Surface Finish Specification For CNC Parts: Ra Values, N Grades, And Production Methods-Changzhou Yuqing Heavy Machinery Co., Ltd

Why Is Surface Finish a Distinct Engineering Parameter?

Surface finish is what the surface looks like and how it interacts with the world. A rough surface holds oil and resists wear; a smooth surface seals against leakage and looks polished. The function of the surface determines the finish it needs, and the function of the surface is rarely the same across a single part.

A bearing journal needs a fine finish to support the bearing and to resist wear. A mounting flange needs a moderate finish to seat against the mating surface. A cosmetic cover needs a specific finish to look right and to hide minor imperfections. A gripping surface needs a coarse finish to provide traction. A single part may require all four.

The CNC machine tool processing capability at Yuqing can deliver a range of finishes; the engineer should specify only the finish that the function requires.

What Is Surface Roughness?

Surface roughness is the measurement of the fine irregularities on a machined surface. It is measured as the vertical deviation from a mean line over a defined sample length.

The three most common parameters:

Parameter	Definition	Application
Ra	Arithmetic average of absolute deviations	General purpose, most common
Rz	Average of the five highest peaks and five lowest valleys	Worn surfaces, sealing
Rt	Maximum peak-to-valley height	Critical surfaces, plating base

Ra is the default parameter in most drawings. Rz is used for sealing surfaces where the peak height matters more than the average. Rt is used for plated surfaces where the plating must cover the peaks.

What Are the Standard Ra Values?

ISO 4287 / ISO 4288 defines a preferred series of Ra values:

Ra (µm)	Ra (µin)	Typical Application	Typical Process
0.1	4	Precision gauges, optical surfaces	Honing, lapping
0.2	8	Bearing seals, hydraulic cylinders	Fine grinding, honing
0.4	16	Bearing journals, hydraulic spools	Cylindrical grinding
0.8	32	Precision bearings, O-ring grooves	Fine turning, grinding
1.6	63	Bearing seats, gasket surfaces	Standard turning, fine milling
3.2	125	General machined surfaces	Standard milling, turning
6.3	250	Rough machined surfaces	Heavy milling, turning
12.5	500	Very rough, non-critical	Saw cut, flame cut
25	1000	As-forged, as-cast	Sand casting, forging

The preferred series doubles at each step (geometric progression of approximately 1.6). A drawing should specify one of these values rather than an intermediate value.

What Are N Grades?

N grades (ISO 1302) describe a range of roughness using a single symbol and a number. The grade is defined by the maximum and minimum Ra values within the grade.

N Grade	Ra Range (µm)	Typical Application
N1	0.025–0.05	Precision gauges
N2	0.05–0.1	Gauges, instruments
N3	0.1–0.2	Hydraulic cylinders
N4	0.2–0.4	Bearing journals
N5	0.4–0.8	Bearing seats
N6	0.8–1.6	General machined surfaces
N7	1.6–3.2	Rough machined surfaces
N8	3.2–6.3	Very rough surfaces
N9	6.3–12.5	As-forged
N10	12.5–25	As-cast
N11	25–50	Very rough cast
N12	50–100	Unfinished

N grades are useful when the function tolerates a range of finishes within a single grade, allowing the shop flexibility without inspection dispute.

What Is Lay?

Lay is the direction of the predominant surface pattern, left by the cutting tool. Lay matters for sealing surfaces (perpendicular lay leaks), for fatigue surfaces (parallel lay can initiate cracks), and for cosmetic surfaces (visible lay affects appearance).

Lay symbols per ISO 1302:

Symbol	Lay Direction
=	Parallel to the surface boundary
⊥	Perpendicular to the surface boundary
C	Circular
R	Radial
M	Multi-directional (cross-hatched)
P	Particulate (non-directional)

For a cylinder that must seal against an O-ring, the lay should be perpendicular to the axis (⊥) so the O-ring crosses the lay pattern. For a shaft that rotates against a seal, the lay should be circumferential (C) for the same reason.

How Is Surface Finish Specified on the Drawing?

The drawing should specify the finish in three layers:

General finish note: applied to all surfaces unless overridden. Defines the default expectation.

Surface symbol with Ra value: applied to specific surfaces with different requirements.

Notes for special finishes: applied where a special process is required (grinding, honing, polishing).

A typical general finish note is "All surfaces Ra 3.2 µm unless otherwise specified." This allows the shop to deliver all surfaces at 3.2 µm without further marking.

