A machined surface carries the marks of the tool that made it, and how it looks depends on the cutter, the feed per tooth, the toolpath and the material. Everything after that is a finishing operation with its own cost and lead time, and the useful distinction is between operations that change the surface itself and coatings that add a layer to it.
As machined, and what that means
The default. A finishing pass leaves a consistent tool mark pattern, and the direction of that pattern depends on the toolpath, so adjacent faces machined by different strategies can look different. That is normal and it is why cosmetic parts are usually blasted or grained: to remove the variation rather than to remove roughness.
Mechanical finishes
Bead blasting gives a uniform matte surface and hides tool marks, and it is the usual precursor to anodising. Tumbling and vibratory finishing deburr and lightly polish small parts in bulk and are cheap per part. Graining gives a directional brushed appearance. Hand polishing and lapping reach the finest surfaces at the highest cost.
Coatings and conversions
Anodising on aluminium, black oxide on steel, plating and powder coating are all layers or conversions applied to the finished surface, and all of them interact with dimensions to some degree. They also require the underlying finish to be right first, because a coating reveals rather than hides what is beneath it. Blast and then anodise is a sequence, not two options.
Specifying it without wasting money
Say what each face needs, not what the part needs. Cosmetic faces get a finish; internal and hidden faces stay as machined. Say whether the requirement is appearance or roughness, since those are different specifications. And name the sequence where a coating follows a mechanical finish, because the shop will otherwise choose one and it may not be yours.
Questions people ask about cnc surface finish
What does as machined mean?
The surface left by the finishing cut, carrying the tool's mark pattern. It varies between faces machined by different strategies, which is normal.
What is bead blasting for?
Producing a uniform matte surface that hides tool marks and variation between faces. It is also the standard preparation before anodising.
Is anodising a finish or a coating?
A conversion of the aluminium surface that grows a layer, so it behaves like a coating dimensionally. It reveals rather than hides the surface beneath, so the mechanical finish comes first.
How should I specify machining finish on a drawing?
Per face, and by intent. Say which faces are cosmetic and what they need, and let hidden faces stay as machined.