Surface roughness, measured by the Ra (Arithmetic Average Deviation) value, is a critical indicator of CNC machining quality—it directly impacts part functionality, wear resistance, and fit. Whether you’re making consumer goods, industrial components, or medical implants, knowing the achievable Ra range of CNC processing and how to control it is essential. This article breaks down the RA values that CNC processing can reach across common methods, explains key influencing factors, and shares practical selection strategies.
1. RA Value Ranges by Common CNC Machining Methods
Different CNC machining techniques—from rough turning to ultra-precision grinding—deliver vastly different Ra values. Below is a detailed table of achievable ranges, tailored to help you match methods to your roughness needs.
| CNC Machining Method | Sub-Method | Achievable RA Value Range (μm) | Typical Application Scenarios |
| CNC Turning | Rough Turning | 20 – 10 | Initial shaping of metal blanks; parts with no surface finish requirements (e.g., temporary structural supports) |
| Semi-Finishing/Finishing Turning | 1.6 – 0.8 | General-purpose parts (e.g., low-speed shafts, non-critical housings) | |
| Mirror Turning (Diamond Tools for Non-Ferrous Metals) | 0.04 – 0.01 | High-gloss, precision parts (e.g., aluminum decorative components, optical instrument parts) | |
| CNC Milling | Rough Milling | 6.3 – 2.5 | Large structural parts (e.g., machine frames, bracket blanks) |
| Finish Milling | 1.6 – 0.63 | Fitted parts (e.g., sliding guides, gear housings) | |
| Super Fine Milling (High-Speed, Small Feed) | 0.4 | Precision mechanical components (e.g., high-speed bearing seats) | |
| CNC Boring | Ordinary Boring | 2.5 – 0.63 | Hole machining for general parts (e.g., hydraulic cylinder bores) |
| Fine Boring | 0.32 – 0.08 | High-precision holes (e.g., engine cylinder bores, precision valve holes) | |
| Grinding | Precision Grinding | 0.16 – 0.04 | High-wear parts (e.g., bearing races, tool bits) |
| Ultra-Precision Grinding | < 0.01 | Ultra-high-precision components (e.g., medical implant surfaces, semiconductor parts) |
2. Practical RA Value Selection: Balancing Function, Cost, and Scenarios
Not all parts need ultra-low Ra values—over-processing wastes time and money. Below is a guide to standard RA options and their cost implications, aligned with real-world use cases.
2.1 Standard RA Grades for CNC Processing
| RA Value (μm) | Grade Type | Key Application Scenarios | Cost Impact (vs. Ra 3.2 μm) |
| 3.2 | Economy Grade | General consumer parts (e.g., plastic toy components, simple brackets); light-load, low-speed moving parts. Surface has slight knife marks but no functional impact. | Base cost (0% increase) |
| 1.6 | Functional Grade | Tightly fitting or stressed parts (e.g., sliding guides, low-speed rotary shafts); requires high-speed cutting and fine feed. | ~3% cost increase |
| 0.8 | High-Grade | Bearings, high-stress concentration areas (e.g., gear teeth roots); improves wear resistance and fatigue life. | ~5% cost increase |
| 0.4 | Ultra-Fine Grade | High-precision bearings, medical implants (e.g., artificial joints); demands strict surface smoothness to avoid tissue irritation or friction damage. | 11–15% cost increase |
| < 0.01 | Ultra-Precision Grade | Semiconductor parts, optical components; only achievable via ultra-precision grinding. | 50–100% cost increase |
2.2 Example: How to Choose RA for an Automotive Shaft
- If the shaft is a non-critical auxiliary component (e.g., a cover support shaft): Choose Ra 3.2 μm (economy, no unnecessary cost).
- If the shaft is a rotating part with a sliding fit (e.g., a transmission auxiliary shaft): Choose Ra 1.6 μm (balances function and cost).
- If the shaft is a high-speed bearing journal (e.g., an engine crankshaft): Choose Ra 0.8 μm (ensures wear resistance and long life).
3. 3 Key Factors That Affect CNC Processing Surface Roughness
To achieve your target RA value consistently, you need to control these three critical variables:
3.1 Tool Selection & Cutting Parameters
- Tool Edge Accuracy: Dull or low-precision tools leave deeper tool marks, increasing Ra values. Use sharp, high-hardness tools (e.g., carbide tools for steel, diamond tools for non-ferrous metals).
- Cutting Speed: Higher speed (within material limits) reduces friction between tool and workpiece, creating a smoother surface. For example, increasing turning speed from 100 m/min to 300 m/min can lower Ra from 1.6 μm to 0.8 μm.
- Feed Rate: Smaller feed rates (e.g., 0.1 mm/rev vs. 0.3 mm/rev) reduce the distance between tool paths, minimizing surface irregularities.
3.2 Workpiece Material Properties
- Non-Ferrous Metals (e.g., aluminum alloys, copper): Soft and easy to machine, making it simple to achieve low Ra values (e.g., Ra 0.04 μm via mirror turning).
- Ferrous Metals (e.g., carbon steel, stainless steel): Harder and more prone to tool wear, requiring stricter process control (e.g., higher tool hardness, optimized cooling) to reach Ra < 0.8 μm.
3.3 Post-Processing Techniques
Post-processing can further improve surface roughness, but note its impact on dimensional tolerances:
- Polishing: Reduces Ra by 50–70% (e.g., from 1.6 μm to 0.4 μm) but may slightly reduce part size.
- Sanding: Suitable for removing minor tool marks (e.g., lowering Ra from 3.2 μm to 1.6 μm) but is labor-intensive.
- Electroplating: Creates a smooth coating (e.g., chrome plating) to lower Ra, but adds cost and requires strict environmental controls.
4. Yigu Technology’s Perspective on CNC Processing Surface Roughness
At Yigu Technology, we often see clients overspecify RA values—for example, choosing Ra 0.4 μm for a non-critical bracket that only needs Ra 3.2 μm, increasing costs by 15% unnecessarily. Our advice: Start with the functional requirement, not the lowest possible Ra. For most industrial parts, Ra 1.6–0.8 μm balances performance and cost. We also help clients optimize processes: For a recent automotive client, adjusting their milling feed rate from 0.2 mm/rev to 0.1 mm/rev (while keeping speed constant) lowered Ra from 1.6 μm to 0.8 μm without extra cost. For ultra-precision needs (e.g., medical implants), we combine fine boring with polishing to hit Ra 0.4 μm consistently, ensuring both quality and cost efficiency.
FAQ: Common Questions About CNC Processing Surface Roughness RA
- Q: Can CNC turning achieve Ra < 0.01 μm like ultra-precision grinding?
A: No. Even with diamond tools, mirror turning (the most precise CNC turning method) only reaches Ra 0.01–0.04 μm. Ra < 0.01 μm requires ultra-precision grinding, which uses abrasive particles to remove material at the micron level.
- Q: Will increasing cutting speed always lower the RA value?
A: No. Beyond a certain limit, excessively high speed causes tool overheating and wear, which increases Ra. For example, turning aluminum at > 500 m/min may melt the material surface, creating irregularities. Always follow material-specific speed guidelines.
- Q: How do I verify if a CNC shop can actually achieve the RA value I need?
A: Ask for a sample part machined with the same material and process as your project. Use a surface roughness tester (e.g., a portable Ra meter) to measure the sample’s Ra value—don’t rely solely on the shop’s claims. For critical parts (e.g., medical implants), request a third-party inspection report.