- Home
- /
- Surface Treatment
- /
- Mechanical Finishing
- /
- Brushing / Hairline
Brushing &
Hairline
Finish
Uniform, controlled abrasive grain texture on aluminium, stainless steel, and titanium — the surface that defines premium consumer electronics, precision instrument housings, and architectural panels. Consistent grit, consistent direction, consistent Ra. Reference-sample matched on every batch.
Controlled Directional Grain —
Not Random, Not Approximate
Brushing (hairline finishing) produces a uniform, directional scratch pattern on a metal surface — an abrasive belt or pad moves in a single, controlled direction, creating parallel grain lines that all run the same way. The result is the characteristic fine linear texture seen on premium aluminium laptops, professional audio panels, medical device housings, and precision instrument fronts.
The critical distinction: polishing aims to eliminate surface texture and minimise Ra. Brushing aims to create and control surface texture — maintain a specific Ra at a consistent level (Ra 0.4–1.6 μm depending on grit), with every scratch running in the same direction. A brushed surface polished in the wrong direction — even at a lower Ra — loses the grain appearance entirely and must be re-brushed.
Grit selection is the primary specification. 120 grit produces a coarse industrial satin; 320–400 grit produces the ultra-fine hairline of premium electronics and medical instruments. We match every batch to a physical reference sample signed off by the customer — under the same lighting angle and viewing distance used in final inspection.
Four Standard Grades —
Each with a Distinct Visual Character and Application
120 Grit — Coarse Satin (ASTM A480 No.4)
The boldest standard brushing grade — a clearly visible coarse grain that reads as deliberately textured under any lighting. On stainless steel (304/316L), 120 grit produces the ASTM A480 No. 4 directional satin finish — the most widely specified stainless steel surface finish in the world for food-processing equipment, architectural panels, and industrial housings. Widely acceptable for CIP (clean-in-place) surfaces per 3-A Sanitary Standards.
On aluminium, 120 grit is typically used as a preparatory step before progressing to a finer grit, or as the final finish on industrial enclosures and structural panels where robustness matters more than aesthetic refinement. Not typically specified for pre-anodise on appearance-grade parts — anodising over 120 grit produces a noticeably coarse texture in the finished part.
240 Grit — Standard Satin (Pre-Anodise Production Standard)
The most common brushing grade for aluminium parts before Type II anodising — fine enough for a consistent, even anodise colour with no patchy uptake, and economical enough for high-volume production. The Ra 0.8 μm produced by 240 grit anodises evenly over large flat areas — no mottling or streaking. After clear (natural) anodise, the result is a consistent silver-grey satin that reads as professional and well-finished.
On aluminium appliance panels, commercial kitchen equipment, and industrial enclosure fronts that need to look good without the cost of a premium finish, 240 grit pre-anodise is the right specification. It is also the correct grade for aluminium parts that will receive coloured Type II anodise (black, gold, red, blue) — the grain uniformity at 240 grit ensures consistent dye uptake and even colour across the whole surface.
320 Grit — Fine Hairline (Premium Electronics Standard)
The most frequently specified grade for premium consumer electronics, professional audio, medical device housings, and precision instrument panels. The 320 grit grain is fine enough to appear almost mirror-like under diffuse light but clearly directional under raking light — producing the refined look customers describe as the 'premium laptop' or 'professional instrument' texture. After Type II clear anodise over 320 grit brushed aluminium, the result is a consistent, controlled silver-grey that communicates quality.
We hold signed physical reference samples for all 320 grit production parts and compare every batch to the reference under standardised 45° raking light. If the grain is inconsistent — visible pressure marks, uneven grit depth, or direction change — the part is re-brushed. The sample sign-off process, combined with documented fixture direction and controlled belt pressure, ensures every shipment matches the reference exactly.
400 Grit — Ultra-Fine Hairline
The finest standard brushing grade — Ra 0.4 μm produces a surface that approaches a satin polish under diffuse light, with only a very subtle grain visible under raking light. Specified for the highest-grade consumer products, Class 1 medical device panels, and precision optical instrument housings where the brushed texture must be barely perceptible but still present — avoiding the 'mirror' look while maintaining the premium texture.
400 grit is also specified on titanium parts that will be anodised for colour — titanium's strong anodise interference colours (gold at 20V, blue at 40V, purple at 60V) appear more vivid and uniform over a 400 grit surface than a coarser one. The finer the pre-anodise surface, the more consistent the interference colour over complex 3D titanium geometry.
Brushing Specifications — What We Confirm Before and Verify After
Every parameter documented on the route card and verified at final inspection against the signed reference sample.
| Parameter | Specification | How We Control It | Measurement |
|---|---|---|---|
| Grit Grade | 120 / 180 / 240 / 320 / 400 grit Per drawing callout or reference sample specification |
Grit grade on route card. Same grade belt throughout the batch — no mid-batch belt changes without supervisor sign-off. | Visual comparison to signed reference sample under 45° raking light. Every batch checked before release. |
| Grain Direction | Linear parallel, linear perpendicular, or radial Specified on route card or drawing |
Fixture orientation documented with diagram on route card. Same fixture used for every part in every batch. | Visual check — all grain lines must run in the same direction across the entire brushed surface area. |
| Surface Roughness Ra | Ra 0.4 μm (400g) to Ra 1.6 μm (120g) Per grit grade and material |
Controlled by grit grade and belt pressure. Consistent belt pressure maintained — documented by operator sign-off. | Contact profilometer (ISO 4287) on parts with explicit Ra callout on drawing. Reference sample comparison for all others. |
| Material Removal | 0.005–0.02 mm per surface Brushing at standard grit grades |
Negligible for most applications. Precision bores and threads masked before brushing adjacent surfaces. | Not measured routinely — masking confirmation check. Dimension check on masked precision features if called out. |
| Reference Sample | Signed customer sample, batch-matched Required for all appearance-grade brushed parts |
Sample held in QC archive. Same lighting setup (45° raking light, 600 lux) used for every batch comparison. | Batch sign-off sheet: inspector, date, lighting confirmed, pass/fail vs reference. Retained in quality records. |
| PE Film Protection | Applied within 5 min of completing brushed finish All brushed aluminium and titanium parts |
Film application is a mandatory step in the brushing work instruction. Route card sign-off required after film application. | Visual check that film covers all brushed surfaces completely. Part cannot be packaged without film confirmed. |
Real Brushing Projects — Specification, Challenge, and Result
Three production brushing jobs with actual grit grades, customer requirements, challenges, and final outcomes.
