CNC milling is a precision machining process that controls the relative movement between the milling cutter and the workpiece via computer programs to achieve material removal and part shaping. Different part features (such as flat surfaces, grooves, curved surfaces, threaded holes, etc.) correspond to different types of milling operations. Each operation has clear technical specifications for tool selection, cutting path planning, and process parameter setting, which directly affect machining accuracy, efficiency, and part quality. The following is a detailed technical analysis of 6 most commonly used CNC milling operations.
1. Face Milling: Basic Operation for Flat Surface Machining
Core Technical Principle
Face milling focuses on machining flat surfaces of workpieces. It uses a disc-shaped face mill (with the cutter axis perpendicular to the workpiece machining surface). Multiple cutting edges on the bottom of the cutter rotate to remove excess material from the workpiece surface, ensuring the machined surface meets preset flatness and surface roughness requirements. This operation is mainly used for rough machining (removing large material allowances) and finish machining (ensuring surface accuracy) of workpiece blanks. During cutting, the cutter feeds linearly along the workpiece surface, enabling efficient machining of large-area flat surfaces.
Application Scenarios & Typical Parts
- Basic parts requiring flatness: Top/bottom reference surfaces of metal middle frames for mobile phones, top surfaces of automotive engine blocks, table surfaces of CNC machine tools;
- Reference surface machining for subsequent processes: Preprocessing of template flat surfaces before mold cavity machining, precision finishing of assembly surfaces for mechanical parts.
Key Process Parameters & Operation Points
- Tool Selection: For aluminum and aluminum alloys (hardness HB 60-120), use 4-6 flute carbide face mills; for stainless steel (HB 150-300), use 8-10 flute ultra-fine grain carbide cutters to improve cutting stability;
- Cutting Parameters: For rough machining: depth of cut 0.5-2 mm, feed rate 800-1500 mm/min, cutting speed 150-300 m/min; for finish machining: depth of cut 0.1-0.5 mm, feed rate 500-1000 mm/min, cutting speed 200-350 m/min;
- Accuracy Control: Achieve spindle dynamic balance (G0.4 grade or higher) and machine level calibration (flatness ≤ 0.001 mm/m) to ensure machined flatness ≤ 0.01 mm/100 mm and surface roughness Ra ≤ 1.6 μm.
2. End Milling: Forming Machining for Grooves and Step Structures
Core Technical Principle
End milling uses a cylindrical end mill (with the cutter axis perpendicular to the workpiece machining surface). It leverages the combined action of cutting edges on the bottom and side of the cutter to simultaneously machine multiple features such as flat surfaces, grooves, and steps. When machining grooves, the cutter feeds along the preset groove width and depth path to remove material inside the groove; when machining steps, it controls the cutter depth in the Z-axis direction to form step surfaces of different heights. This operation is suitable for forming open grooves and step structures.
Application Scenarios & Typical Parts
- Parts with open grooves: Key installation grooves on keyboard bases, guide grooves of wardrobe slide rails, wiring grooves on motor end covers;
- Parts with step structures: Step surfaces of mechanical bearing housings, sealing steps of hydraulic valve bodies, assembly steps of equipment housings.
Key Process Parameters & Operation Points
- Tool Selection: Select end mill diameter based on groove width (cutter diameter is 0.1-0.2 mm smaller than groove width to reserve finish machining allowance); for deep grooves (groove depth > 5× cutter diameter), use long-neck end mills;
- Cutting Parameters: For aluminum parts: feed rate 500-1000 mm/min, cutting speed 150-250 m/min; for stainless steel parts: feed rate 300-600 mm/min, cutting speed 50-100 m/min;
- Process Optimization: For deep groove machining, adopt layered cutting (depth of cut per layer ≤ 1/3 of cutter diameter) to avoid chatter caused by excessive cutter overhang, ensuring groove wall perpendicularity ≤ 0.01 mm/100 mm.
3. Peripheral Milling: Forming Machining for Workpiece Outer Contours
Core Technical Principle
Peripheral milling focuses on machining the outer contours of workpieces. It uses end mills or formed mills, and the side cutting edges of the cutter feed along the workpiece’s outer contour path to remove excess material outside the contour, shaping the workpiece into a preset form (such as circular, polygonal, or irregular shapes). This operation requires strict control of the fitting accuracy between the cutter path and the contour, and is usually divided into two steps: rough milling (removing most allowances and reserving 0.1-0.2 mm finish machining allowance) and finish milling (ensuring contour dimensional accuracy).
