardcoat run leaves no room inside an H7 or g6 band. On Type II with a band over 30 μm, the CAM offset holds it and masking only adds cost. The exception is a masked bore that also needs corrosion protection on its wall, where a post-anodize hone is the better route.

4.2 Where rack marks are allowed to land

Rack contact leaves bare aluminum because the fixture carries current into the part and the film cannot form under the contact point. MIL-A-8625F section 6.2 lists rack point location among the items the purchase document supplies when the location matters to part function, so the standard expects you to name it.

Name a zone, not a point. Mark an area on a non-cosmetic face, away from any fastener seat or sealing groove, then state that contact outside that zone is a reject. Leaving it unstated hands the choice to the finisher, and on an A-surface cover panel that shows as two bare witness marks. Run the zone past the DFM design guidelines for machined features before release.

5. The three drawing notes that make this contractual

Three notes turn the Three-S Handoff into something a finisher is bound by. Each closes a decision the shop would otherwise make for you. The MIL-A-8625F type, class, color, and sealing block sits above these, in the 6061 grade and finish selection guide.

Note 1 fixes the datum in time. Without it the machinist reads the dimensions as finished sizes, cuts to nominal, and the lot sorts after the bore closes.

ALL DIMENSIONS APPLY AFTER ANODIZE UNLESS NOTED.

PRE-ANODIZE MACHINING ALLOWANCE PER FEATURE IS THE

SUPPLIER’S RESPONSIBILITY BASED ON THE SPECIFIED

COATING THICKNESS.

Note 2 is the mask list, named by balloon or hole callout. An omission is an instruction.

MASK BEFORE ANODIZE, NO COATING PERMITTED:

ALL TAPPED HOLES, BORE Ø20 H8 (ITEM 4), DATUM PAD A.

MASKING WITNESS EDGE ACCEPTABLE WITHIN 0.5 mm.

Note 3 is the rack zone, given as an area with a reject condition, since bare contact marks are unavoidable and only their location is negotiable.

RACK CONTACT PERMITTED ONLY WITHIN ZONE R (SEE DETAIL B).

CONTACT MARKS ON SURFACES MARKED A-SURF ARE CAUSE

FOR REJECTION.

Engineering note. When a part ships above roughly 50 pieces and repeats, write all three notes at release, because the per-lot cost of a masking discussion and a rack decision recurs every time and shows up in low volume production runs as quote variance. On a one-off appearance model with no fit features, Note 1 alone carries the job. The exception is a cosmetic housing with a Pantone target, where a bare witness mark fails the part whatever the sizes measure, which is the norm on consumer electronics housings with cosmetic A-surfaces.

6. Pre-anodize release checklist

Run this before the drawing goes out, not before the parts ship. Every line is a place a 6061 lot has been sorted or scrapped.

Size

1. Coating type and class stated, with a thickness value where a fit feature depends on it
2. Every fit-class bore, shaft, slot, and thread listed with its total tolerance band
3. Bands under 25 μm flagged for masking instead of allowance
4. Allowance direction set per feature, bores oversize and shafts undersize, not one global offset
5. Prep removal subtracted before the offset, about 12 μm if a caustic etch is in the route
6. External corners broken 0.3 to 0.5 mm on any part going to Type III

Surface

1. Ra called out on cosmetic faces, with the prep method named
2. Color-matched assemblies grouped into one anodize batch, with a ΔE limit on the drawing
3. Toleranced and cosmetic requirements separated onto different faces, or the boundary accepted

Shield

1. Mask list complete by balloon number, since unlisted features get full coverage
2. Rack zone drawn as an area on a non-cosmetic, non-sealing face, with a reject condition stated
3. Recovery path named for every Type III fit feature, honing or masking

7. Frequently Asked Questions

How much does anodizing add to a 6061 part?

Type II adds 5 to 25 μm per surface on 6061 and Type III adds 25 to 75 μm, per the film thickness ranges in MIL-A-8625F. Direction decides what that means for your feature. An external surface grows by the film value, so a shaft gains the full amount on diameter. An internal surface grows toward the axis, so a bore loses the full amount on diameter. A single flat face moves outward by half. Enclosed geometry runs thicker than open faces, reaching 10 to 50 μm on a Type II run inside a blind pocket. Sharp corners run around 80 percent of flat face buildup.

How much should I oversize a bore before anodizing?

