In CNC machining, surface roughness isn’t just about how a part looks—it directly impacts how it works. A rough surface on a bearing can cause excessive wear, while a too-smooth surface on a grip might make it slippery. For designers and manufacturers, choosing the right surface roughness (measured by Ra values) balances performance, cost, and production time. This guide breaks down what surface roughness is, how to achieve different Ra grades, which grade to pick for your project, and real-world examples to avoid costly mistakes.
What Is Surface Roughness in CNC Machining?
First, let’s clear up confusion: surface roughness and surface finish are not the same.
- Surface Roughness: The tiny, irregular peaks and valleys on a part’s surface right after CNC machining. It’s measured by Ra (Average Roughness), which calculates the average distance between the highest peaks and lowest valleys (in microns, μm—1 μm = 0.001 mm).
- Surface Finish: The final look/feel of a part after post-processing (e.g., anodizing, sandblasting, or electroplating). Post-processing can improve appearance, but it doesn’t erase the base surface roughness from machining.
Why Ra Matters
Even small changes in Ra affect a part’s performance:
- Friction: A rough surface (high Ra) creates more friction—bad for moving parts like gears. A smooth surface (low Ra) reduces friction but can be too slippery for grips.
- Wear: Rough surfaces wear out faster. For example, a shaft with Ra 3.2 μm will wear a bearing 2x faster than one with Ra 0.8 μm.
- Fit: Tight-fitting parts (e.g., a piston in a cylinder) need low Ra to avoid jamming or leaks.
Example
A medical device maker used a 3.2 μm Ra finish for a surgical tool’s handle. The rough surface was hard to clean (bacteria trapped in valleys), so they switched to 1.6 μm Ra. The smoother surface was easier to sanitize, and it still provided enough grip for surgeons.
Key Ra Grades for CNC Machining: What They Mean & When to Use Them
The manufacturing industry (per standards like ISO 4287) uses specific Ra values for CNC machining. The four most common grades—from rough to smooth—are 3.2 μm, 1.6 μm, 0.8 μm, and 0.4 μm. Below’s a detailed breakdown of each, including cost, use cases, and machining tips.
Ra Grade Comparison Table
| Ra Value | Visual/Feel | Best Applications | Machining Requirements | Cost Impact (vs. 3.2 μm Ra) |
|---|---|---|---|---|
| 3.2 μm | Visible cut marks; slightly rough to the touch | – Consumer parts (e.g., plastic toy components, simple brackets) – Parts with light stress/load (e.g., low-weight shelves) – Default finish for most non-critical parts | High speed, fine feed, light cutting | Base cost (0% increase) |
| 1.6 μm | Slightly visible cut marks; smoother to the touch | – Tight-fitting parts (e.g., small gears, sliding door tracks) – Parts with light vibration (e.g., small electric motor components) – Food-safe parts (easier to clean than 3.2 μm) | Controlled high speed, very fine feed, minimal cutting force | +2.5% (standard aluminum); higher for complex parts |
| 0.8 μm | Hardly visible cut marks; very smooth | – Stress-concentrated parts (e.g., airplane wing brackets) – Light-load bearings (occasional movement) – Parts needing paint/adhesive (smooth surface helps bonding) | Tight speed/feed control; multiple light passes | +5% (standard aluminum); rises with complexity |
| 0.4 μm | Near-mirror finish; no visible marks | – High-stress parts (e.g., engine crankshafts) – Fast-rotating components (e.g., high-speed bearings, shafts) – Medical implants (smooth surface prevents tissue irritation) | Multiple fine passes; specialized tools (e.g., diamond-tipped cutters); strict quality control | +11–15% (standard aluminum); significantly higher for complex parts |
Real-World Case Study: Choosing Ra for a Car Transmission Gear
A car manufacturer tested three Ra values for transmission gears:
- 3.2 μm Ra: Gears made noise and wore out after 50,000 km.
- 1.6 μm Ra: Noise reduced, but wear still occurred at 80,000 km.
- 0.8 μm Ra: Minimal noise, wear-free up to 150,000 km.
The manufacturer chose 0.8 μm Ra—even though it added 5% to cost, it reduced warranty claims by 40%.
How to Achieve Your Desired Ra Value
Getting the right Ra isn’t just luck—it depends on machining tools, settings, and material. Here’s what you need to know:
1. Choose the Right Cutting Tool
- High Ra (3.2 μm): Standard carbide end mills work well. They’re cheap and fast.
- Low Ra (0.4 μm): Use sharp, high-quality tools (e.g., diamond-tipped or coated carbide). Dull tools leave rough surfaces.
