CNC machining surface roughness is the microscopic peak-and-valley height left on a part after cutting, measured mainly by Ra and Rz in micrometres. It is set by feed, nose radius, tool condition, vibration and cooling, and it governs sealing, friction, wear and fatigue life.
Ra is the arithmetic mean of the profile deviation over the sampling length, and it is the most common callout. Rz is the maximum profile height, so it reacts strongly to a single deep valley. The two cannot be converted into each other: ISO 21920 gives no formula, and the shop estimate of Rz at four to seven times Ra is process dependent.
Surface roughness and dimensional tolerance are two separate systems. Dimensions are judged with a CMM and statistical process control, while surfaces are judged with a profilometer.
Surface integrity adds waviness and lay to roughness. The precision machining processes at Yujiaxin Tech fold all three into first-article approval, with deburring and cleaning.
Geometry sets a floor for CNC machining surface roughness. The milling and turning formula is Ra ≈ f²/(32·rε), where f is feed per revolution and rε is the nose radius in mm; multiply by 1000 to get micrometres. With f = 0.15 and rε = 0.8, Ra is about 0.88 μm.
Two rules matter most. Feed enters as a square, so halving feed cuts Ra to roughly one quarter. Nose radius enters linearly, so doubling the radius roughly halves Ra. A wiper insert can push Ra lower at the same feed.
The formula is a floor, not a forecast. Real surfaces usually land at 1.5 to 3 times the theoretical value, because tool wear, vibration and built-up edge each add extra peaks and valleys.
As flank wear VB grows, rubbing and ploughing at the edge increase, and Ra climbs with it. Once VB passes roughly 0.3 mm, the surface starts to degrade.
At low cutting speed, workpiece material can weld to the edge and form a built-up edge (BUE). When it breaks away with the chip, it tears the finished surface. The fix is higher cutting speed, not lower feed.
Coatings and edge preparation change the result as well. Aluminium parts often use DLC or TiB₂, stainless steel uses TiAlN or AlTiN, and titanium uses TiSiN, all with a sharp edge to limit adhesion and tearing. Edge wear shifts finish and size at once, and the dimensional side is covered in how dimensional tolerance control works alongside finish.
When the surface shows fish-scale or wavy marks, chatter is usually the cause rather than a heavy feed. Lowering the feed rarely helps, so the fix has to come from rigidity.
A tool that overhangs more than three times its diameter behaves like a tuning fork. Slender and thin-wall parts are most sensitive, so a tailstock or steady rest helps. Spindle runout above 2 μm writes regular chatter marks into the surface.
Reducing setups through multi-axis motion also cuts tool marks, which matters more on complex faces than small parameter changes.
Flood cooling lowers cutting temperature, suppresses BUE and flushes chips out of the cut. A chip that drags across the finished face a second time leaves a mark, so chip evacuation and breaker geometry matter.
Stainless steel and titanium often run with high-pressure through-tool coolant at 30 to 70 bar, which delivers fluid close to the edge. Minimum quantity lubrication (MQL) can lower Ra and cut fluid use, while dry cutting usually pushes Ra up.
Soft materials such as pure aluminium and copper gum up and scratch easily, so they need a sharp edge and higher cutting speed. Stainless steel 316L work-hardens and tends to tear, while titanium conducts heat poorly and is prone to burning and burrs.
Microstructure matters too. A fine-grain or quenched-and-tempered steel finishes more cleanly at the same parameters, and aluminium usually finishes 20% to 30% smoother. For the link between surface integrity and fatigue, see ASM International. Achievable Ra by material and the matching surface spec are listed on precision metal components and their finishes.
Material grade and heat treatment set how far CNC machining surface roughness can be pushed at fixed cutting parameters.
A stylus profilometer drags a diamond tip across the surface and reports Ra, Rz and Rt, with resolution down to 0.01 μm; the principle and SI traceability are described in the NIST note on the stylus profilometer. Optical systems use white-light interferometry or confocal microscopy, stay non-contact, and suit soft materials and coatings.
Sampling length and cut-off change the reading, so buyer and supplier must agree on one setting, for example 0.8 mm. The stylus must travel across the lay, or the value reads low.
A comparator block gives a fast visual check on the shop floor, but arbitration still runs on the profilometer. surface inspection and quality control at Yujiaxin Tech link first-article, in-process and outgoing checks into one loop.
Different functional faces call for different CNC machining surface roughness values, and one part often carries several callouts. A static gasket face usually sits at Ra 1.6 to 3.2 μm, with a lay direction that helps the seal.
An O-ring groove runs at Ra 0.8 to 1.6 μm and must carry an Rz limit as well, for example Rz ≤ 6.3 μm. One deep valley can cause a leak even when Ra passes, which is why Rz is called out here.
A hydraulic bore or dynamic seal face usually needs Ra 0.2 to 0.4 μm and is honed; a bearing seat or mating shaft sits at Ra 0.4 to 0.8 μm; a general structural face is fine at Ra 3.2 μm. For how such parts are made, see CNC machining for new energy vehicle parts.
Within one process, moving Ra from 3.2 to 1.6 and then to 0.8 adds roughly 25% to 50% of time at each step. Below Ra 0.4 μm the part usually leaves the cutting machine for grinding, honing, burnishing or polishing. Milling alone generally reaches Ra 0.8 to 1.6 μm.
Over-specifying Ra is the most common hidden cost, and most parts only need Ra 1.6 to 3.2 μm.
Three questions come up most. Which parameter should you pick, Ra or Rz? Sealing faces lean on Rz, while mating faces lean on Ra.
Does halving the feed really cut Ra by three quarters? Only when the tool and rigidity are near ideal, so real gains are smaller.
Why does the surface worsen when the program has not changed? Check tool wear and BUE first, then workholding rigidity.
CNC machining surface roughness is a sum of variables, not luck. To confirm whether your drawing can be met in one pass, gather the tolerance, surface symbols and lay direction. Then send your drawing and finish requirements to our engineers, and Yujiaxin Tech will return achievable Ra per face and a plan.
info@szyujiaxin.com 
中文
English
日本語
Deutsch
Русский
SHENZHEN YUJIAXIN TECH CO.,LTD.