Finish is not decoration. It controls corrosion resistance, wear, friction, solderability, appearance and how well paint or adhesive bonds. Specifying finish by function rather than by habit avoids paying for properties the part will never use - and avoids a part that fails in service because a surface property was never stated.
Mechanical finishes
As-machined leaves visible tool marks; bead blasting produces a uniform matte surface; vibratory finishing deburrs and polishes edges. Tumbling is the usual way to remove sharp edges from volume parts cheaply.
Mechanical finishes change the surface without changing the material, so they have no dimensional consequence beyond a few micrometres of material removal. That makes them the safe default when a tolerance sits on the finished surface.
Chemical and electrochemical finishes
Anodising grows a controlled oxide layer on aluminium for corrosion and wear resistance and can be dyed. Passivation removes free iron from stainless surfaces to restore corrosion resistance after machining. Plating (nickel, zinc, chrome) adds hardness, corrosion resistance or conductivity.
Each has dimensional consequences: anodising grows the part slightly, plating adds a measurable layer. Hard anodising can add tens of micrometres per surface, which is enough to move a tight bore out of tolerance if it is not accounted for.
Specify by function, not by habit
State what the surface must do - resist salt spray, withstand abrasion, take paint, conduct, look uniform - and let the finish follow. Where a tight tolerance sits on a finished surface, finish thickness must be accounted for in the drawing.
This is also how you avoid over-specifying. A part that lives indoors and never handles abrasion rarely needs hard anodising, and specifying it anyway adds cost, lead time and a tolerance risk for no benefit.
Cosmetic standards need to be written down
Appearance is subjective unless it is defined. Two inspectors can disagree about whether a bead-blasted surface is uniform, and a customer can reject a part that meets every dimensional requirement.
The practical fix is a written cosmetic standard with an agreed limit on marks, colour variation and texture, ideally backed by a signed sample. It removes an argument that has nothing to do with whether the part works.
Finishes applied as production processes
- Anodised aluminium housings, panels and frames
- Passivated stainless parts for food, medical and marine use
- Zinc and nickel plating for corrosion resistance on steel
- Bead blasting and tumbling for uniform cosmetic surfaces
- Powder coating for enclosures and welded assemblies
What most often goes wrong with finishing
Anodising and plating grow the part. If a tight bore is finished afterwards, it will measure out of tolerance even though the machined part was perfect.
A plated or coated part keeps its burrs, now harder and more firmly attached. Deburring has to happen before the finish operation, not after.
Without an agreed appearance limit, cosmetic rejection becomes a judgement call. A signed sample removes the ambiguity.
Questions we get asked about surface finishing
Does anodising change part dimensions?
Yes. The oxide layer grows the surface, and hard anodising grows it considerably more than decorative anodising. Where a tolerance sits on an anodised surface, the expected growth has to be written into the drawing or the finished part will be out of specification.
What is the difference between passivation and pickling?
Passivation removes free iron contamination from a stainless surface so the natural passive layer can re-form; pickling is a stronger chemical treatment that also removes scale and a thin layer of base metal. Passivation is normally sufficient after machining.
Can you finish a part after heat treatment?
Usually yes, and it is often the correct sequence - machine soft, heat treat, then finish surfaces that must stay dimensionally correct. Masking may be needed where a surface must not be coated, such as a locating face or an electrical contact.
How do I know which finish to specify?
Describe the environment and what the surface must do - salt spray, abrasion, conductivity, paint adhesion, appearance. That description is enough to shortlist two or three finishes, and the final choice is usually settled on total cost.
- Deburring on every part
- Finish thickness considered in tolerances
- Corrosion and wear performance stated
- Cosmetic standard agreed before production
Quote a finished part
Tell us the environment the part works in and what the surface has to do. We will recommend the finish that meets it, not the most expensive one.