CNC Surface Machining: Comprehensive Guide to Types, Charts and Tests
Sep 05, 2025
Surface quality is a key indicator for measuring the precision of CNC machined parts. It involves three aspects: roughness (microscopic unevenness), waviness (macroscopic periodic unevenness), and texture (machining tool path direction).
I. Surface Processing Types (How to Achieve)
Different processing operations and strategies can achieve different surface finishes. The following is arranged in order from coarse to fine.
Typical achievable roughness (Ra) description of processing types and applicable scenarios
Rough machining of 12.5 μm - 3.2 μm uses a large cutting depth and high feed rate to quickly remove the material, leaving obvious tool marks and a poor surface. When the parts are initially formed, machining allowances are reserved for non-critical surfaces.
Semi-finishing is 3.2 μm - 1.6 μm to prepare for finishing, remove the marks of rough machining, and ensure an appropriate allowance for finishing. The final processing of most non-mating surfaces, installation surfaces, etc.
Conventional finishing of 1.6 μm - 0.8 μm adopts small cutting depth, small feed rate and high rotational speed. The knife marks are visible to the naked eye but smooth to the touch. The most common precision requirements are used for static mating surfaces, sealing surfaces, bearing housings, etc.
High-precision finishing of 0.8 μm - 0.4 μm requires optimized parameters, sharp cutting tools, high-rigidity machine tools and effective cooling. The surface is extremely smooth. Dynamic mating surfaces, hydraulic cylinder walls, and high-load bearing surfaces.
Superfinishing of 0.4 μm - 0.1 μm requires the use of single crystal diamond tools, extremely high machine tool accuracy and a stable environment (constant temperature). Optical components, precision instrument surfaces, silicon wafer processing.
Manual polishing/grinding < 0.1 μm: Remove the knife marks by hand or mechanical means such as sandpaper or oilstone to achieve a mirror-like effect. Appearance parts, mold cavities, surfaces of food and medical equipment.
Ii. Symbols, Charts and Annotations (How to Specify)
Engineers clearly specify the requirements on the drawing through surface roughness symbols.
1. Basic symbols
Explanation of symbol meanings
√ Basic symbols indicate that the surface can be obtained through any process and are meaningless to use alone.
Youdaoplaceholder0 is the most commonly used to remove materials. It indicates that the surface is obtained by removing the material through processing methods such as milling, turning and drilling.
"Non-removal of material refers to surfaces formed through casting, forging, rolling, etc., which do not require processing."
2. Complete annotation (taking the removal of material symbols as an example) :
` ` `
[a] - Roughness parameters and values (such as Ra 0.8)
[b] - Processing methods (such as "milling ")
[c] - Texture direction symbols (such as "=")
[d] - Machining allowance (e.g. 0.3mm)
[e] - Sampling length (e.g. 0.8mm)
3. Common Annotation examples:
· ⌝ Ra 1.6: the most common form. It indicates that the maximum surface roughness Ra value is 1.6 μm by the method of removing the material.
· ⌝ Ra max 3.2: the Ra value shall not exceed 3.2 μm.
· ⌝ Ra 0.8 / Rz 3.2: both Ra and Rz values are specified.
· ⌝ Rz 10 N8: marked with "N grade", N8 corresponds to Rz 10μm.
4. Surface texture direction symbol: The texture direction is crucial for sealing and motion coordination. The symbol is marked on the extension line.
Schematic diagram of symbol meaning
The tool path direction of the projection plane parallel to the view is parallel to the boundary of the plane it is on
Perpendicular to the projection plane of the view, the direction of the tool path is perpendicular to the boundary of the plane where it is located
The X-cross texture tool path is in a cross shape (such as milling back and forth)
M multi-directional without a dominant direction (such as point milling)
The C approximate concentric circles are produced by turning
R-approximate radiation is produced by end face turning or end face milling.
Iii. Surface Roughness Testing (How to Verify)
After processing is completed, professional instruments should be used for objective measurement to verify whether it meets the requirements of the drawings.
1. Contact profilometer (needle tracing method)
· Principle: This is the most classic and authoritative method. An extremely sharp diamond probe (with a tip radius of approximately 2μm) gently slides across the surface of the workpiece. The vertical displacement is converted into an electrical signal, which is then amplified and calculated to obtain parameters such as Ra and Rz.
· Equipment: Surface roughness measuring instrument.
· Advantages: Precise measurement, compliance with national standards, and capable of measuring various complex shapes.
· Disadvantages: It is a contact measurement, which may scratch extremely soft materials and has a relatively slow measurement speed.
2. Non-contact optical profiler
· Principle: By using techniques such as light interference, confocal microscopy or white light scattering, a 3D surface topography is constructed by analyzing the reflection of light on the surface, thereby calculating the roughness.
· Advantages: Fast speed, no scratching of workpieces, and capable of measuring extremely soft materials.
· Disadvantages: Sensitive to surface reflective characteristics (difficult to measure transparent and highly reflective materials), and the equipment is usually more expensive.
3. Compare Sample Blocks (Quick and Practical Method)
· Principle: A set of standard sample blocks with known Ra values are used. Through fingernail touch perception and visual comparison, the surface to be measured is compared with the sample blocks to estimate the approximate roughness range.
· Advantages: Extremely low cost, fast and convenient, suitable for workshop sites.
· Disadvantages: It is highly subjective and has poor accuracy. It can only be used for rough estimation and preliminary judgment and cannot be used as the basis for final acceptance.
Suggested measurement process
1. Drawing analysis: Clearly identify the parameters to be measured (such as Ra) and their theoretical values.
2. Clean the surface: Ensure that the tested area is free of oil stains, dust and burrs.
3. Selection method:
· Quick online check → Use comparison blocks.
· Final quality inspection → Use a contact profilometer.
For soft or mirror-finished workpieces, consider non-contact optical measurement.
4. Conduct measurements: Take the average of multiple measurements at different positions on the surface to ensure the representativeness of the results.
5. Recording and Judgment: Record the measured values and compare them with the requirements of the drawings to make a judgment of qualified or unqualified.
Only by combining the correct processing technology, clear drawing marking and scientific measurement verification can the surface quality of CNC parts be fully controlled.
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