Surface Finish Guide for CNC Machined Parts: Ra Values, Methods & Standards

                                                                                                                                     

Surface finish — also called surface roughness or surface texture — is one of the most important yet often misunderstood parts of CNC machining. It affects how a part looks, how it feels, how it works in assembly, how it resists wear and rust, and ultimately how much it costs.

Many engineers specify surface finish wrong — either too loose (causing part failures) or too tight (raising costs for no reason). This complete guide explains everything you need to know about surface finishes for CNC machined parts: Ra values, measurement methods, achievable finishes by process and material, and how to specify surface finish correctly on your drawings.

Based on 25+ years of precision manufacturing experience at Eastmaster Manufacturing Limited in Shenzhen, China.

What Is Surface Finish? Understanding Ra, Rz, and Surface Texture

Surface finish describes the texture of a machined surface at a tiny, close-up level. No machined surface is perfectly smooth — every process leaves behind a pattern of peaks and valleys. The key numbers used to measure surface finish are:

Ra — Simple Average Roughness

Ra (also written as Rₐ) is the most commonly used surface finish number. It is the simple average of all the ups and downs of the surface profile from the center line over a sample length.

    Measured in micrometers (μm) or microinches (μin)

    Lower Ra = smoother surface

    Most commonly used surface finish number on engineering drawings

    Does not tell peaks and valleys apart — two surfaces with the same Ra can feel very different

Rz — Maximum Height of the Profile

Rz measures the biggest height difference between the highest peak and the deepest valley within the sample length. It is more sensitive to extreme highs and lows than Ra.

    Usually 4-8× larger than the Ra value for the same surface

    Common in European and ISO drawings

    Better for jobs where peak-to-valley height matters (sealing, coating)

Other Parameters

    Rq (RMS): Root mean square roughness — similar to Ra but uses a different math formula (Rq ≈ 1.1 × Ra)

    Rmax: Biggest peak-to-valley height over the full check length

    Rsk (Skewness): Describes whether peaks or valleys dominate — positive = more peaks, negative = more valleys

    Rku (Kurtosis): Describes how sharp the peaks are — high = sharp peaks

Ra Value Chart: From Rough to Mirror Finish

The table below shows common Ra values, what they look and feel like, typical machining methods, and uses:

Ra (μm)	Ra (μin)	Surface Look	Typical Process	Uses
25	1000	Very rough, deep tool marks	Rough turning/milling	Non-working surfaces
12.5	500	Rough, visible tool marks	Rough machining	Structural, non-critical
6.3	250	Moderate, tool marks visible	Standard milling/turning	General engineering
3.2	125	Smooth-ish, faint tool marks	Finish machining	General machined parts
1.6	63	Smooth, tool marks barely visible	Fine machining	Moving parts, sliding fits
0.8	32	Very smooth, no visible marks	Precision machining/grinding	Sealing faces, bearings
0.4	16	Near mirror, faint reflection	Fine grinding/polishing	Optical, medical, hydraulic
0.2	8	Mirror-like reflection	Lapping/superfinishing	Optical mirrors, seals
0.1	4	Perfect mirror	Polishing/lapping	Laser optics, precision optics
0.05	2	Super mirror	Magnetorheological polishing	Semiconductor, laser

Rule of thumb: Each step down (halving Ra) usually raises cost by 30-50%.

Achievable Surface Finish by CNC Machining Process

Different CNC processes produce different surface finish ranges. Here is what is usually possible:

CNC Process	Standard Finish	Best Achievable	Key Factors
CNC Turning (standard)	Ra 1.6-3.2 μm	Ra 0.4 μm	Tool nose radius, feed rate
CNC Turning (diamond)	Ra 0.1-0.4 μm	Ra 0.025 μm	Single crystal diamond tool
CNC Milling (standard)	Ra 1.6-3.2 μm	Ra 0.8 μm	Stepover, tool shape
CNC Milling (high-speed)	Ra 0.4-0.8 μm	Ra 0.2 μm	Spindle speed, small stepover
Surface Grinding	Ra 0.4-0.8 μm	Ra 0.1 μm	Wheel grit, feed rate
Cylindrical Grinding	Ra 0.2-0.4 μm	Ra 0.1 μm	Wheel speed, dwell time
Lapping	Ra 0.05-0.1 μm	Ra 0.025 μm	Abrasive, pressure, time
Electropolishing	Ra 0.2-0.4 μm	Ra 0.1 μm	Current density, time