Application Scenarios & Typical Parts
- Parts with regular outer contours: Metal housings of bicycle pedals (circular contour), flange plates (annular contour), square connecting blocks (rectangular contour);
- Parts with irregular outer contours: Drone propeller mounting bases, metal fixing rings for camera lenses, special-shaped connecting structures of automotive components.
Key Process Parameters & Operation Points
- Tool Selection: For linear contours, use 4-flute end mills; for complex curved contours, use 2-3 flute end mills to improve path followability;
- Path Planning: Adopt climb milling (cutter rotation direction is consistent with feed direction) during finish milling to reduce tool wear and workpiece surface scratches; use arc transitions at contour corners (fillet radius ≥ cutter radius) to avoid sudden changes in cutting force;
- Accuracy Control: Use contour control functions of CNC systems (such as NURBS interpolation) to ensure contour dimensional tolerance ± 0.005 mm and profile tolerance ≤ 0.003 mm.
4. Pocket Milling: Forming Machining for Closed Grooves
Core Technical Principle
Pocket milling is used to machine closed grooves (i.e., “pocket” structures with no openings around the groove) on workpieces. It uses end mills or pocket mills, and removes material inside the pocket via “helical plunge cutting” or “layered circular cutting” paths. Helical plunge cutting (the cutter gradually plunges into the workpiece along a helical path) avoids cutter chipping caused by direct penetration into the material; layered circular cutting (cutting inward layer by layer along the pocket contour) ensures surface accuracy and dimensional consistency of the pocket walls. This operation is suitable for machining deep pockets and complex pocket structures.
Application Scenarios & Typical Parts
- Parts with functional pockets: Pointer mounting pockets on watch dials, fixing pockets for mobile phone batteries, valve core receiving pockets of hydraulic valves;
- Mold pocket parts: Product forming pockets of plastic molds, die cavities of stamping dies, casting pockets of die-casting molds.
Key Process Parameters & Operation Points
- Tool Selection: For pockets with diameter > 10 mm, use end mills; for pockets with diameter ≤ 10 mm or narrow gaps (width < 5 mm), use solid carbide micro end mills (diameter 0.5-10 mm);
- Cutting Parameters: For rough machining: depth of cut per layer 0.3-1 mm, feed rate 400-800 mm/min; for finish machining: depth of cut per layer 0.1-0.3 mm, feed rate 200-500 mm/min;
- Chip Evacuation Control: Use high-pressure coolant (pressure 3-5 MPa) to flush the cutting area directionally, avoiding secondary cutting caused by chip accumulation inside the pocket and ensuring surface roughness of pocket walls Ra ≤ 0.8 μm.
5. Contour Milling: Precision Machining for Curved Surfaces and Complex Curves
Core Technical Principle
Contour milling focuses on machining curved surfaces, arcs, and complex curve features of workpieces. It uses formed cutters such as ball end mills and bull nose mills, and controls the cutter to feed along the curve path via multi-axis linkage of the CNC system (usually 3-axis linkage; 5-axis linkage is required for complex curved surfaces). This ensures continuous contact between the cutter’s cutting edges and the curved surface, achieving smooth surface forming. This operation requires interpolation algorithms (such as NURBS interpolation) to ensure fitting accuracy between the cutter path and the designed curved surface, reducing the “step effect”.
Application Scenarios & Typical Parts
- Parts with smooth curved surfaces: Curved surfaces of automotive door handles, fitting curved surfaces of headphone housings, grip curved surfaces of fitness equipment;
- Parts with complex curves: Aerodynamic curved surfaces of aero-engine blades, human-fitting curved surfaces of medical devices, complex parting surfaces of molds.