2. Tune Machining Settings
- Speed (RPM): Higher speed = smoother surface (reduces tool vibration). For aluminum, use 3,000–5,000 RPM for 3.2 μm Ra; 5,000–8,000 RPM for 0.4 μm Ra.
- Feed Rate: Slower feed = smoother surface. For 3.2 μm Ra, use 100–200 mm/min; for 0.4 μm Ra, drop to 50–100 mm/min.
- Depth of Cut: Shallow cuts (0.1–0.2 mm) leave less roughness than deep cuts (0.5+ mm).
3. Material Matters
Soft materials (e.g., aluminum, plastic) are easier to smooth than hard materials (e.g., steel, titanium):
- Aluminum: Can reach 0.4 μm Ra with standard tools.
- Steel: Needs coated tools and slower speeds to hit 0.8 μm Ra.
- Titanium: Requires specialized tools to get below 1.6 μm Ra.
Example
A shop tried to machine titanium parts to 0.8 μm Ra with standard carbide tools. The surface was too rough (1.2 μm Ra). They switched to titanium-coated tools and slowed the feed rate—finally hitting 0.8 μm Ra.
CNC Milling vs. Turning: How They Affect Surface Roughness
CNC milling (cutting with rotating tools) and CNC turning (spinning the part while cutting) produce different surface roughness—even with the same Ra target. Here’s why:
| Process | How It Works | Surface Roughness Notes | Best For |
|---|---|---|---|
| CNC Milling | Tool rotates and moves across the part. Leaves a “scalloped” surface (from tool rotation). | – Harder to get very low Ra (0.4 μm) because of scallops. – Scallops are more visible on large flat surfaces. | Complex parts with holes, slots, or 3D shapes (e.g., engine blocks, brackets). |
| CNC Turning | Part spins; tool stays stationary. Leaves a smooth, circular pattern. | – Easier to get low Ra (0.4 μm) because of consistent tool contact. – Smoother surfaces on cylindrical parts (e.g., shafts, bolts). | Cylindrical parts (e.g., bearings, pins, pipes). |
Case Study: Milling vs. Turning a Shaft
A manufacturer needed a 10 mm steel shaft with 0.8 μm Ra.
- Milling: Took 3 passes to hit 0.8 μm Ra; surface had faint scallops.
- Turning: Hit 0.8 μm Ra in 1 pass; surface was smooth and uniform.
They chose turning for the shaft—it was faster and produced a better surface.
When to Avoid Over-Smoothing (And Save Money)
Low Ra (e.g., 0.4 μm) sounds great, but it’s not always necessary. Over-smoothing wastes time and money:
- Non-Moving Parts: A decorative bracket doesn’t need 0.4 μm Ra—3.2 μm works fine and costs less.
- Grips/Handles: Too-smooth surfaces (0.4 μm Ra) are slippery. A 1.6 μm Ra finish provides better grip at a lower cost.
- Post-Processed Parts: If you’re anodizing a part, 3.2 μm Ra is enough—the anodize will hide rough marks.
Example
A furniture company planned to use 0.8 μm Ra for wooden chair legs (CNC-machined). They realized the legs would be painted, so they switched to 3.2 μm Ra. They saved 5% per leg and the paint covered the rough marks perfectly.
Yigu Technology’s Perspective on CNC Machining Surface Roughness
At Yigu Technology, we believe surface roughness (Ra) is a balance of function and cost. Too many clients overspecify (e.g., 0.4 μm Ra for a non-critical bracket) and pay extra for no benefit. We help them match Ra to their part’s use: for example, a client making garden tool handles switched from 1.6 μm to 3.2 μm Ra, saving 2.5% per unit with no loss in grip. We also share machining tips—like using turning for cylindrical parts—to hit Ra targets faster. Surface roughness isn’t just a number; it’s about making parts that work well without breaking the bank.
FAQ
- Can I get a Ra value lower than 0.4 μm for CNC machining?
Yes, but it’s rare and costly. Values like 0.2 μm or 0.1 μm require specialized processes (e.g., lapping or honing) after CNC machining. These add 20–50% to cost and are only used for ultra-precision parts (e.g., aerospace engine components or high-end medical implants). - Why is CNC turning better than milling for smooth cylindrical parts?
Turning keeps the part spinning at a consistent speed, so the cutting tool makes even contact with the surface—no scallops like milling. For a 10 mm shaft, turning can hit 0.4 μm Ra in 1 pass, while milling needs 3+ passes and still leaves faint scallops. - Does post-processing (like anodizing) improve surface roughness?
No—post-processing changes appearance but doesn’t reduce the base Ra. Anodizing a 3.2 μm Ra aluminum part will make it look shiny, but the underlying peaks and valleys (3.2 μm) are still there. If you need a smooth surface, you must machine it to a low Ra first—post-processing can’t fix a rough base.