Two lines can run the same stainless coil through the same grit sequence and come out with two different surfaces. One has a clean, continuous grain that takes PVD coating evenly. The other shows banding under a raking light, or a bright streak along one edge that only appears after coating — when it is far too late to fix.
The abrasive is rarely the problem. What separates the two is contact pressure, belt speed, and how the material is held against the head. This guide covers the grit sequence, the four parameters that actually control the grain, and what each defect tells you about which one is off.
What a hairline finish actually is
Hairline (HL) is a unidirectional ground finish. A coated abrasive belt runs against the sheet or coil in one direction only, leaving fine, continuous lines that follow the rolling direction. It is not a finer version of No.4 — it is a different surface, produced with a different head setup and a different grit progression.
The practical difference shows up in three places: roughness, reflectivity, and how the surface behaves downstream.
| Finish | Typical Ra | Grain | Where it is used |
|---|---|---|---|
| No.4 | 1.0 – 1.5 μm | Short, coarse, non-directional | Kitchen equipment, general fabrication |
| Hairline (HL) | 0.4 – 0.8 μm | Long, continuous, directional | Elevator panels, appliance fronts, cladding |
| SB / vibration | 0.2 – 0.5 μm | Non-directional, matte | Appliances, architectural interiors |
| 8K mirror | under 0.05 μm | None (reflective) | Decorative mirror panels |
Those ranges are typical values, not specifications — the number you land on depends on grit, pressure and how many passes you run. What matters is consistency across the coil, not hitting an exact figure.
Hairline is also the standard base surface for PVD coating and anti-fingerprint treatment. A grain that looks acceptable in daylight can still fail after coating, because the coating amplifies any variation in the underlying texture. That is why process control matters more here than on a decorative No.4.
The grit sequence
Hairline is built in stages, and each stage has one job. Skipping a stage to save a pass is the most common reason a line produces a grain that looks right but measures wrong.
Stage 1 — Foundation grinding
This pass removes rolling marks, light scratches and the original mill finish (2B, BA, No.1). Typical grit: #120 to #180. The goal is a flat, uniform scratch pattern with no residual features from the incoming surface. If you can still see the original 2B texture after this stage, the pressure is too low or the belt is worn.
Stage 2 — Grain definition
This is where the hairline actually forms. Typical grit: #240 to #320. The lines produced here are the ones the customer sees, so the belt must be fresh, the pressure constant, and the feed rate steady. Any variation at this stage cannot be corrected later without removing the grain entirely.
Stage 3 — Refinement
Optional on most architectural work, essential for PVD base surfaces. Typical grit: #400, occasionally #600 for very fine grain. This pass reduces Ra without destroying the directional pattern. Over-polishing here is a real risk — push it too far and the grain washes out into a semi-mirror with visible swirl.
One rule that saves a lot of scrap: never reverse direction between stages. Every pass runs the same way as the rolling direction. A single reversed pass leaves cross scratches that no amount of subsequent grinding will hide, because they sit at an angle to the grain.
The four parameters that control the grain
Grit gets the attention, but on a running line the surface is decided by four settings. When a customer sends a sample that "doesn't match the last batch", one of these has drifted.
1. Contact pressure
Typically held in the 0.2 – 0.5 MPa range for hairline heads, applied through pneumatic or hydraulic cylinders. Too low and the belt skates, leaving a shallow grain that reads as uneven gloss. Too high and the belt loads up, generating heat and producing a darker, smeared surface. The number matters less than the stability — a ±10% swing across the sheet width will show as banding under raking light.
2. Belt speed
Most hairline heads run belts at 18 – 28 m/s. Higher belt speed gives a finer, brighter cut at the same grit. Lower speed cuts deeper and darker. If you change belt speed without changing grit, you change the finish — which is why speed and grit should be treated as one setting, not two.
3. Feed rate
Sheet lines typically feed at 4 – 12 m/min, continuous coil lines run considerably faster, in the 20 – 60 m/min range. Feed rate controls how long each area sits under the head. Faster feed means fewer scratches per unit area, which reads as a lighter grain. Slower feed means more overlap and a denser, darker finish.
4. Belt tension and tracking
Tension holds the belt flat against the contact roll, tracking keeps it from wandering. A belt that oscillates even slightly produces a grain with a subtle wave that is nearly invisible on the line and very visible on a finished panel. Automatic tension and tracking are not convenience features — on a production line they are what makes the previous three parameters repeatable.
Sheet or coil: two different machines
This is where a lot of buying decisions go wrong. Sheet and coil hairline lines solve different problems and are not interchangeable.
A sheet polishing line handles cut-to-length panels, usually 1,000 – 1,500 mm wide and 0.5 – 3 mm thick. Panels are fed individually, which means the head sees a start and an end on every piece. The critical control is edge quality: the moment the head engages and disengages, pressure spikes, and that spike is what produces the bright band 20 mm in from each end that customers reject.