Premium Audio Equipment Panels — 320g Brushed + Type II Clear Anodise
Stainless Steel Equipment Panels — ASTM A480 No. 4 Finish, 3-A Hygienic
Titanium Instrument Handles — 400g Brushed + Colour Anodise (Blue/Gold)
Brushing by Material — Grit, Ra, and Application Notes
Brushing / Hairline — Technical Answers
Specific technical questions about specifying and producing brushed and hairline finishes on CNC machined parts.
320 grit for premium appearance-grade panels (consumer electronics, professional audio, medical instruments). 240 grit for standard production industrial panels. Coarser than 240 grit (120–180g) is generally not recommended for pre-anodise on appearance-grade parts — the anodise process accentuates the coarser grain and the result looks unrefined on flat panels.
For coloured anodise (black, gold, blue, red): 240 grit is the minimum for even dye uptake. Finer grit (320g) produces more consistent colour uniformity. Coarser grit produces visually uneven dye absorption that appears as mottled or streaked colour.
For hard anodise (Type III): the pre-brush surface matters less as the hard anodise layer obscures the fine grain texture. 240 grit is adequate for hard anodise pre-treatment.
For flat faces and simple curves: machine brushing with a belt running in a fixed direction — fully consistent. The fixture holds the part at a fixed angle to the belt, and the direction is documented on the route card.
For complex 3D geometry with multiple faces at different angles: the part is repositioned in the fixture for each face, with the direction of each face brushing documented separately. The join lines between adjacent faces (where the brush direction changes) are either hidden by design (internal corners, under edges) or the part geometry is designed so all visible faces can be brushed in one direction.
For impossible cases (e.g., deeply recessed areas that a belt cannot reach): hand brushing with a Scotch-Brite pad is used as a supplement. Hand brushing is less consistent than machine brushing — we advise at DFM if any face will require hand brushing so the customer knows what level of uniformity to expect.
Yes — brushing then anodising is the standard production sequence for premium aluminium parts. The brushed surface is the pre-anodise finish, not a post-anodise operation. The sequence is: CNC machine → brush → clean/degrease → Type II anodise → seal.
Critical: the brushed aluminium surface must be free of oil, fingerprints, and loose abrasive particles before entering the anodise bath. We ultrasonic clean all brushed parts before anodising. The PE protective film is removed just before the cleaning step. Any contamination on the brushed surface will appear as a spot, streak, or mark in the anodised finish.
Important: do not apply the brushed finish to already-anodised parts — the abrasive will cut through the hard anodise layer unevenly and produce a scratched appearance, not a brushed one. Strip anodise first, then brush, then re-anodise.
Standard drawing callout options: 'Brushed finish 320g, linear direction parallel to long axis, Ra 0.6 μm max, pre-anodise' (complete specification). Alternatively, the ISO 1302 surface finish symbol with Ra value and a drawing note: 'Brushed 320 grit before anodise, direction as indicated by arrow on drawing.'
If you have a physical reference sample: 'Surface finish to match supplied reference sample Ref. No. XXX-B01' — we register the sample in our QC system and match to it. This is the most reliable specification method for appearance-grade parts, as it avoids ambiguity about Ra values and grain appearance.
If in doubt, send us your drawing and a description of the desired appearance — we will confirm the correct grit grade and callout format at DFM.
The anodise process creates a transparent oxide layer that acts as a light-refracting film over the brushed surface. The anodised brushed grain appears slightly different from the pre-anodise brushed surface — typically slightly more reflective and with a cleaner, crisper grain. This is normal and expected.
Problems that cause unexpected post-anodise appearance: Mottled or uneven colour — caused by non-uniform brushing (pressure variations), contamination on the brushed surface before anodise, or alloy content variation in the aluminium. Loss of grain visibility — caused by over-anodising (too thick a layer) or high anodise bath temperature, which produces a more opaque oxide that fills the grain valleys. Patches — caused by fingerprints or oil on the brushed surface before anodise.
We quality-check the brushed surface before releasing it for anodising — any surface defects are re-brushed at this stage, before the anodise tank, not after.
Brushing: directional, consistent, controlled grain — Ra 0.4–0.8 μm for 240–400g grades. After anodise: consistent directional sheen, premium appearance, 'hairline' grain. Preferred for flat panels, enclosures, and any part where the grain direction contributes to the visual design.
Sandblasting (glass bead): non-directional, uniform matte texture — Ra 1.6–2.4 μm. After anodise: consistent non-directional matte or satin. Preferred for complex 3D geometry where brushing direction cannot be controlled consistently, and for parts where a matte (rather than directional satin) appearance is specified.
Rule of thumb: flat panels → brush. Complex 3D shapes → blast. Both produce consistent anodise colour uptake when done correctly. The choice is primarily aesthetic and geometric, not functional.