Surface Finish by Material

Material properties greatly affect what surface finish you can get. Here is what to expect:

Metals

    Aluminum (6061/7075): Ra 0.4-0.8 μm standard; Ra 0.1 μm with diamond tooling — excellent ease of machining gives outstanding finishes

    Stainless Steel (304/316): Ra 0.8-1.6 μm standard; Ra 0.2 μm with grinding — work hardening makes ultra-smooth finishes harder

    Carbon Steel: Ra 0.8-1.6 μm standard; Ra 0.2 μm with grinding — good ease of machining, predictable results

    Titanium: Ra 0.8-1.6 μm standard; Ra 0.4 μm possible — sticking tendency limits finish quality

    Brass/Copper: Ra 0.2-0.8 μm standard; Ra 0.05 μm possible — soft materials polish very well

Engineering Plastics

    PEEK: Ra 0.4-0.8 μm (unfilled); Ra 0.8-1.6 μm (filled) — sharp tools are a must

    POM (Delrin): Ra 0.2-0.8 μm — excellent ease of machining gives very smooth surfaces

    PTFE: Ra 0.4-1.6 μm — soft material needs special tooling

    PC (Polycarbonate): Ra 0.4-0.8 μm — can reach optical-quality finishes

    ABS: Ra 0.8-1.6 μm — vapor polishing can reach near-mirror finishes

Factors Affecting Surface Finish in CNC Machining

Many factors affect the final surface finish of a CNC machined part. Understanding these helps you save money:

    Tool Nose Radius (Turning): Larger nose radius gives smoother finishes but increases cutting force. For fine finishes, use 0.8-1.2 mm nose radius.

    Feed Rate: The single biggest factor. Surface roughness goes up with the square of feed rate. Halving the feed rate cuts Ra by about 75%.

    Cutting Speed: Higher speeds usually improve finish by reducing built-up edge and vibration. But too high a speed causes tool wear and heat damage.

    Stepover/Stepdown (Milling): Smaller stepover gives finer finishes. For mirror finishes, stepover may be as low as 5% of tool diameter.

    Tool Sharpness: Dull tools tear instead of cut, leaving rougher surfaces. Sharp or freshly honed tools always give better finishes.

    Tool Material & Coating: Diamond-coated tools on aluminum give the finest finishes. Uncoated carbide is standard for most steels.

    Coolant/Lubrication: Good coolant reduces heat and prevents built-up edge, improving surface quality.

    Machine Rigidity: Vibration and chatter are the enemies of fine finishes. Rigid machines, short tool overhang, and good workholding are key.

    Material Properties: Hard, brittle materials usually give better finishes than soft, gummy ones. Free-machining alloys (with sulfur or lead) improve finish.

Post-Machining Surface Treatments

When CNC machining alone cannot reach the desired surface finish, extra treatments can fill the gap:

Mechanical Finishing

    Vibratory Tumbling / Barrel Finishing: Batch finishing for deburring and smoothing. Reaches Ra 0.4-0.8 μm. Good for large batches.

    Belt Sanding / Polishing: Manual or automated belt sanding for flat surfaces. Reaches Ra 0.2-0.8 μm.

    Buffing / Polishing: Mirror-finishing using progressively finer abrasives. Reaches Ra 0.05-0.2 μm.

    Lapping: Precision flat surface finishing using abrasive slurry. Reaches Ra 0.05-0.1 μm with excellent flatness.

    Shot Peening / Sandblasting: Creates uniform matte texture for looks or paint adhesion. Does not reduce Ra but gives a consistent look.

Chemical / Electrochemical Finishing

    Electropolishing: Removes surface material using electricity, smoothing peaks first. Great for stainless steel. Reaches Ra 0.2-0.4 μm.

    Chemical Polishing: Dissolves surface material evenly. Simpler than electropolishing but less controlled.

    Vapor Polishing (for plastics): Acetone vapor smooths plastic surfaces by melting the top layer. Reaches near-mirror finish on ABS, PC.

    Etching / Passivation: Does not improve roughness but creates uniform surface chemistry for rust resistance.

