How to Select Proper Contact Points for Anodizing CNC Machined Aluminum Parts

In the production of CNC machined aluminum parts, anodizing is a frequently applied surface treatment process that improves corrosion resistance and wear resistance while providing a consistent appearance. However, the anodizing process involves direct contact between the part and the anodizing fixture or rack, which leads to localized marks or interruptions in the anodic coating. These areas, known as contact points or rack marks, are unavoidable due to the need for electrical current conduction during anodizing. Proper selection of these contact points is critical in minimizing visual impact, preserving functionality, and maintaining dimensional accuracy in precision parts.

This article outlines the technical considerations involved in determining appropriate contact points on CNC machined aluminum parts before anodizing, especially for custom and complex geometries.

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Understanding Contact Points in Anodizing

During anodizing, aluminum parts are fixed onto conductive racks or fixtures using clamps, hooks, or screws. These fixtures are connected to the power source that supplies direct current, enabling the formation of the anodic oxide layer on the exposed surfaces.

However, the areas where the part touches the rack do not receive the anodic coating. These regions remain untreated, and depending on the application, may be visible or hidden. Therefore, selecting appropriate contact points is not only a technical issue but also an aesthetic and functional consideration.

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Key Considerations for Choosing Contact Points

1. Functionality of the Surface

One of the primary rules is to avoid placing contact points on functional surfaces such as sealing faces, sliding surfaces, tight tolerance features, or any area where conductivity, fit, or smoothness is essential. Contact marks in these areas may disrupt the performance or assembly of the final product.

Instead, non-critical surfaces, such as internal faces, mounting holes, or areas later concealed during assembly, should be preferred.

2. Part Geometry

The geometry of the machined part significantly affects the racking method. For example:

- Flat parts can be clamped along the edge or through holes if available.
- Cylindrical parts may be clamped from one end or suspended through the bore.
- Thin-walled or lightweight parts must be handled carefully to avoid deformation or bending during racking.

In complex shapes, racking contact might be required at multiple locations to ensure stability during anodizing.

3. Electrical Current Requirements

The anodizing process requires a continuous electrical path between the power source and the aluminum surface. Inadequate contact or poor conductivity at the contact point can result in incomplete coating, discoloration, or process failure.

The surface area of the contact point must be sufficient to allow for proper current flow, especially for thicker coatings or hard anodizing, which require higher current densities.

4. Load Stability and Rack Integrity

The racking method must support the part securely during movement through the anodizing line. If the part is heavy or has an asymmetric shape, improper contact points can lead to shifting, which may cause inconsistent coating thickness or mechanical damage.

In high-volume production, repeatable racking fixtures may be used to ensure consistent contact point placement.

5. Cosmetic Requirements

When aesthetics are a concern—such as for consumer electronics or visible structural parts—contact points should be placed in hidden areas or areas that will later be machined or assembled out of view. In some cases, customers may accept small marks, provided they are consistent and limited in size.

Some surface treatment providers use specialized masking techniques to minimize the appearance of rack marks, but this may increase cost and lead time.

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Communicating Contact Point Preferences with Clients

When manufacturing custom CNC aluminum parts, contact point decisions should be discussed early in the production cycle, especially if anodizing is part of the requirement.

Clients should be encouraged to:

- Mark preferred or restricted contact areas on technical drawings.
- Indicate critical surfaces and visible areas.
- Provide feedback on acceptable cosmetic outcomes.

If drawings are unclear, the manufacturer should confirm contact areas before anodizing begins.

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Documentation and Internal Control

To ensure quality and repeatability:

- Standard racking instructions can be documented for recurring part numbers.
- Photographs or diagrams of contact locations may be included in the production record.
- In-house anodizing or subcontracted suppliers should follow consistent racking practices aligned with the agreed standards.

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Summary

Contact point selection is a critical but often overlooked step in preparing CNC machined aluminum parts for anodizing. Proper identification and planning of these areas can reduce rework, avoid rejected parts, and enhance both functionality and visual appeal. Manufacturers should evaluate geometry, functionality, current requirements, and client expectations to determine optimal racking strategies. Open communication with the customer, combined with consistent internal practices, contributes to reliable outcomes and improved part quality.

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FAQ: Can anodizing be done without leaving any rack marks?

No, anodizing always requires at least one conductive contact point to pass electric current through the part. These points will remain uncoated after the process. While it is technically impossible to completely eliminate rack marks, their visibility and impact can be minimized by:

- Selecting non-critical surfaces for contact,
- Using appropriate racking tools with small contact areas,
- Applying post-processing steps such as touch-up or blending,
- Designing parts with designated hidden or masked areas.

It is important for buyers and engineers to account for rack marks during the design stage and to communicate any specific cosmetic or functional concerns with the CNC machining parts manufacturer before anodizing begins.