Introduction
When you order CNC machined parts that require secondary finishing — whether that is anodising, threading, bead blasting, or passivation — the machining and finishing are not separate jobs. They are interdependent. Tolerances set during CNC milling or CNC turning must account for the dimensional changes that happen during finishing, or the parts will not assemble correctly.
This is one of the most common sources of rework across the machining industry in Australia. A bore machined to final size before anodising will shrink out of tolerance. A thread cut to standard pitch diameter before hardcoat will seize. A thin-walled part sent to bead blasting without masking will warp. These are not edge cases — they are the everyday realities of CNC machining services that include secondary operations.
This guide covers the CNC engineering fundamentals that design engineers and procurement teams need to understand before ordering CNC machining with secondary finishes. It draws on Australian standards including AS 1231-2000 for anodising, ASTM A967 and AMS 2700 for passivation, and current regulatory requirements for surface treatment chemicals across machining Australia.
Summary
Key Takeaways
- Anodising changes your dimensions. Type II sulphuric anodising grows outward by roughly one-third of the coating thickness per side. Type III hardcoat grows by roughly half. Parts must be machined to green-state dimensions to hit final tolerances (AS 1231-2000; RivCut, 2025).
- Threads are especially sensitive. The 4x Pitch Diameter Rule means a 25 µm hardcoat adds 0.10 mm to external thread pitch diameter. Cut threads undersize or oversize before coating, or they will not assemble.
- Bead blasting can warp thin parts. Walls under 0.5 mm are at high risk of deformation under blasting pressure. Bores and threads holding ±0.05 mm or tighter must be masked.
- Citric acid passivation is replacing nitric. For stainless steel, citric acid (ASTM A967 Citric 1–5) avoids the flash-attack etching risk of nitric acid while meeting the same corrosion-resistance requirements.
- New Australian regulations affect finishing. The PFAS ban (1 July 2025) and Safe Work Australia’s WEL transition (1 December 2026) are changing which coatings and pretreatments are available domestically.
- Specify finishing requirements at quoting stage. A CNC machining partner who understands your secondary operations can machine to the correct green-state dimensions from the start, avoiding rework.
1. Anodising: Dimensional Growth and How to Machine for It
Anodising is an electrochemical process that converts the surface of an aluminium part into a hard, corrosion-resistant oxide layer. Unlike paint or plating, the coating is not deposited on top — it grows partly outward and partly inward by converting the substrate metal itself (AS 1231-2000). This means every anodised part changes dimensions, and every CNC machining tolerance must account for that change.
Type II vs Type III: What Changes
In standard Type II sulphuric anodising, the coating is typically 5 to 25 µm thick. Approximately one-third of that thickness grows outward from the original surface, while two-thirds penetrates inward. For a 20 µm Type II coating, the external dimension grows by roughly 0.007 mm per side (RivCut, 2025).
Type III hardcoat anodising is denser and thicker — typically 25 to 75 µm. The growth ratio shifts to approximately half outward and half inward. A 50 µm hardcoat adds roughly 0.025 mm per side to external dimensions. This is the finish commonly specified for defence, mining, and heavy-wear applications where surface hardness and abrasion resistance matter.
In Australia, anodised finishes are specified under AS 1231-2000 (Aluminium and aluminium alloys — Anodic oxidation coatings), which defines thickness grades from AA10 through AA25 based on atmospheric exposure. AA20 is the baseline for architectural and engineering applications, while AA25 is required for harsh coastal environments.
Green-State Machining
To achieve correct final dimensions after anodising, parts must be machined to a green state — pre-anodise dimensions that compensate for the outward growth. External features (shafts, block widths) are machined undersize. Internal features (bores, pockets) are machined oversize. The calculation is straightforward: subtract or add the outward growth per side from your target dimension. Getting this wrong is one of the most common failure points in metal machining jobs that include anodising.