A specific surface is marked with the surface finish symbol and the Ra value:

┌───────

│ Ra 0.8

▼

──────────

The symbol's long line is the surface being specified; the small triangle points to the surface. The Ra value (or N grade) is written on the symbol.

How Is Surface Finish Produced?

The finish is determined by the cutting process, the tool geometry, the cutting parameters, the material, and any post-process operations.

Process	Typical Ra (µm)	Notes
Turning	0.4–6.3	Depends on feed, nose radius, speed
Milling	0.8–6.3	Depends on feed per tooth, cutter geometry
Drilling	1.6–12.5	Standard twist drill; reaming for finer finish
Reaming	0.4–1.6	For precise holes
Boring	0.4–3.2	For large precise holes
Grinding	0.1–1.6	For hard materials and fine finishes
Honing	0.05–0.8	For cylinder bores
Lapping	0.025–0.2	For gauges, sealing
EDM	0.4–3.2	For hard materials, complex shapes
Polishing	0.05–1.6	Cosmetic or sealing
Bead blasting	1.0–6.3	Cosmetic matte finish

A drawing that specifies Ra 0.4 µm on a feature that will be produced by standard milling is a drawing that requires a secondary grinding or honing process — and the cost of that secondary process must be in the budget.

How Does Material Affect Surface Finish?

The same process produces different finishes in different materials:

Material	Aluminum	Mild Steel	Stainless	Titanium
Standard turning	Ra 0.8	Ra 1.6	Ra 1.6	Ra 1.6
Fine turning (low feed)	Ra 0.4	Ra 0.8	Ra 0.8	Ra 0.8
Standard milling	Ra 1.6	Ra 3.2	Ra 3.2	Ra 3.2
Fine milling	Ra 0.8	Ra 1.6	Ra 1.6	Ra 1.6

Softer materials (aluminum) cut more cleanly and produce finer finishes for the same tool and parameters. Harder materials (titanium) require sharper tools and slower cutting to achieve equivalent finishes.

How Is Surface Finish Measured?

Surface finish is measured with a profilometer (stylus instrument) or a portable roughness tester. The measurement is performed perpendicular to the lay direction, over a defined sample length.

Measurement Type	Typical Use	Cost
Profilometer (lab)	High-precision measurement, calibration	High
Portable roughness tester	Shop floor inspection	Moderate
Visual / tactile comparison	Coarse finishes, rough inspection	Low
Replica tape (Press-O-Film)	Field measurement, internal surfaces	Low

For critical surfaces (bearing seats, sealing surfaces), a profilometer measurement is required. For general surfaces (mounting flanges, brackets), a tactile comparison against a reference sample may be sufficient.

What Is the Relationship Between Finish and Tolerance?

The achievable finish is loosely correlated with the achievable tolerance, but they are independent specifications:

A surface can be held to ±0.05 mm tolerance with Ra 3.2 µm finish (typical for as-cast surfaces that are machined for size only).

A surface can be held to ±0.5 mm tolerance with Ra 0.1 µm finish (typical for honed hydraulic bores).

The drawing should specify both, but they are independent. A shop that quotes a tight tolerance with a coarse finish is being optimistic; a shop that quotes a tight tolerance with a fine finish is being conservative.

How Is Surface Finish Specified for Sealing?

Sealing surfaces (O-ring grooves, gasket faces, hydraulic spools) require finishes that hold the seal under pressure:

Application	Typical Ra (µm)	Lay Direction	Notes
Static O-ring (radial)	0.4–0.8	⊥ (perpendicular to axis)	Prevents spiral leak
Static O-ring (axial)	0.2–0.4	M or random	Even compression
Dynamic O-ring (rotary)	0.05–0.2	C (circumferential)	Prevents seal wear
Hydraulic spool	0.1–0.4	C	Leak-free sliding
Gasket face (soft gasket)	1.6–3.2	M	Conforms to gasket
Gasket face (metal gasket)	0.4–0.8	M	Seats against small features

The sealing function dictates the finish. A drawing that calls out Ra 3.2 µm on a hydraulic spool will leak; a drawing that calls out Ra 0.05 µm on a soft-gasket face is over-specified.

How Is Surface Finish Specified for Fatigue?

Fatigue cracks initiate at surface defects. A smoother surface resists crack initiation:

Surface Condition	Relative Fatigue Life
Polished (Ra < 0.4 µm)	100% (baseline)
Ground (Ra 0.8 µm)	80–90%
Fine turned (Ra 1.6 µm)	70–85%
Standard turned (Ra 3.2 µm)	60–75%
Rough turned (Ra 6.3 µm)	50–65%

For fatigue-critical surfaces (springs, rotating shafts, loaded lugs), the drawing should specify a fine finish and may also specify compressive residual stress (shot peening, roller burnishing) to extend fatigue life further.

