How do product designers decide what surface finishes to require, and how should CNC machinists meet the requirement?

There are a variety of factors, and they may even vary for different stages in the manufacture of a particular part. 

1, Extra material consideration

For example, we may have a surface finish requirement on a casting that ensures the finish (think of it as the deviations of the surface of the casting from an ideal) is good enough that the allowance made for extra material that will be machined off in a future step is sufficient. If the casting is too imperfect, the trough of imperfection may be below the expected machined surface of that future step.

2, Friction

Another important consideration is friction. Reducing surface roughness typically reduces friction which can be critical to reducing wear and increasing the efficiency of sliding parts.

3, Function and usage of the parts 

The required surface finishes are very much determined by the function and use of the component.  For example, propellers of ships start out with fairly high surface finish standards but in actual use, they erode pretty quickly. Optical and especially components used with X-Rays have some of the finest surface finish requirements achievable.

The goal of the designer is to specify surface finishes that are as coarse as possible but will still function within the part's desired operating parameters. The goal of the machinist is to achieve surface finishes on parts that are as good as those required by the designer, but not better as that results in the cheapest to manufacture parts. It's important for the designer and manufacturer to agree on exactly which parameters (Ra, Rz, etc..) are to be used for inspecting and part acceptance. While there are commonly accepted conversions between these, actually using the same parameters for the inspection that the designer used with the design always yields the best results.

Xu Feng is more than a supplier of parts. It expects to serve with a humble heart and manufacture with fine workmanship. Besides offering custom parts with high precision. It is eager to work with its customers on professional consultation and adjustment on trial feedback. so if you have any questions on the part surface, contact us for a detailed answer.