Rugosidade da superfície, medido pelo Rá (Desvio Médio Aritmético) valor, é um indicador crítico de Usinagem CNC qualidade – afeta diretamente a funcionalidade da peça, resistência ao desgaste, e ajuste. Esteja você fabricando bens de consumo, componentes industriais, ou implantes médicos, knowing the achievable Ra range of CNC processing and how to control it is essential. Este artigo detalha o 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) | Cenários de aplicação típicos |
| CNC virando | Rough Turning | 20 – 10 | Initial shaping of metal blanks; parts with no surface finish requirements (Por exemplo, temporary structural supports) |
| Semi-Finishing/Finishing Turning | 1.6 – 0.8 | Peças de uso geral (Por exemplo, low-speed shafts, non-critical housings) | |
| Mirror Turning (Diamond Tools for Non-Ferrous Metals) | 0.04 – 0.01 | High Gloss, peças de precisão (Por exemplo, aluminum decorative components, optical instrument parts) | |
| Fresagem CNC | Rough Milling | 6.3 – 2.5 | Large structural parts (Por exemplo, quadros de máquina, bracket blanks) |
| Finish Milling | 1.6 – 0.63 | Fitted parts (Por exemplo, sliding guides, Altas de equipamento) | |
| Super Fine Milling (High-Speed, Small Feed) | 0.4 | Componentes mecânicos de precisão (Por exemplo, high-speed bearing seats) | |
| CNC chato | Ordinary Boring | 2.5 – 0.63 | Hole machining for general parts (Por exemplo, hydraulic cylinder bores) |
| Fine Boring | 0.32 – 0.08 | High-precision holes (Por exemplo, engine cylinder bores, precision valve holes) | |
| Moagem | Moagem de precisão | 0.16 – 0.04 | Peças de desgaste alto (Por exemplo, raças de rolamento, tool bits) |
| Ultra-Precision Grinding | < 0.01 | Ultra-high-precision components (Por exemplo, medical implant surfaces, Peças de semicondutores) |
2. Practical RA Value Selection: Balancing Function, Custo, 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 | Principais cenários de aplicação | Impacto de custo (vs.. Rá 3.2 μm) |
| 3.2 | Economy Grade | General consumer parts (Por exemplo, Componentes de brinquedos de plástico, Suportes simples); light-load, low-speed moving parts. Surface has slight knife marks but no functional impact. | Custo base (0% aumentar) |
| 1.6 | Functional Grade | Tightly fitting or stressed parts (Por exemplo, sliding guides, low-speed rotary shafts); requires high-speed cutting and fine feed. | ~3% cost increase |
| 0.8 | High-Grade | Rolamentos, high-stress concentration areas (Por exemplo, gear teeth roots); improves wear resistance and fatigue life. | ~5% cost increase |
| 0.4 | Ultra-Fine Grade | Rolamentos de alta precisão, implantes médicos (Por exemplo, articulações artificiais); demands strict surface smoothness to avoid tissue irritation or friction damage. | 11–15% cost increase |
| < 0.01 | Ultra-Precision Grade | Peças de semicondutores, componentes ópticos; only achievable via ultra-precision grinding. | 50–100% cost increase |
2.2 Exemplo: How to Choose RA for an Automotive Shaft
- If the shaft is a non-critical auxiliary component (Por exemplo, a cover support shaft): Escolher Rá 3.2 μm (economy, no unnecessary cost).
- If the shaft is a rotating part with a sliding fit (Por exemplo, a transmission auxiliary shaft): Escolher Rá 1.6 μm (balances function and cost).
- If the shaft is a high-speed bearing journal (Por exemplo, an engine crankshaft): Escolher Rá 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 Seleção de ferramentas & Parâmetros de corte
- Tool Edge Accuracy: Dull or low-precision tools leave deeper tool marks, increasing Ra values. Use Sharp, Ferramentas de alta resistência (Por exemplo, carbide tools for steel, diamond tools for non-ferrous metals).
- Velocidade de corte: Higher speed (within material limits) reduces friction between tool and workpiece, creating a smoother surface. Por exemplo, increasing turning speed from 100 m/min para 300 m/min can lower Ra from 1.6 μm para 0.8 μm.
- Taxa de alimentação: Smaller feed rates (Por exemplo, 0.1 mm/rev vs. 0.3 mm/rev) reduce the distance between tool paths, minimizing surface irregularities.
3.2 Workpiece Material Properties
- Metais não ferrosos (Por exemplo, ligas de alumínio, cobre): Macio e fácil de máquina, making it simple to achieve low Ra values (Por exemplo, Rá 0.04 μm via mirror turning).
- Ferrous Metals (Por exemplo, aço carbono, aço inoxidável): Harder and more prone to tool wear, requiring stricter process control (Por exemplo, higher tool hardness, optimized cooling) to reach Ra < 0.8 μm.
3.3 Técnicas de pós-processamento
Post-processing can further improve surface roughness, but note its impact on dimensional tolerances:
- Polimento: Reduces Ra by 50–70% (Por exemplo, de 1.6 μm para 0.4 μm) but may slightly reduce part size.
- Lixar: Suitable for removing minor tool marks (Por exemplo, lowering Ra from 3.2 μm para 1.6 μm) but is labor-intensive.
- Eletroplatação: Creates a smooth coating (Por exemplo, cromo) to lower Ra, but adds cost and requires strict environmental controls.
4. Yigu Technology’s Perspective on CNC Processing Surface Roughness
Na 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, Aumentando custos por 15% unnecessarily. Nosso conselho: Start with the functional requirement, not the lowest possible Ra. Para a maioria das partes industriais, 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 para 0.8 μm without extra cost. For ultra-precision needs (Por exemplo, implantes médicos), we combine fine boring with polishing to hit Ra 0.4 μm consistently, ensuring both quality and cost efficiency.
Perguntas frequentes: Common Questions About CNC Processing Surface Roughness RA
- P: Can CNC turning achieve Ra < 0.01 μm like ultra-precision grinding?
UM: Não. Even with diamond tools, mirror turning (the most precise CNC turning method) only reaches Ra 0.01–0.04 μm. Rá < 0.01 μm requires ultra-precision grinding, which uses abrasive particles to remove material at the micron level.
- P: Will increasing cutting speed always lower the RA value?
UM: Não. Beyond a certain limit, excessively high speed causes tool overheating and wear, which increases Ra. Por exemplo, turning aluminum at > 500 m/min may melt the material surface, creating irregularities. Always follow material-specific speed guidelines.
- P: How do I verify if a CNC shop can actually achieve the RA value I need?
UM: Ask for a sample part machined with the same material and process as your project. Use um testador de rugosidade de superfície (Por exemplo, um medidor Ra portátil) para medir o valor Ra da amostra – não confie apenas nas afirmações da loja. Para peças críticas (Por exemplo, implantes médicos), solicitar um relatório de inspeção de terceiros.