Handling After Anodising
AS 1231-2000 Appendix D specifies that anodic coatings must be protected from corrosive agents during transport and installation. Concrete, plaster, and mortar splashes must be cleaned immediately with water. Brick-cleaning acid must never contact anodised aluminium — the surface should be saturated with water beforehand if adjacent masonry is being cleaned. These are not theoretical concerns; they are common on-site failures for components used in construction and infrastructure projects.
2. Threading: Standards, Tolerances, and the Anodising Trap
Threading is one of the most sensitive CNC machining operations when secondary finishes are involved. The V-profile geometry of standard threads means coating growth affects the pitch diameter by a multiplier, not just a simple addition.
Thread Standards in Australia
CNC machining services in Australia work across multiple thread standards: Metric (ISO 261/965) with a 60° thread angle is the default. BSP (British Standard Pipe) governed by AS ISO 7 uses a 55° Whitworth profile and is the dominant pipe thread standard in Australia. NPT (National Pipe Taper) uses a 60° angle with flat crests.
A common procurement risk is the BSP vs NPT trap. Because both BSPT and NPT use a 1:16 taper, a male NPT thread will partially engage a female BSPT connection. But the different thread angles (60° vs 55°) mean the joint will leak under pressure. Always specify the thread standard explicitly on your drawing when ordering CNC machine services or CNC machining service work.
The 4x Pitch Diameter Rule
This is the critical trap. Standard 60° threads have flanks oriented at 30° to the radial axis. Because the anodic coating grows perpendicular to all exposed surfaces, the pitch diameter change is exactly four times the outward build-up per surface. For a 25 µm Type III hardcoat with 50% outward growth (12.5 µm per surface), the pitch diameter shifts by 0.05 mm per side — 0.10 mm total. This is enough to turn a Go-gauge thread into a No-Go failure.
To compensate, CNC programmers must apply a pitch diameter offset during CNC turning or milling: cut external threads undersize and internal threads oversize by the calculated 4x value. If your CNC machinist in Melbourne does not account for this at the programming stage, the parts will need rework after coating.
Thread Design for Manufacturability
Beyond anodising compensation, standard threading DFM rules reduce tool wear and part rejection on any CNC machining job. Limit thread length to 1.5 to 3 times the nominal diameter — engagement beyond 3x provides negligible additional strength. Drill blind holes at least 1.5 to 2 pitches deeper than the required thread depth to clear the tap's chamfered lead. Include a 90° or 120° countersink at every threaded hole entry. And for Type III hardcoat applications, use rounded thread profiles (minimum 0.1 mm root radius) to prevent the brittle oxide layer from cracking at sharp V-thread roots.
3. Bead Blasting: When Surface Prep Becomes a Dimensional Risk
Bead blasting is a standard step for producing uniform matte finishes on CNC machined aluminium parts, particularly before Type II anodising. But it is a mechanical process that removes and deforms material, and it can compromise precision features if applied without controls.
Glass Beads vs Aluminium Oxide
Glass bead media (Mohs hardness 5–6) acts through micro-peening: it smooths tooling marks and produces a uniform satin finish without aggressively removing material. Aluminium oxide media (Mohs hardness 8–9) cuts into the surface, creating a deeper profile suitable for coating adhesion but significantly altering dimensions and surface roughness. For cosmetic CNC machining Melbourne parts, glass bead blasting is the standard pre-anodise step.
What to Mask and What to Watch
Three risks require management on any blasting operation. Edge rounding: high-pressure peening naturally deforms and rounds sharp external edges. If a feature requires a sharp corner or sealing land, it must be masked. Thin wall deformation: parts with wall thicknesses below 0.5 mm are at high risk of warping under compressive blasting stress. Pressure must be restricted or fixtures used to support the part. Tolerance erosion: any bore, thread, or locating face holding a tolerance of ±0.05 mm or tighter must be masked before blasting. These are critical considerations for CNC manufacturing workflows where post-machining finishes are part of the specification.