Key Process Parameters & Operation Points
- Tool Selection: For high-precision curved surfaces, use ball end mills (nose radius is selected based on curved surface curvature, usually 0.2-5 mm); for curved surfaces with fillets, use bull nose mills (fillet radius 0.1-3 mm);
- Cutting Parameters: Feed rate 200-600 mm/min, cutting speed 80-200 m/min (the smaller the curved surface curvature, the lower the speed to avoid cutter vibration);
- Accuracy Control: Reduce interpolation step size (usually ≤ 0.1 mm) and optimize cutter paths (adopt “contour-parallel machining” or “radial machining” strategies) to ensure curved surface profile tolerance ≤ 0.005 mm and surface roughness Ra ≤ 0.4 μm.
6. Drilling & Tapping: Integrated Machining for Holes and Threads
Core Technical Principle
Drilling and tapping is a combined operation for hole feature machining: first, a drill mill (or twist drill) is used to machine a straight hole at the preset position of the workpiece (hole diameter is determined based on the subsequent tapping specification); then, a tap is replaced to rotate and feed along the hole axis, machining internal threads on the hole wall. This operation requires ensuring the coaxiality and perpendicularity of the drilled hole to avoid tap breakage or substandard thread accuracy during tapping. Some CNC milling machines can achieve continuous drilling and tapping via automatic tool change systems.
Application Scenarios & Typical Parts
- Parts with threaded connection holes: Screw holes on metal connectors for furniture, motherboard fixing holes of computer cases, cover connection holes of equipment housings;
- Parts with functional threaded holes: Threaded holes of hydraulic joints, connection threaded holes of pipe flanges, mounting threaded holes of motor bases.
Key Process Parameters & Operation Points
- Drilling Parameters: Determine the drill diameter based on the thread specification (e.g., M4 thread corresponds to a drill diameter of 3.3 mm, M6 thread corresponds to a drill diameter of 5 mm); feed rate 100-300 mm/min, cutting speed 30-80 m/min;
- Tapping Parameters: Adopt rigid tapping (strict synchronization between spindle rotation and feed); feed rate = spindle speed × thread pitch (e.g., for M4×0.7 thread, when spindle speed is 1000 rpm, feed rate is 700 mm/min);
- Quality Control: Check the perpendicularity of the drilled hole (≤ 0.01 mm/100 mm) before tapping; use thread plug gauges to test thread accuracy (usually 6H/6g grade) after tapping to avoid loose or tight threads.
Operation Type Selection Guide: Process Matching Based on Part Features
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Part Feature to Be Machined
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Recommended Milling Operation Type
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Key Technical Requirements
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Typical Application Cases
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Flat surfaces (flatness required)
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Face Milling
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Flatness ≤ 0.01 mm/100 mm, Ra ≤ 1.6 μm
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Top surfaces of engine blocks, machine tool table surfaces
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Open grooves/steps
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End Milling
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Groove wall perpendicularity ≤ 0.01 mm/100 mm, Ra ≤ 3.2 μm
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Keyboard key grooves, bearing housing step surfaces
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Outer contours (regular/irregular)
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Peripheral Milling
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Profile tolerance ≤ 0.003 mm, dimensional tolerance ± 0.005 mm
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Flange plates, special-shaped connecting blocks
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Closed pockets/grooves
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Pocket Milling
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Surface roughness of pocket walls Ra ≤ 0.8 μm
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Watch dial pockets, mold forming cavities
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Curved surfaces/complex curves
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Contour Milling
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Curved surface profile tolerance ≤ 0.005 mm, Ra ≤ 0.4 μm
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Automotive door handle curved surfaces, engine blades
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Threaded holes (including pre-drilled holes)
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Drilling & Tapping
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Thread accuracy 6H/6g grade, hole perpendicularity ≤ 0.01 mm/100 mm
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Equipment mounting holes, hydraulic joint threaded holes
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Conclusion
The selection of CNC milling operations must be comprehensively determined based on part features, material properties, and accuracy requirements. For example, contour milling combined with ball end mills is preferred for machining stainless steel curved surfaces, while face milling can be used for efficient machining of aluminum flat surfaces. In actual production, a single part often requires the collaboration of multiple operations (e.g., mobile phone middle frames require face milling, peripheral milling, and drilling & tapping). Rational process sequencing (rough machining → semi-finish machining → finish machining) and parameter optimization are necessary to balance machining efficiency and quality.
If you need to learn more about equipment selection, tool life management, or troubleshooting for a specific operation, feel free to leave a comment. Targeted technical analysis will be provided in subsequent content.
