Surface Roughness (Ra) Chart: How to Specify Finish on CNC Parts

"Just make it smooth" is the most expensive sentence on a machining drawing. Surface roughness (Ra) is a measurable, controllable specification — and every step you tighten it adds grinding, lapping or polishing time to the quote. This guide explains what Ra actually means, gives you a usable chart, and shows how to specify finish so you pay for what the part needs and nothing more.

What Ra Actually Measures

Ra is the arithmetic average of the microscopic peaks and valleys on a surface, measured in micrometres (µm) or microinches (µin). Two things to understand before you write it on a drawing:

• Ra says nothing about direction. A turned surface and a milled surface can have identical Ra but completely different sealing behaviour, because the lay (tool marks) runs in different directions.
• Ra is an average, so outliers hide. A surface with one deep scratch and otherwise perfect finish can still pass an Ra check. Where that matters, specify Rz (maximum height) or Rmax instead.

Surface Roughness Chart

Process	Typical Ra (µm)	Typical Ra (µin)	Appearance / use
Rough milling / sawing	12.5 – 25	500 – 1000	Visible tool marks; clearance faces, weld prep
Standard CNC milling / turning	3.2 – 6.3	125 – 250	Default as-machined finish; most brackets, housings
Fine CNC milling / turning	1.6 – 3.2	63 – 125	Good finish, minimal marks; mating faces
Fine finishing pass / reaming	0.8 – 1.6	32 – 63	Smooth; bearing bores, seal seats
Cylindrical grinding	0.4 – 0.8	16 – 32	Precision shafts, hardened parts
Fine grinding / honing	0.1 – 0.4	4 – 16	Hydraulic cylinders, precision spindles
Lapping / polishing	0.025 – 0.1	1 – 4	Optical, sealing, mirror finishes

The Cost Curve

Roughness does not scale linearly with cost — it scales worse. Taking a milled aluminium part from 3.2 µm to 1.6 µm might add 10% to cycle time. Pushing the same part to 0.4 µm may require a separate grinding operation, a new setup and an inspection step, and can double the price. Below about 0.1 µm you are into lapping and polishing, where cost is driven almost entirely by hand time.

The practical rule: specify the coarsest finish that meets the function. If a face only carries a cosmetic cover, 6.3 µm is fine. If it seals an O-ring, 1.6 µm is usually right. If it is a bearing bore, 0.8 µm. Do not default the whole drawing to 0.8 µm because it "looks professional".

Comparing to Other Surface Standards

Standard	Roughness value	Approx. Ra
N5	Ra 0.4 µm	0.4 µm
N6	Ra 0.8 µm	0.8 µm
N7	Ra 1.6 µm	1.6 µm
N8	Ra 3.2 µm	3.2 µm
N9	Ra 6.3 µm	6.3 µm
SPI A1–A3	Mirror polish	0.025 – 0.1 µm
SPI B1–B3	Semi-gloss	0.1 – 0.4 µm
SPI C1–C3	Matte	0.4 – 1.2 µm
SPI D1–D3	Textured / dull	1.2 – 6.3 µm

The SPI scale is written for moulds and tooling, but buyers often borrow it for machined cosmetic parts. If you mean SPI, say SPI — do not write "Ra similar to SPI B1".

Anisotropy: Where Ra Misleads You

Two surfaces can read Ra 1.6 µm and behave nothing alike:

• A turned shaft has circumferential lay — tool marks run around the part. Great for a rotating seal, poor for a face seal, because the sealing line crosses the grooves.
• A milled face has parallel lay from the tool path. Good for a static gasket, poor for a sliding surface that will drag across the marks.
• Cross-hatched or random lay from lapping holds oil and is the standard for cylinder walls and slideways.

So specify lay direction as well as Ra whenever the surface seals, slides or wipes. On the drawing this is a symbol, not a sentence — which is exactly why our guide to machining surface finish symbols is worth five minutes.

How to Specify Finish Properly

1. Mark only the faces that need it. A blanket "Ra 0.8 all over" note is the single biggest avoidable cost on most drawings.
2. Use the standard note — "Unless otherwise specified, Ra 3.2 µm" — and then call out exceptions on individual faces.
3. Add lay direction where the surface seals or slides.
4. Add Rz where a single deep defect would cause a leak or a fatigue crack.
5. State the inspection method if you intend to check it: profilometer trace length, or visual comparison with a roughness specimen.

Get this right and you remove the two most common causes of quote disputes and rejected parts.

How Renjiu Achieves Tight Finishes

We produce as-machined finishes to Ra 3.2 µm as standard, Ra 1.6 µm with a finishing pass, and Ra 0.4 µm and below via grinding and lapping partners under the same quality system. Finish is verified during first-article inspection as part of our QC process. Post-processing such as anodising, plating and passivation changes both roughness and dimension, so tell us the final finish when you request the quote. Upload the drawing through our RFQ form and we will flag any callout that is going to drive cost before we quote it.