Precision Alloy Completion Service: How to Specify the Last Step

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  Key takeaways
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  Matching finish to alloy and function
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  What a precision alloy completion service actually includes
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  How coating thickness changes your tolerance stack
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  Where different alloys force different completion choices
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  Five checks before you release a supplier
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  Common mistakes that cost a production lot
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  How to run a completion order step by step

A precision alloy completion service is everything that happens after the last cutting pass and before the part reaches your dock. That covers edge breaking, surface preparation, masking, coating, marking and final inspection. It is not one process. It is a sequence, and the order matters. Blast before anodize. Mask before plating. Mark after coating if the mark must survive handling.

Buyers often treat this stage as a line item to negotiate down. The better move is to define it early. Once the part is machined, you cannot add a chamfer to a coated edge without rework. You cannot re-mask a plated thread to fix a thickness problem. Completion decisions made at the quoting stage cost a fraction of decisions made after the first article arrives.

The reason this stage matters more on alloy parts than on mild steel is the material cost. A 5-axis titanium or Inconel component may carry weeks of machining time. Scrapping it during finishing is expensive. That is why we run completion as a controlled process with its own traveler, not as a favor bolted onto the machining order.

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  Deburring and edge breakManual and abrasive flow where the drawing calls for a specific edge condition.
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  MaskingThreads, dowel holes, seal faces and electrical contact areas.
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  Coating or platingAnodize, electroless nickel, zinc, silver, gold, powder coat, black oxide.
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  VerificationThickness, hardness, roughness and visual checks before packing.

Any conversion coating adds material. Clear anodize on aluminum typically adds 5–12 μm per surface. Hardcoat runs thicker, often 25–50 μm. Electroless nickel is usually specified at 10–25 μm and can sit at 50 μm for wear surfaces. Those numbers are small until they land on a bore or a press fit.

Consider a shaft ground to 20.000 mm with a slip fit into a reamed hole. Add 15 μm of electroless nickel to each surface and the effective clearance drops by 30 μm. On a nominal 20 H7/g6 fit, that can consume most of the allowance. The part still measures correctly before plating and seizes after.

The fix is not to avoid coating. It is to pre-machine the dimension. Tell the finisher the final requirement and let them specify the pre-plate size. For hardcoat on a precision bore, we usually leave 0.03–0.05 mm of stock and cut the bore back after coating if the drawing allows. If it does not, mask the bore. Masking is cheaper than a scrapped titanium housing.

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  Anodize Type II5–12 μm per surface on aluminum.
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  Hardcoat Type III25–50 μm per surface; brittle on edges under 0.3 mm.
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  Electroless nickel10–25 μm typical, uniform on complex geometry.
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  Zinc plating5–15 μm; acid pickling can shift thin-wall dimensions.

Aluminum is the forgiving case, with limits. 6061 and 6082 anodize evenly and hold color well. 7075 takes hardcoat and gives a good wear surface. 2024 is the problem child. Its copper content makes clear anodize come out dark and blotchy, and the result varies by heat lot. If the part is visible, plan on a dye or accept the color shift in writing.

Titanium does not anodize the way aluminum does. Type II and Type III anodize are aluminum processes. Titanium anodizing is a different bath and produces a thin oxide that is used mainly for color coding or for a light wear layer. For Ti-6Al-4V parts that need real wear resistance, we steer customers toward a coating specified for titanium or toward leaving the surface as machined and controlling roughness instead.

Inconel and other nickel alloys are usually finished by blasting, tumbling or polishing rather than by conversion coating. The material resists most chemical baths well, which is helpful, but it also work-hardens. Aggressive tumbling can round edges that the drawing wanted sharp. Specify edge condition before the finisher touches the part.

Stainless is the case where passivation matters most. 303 and 316L both benefit, and 17-4PH needs it after machining if the surface has been contaminated by carbon steel tooling. Passivation is not a cosmetic step. It removes free iron and restores the chromium oxide layer. Ask which specification is being used, because citric and nitric passivation are not interchangeable.

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  2024 aluminumExpect color variation on clear anodize; specify dye or accept in writing.
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  Ti-6Al-4VUse titanium-specific processes, not standard aluminum anodize.
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  InconelMechanical finishing is usually the practical route.
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  StainlessConfirm passivation type and specification before release.

First, ask whether finishing is done in-house or outsourced. Both can work. What matters is who controls the schedule and who owns the nonconformance. If your supplier sends parts to a third-party plater, the delivery date you were given may be the machining date, not the ship date.

Second, ask for a first article with finish data. Thickness readings on three points of the part, hardness for hardcoat, and roughness for polished or blasted surfaces. A certificate that lists a specification number without measurements tells you almost nothing about what happened to your parts.

Third, confirm the masking plan in writing. Which holes, threads and faces are protected, and with what method. Silicone plugs, tape and lacquer all behave differently in a hot bath. A plug that falls out mid-process leaves a plated thread that will not accept a fastener.

Fourth, check the inspection step. We run 100% inspection before shipment, with raw material check, in-process monitoring and final inspection, and reports on request. That is the level of detail to ask for. If a supplier cannot describe their in-process checks, they are probably relying on the final look of the part.

Fifth, ask what happens when something goes wrong. Rework options for a mis-masked anodize, strip and recoat limits, and who pays. Aluminum can usually be stripped and re-anodized once before the surface degrades. Plated steel can sometimes be stripped. Coated titanium often cannot. Know the limit before you need it.

The most expensive mistake is leaving the finish callout as 'as machined' and then adding a coating requirement after the parts are cut. That forces a decision between rework and scrap, and it almost always delays the program. Put the completion spec on the drawing before the first chip is made.

The second is ignoring how masking interacts with geometry. A deep blind hole with a 2 mm entry cannot be reliably masked or plated. The finisher will either plug it badly or leave it open and change the bore size. If the hole is critical, design it for the process or plan to machine it after coating.

The third is treating a certificate as an inspection report. A certificate states that a process was followed. A report states what was measured on your parts. For aerospace, automotive and medical work, the difference matters during an audit.

Finally, do not assume that a supplier with strong machining capability also has strong finishing control. They are different skill sets. Ask who runs the finishing line, how long they have run it, and what their first-pass yield looks like on your alloy. On titanium and Inconel, the honest answer is often that some processes are better outsourced to a specialist.