A coil polishing line runs continuously, so there is no entry or exit transient — but the material never stops moving, which makes tension control the dominant variable. Coil lines also have to handle strip shape: a coil with edge wave will produce a grain that is heavier in the centre than at the edges, no matter how well the head is set.
If your output is mixed, the honest answer is that one machine will do one job well and the other adequately. Decide which product carries your margin before choosing the line.
Reading the surface: what each defect tells you
Defects are diagnostic. Each one points at a specific parameter, and once you know which, the fix is usually a setting rather than a rebuild.
| Symptom | Most likely cause | Check first |
|---|---|---|
| Banding — alternating light and dark stripes across the width | Pressure fluctuation, or uneven belt wear | Cylinder pressure stability, belt running hours vs. rated life |
| Cross scratches at an angle to the grain | Debris on a guide roll, or rolls running out of sync | Roll surfaces, drive synchronisation |
| Bright streak down one edge | Local over-pressure, or a belt joint passing through | Parallelism of the contact roll, belt splice quality |
| Chatter — regular ripple along the feed direction | Resonance between drive and frame, or a worn bearing | Spindle bearings, whether the ripple frequency tracks line speed |
| Uneven gloss across the sheet | Insufficient coolant, or hardness variation in the incoming material | Coolant flow and concentration, incoming coil certificate |
| Grain disappears after PVD | Ra too low — surface too smooth for the coating to key into | Ra measurement before coating, not after |
The last row is the one that costs the most money. A surface that measures 0.2 μm looks superb in the inspection bay and fails at the coater, because there is not enough texture for the coating to bond. If the panel is going to PVD, target the middle of the Ra range and verify with a roughness tester, not by eye.
Quality control that actually catches problems
Visual inspection under normal light catches almost nothing on a hairline surface. Three checks do the work:
- Raking light. Hold a light source at a low angle across the panel. Banding and chatter that are invisible straight-on become obvious. This is the single most useful check and costs nothing.
- Ra measurement at three points. Centre and both edges, same direction as the grain. A reading that varies by more than about 0.15 μm across the width means the head is not applying uniform pressure.
- Gloss at 60°. Consistency of gloss across a batch tells you whether the process held steady over the run, which Ra alone will not show.
For export orders, add a retained sample. When a customer disputes a batch six weeks later, a physical reference from the approved run ends the conversation faster than any measurement report.
Choosing a hairline machine
Most specification sheets list the same headline numbers. The differences that matter in production are less visible:
- Whether the head is fixed or floating. A floating head follows material thickness variation, a fixed head does not, which shows up as pressure variation on coil with thickness tolerance.
- How the belt is changed. A belt change that takes 20 minutes limits you to long runs. One that takes 5 minutes lets you switch grit mid-shift, which is what small mixed orders actually need.
- Whether tension and tracking are automatic. Manual adjustment works on a sheet line running one product. On a coil line running three, it does not.
- Coolant handling. Filtration and temperature control determine whether the finish stays consistent through an eight-hour shift.
Speed is the number buyers ask about and the number that matters least. A line that runs 20% slower and holds its parameters will out-produce a faster line that needs re-setting every few coils.
Frequently asked questions
Is hairline the same as No.4?
No. No.4 is a shorter, coarser, non-directional grain around Ra 1.0 – 1.5 μm. Hairline is longer, finer and strictly directional, around Ra 0.4 – 0.8 μm. They are produced on different heads and are not interchangeable on a specification. If you need the visual difference explained side by side, see the No.4 finish page and the hairline finish page.
Which stainless grades can be hairline finished?
304, 316L, 430 and 201 are all routine. The variable is not the grade but the hardness and surface condition — 430 work-hardens faster and needs lighter pressure, while 201 from different mills can behave quite differently under the same settings. Always run a trial panel from the actual coil.
Can one machine run both sheet and coil?
Some lines are built to do both, and they compromise. Sheet feeding needs a start-stop cycle with edge control, coil needs continuous tension. If your volume in either format is meaningful, a dedicated line will hold tolerance better.
Can hairline be processed further?
Yes — PVD, anti-fingerprint coating, laser engraving, etching and protective film are all common. The constraint is Ra: too smooth and the coating will not key, too rough and it shows through. Agree the Ra target with the coater before you grind the first panel.
How thick can the material be?
Most sheet hairline lines handle 0.5 – 3 mm, coil lines typically run 0.3 – 2 mm. Beyond that, the head pressure needed to cut starts to distort the strip, and you are into a different machine class.
Where this fits in a finishing line
Hairline is usually one stage in a longer sequence — grinding before it, buffing or coating after it. If you are laying out a full line rather than adding a single head, the order of stages and the transfer between them matter as much as any individual machine. The elevator panel application page walks through one complete configuration, and the hairline and SB buffing machine covers the case where both finishes are needed on the same line.
If you are still deciding between finishes rather than tuning a process, start with the No.4 finish guide — it explains the grit logic that hairline builds on.