Coating / Plating

    Anodizing (Aluminum): Creates hard, uniform oxide layer. Can be Type II (decorative) or Type III (hard). Slightly raises surface roughness.

    Powder Coating: Thick polymer coating that hides small surface flaws. Typical thickness 60-120 μm.

    Chrome / Nickel Plating: Hard, wear-resistant metal coating. Fills small surface defects. Can be polished to mirror finish.

    PVD/DLC Coating: Thin (1-5 μm), very hard coating. Keeps base surface finish while adding wear resistance.

How to Specify Surface Finish on Drawings

Correct surface finish specification is key for clear communication with your CNC machining supplier. Follow these guidelines:

ISO Surface Finish Symbols

    √ (basic symbol): Surface made by any method

    √ with bar through it: Material removal required (machining)

    √ with circle: Material removal not allowed (cast/forged surface as-is)

    Ra value after the symbol: e.g., √ Ra 1.6 means Ra ≤ 1.6 μm

    If no value given, default is usually Ra 3.2 μm or Ra 6.3 μm

Best Practices

    Specify surface finish only on working surfaces — every extra surface with a tight finish raises cost

    Use the roughness symbol with material removal bar (√ with bar) on all machined surfaces

    For general surfaces, use a default note: 'Unless otherwise specified, surface finish Ra 3.2 μm'

    For critical surfaces, specify both Ra and, if needed, Rz or Rmax for full surface control

    Note the sampling length (cutoff) if different from default — usually 0.8 mm for Ra ≤ 2 μm

    Specify the lay direction (marking direction) when important for function — e.g., for sealing or oil retention

    Talk to your supplier early — they may suggest cheaper options that still work

How Surface Finish Affects CNC Machining Cost

Surface finish is a big cost driver in CNC machining. Understanding the cost impact helps you make smart choices:

Surface Finish	Relative Cost	Process Needed	When to Specify
Ra 6.3-3.2 μm	1.0× (baseline)	Standard machining	General non-critical surfaces
Ra 1.6-0.8 μm	1.2-1.5×	Fine machining, slower feed	Mating surfaces, sliding fits
Ra 0.8-0.4 μm	1.5-2.5×	Precision machining or light grinding	Sealing faces, bearing seats
Ra 0.4-0.1 μm	2.5-5.0×	Grinding, polishing, lapping	Optical, hydraulic, medical
Ra < 0.1 μm	5.0-20×	Superfinishing, lapping	Laser optics, semiconductor

Key cost-saving tips:

    Only specify tight surface finishes on surfaces that truly need them

    Consider post-machining treatments as cheaper ways to get fine finishes

    Larger surfaces cost more to finish fine — specify fine finish only on the actual working area

    Some materials reach fine finishes more cheaply than others (aluminum > stainless steel > titanium)

Surface Finish Specification Checklist

    Only working surfaces have tight surface finish requirements

    Default surface finish set for non-critical surfaces (usually Ra 3.2 μm)

    Surface finish values are possible with the chosen CNC process

    Measurement method is practical (profilometer, comparator, Ra tester)

    Material chosen can reach the specified finish

    Post-machining treatments considered as cheaper options

    Cost impact of surface finish reviewed

    Surface finish symbol and Ra value properly shown on drawing

    Lay direction noted if it matters for function

    Sampling length (cutoff) set for non-standard measurements

Why Choose Eastmaster for Superior Surface Finishes

At Eastmaster Manufacturing Limited, reaching excellent surface finishes is part of our standard CNC machining capability.

    High-speed CNC machining centers with spindles up to 24,000 RPM for great surface finishes

    Diamond tooling capability for ultra-fine finishes on aluminum and copper alloys

    In-house grinding, lapping, and polishing for mirror finishes

    Surface roughness testers (Ra, Rz) for accurate measurement and checking

    Trusted partners for anodizing, plating, powder coating, and other surface treatments

    ISO 9001 and ISO 13485 certified quality management

    D-U-N-S verified: 66-849-8033

Contact Eastmaster with your drawing requirements. Our engineering team will review your surface finish needs, suggest the most cost-effective approach, and provide a detailed quote — usually within 24 hours.

• Email: info@eastmaster.com 
• Phone: +86 755 22676100
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