After the CNC Cut: Anodizing, Plating, Heat Treat and Outsourced Finishing Risk
Anodizing grows a surface, plating adds a layer, and heat treat moves the whole part — none of them leaves your machined dimensions untouched. The rule: design the finish into the drawing before machining, because a part machined to final size and then anodized is already out of tolerance on its coated features.
A CNC part leaves the machine as bare metal with a specific size, surface finish, and residual stress state. Almost every production part then goes through something else: anodize for corrosion and color, plating for solderability or wear, heat treat for strength, bead blast for texture, passivation for stainless. Each step changes the part — in dimension, in surface, or in metallurgy — and each change carries risk that the shop floor controls only partially. This guide covers the common finishing operations, what they do to your dimensions, and where the real risk sits when finishing happens outside the machine shop.
What Each Finish Does to Your Dimensions
The first fact to internalize: finishes add or remove material in controlled but real amounts. Hard anodize (Type III) builds a coating 25–100 µm thick, roughly half of which grows inward from the original surface — so a bored hole loses about half the coating thickness on its radius. Electroless nickel plates uniformly onto every surface including threads and blind holes, at 2.5–50 µm depending on spec. Chromate conversion and passivation are thin enough to ignore dimensionally but still change the surface chemistry. Even bead blasting removes a few microns and rounds sharp edges.
| Finish | Typical thickness added | Dimensional effect | Notes |
|---|---|---|---|
| Type II anodize | 5–25 µm coating | ~half inward on each surface | Seal before critical fits |
| Type III hard anodize | 25–100 µm | Significant inward growth | Machine finish allowance first |
| Electroless nickel | 2.5–50 µm per spec | Uniform on all surfaces | Follows threads and bores |
| Zinc/nickel plating | 5–15 µm | Grows external, shrinks internal | Pre-plate allowance needed |
| Passivation (stainless) | Negligible | None measurable | Chemical only |
| Bead blast | Removes ~2–5 µm | Rounds edges, roughens | Do before final critical cuts |
The dimensional rule is simple: identify coated surfaces on the drawing, and machine them to size minus the coating that will grow inward — or machine after coating where the coating is thin enough. Threads are the classic failure: plated threads that were machined to gage size before plating fail the gage after, because the plating consumes the clearance. Shops that know the finish in advance cut the thread to a pre-plate class and save the batch.
Heat Treat: Before or After Machining?
Heat treat is where dimensional control gets genuinely hard, because the process does not just change the surface — it changes the whole part. Steel that is machined soft and then hardened will distort: stresses relax, phases transform, and parts that were perfectly straight come out of the furnace bent by measurable amounts. Quenching is the violent part — thin sections and asymmetric geometry distort most. The standard engineering answer is to machine in the soft or pre-hardened state, heat treat, then finish-grind or finish-machine the critical features. Our CNC turning and milling work routinely runs this sequence: rough machine, send out for heat treat, then finish the datums and bores that must hold ±0.01 mm.
| Material route | Sequence | Tolerance risk |
|---|---|---|
| Steel, low precision | Machine to finish, heat treat, accept distortion | ±0.1 mm or worse on long parts |
| Steel, critical features | Rough machine, heat treat, finish machine | Controlled by finish pass |
| Aluminum age hardening (T6) | Machine from T6 stock, no re-treat | Stable if stock is correct temper |
| Case hardening | Machine, case harden, grind ID/OD | Grinding restores geometry |
| Stress relief | Rough, stress relieve, finish | Removes machining stress movement |
The cost logic favors doing heat treat once and machining around it, not twice. If the drawing calls a hardened steel part with a ±0.005 mm bore, expect the process to be: machine near-net, harden, then grind or bore the hole to final. A supplier who quotes that sequence understands the physics; one who promises to hold that bore through a quench is either lucky or not actually heat treating.