How Is Surface Finish Specified for Cosmetics?

Cosmetic surfaces require finishes that look right under the intended lighting:

Lighting	Recommended Finish	Notes
Direct sunlight / bright light	Ra 0.4–0.8	Reveals scratches
Indoor lighting	Ra 0.8–1.6	Standard appearance
Diffuse lighting	Ra 1.6–3.2	Tolerant of minor marks
Backlit / transmitted	Ra 0.4–0.8	Shows distortion
Painted	Ra 3.2–6.3	Paint hides minor marks

A drawing that specifies a cosmetic finish must also specify which surfaces are cosmetic, which are non-cosmetic, and what inspection method is used (visual comparison, profilometer, or both).

What Is the Cost Impact of Surface Finish?

Tighter finishes are more expensive:

Ra (µm)	Relative Cost Factor
6.3	1× (baseline)
3.2	1× (typical for CNC)
1.6	1.5× (slower feeds, sharper tools)
0.8	2–3× (fine turning, special tooling)
0.4	3–5× (grinding, longer cycle)
0.2	5–10× (fine grinding, honing)
0.1	10–20× (lapping, specialized process)

The cost increases because tighter finishes require slower cutting, finer feeds, sharper (more frequent) tooling, secondary processes, and longer inspection time.

How Are Surface Finish Requirements Communicated?

The drawing communicates the requirement, but the shop also needs:

The measurement method (profilometer, comparator, replica tape).

The sample length and the number of measurements.

The acceptance criterion (no measurement above the limit, average below the limit, or other).

The location of measurements (the worst area, a defined zone, or every instance).

A drawing that simply says "Ra 0.8 µm" without the measurement method and acceptance criterion allows the shop to use whatever is convenient. The result may be a part that the inspector passes but the customer rejects.

Conclusion

Surface finish specification is the engineering discipline that defines the texture and roughness of each surface for its function. The specification uses Ra (or N grade) for general purpose, paired with lay direction for critical surfaces, and is supported by the measurement method and acceptance criterion. The CNC machine tool processing and structural parts processing capabilities at Yuqing deliver a range of finishes; engineers should specify only the finish that the function requires, no finer, and pair it with the process that achieves it.

Frequently Asked Questions

What is the difference between Ra and Rz?

Ra is the arithmetic average of the absolute deviations from the mean line. Rz is the average of the five highest peaks and five lowest valleys within the sample length. Ra is more forgiving (averages out anomalies); Rz is more sensitive to peaks (important for sealing surfaces).

What is a typical surface finish for as-milled aluminum?

Ra 3.2 µm is typical for standard milling of aluminum with sharp carbide tools at moderate feeds. Ra 1.6 µm is achievable with finer feeds and sharper tools.

How does surface finish affect fatigue life?

Smoother surfaces resist fatigue crack initiation. A polished surface (Ra 0.4 µm) typically has 80–100% of the baseline fatigue life; a standard turned surface (Ra 3.2 µm) may have only 60–75%.

What is the lay direction for an O-ring sealing surface?

For a radial O-ring (cylinder into a bore), the lay should be circumferential (C) — perpendicular to the axis — so the O-ring rolls over the lay pattern as it expands. For an axial O-ring (face seal), the lay should be random (M) or parallel to the seal.

How is surface finish measured on a shop floor?

A portable roughness tester is the standard shop-floor instrument. The tester moves a stylus across the surface and displays the Ra value directly. The measurement is taken perpendicular to the lay direction.

What is the difference between N grade and Ra?

Ra is a single value (e.g., Ra 0.8 µm). N grade is a range (e.g., N5 = 0.4 to 0.8 µm). N grades give the shop flexibility while still defining the surface quality.

Can surface finish be improved after machining?

Yes. Secondary processes such as grinding, honing, lapping, polishing, and buffing can improve the finish of an already-machined surface. The drawing should specify the secondary process if the in-process finish is not sufficient.

What is the role of surface finish in corrosion resistance?

Smoother surfaces resist corrosion because they have fewer crevices for water and contaminants to collect. For marine or chemical environments, a smoother finish (Ra 0.8 µm or finer) plus a coating (anodizing, plating) is the standard protection.

How is bead blasting used for cosmetic finishes?

Bead blasting produces a uniform matte finish (Ra 1.0–6.3 µm) by impacting the surface with glass or ceramic beads. It hides minor machining marks and provides a consistent appearance across production parts.