Oversize the bore by the full specified film thickness, then cut to the tight half of the tolerance band. A Ø20.000 mm H8 bore with a +0.033/0 band taking a 20 μm Type II film becomes a pre-anodize target of Ø20.020 mm, machined between 20.020 and 20.033 mm so that film variation on the high side still lands in tolerance after the bore closes. Subtract any prep removal first, since a caustic etch pulls about 12 μm off every exposed face. This method stops working when the band is narrower than about 30 μm or when the coating is Type III, where masking replaces the allowance.

Do threads need to be masked before anodizing?

Mask threads on almost every anodized 6061 part, because radial buildup on a 60 degree flank shifts pitch diameter by four times the radial film thickness. A 50 μm Type III film adds 25 μm on radius and pulls 100 μm off an internal pitch diameter, which no 2B class thread has room to absorb. Type II at 20 μm still removes 40 μm of pitch diameter and binds a go gauge on a tight thread. The two working routes are plug masking before the tank or tapping after anodize. Plug masking adds a per-part charge and a masking setup, and unlisted holes receive full coverage.

Why did my bore come back undersize after hard anodize?

The bore closed by the full film thickness, and hardcoat film varies enough across a rack to consume a fit band on its own. Type III runs 25 to 75 μm, so a bore set at nominal comes back 25 to 75 μm small on diameter. Rack spread on a job targeting 50 μm moves around 12 to 20 μm between parts at the top and bottom of the fixture, because current density is not identical across the rack. An H7 or g6 band cannot hold that. Recovery on a through bore with no runout callout runs through honing 12 to 25 μm of oxide back off with silicon carbide or diamond.

What Ra should a 6061 part be before Type II anodize?

Target Ra 0.8 to 1.6 μm on any face that will be seen through a clear or dyed Type II film. An as-machined 6061 face runs Ra 1.6 to 3.2 μm, which shows tool marks because dye loads deeper into the valleys of the machined profile than into the peaks. A face-milled panel comes back with shade stripes and a bored housing comes back with concentric rings, both inside dimensional tolerance and both rejected on appearance. Reaching Ra 0.8 to 1.6 μm takes a sharp cutter, a light stepover, and one cutting direction across the face. Bead blasting reaches an even matte instead, at a fixed line cost.

Can you machine a part after it has been hard anodized?

Machining Type III oxide means honing or lapping, not milling, because the film is Al₂O₃ and behaves like a ceramic against carbide. Silicon carbide or diamond hones remove 12 to 25 μm from a bore and return it to size. The route works on a through bore carrying only a diameter tolerance. It fails on a blind bore, and it fails on any bore that also carries position, concentricity, or runout, because the film builds unevenly and the axis moves with it. Stripping and re-anodizing is the other option, though it adds a full finishing cycle to the lead time and a second dimensional risk.

Do rack marks have to show on an anodized part?

Bare rack contact marks are unavoidable, and only their location is negotiable. The fixture carries current into the part, so no film forms under the contact point. MIL-A-8625F section 6.2 lists rack point location among the items the purchase document supplies when that location matters to part function, which means the standard expects the drawing to state it. Mark a zone, not a point, place it on a non-cosmetic face away from any sealing groove or fastener seat, and add a reject condition for contact outside that zone. Leaving it unstated hands the choice to the finisher, and on a visible cover panel that shows as two bare witness marks.

Does anodizing on 6061 vary batch to batch?

Color holds to ΔE 2 or better inside one controlled run and can drift past that between runs separated by weeks. Bath chemistry, dye concentration, current density, and aluminum lot variation all move the final shade. 6061 gives a narrower spread than most machinable grades because copper stays at 0.15 to 0.40 percent per ASTM B209/B209M-21a, low enough to take dye evenly. Parts that have to match visually go into one batch, and the drawing states that requirement alongside a ΔE limit. Run a first article color sample and get it approved before a production lot for any part with a Pantone or brand color target.

Related Resources

Related Capability: Anodizing Type II and Type III on aluminum  Film thickness ranges, masking rules, tank capacity, and the 3 to 7 day standard finish window.

Related Article: 6061 aluminum grade and variant selection  Where the MIL-A-8625F type, class, color, and sealing callout block gets written.

Related Material Guide: Aluminum grade selection logic  Which aluminum grades take dye evenly and which show grain through the film.

Related Standards Reference: Machining tolerances and standards  The ISO 2768-m and precision bands your allowance has to fit inside.

Related Case Study: DFM mistakes that delay CNC prototypes  What late masking and finish decisions cost in rework and lead time.

Fix the coating type and thickness first, then work the three checks in order, because an allowance set before the prep method is chosen has to be calculated twice. Send the STEP file and drawing with the twelve items resolved and a DFM review comes back within 12 hours with the allowance already applied per feature.