How Should Surface Finishing for CNC Parts Be Selected?

Surface finishing for CNC parts must be decided at the drawing stage; otherwise, subsequent processing and measurement will encounter problems. Anodizing generates an oxide layer on aluminum surfaces, which changes dimensions; electroplating and electroless nickel deposit a thin film on metal surfaces, which also affects hole diameters and thread fits; heat treatment changes the metallurgical structure and causes dimensional deformation. All these changes must be compensated for during CNC machining. When Yuan Shun Li takes on OEM orders, it is common for buyers to provide only a 3D file without specifying surface finishing, only to discover after trial production that dimensions have exceeded tolerance. It is recommended that buyers provide the material grade, application, functional requirements, and acceptance criteria together at the inquiry stage, so the machining shop can reserve dimensional allowances for downstream processes during the CNC stage. If the drawing only states "surface finishing per customer instruction," it is practically considered an incomplete drawing, and the machining shop has the right to request additional information or decline to quote.

Anodizing is mainly divided into three types: sulfuric acid anodizing, hard anodizing, and dye anodizing. Choosing the wrong type directly affects the function and appearance of CNC parts. Sulfuric acid anodizing is suitable for general aluminum alloy appearance parts, with a thinner film that can be dyed later; hard anodizing has a thicker film, higher corrosion and wear resistance, and is commonly used for pneumatic, hydraulic, and moving parts, but it significantly changes dimensions, so a margin of at least 0.02 mm per side must be reserved during machining; dye anodizing is used for appearance parts requiring brand colors or identification colors. Among the aluminum alloy CNC parts Yuan Shun Li handles, common applications include optical equipment housings, automotive and motorcycle aftermarket parts, and industrial enclosures, which mostly go through sulfuric acid anodizing or hard anodizing. If a buyer only specifies "anodizing" without indicating the type, the machining shop will use general sulfuric acid anodizing, which can easily lead to film thickness disputes during acceptance. It is recommended to clearly specify the film thickness range, hardness requirements, and whether dyeing is needed on the drawing, and to note the datum surface and measurement locations.

Heat treatment changes the metallurgical structure and dimensions of metal, which is the key reason why compensation allowances must be reserved when machining CNC parts. Processes such as carburizing, quenching, and tempering cause steel parts to expand or contract, and the amount of deformation varies depending on the material grade, part shape, and heat treatment temperature. In practice, common deformation amounts range from 0.01 mm to 0.05 mm, with long, thin, or thin-walled parts deforming even more. When handling tool steel and alloy steel CNC parts, Yuan Shun Li reserves a heat treatment deformation allowance after rough machining, and then performs finish machining and grinding after heat treatment to ensure the final dimensions fall within tolerance. If buyers mark "machine after heat treatment" or "heat treatment after finish machining" on the drawing, it directly affects the machining process and cost. It is recommended that buyers specify the position of the heat treatment process when requesting a quote and require the machining supplier to provide dimensional measurement reports before and after heat treatment, to avoid return disputes caused by dimensional changes during subsequent acceptance.

When writing surface treatment into drawings and acceptance standards, it must include film thickness, hardness, roughness, measurement locations, and acceptance methods—none of these can be omitted. Many buyers' drawings only specify "surface treatment: anodizing" or "zinc plating" without noting the film thickness range, hardness requirements, and measurement datums, leading to different understandings of acceptance standards between the machining supplier and the buyer. When undertaking OEM orders, Yuan Shun Li commonly encounters disputes including film thickness out of range, insufficient hardness, color differences, and inconsistent measurement locations. It is recommended that buyers clearly mark on the drawing: the type of surface treatment, film thickness range (e.g., 10–15 µm), hardness requirement (e.g., HV 300 or above), surface roughness (e.g., Ra 0.8 µm), measurement location (e.g., datum surface A), and acceptance method (e.g., film thickness gauge, hardness test, visual color comparison). If the surface treatment is outsourced to a specialized supplier, buyers should require the machining supplier to provide quality certificates and measurement reports for the downstream processes, and specify the responsibility allocation.

When post-processing surface treatment is outsourced, the allocation of quality responsibility is the most common source of disputes in CNC parts procurement. In practice, the common arrangement is for the CNC machining shop to be responsible for dimensions and tolerances, while the post-processing surface treatment shop is responsible for coating thickness and appearance. However, the interface responsibility between the two is often unclear. When handling such cases, Yuan Shun Li clearly distinguishes the scope of responsibility between CNC machining and post-processing at the quotation stage, and requires buyers to specify the acceptance criteria and responsibility allocation on the drawings. It is recommended that buyers confirm at the inquiry stage: who inspects dimensional tolerances, who inspects coating thickness and hardness, who judges appearance color deviation, and who bears responsibility in the event of returns. Many buyers require the machining shop to provide a "turnkey" service, covering everything from CNC machining to post-processing surface treatment. In this model, the machining shop assumes full quality responsibility, but the cost is reflected in the quotation. If the buyer chooses to outsource the post-processing steps themselves, they must bear the quality risks of surface treatment and inspect both CNC dimensions and surface treatment quality separately during acceptance.