How to Specify Surface Treatment on Drawings and Acceptance Criteria
Surface treatment must be specified on drawings and acceptance criteria; otherwise, the machining shop can only rely on experience and guesswork, which often leads to disputes during the acceptance stage. Before placing an order, buyers should clearly mark on the engineering drawing the coating type, thickness, surface roughness, salt spray hours, and measurement locations, and state the inspection basis and sampling ratio in the purchase order. As a CNC machining shop, Yuan Shun Li executes the surface treatment specifications designated by the customer and performs final inspection before shipment, but the more complete the drawing information, the more accurate the quotation and delivery time.
Which Surface Treatment Fields Should Be Marked on the Drawing So the Machining Shop Gets It Right the First Time
The surface treatment fields that should be marked on the drawing include at least five items: coating type name, coating thickness, surface roughness, corrosion resistance test conditions, and measurement locations. For the coating type name, it is recommended to use common codes, for example, "Type II or Type III" for anodizing aluminum alloys, "Type III hard" for hard anodizing, "Zn" for zinc plating, and "Ni" for nickel plating, avoiding in-house custom codes. The unit for coating thickness should be consistent, commonly μm or mil, and buyers should specify whether it is "minimum thickness" or "typical thickness," because the acceptance criteria for the two are completely different. The surface roughness Ra value should be written to one decimal place, for example, Ra 0.8 µm or Ra 0.4 µm, which is particularly critical when connecting the surface treatment layer to the CNC-machined surface roughness of Ra 0.3 µm. For corrosion resistance test conditions, the salt spray test hours and acceptance criteria should be clearly stated, for example, "neutral salt spray 96 hours, no red rust," which is the field most likely to cause disputes in the automotive and electronics industries. Measurement locations should be indicated on the drawing with arrows or dimension lines to prevent the machining shop from choosing measurement points on its own, which could lead to data deviations.
How surface finishing clauses should be written on purchase orders to protect the buyer in disputes
Surface finishing clauses on purchase orders should be split into three sections—"specification reference," "inspection method," and "non-conformance handling"—to protect the buyer's rights in the event of a dispute. The specification reference section should state the drawing number, the version of the surface finishing standard, and the coating type code, for example, "Per drawing ABC-001 Rev. C, anodizing Type II, coating thickness 10–15 μm." The inspection method section should specify whether inspection is performed by the machining shop, a third party, or the buyer's resident inspector, and state the format and submission timing of the inspection report. The non-conformance handling section should pre-agree on the process and cost allocation for returns, rework, allowances, or scrapping, to avoid time-consuming negotiations afterward. For OEM and Tier-1 buyers, these three clauses are indispensable, because surface finishing defects are often only discovered after final product assembly. If the purchase order does not specify the handling method, the machining shop may claim that it has passed incoming inspection and in-process checks and refuse responsibility. Yuan Shun Li performs final inspection before shipment according to the inspection criteria specified by the customer, but the contractual allocation of responsibility must still be clearly defined by the buyer in the purchase order.