How Surface Finishing Affects Final Tolerances
Surface finishing adds or removes a layer of material on the part surface, so the final dimensional and geometric tolerances must account for the finishing thickness and its own tolerance window. If buyers overlook this layer when reviewing drawings and requesting quotes, parts that measure within tolerance may actually fall outside the dimensional tolerance required by the original drawing.
Why Surface Finishing Changes Final Tolerances
Common surface finishing processes such as anodizing, electroplating, chemical conversion coating, painting, or powder coating form an additional layer on the part surface. This layer increases external dimensions and reduces internal hole dimensions, and the coating thickness itself has process tolerances. Even if CNC machining itself stays within ±0.01 mm, the final dimensions after subsequent processing can still shift outside the original tolerance band. If buyers define tolerances based on the as-machined state but apply surface finishing before shipment, dimensional disputes can easily arise during incoming inspection.
Four Things Buyers Should Note on Drawings
Specify final tolerances after surface finishing
The tolerance basis should be clearly stated as "after surface finishing" to avoid discrepancies in how machining and inspection teams interpret the reference.
Specify coating thickness and its tolerance
For example, specifying anodizing at 10–15 μm gives the machining supplier a clearer basis for reserving the correct machining dimensions than simply writing "anodizing."
Distinguish functional and cosmetic dimensions
Mating surfaces and sealing surfaces are functional and must be tightly controlled; appearance surfaces can be relaxed appropriately.
Mark datums and measurement locations
Avoid placing measurement points where coating thickness is uneven, which could lead to inspection disputes.
Practical Approach When Communicating with the Machining Supplier
Provide the surface finishing process type and coating thickness specification during the RFQ stage so the machining supplier can reserve the corresponding allowance during CNC machining. If a part has both functional and cosmetic tolerances, it is recommended to mark them separately on the drawing and ask the supplier to provide measurement reports both before and after surface finishing. Yuan Shun Li can reserve dimensions according to the specified coating thickness during CNC machining and perform final inspection based on the customer's specified datums before shipment; the actual achievable tolerance window depends on the material, geometry, and surface finishing process, and must be confirmed against the actual specification.
Need to Include Surface Finishing in Tolerance Evaluation?
Feel free to provide your drawings and surface finishing specifications, and we will reserve the correct dimensional allowance for you during the CNC stage.