Where Outsourced Finishing Risk Actually Lives
Few CNC shops run anodize and plating tanks in-house — those are specialized, environmentally regulated operations, and most are done by subcontractors. The risk is not that the finisher is incompetent; it is the handoff. Parts leave the machine shop, go to a plater who racks them, and return with: coating on surfaces that should have been masked, thickness that varies from rack position, hydrogen embrittlement on high-hardness steel, or simple damage from handling. Each of these is a quality event that the machine shop did not create but is expected to catch.
| Outsourcing risk | How it shows up | What prevents it |
|---|---|---|
| Wrong coating thickness | Failed gage fit, dimensional drift | Written spec, coating thickness report |
| Plating in masked-off areas | Solder failure, thread fit issues | Correct masking on drawing |
| Hydrogen embrittlement | Delayed cracking of hardened parts | Bake per spec after plating |
| Handling damage | Dents, scratches on finished surfaces | Protective packaging, inspection on receipt |
| Mixed batches | Wrong color or coating on some parts | Lot control, clear part marking |
The buyer's defense is contractual and documentary: specify the finish by standard (MIL-A-8625 for anodize, ASTM B733 for electroless nickel, AMS or ISO equivalents for plating), require a coating thickness report with the batch, and define masking on the drawing. When we manage finishing for a customer, the inspection step after the parts return from the finisher is part of the deliverable — the batch ships with dimensional and coating checks, not just the machinist's word. Our CNC precision components line and the wider CNC machining operation treat the finishing subcontractor as part of the quality chain, with the same reporting discipline applied to the CNC cuts themselves.
Designing for Finish, Not Against It
The cheapest way to avoid finishing problems is to design for them in the first place. Ask three questions at the drawing stage. First, does this surface need the finish at all — an internal mounting face does not need cosmetic anodize, and a part that only needs corrosion protection may take a cheaper chromate or passivation. Second, what is the coating thickness budget — call the finish class so the machinist can leave the right allowance instead of guessing. Third, what is the thermal or embrittlement exposure — hardened parts that get plated need the bake specified, and parts that must not grow need those surfaces masked or machined after coating.
Edge cases deserve the same discipline. Sharp edges plate heavily and anodize unevenly; the drawing should call a small chamfer or radius on coated edges. Blind threads trap plating chemistry and need thorough rinsing or masking. And when a part mixes a coated cosmetic surface with a precision bare bore, plan for two operations: finish the bore after coating, or mask it before. This is the same logic as our plastics machining guide makes about material behavior — the finish is part of the material system, and ignoring it costs a batch.
Who Owns the Finish?
Finally, decide who coordinates the finish chain before you order, not after a problem appears. A parts buyer managing anodize separately from machining saves a little and inherits the handoff risk. A shop that coordinates machining, heat treat, and finishing from one drawing owns the sequence and the inspection — which is why we prefer to manage the full route from CNC turning and milling through outsourced finishing, with every step documented and every batch checked on return. The same logic applies when the material is exotic: a titanium part going for color anodize or passivation needs the same process control, as our titanium machining guide details.
Send the drawing with the finish called out by standard, the coating thickness specified, and the critical surfaces identified. The quote will then include the finish allowance, the coordination, and the inspection — and the parts that arrive will match the drawing after finishing, not before it.
Email sc@bquq.com or WhatsApp +86 137 1315 7787 with your PDF/DXF/STEP file. An engineer reviews it and replies with price, lead time and DFM notes on working days.
Frequently Asked Questions
Q: Does anodizing change the dimensions of my CNC part?
A: Yes. Anodic coating grows roughly half inward from the original surface, so a Type III hard coat of 50 µm removes about 25 µm per surface from bores and external dimensions. Machine coated features with a finish allowance, or size them after coating.
Q: Should I machine before or after heat treat?
A: Both, in sequence. Machine near-net in the soft state, heat treat, then finish-machine or grind only the critical features. Machining fully hardened material is expensive, and holding tight tolerances through a quench distortion is usually impossible.
Q: What is the risk of plating hardened steel parts?
A: Hydrogen embrittlement. Plating processes introduce hydrogen that can crack high-hardness steel hours or days later. Specified baking after plating relieves it. If your parts are hardened above roughly 40 HRC and get plated, require the bake in the process spec.
Q: How do I spec a finish so the supplier cannot guess?
A: Name the standard and the class — for example MIL-A-8625 Type II Class 2 for dyed anodize, or ASTM B733 for electroless nickel with the thickness grade. State which surfaces are masked and require a coating thickness report with the batch.
Q: Who is responsible when outsourced finishing damages parts?
A: The party managing the contract. If you send machined parts directly to a plater, you own the handoff risk. If your machining supplier coordinates the finish, they own the sequence and should inspect parts on return — ask for that inspection report with your batch.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
