Manufacturing Process
Polishing
抛光
Refining a metal surface for appearance or cleanliness - linishing, brushed and satin finishes, mirror polishing and electropolishing through audited partners.
Process Steps
- Finish standard agreed
- Grit sequence set
- Surface refined in stages
- Finish and cleanliness verified
Two different jobs called polishing
Direct answer: polishing either changes how a part looks or changes what a surface will do. The first is about grain, gloss and consistency between parts; the second is about roughness, cleanliness and how the surface behaves in service. They use different processes and they are specified in different ways — so a drawing that says "polished" without a number will be quoted differently by every supplier.
Finishes and how to specify them
| Finish | What it looks like | How to specify it | Typical use |
|---|---|---|---|
| Linished / brushed | A directional satin grain, uniform across the part | Grit or grade plus the grain direction and whether welds must be blended first | Panels, covers, handles and enclosures |
| Satin / scratch finish | Matte, low reflection, controlled pattern | Grit range and the sample that defines it | Visible stainless parts, instrument bodies |
| Polished (gloss / mirror) | Bright, low roughness, reflective | An Ra value, or a mirror grade agreed against a sample | Visible trim, optical and instrument parts |
| Vibratory / tumbling | Uniform, softened edges, no direction | Media and cycle; usually specified as a general edge-break instead of a surface value | Small parts in quantity |
| Electropolishing / passivation | Chemically levelled surface, hygienic and corrosion-resistant | Standard plus an Ra value where the surface is functional | Stainless parts for food, medical and clean service |
Ra: the number behind "polished"
Ra is the average roughness of the surface, in micrometres, and it is the only way to make a cosmetic requirement repeatable across orders. Roughly:
| Ra | What it reads as | Usually reached by |
|---|---|---|
| 3.2 µm | Machined, clearly visible tool pattern | Machining alone, with a clean finish pass |
| 1.6 µm | Fine machined or coarse brushed | Finer machining, or a coarse linishing pass |
| 0.8 µm | Satin, low reflection | Progressive linishing, fine belt |
| 0.4 µm and below | Bright, reflective | Polishing stages, or electropolishing on stainless |
Note that a roughness figure says nothing about appearance: a surface can hit Ra 0.4 and still look patchy if the grain runs in different directions across a weld. Where the part is visible, agree a physical sample as the reference.
Grain direction and weld areas
- Say which way the grain runs — across a panel, along a bend, or radially. It is the single most common cause of a part being rejected on appearance;
- Say whether welds are dressed before finishing. A brushed finish over an undressed weld shows the joint straight through it;
- Allow for the edge. A sharp edge cannot hold a uniform finish, and it is also the edge that cuts the person handling the part — a small edge break is usually part of the finish specification;
- Protect after finishing. A polished face is marked by handling, so parts are packed to protect the finished surface rather than nested bare.
What runs in-house and what does not
Deburring, edge work and the mechanical preparation that a finish needs run in-house, alongside the cutting, forming and welding that made the part. The bulk chemical finishes — plating, passivation and electropolishing — are carried out by audited partner companies and declared as such on the quotation, so the finishing route and its lead time are known before the order is placed rather than discovered afterwards. The partner structure is set out under surface treatment.
Frequently asked
What is the difference between polishing and electropolishing?
Polishing is mechanical: an abrasive removes material and leaves a direction. Electropolishing is chemical: the surface is dissolved evenly, which leaves no direction, reaches into recesses a belt cannot, and improves corrosion resistance. For stainless parts that have to be hygienic, electropolishing is usually the answer.
Can a polished part still be welded?
Yes, but the weld is made first and the finish follows, or the weld area is masked and refinished afterwards. Polishing a finished face and then welding next to it undoes the finish.
How do I specify a brushed finish on a drawing?
Three things: the grit, the grain direction, and whether welds have to be blended into it. Add a sample or a photo where the appearance matters — that removes the argument later.
Does polishing change the dimensions?
It removes material, so where a surface is both polished and toleranced, the finish allowance is added to the machining stock. This is why a surface that is both functional and visible is called out at DFM stage rather than at the end of the job.
Related: metal finishing and secondary operations · grinding · surface treatment · deburring · powder coating