4. Stainless Steel Passivation: Citric vs Nitric and What ASSDA Recommends
Stainless steel forms a passive chromium oxide film naturally, but CNC machining operations like cutting, grinding, and polishing can embed free iron in the surface and disrupt that film. Passivation restores it chemically.
Nitric vs Citric Acid
The two primary methods are governed by ASTM A967 and AMS 2700. Nitric acid passivation is the traditional standard — it dissolves surface iron aggressively but carries a risk of flash-attack etching on sensitised austenitic steels, and some formulations involve dichromate additives that generate hexavalent chromium waste. Citric acid passivation is the modern alternative: it removes iron through chelation rather than aggressive dissolution, produces no chrome waste, and avoids the flash-attack risk. Both methods meet the same corrosion-resistance acceptance criteria when properly executed.
For CNC machining Australia work involving stainless steel, citric acid passivation is increasingly the preferred specification. It is safer for the facility, simpler for waste disposal, and delivers equivalent results for the majority of CNC machining services manufacturer applications.
ASSDA Guidelines for Australian Conditions
The Australian Stainless Steel Development Association (ASSDA) provides specific guidance for preventing tea staining — the superficial coastal corrosion common in Australia's high-salt, high-humidity environments. Key requirements: surface roughness must be maintained below Ra 0.5 µm (equivalent to a 320-grit polish), weld heat tint must be removed by pickling or mechanical means before passivation, and hydrochloric acid must never contact stainless steel on site as it initiates rapid pitting. For fabrication companies and construction teams working near the coast, these are not optional refinements — they are baseline requirements.
5. Regulatory Changes Affecting Surface Treatments in Australia
Two major regulatory shifts are reshaping the metal machining and surface treatment landscape in Australia right now.
PFAS Ban (Effective 1 July 2025)
Australia has prohibited the manufacture, import, and use of three major PFAS chemical groups (PFOS, PFOA, PFHxS) under IChEMS Schedule 7. This directly affects fluoropolymer coatings, anti-graffiti treatments, and some lubricants used in machining and finishing. Procurement teams must audit supply chains to ensure all powder coats, lubricants, and metal pretreatments comply with the ban (Clayton Utz, 2025).
Safe Work Australia WEL Transition (1 December 2026)
From December 2026, Australia transitions from Workplace Exposure Standards (WES) to stricter Workplace Exposure Limits (WEL) for airborne contaminants. Critically, hexavalent chromium — generated during chromate conversion coatings, chrome electroplating, and stainless steel welding — is classified as a non-threshold genotoxic carcinogen. All numeric exposure limits are abolished; facilities must eliminate or reduce exposure to as low as reasonably practicable. This is accelerating the phase-out of chromate pretreatments across the machining industry and driving adoption of non-chrome alternatives and pre-anodising technologies.
6. How Southside Engineering Manages Secondary Operations
As a CNC machining services manufacturer in Melbourne, Southside Engineering machines parts that go on to anodising, passivation, bead blasting, plating, and powder coating every day. We understand that the machining and finishing are one integrated workflow, not two separate jobs.
When you send us a drawing that specifies a secondary finish, we machine to the correct green-state dimensions from the start. We apply the appropriate pitch diameter offsets for anodised threads. We flag features that need masking before blasting. And we work with trusted local finishing partners across CNC Melbourne who understand the same tolerances and standards we do.
Whether you are a machining manufacturer ordering prototypes or a procurement team running high-volume machining production, we coordinate the full process so the parts assemble correctly first time. If you are searching for CNC machining near me that includes finishing coordination, our experienced CNC machinist Melbourne team handles it end to end high-volume machining.
We offer CNC machining, CNC milling, CNC turning, high-volume machining, rapid prototyping, repetition engineering, and assembly and production. As a CNC machining Australia specialist, CNC machinist Melbourne workshop, and CNC machine Melbourne facility, we work across aluminium, stainless steel, carbon steel, brass, bronze, copper, and engineering plastics. 100% Australian owned since 1973.
Need CNC machined parts with secondary finishes? Get a quote or call us on (03) 9587 0405.
