Rugosità superficiale, measured by the Ra (Arithmetic Average Deviation) valore, is a critical indicator of MACCHING CNC quality—it directly impacts part functionality, resistenza all'usura, e in forma. Whether you’re making consumer goods, componenti industriali, o impianti medici, knowing the achievable Ra range of CNC processing and how to control it is essential. Questo articolo analizza il 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) | Scenari applicativi tipici |
| Turning CNC | Rough Turning | 20 – 10 | Initial shaping of metal blanks; parts with no surface finish requirements (PER ESEMPIO., temporary structural supports) |
| Semi-Finishing/Finishing Turning | 1.6 – 0.8 | Parti per scopi generali (PER ESEMPIO., low-speed shafts, non-critical housings) | |
| Mirror Turning (Diamond Tools for Non-Ferrous Metals) | 0.04 – 0.01 | Luogo, parti di precisione (PER ESEMPIO., aluminum decorative components, optical instrument parts) | |
| Macinazione CNC | Rough Milling | 6.3 – 2.5 | Grandi parti strutturali (PER ESEMPIO., frame macchine, bracket blanks) |
| Finish Milling | 1.6 – 0.63 | Fitted parts (PER ESEMPIO., sliding guides, Alloggiamenti degli ingranaggi) | |
| Super Fine Milling (High-Speed, Small Feed) | 0.4 | Componenti meccanici di precisione (PER ESEMPIO., high-speed bearing seats) | |
| Alesatura CNC | Ordinary Boring | 2.5 – 0.63 | Hole machining for general parts (PER ESEMPIO., hydraulic cylinder bores) |
| Fine Boring | 0.32 – 0.08 | High-precision holes (PER ESEMPIO., engine cylinder bores, precision valve holes) | |
| Macinazione | Macinazione di precisione | 0.16 – 0.04 | Parti di abbigliamento alto (PER ESEMPIO., portando razze, tool bits) |
| Ultra-Precision Grinding | < 0.01 | Ultra-high-precision components (PER ESEMPIO., medical implant surfaces, parti di semiconduttore) |
2. Practical RA Value Selection: Balancing Function, Costo, 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 | Scenari applicativi chiave | Impatto dei costi (contro. Ra 3.2 µm) |
| 3.2 | Economy Grade | General consumer parts (PER ESEMPIO., Componenti giocattoli in plastica, Staffe semplici); light-load, low-speed moving parts. Surface has slight knife marks but no functional impact. | Costo di base (0% aumento) |
| 1.6 | Functional Grade | Tightly fitting or stressed parts (PER ESEMPIO., sliding guides, low-speed rotary shafts); requires high-speed cutting and fine feed. | ~3% cost increase |
| 0.8 | High-Grade | Cuscinetti, high-stress concentration areas (PER ESEMPIO., gear teeth roots); improves wear resistance and fatigue life. | ~5% cost increase |
| 0.4 | Ultra-Fine Grade | Cuscinetti ad alta precisione, Impianti medici (PER ESEMPIO., articolazioni artificiali); demands strict surface smoothness to avoid tissue irritation or friction damage. | 11–15% cost increase |
| < 0.01 | Ultra-Precision Grade | Parti di semiconduttore, componenti ottici; only achievable via ultra-precision grinding. | 50–100% cost increase |
2.2 Esempio: How to Choose RA for an Automotive Shaft
- If the shaft is a non-critical auxiliary component (PER ESEMPIO., a cover support shaft): Scegliere Ra 3.2 µm (economy, no unnecessary cost).
- If the shaft is a rotating part with a sliding fit (PER ESEMPIO., a transmission auxiliary shaft): Scegliere Ra 1.6 µm (balances function and cost).
- If the shaft is a high-speed bearing journal (PER ESEMPIO., an engine crankshaft): Scegliere 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 Selezione degli strumenti & Parametri di taglio
- Tool Edge Accuracy: Dull or low-precision tools leave deeper tool marks, increasing Ra values. Usa Sharp, Strumenti ad alta resistenza (PER ESEMPIO., carbide tools for steel, diamond tools for non-ferrous metals).
- Velocità di taglio: Higher speed (within material limits) reduces friction between tool and workpiece, creating a smoother surface. Per esempio, increasing turning speed from 100 M/min a 300 m/min can lower Ra from 1.6 μm a 0.8 µm.
- Velocità di alimentazione: Smaller feed rates (PER ESEMPIO., 0.1 mm/rev vs. 0.3 mm/giro) reduce the distance between tool paths, minimizing surface irregularities.
3.2 Workpiece Material Properties
- Metalli non ferrosi (PER ESEMPIO., leghe di alluminio, rame): Morbido e facile da macchina, making it simple to achieve low Ra values (PER ESEMPIO., Ra 0.04 μm via mirror turning).
- Ferrous Metals (PER ESEMPIO., acciaio al carbonio, acciaio inossidabile): Harder and more prone to tool wear, requiring stricter process control (PER ESEMPIO., higher tool hardness, optimized cooling) to reach Ra < 0.8 µm.
3.3 Tecniche di post-elaborazione
Post-processing can further improve surface roughness, but note its impact on dimensional tolerances:
- Lucidare: Reduces Ra by 50–70% (PER ESEMPIO., da 1.6 μm a 0.4 µm) but may slightly reduce part size.
- Levigatura: Suitable for removing minor tool marks (PER ESEMPIO., lowering Ra from 3.2 μm a 1.6 µm) but is labor-intensive.
- Elettroplazione: Creates a smooth coating (PER ESEMPIO., placcatura cromata) to lower Ra, but adds cost and requires strict environmental controls.
4. Yigu Technology’s Perspective on CNC Processing Surface Roughness
Alla tecnologia Yigu, 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, Aumentare i costi di 15% unnecessarily. Il nostro consiglio: Start with the functional requirement, not the lowest possible Ra. Per la maggior parte delle parti industriali, 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/giro (while keeping speed constant) lowered Ra from 1.6 μm a 0.8 μm without extra cost. For ultra-precision needs (PER ESEMPIO., Impianti medici), we combine fine boring with polishing to hit Ra 0.4 μm consistently, ensuring both quality and cost efficiency.
Domande frequenti: Common Questions About CNC Processing Surface Roughness RA
- Q: Can CNC turning achieve Ra < 0.01 μm like ultra-precision grinding?
UN: 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?
UN: NO. Beyond a certain limit, excessively high speed causes tool overheating and wear, which increases Ra. Per esempio, 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?
UN: Ask for a sample part machined with the same material and process as your project. Use a surface roughness tester (PER ESEMPIO., a portable Ra meter) to measure the sample’s Ra value—don’t rely solely on the shop’s claims. Per parti critiche (PER ESEMPIO., Impianti medici), request a third-party inspection report.