Surface Finishing Options for Machined Parts: A Specification Guide

How Surface Finishes Affect Part Dimensions

Every applied coating adds thickness, while abrasive processes remove material. Understanding these shifts prevents assembly failures. The values below are indicative — confirm against the applicable spec.

Our Eight In-House Surface Finishing Processes

We manage eight specific finishing categories to support CNC milled parts and CNC turned parts. Review our full capabilities for machining and finishing integration.

Anodising Type II & III

Type II provides standard corrosion protection and accepts dyes for colour coding. Type III hard anodising delivers a much thicker, harder ceramic-like layer for high-wear applications. Both processes are exclusive to aluminium substrates.

Powder Coating

This electrostatically applied dry powder is cured under heat to form a thick, durable skin. It offers excellent impact resistance and uniform colour but requires careful masking of precision bores due to its 60–120 µm build-up.

Zinc Plating

Zinc plating acts as a sacrificial layer to protect carbon steel from oxidation. It is highly cost-effective for fasteners and structural brackets where extreme wear resistance is not the primary requirement.

Nickel Plating

Electroless or electrolytic nickel plating provides a hard, uniform layer with excellent chemical resistance. It builds predictably, making it suitable for components requiring both corrosion protection and improved surface hardness.

Passivation

Passivation uses an acid bath to remove free iron from the surface of stainless steel, restoring its natural chromium oxide layer. It changes neither the dimensions nor the appearance of the part.

Polishing

Mechanical polishing smooths surface asperities to achieve low Ra values. It removes a negligible amount of material while significantly reducing friction and improving fatigue life.

Black Oxide

Black oxide converts the surface of ferrous metals into magnetite, adding less than 2 µm of thickness. It provides mild corrosion resistance when oiled and eliminates light reflection without altering fit.

Sandblasting

Sandblasting propels abrasive media against the workpiece to create a uniform matte texture. It removes up to 0.05 mm of material and is frequently used as a pre-treatment before anodising or coating.

Managing Tolerance Conflicts: Pre-Plating vs. Post-Plating

A finish cannot be treated as an afterthought if your drawing calls for tight tolerances. For standard CNC machining on aluminium and steel, we hold ±0.01 mm, and ±0.005 mm is evaluated per project. Grinding achieves ±0.002 mm. However, applying a 50 µm hard anodise layer to a bore machined exactly to nominal size will shrink that bore by 100 µm across the diameter.

When specifying coatings with significant build-up, engineers must decide whether to machine the part undersized (leaving stock for the plater) or machine to final size and mask the critical features. Threads, dowel pin holes, and bearing seats almost always require masking or pre-plating undersizing. If you design for precision tolerances, communicate which datums are functional mating surfaces so our team can plan the fixturing accordingly. Abrasive finishes like sandblasting also demand attention, as removing material uniformly can shift hole centres or thin out delicate walls.

Material and Finish Compatibility

Not every finish works on every substrate. Selecting an incompatible pairing leads to poor adhesion or damaged parts. For a deeper dive into substrate options, see our guide on materials for CNC machining.

Engineering plastics like POM and PEEK do not accept electrochemical treatments. They are limited to mechanical finishes such as polishing or sandblasting to adjust surface roughness.

What to Call Out on Your Drawings

Ambiguous notes cause delays. To ensure your parts are finished correctly on the first run, include these details on your prints:

• Specify the exact standard (e.g., MIL-A-8625 Type II Class 2, rather than just "anodise").
• Define the required coating thickness range if it differs from standard commercial practice.
• Clearly mark all masking areas using cross-hatching or explicit "DO NOT COAT" callouts.
• State the desired surface roughness (Ra) post-finish, not just pre-finish.
• Indicate the preferred colour code (e.g., RAL or Pantone) for powder coating or dyed anodising.
• Note any secondary operations required after finishing, such as reaming masked holes.

Cost Drivers in Surface Finishing

Finishing costs scale differently than machining costs. Batch size matters immensely; running 500 parts through a plating line distributes the setup chemistry and labour far more efficiently than running 10 pieces. We accept orders starting from 1 piece, but unit economics improve at volume.

Racking and fixturing add time. Parts with complex geometries require custom wire fixtures to ensure even current distribution during plating or anodising. Extensive masking—covering threads, sealing faces, or internal bores—is manual labour that directly increases price. Custom colours for powder coating or Type II anodising may require line purges between batches. Finally, secondary machining after finishing, such as tapping a hole that was left masked, adds another operational step.

Requesting a Quote

Our engineering team reviews your CAD files and finish specifications to provide a comprehensive quotation within 24 hours. Once approved, sample lead times are 3–5 days, and batch production runs take 7–15 days depending on complexity. Every batch ships with a dimensional inspection report. We back our processes with ISO 9001:2015 certification and utilize Mitutoyo height gauges, VMS vision measuring systems, and CMMs (500×400×300 mm, ±0.003 mm) to verify results. First Article Inspection (FAI) reports and EN 10204 3.1 material certificates are available upon request. With over 5,000 customers across 40+ countries and 1,000+ product configurations delivered, our quality protocols are proven across automotive, medical, electronics, aerospace, and robotics sectors. Finished parts ship via DHL, FedEx, or UPS based on your destination. Submit your drawings today or explore our advanced equipment list.

FAQ

Does anodising change the dimensions of my aluminium part?

Yes, anodising grows the oxide layer both inward and outward from the original surface. You must account for this buildup, typically 5–25 µm for Type II, when designing mating fits.

Can you apply powder coating to tight-tolerance bores?

We strongly advise against coating precision bores due to the 60–120 µm thickness of powder coating. These areas should be explicitly masked on your drawing prior to processing.

Which surface finishes are compatible with POM and PEEK?

Engineering plastics cannot undergo electrochemical plating or conversion coatings. They are restricted to mechanical surface adjustments like polishing or sandblasting.

How do I specify masking requirements on my drawing?

Clearly highlight the zones to remain uncoated using cross-hatching or bold "MASK" callouts. Providing a separate masking diagram alongside the main print eliminates ambiguity.

What inspection documentation accompanies finished parts?

Every batch includes a dimensional inspection report verifying critical features post-finishing. FAI reports and EN 10204 3.1 material certificates are provided upon request.

How quickly can I receive a quote for finished parts?

We evaluate your CAD models and finish specifications to deliver a formal quotation within 24 hours. Technical questions regarding finish feasibility are answered within the same timeframe.