What are the defects of machined parts?

▶  1.1 Surface scratches and burrs

●  Manifestations: Linear scratches of varying lengths and depths appear on the part surface; small protrusions or sharp edges (burrs) form at part edges, the junctions of processed surfaces, or holes. These defects are easily visible to the naked eye and can be further confirmed with a 5-10x magnifying glass.

●  Main causes:

          Worn or impurity-contaminated processing tools scratch the part surface during cutting.

          Mismatched cutting parameters (e.g., too high or too low cutting speed, inappropriate feed rate) lead to incomplete cutting.

          Improper handling of parts after processing, such as collisions between parts or between parts and tooling during storage or transfer.

●  Impact: Scratches reduce the part’s surface finish, and severe scratches can damage the surface protective layer (e.g., anti-rust coating), leading to rusting. Burrs pose safety risks—they may scratch operators during assembly—and can cause poor fitting between parts, lowering assembly accuracy and even affecting the normal operation of the assembled product.

▶  1.2 Surface discoloration and oxidation

●  Manifestations: The part surface shows abnormal colors (yellow, black, gray, etc.) that differ from the raw material’s normal color; a loose oxide layer forms on the surface, which can be wiped off or peeled off by hand.

●  Main causes:

          Excessively high cutting temperatures during processing cause the part surface to react with oxygen in the air, forming oxides.

          Deteriorated coolants or failure to clean coolants in time after processing result in chemical corrosion on the part surface.

          Long-term storage of parts in humid, high-temperature, or poorly ventilated environments accelerates surface oxidation.

●  Impact: Discoloration and oxidation degrade the part’s surface quality and product appearance, which may lead to customer complaints. In severe cases, they reduce the mechanical properties of the part surface, such as surface hardness and wear resistance, shortening the part’s service life.

Dimensional accuracy is a core indicator for measuring machined part quality. According to CNC machining industry standards, parts must meet the dimensional tolerance requirements specified in design drawings. If dimensional deviation exceeds the allowable range, it will directly cause assembly failure or impair the final product’s performance—this is one of the most common reasons for product returns in the machining industry.

▶  2.1 Dimensional deviation (too large or too small)

●  Manifestations: The actual size of the part (e.g., outer diameter, inner hole diameter, length, thickness) is larger or smaller than the design drawing’s specified size and exceeds the allowable tolerance range. For example, a part with a designed outer diameter of 50mm ±0.02mm may have an actual outer diameter of 50.05mm (too large) or 49.97mm (too small).

●  Main causes:

          Wear of processing tools during use reduces cutting accuracy (e.g., a worn turning tool cannot cut to the designed size).

          Inaccurate positioning of parts on the machine tool, such as unstable clamping (parts shift during cutting) or incorrect positioning datums (datums do not match the design requirements).

          Errors in processing program settings, such as wrong coordinate values for the tool path or miscalculations of cutting depth.

          Thermal deformation of the machine tool or parts during processing (e.g., the machine tool’s spindle heats up and expands, or the part heats up and deforms) affects dimensional accuracy.

●  Impact: Dimensional deviation directly leads to assembly problems. For instance, a part with an excessively large outer diameter cannot be installed into the matching hole; a part with an excessively small inner hole cannot fit the matching shaft. If the part length is too short or too long, it will disrupt the part’s assembly position in the product, causing the product to fail to work normally.

▶  2.2 Shape and position tolerance out of tolerance

●  Manifestations: The part’s shape (e.g., flatness, roundness, straightness) or the relative position between surfaces (e.g., parallelism, perpendicularity, coaxiality) fails to meet design drawing requirements. Common examples include an uneven part surface with concavities or convexities, or misalignment between the inner hole axis and the outer circle axis (coaxiality out of tolerance).

●  Main causes:

Insufficient rigidity of the machine tool—vibration of the machine tool itself during processing distorts the part shape.

Wear or deformation of the machine tool’s guide rails, which causes deviations in the tool’s movement path.

Excessively large clamping force that deforms the part (especially for thin-walled or low-rigidity parts).

Improper processing methods, such as using single-point cutting to process long shafts, which easily leads to straightness out of tolerance.

●  Impact: Out-of-tolerance shape and position tolerance cause uneven force on parts during use, increasing wear and shortening service life. For high-speed rotating parts (e.g., transmission shafts), coaxiality out of tolerance will cause vibration and noise during rotation, threatening the product’s stability and safety.

Internal structure defects of machined parts are not easily detectable through external observation, but they severely affect the parts’ mechanical properties and reliability. In severe cases, they may cause parts to break suddenly during use—this is a major safety hazard for key equipment parts (e.g., engine components, hydraulic valves).

▶  3.1 Internal cracks

●  Manifestations: Fine cracks exist inside the part, which can only be detected through non-destructive testing methods such as ultrasonic testing or X-ray testing. These cracks may be distributed along the material’s grain boundaries or across grains.

●  Main causes:

          The raw material itself has internal defects, such as pre-existing cracks or inclusions.

          Excessively high internal stress in the part during processing, such as rapid cooling after high-temperature cutting (which creates thermal stress) or uneven stress distribution caused by improper cutting paths.

          Excessive impact or extrusion force on the part during clamping (e.g., over-tightening the chuck) or transportation (e.g., dropping parts).

●  Impact: Internal cracks significantly reduce the part’s strength and toughness. When the part is subjected to external force during use, the cracks will expand rapidly, leading to sudden part breakage. For key parts like engine crankshafts or mechanical transmission gears, this kind of breakage may cause the entire equipment to fail, resulting in economic losses or safety accidents.

▶  3.2 Internal inclusions and pores

●  Manifestations: Non-metallic inclusions (e.g., oxides, sulfides) or small holes (pores) exist inside the part. The size of inclusions and pores varies—some are concentrated, while others are scattered.

●  Main causes:

          Impure raw materials—non-metallic substances are mixed into the metal during smelting.

          Cutting fluid or air is trapped in the part’s internal structure during processing (e.g., during deep-hole drilling or tapping).

          Inadequate degassing of the part during heat treatment, leading to pore formation.

●  Impact: Internal inclusions cause stress concentration inside the part, reducing its fatigue resistance (parts are prone to damage under repeated loads). Pores lower the part’s density, impairing mechanical properties such as tensile strength and compressive strength. For sealed parts (e.g., hydraulic cylinder blocks, valve cores), pores will cause leakage, affecting the equipment’s hydraulic or pneumatic performance.

To promptly identify machined part defects and ensure part quality, CNC machining enterprises need to adopt scientific and effective detection methods. The following are practical detection solutions for different types of defects, suitable for enterprises of all sizes.

▶  4.1 Appearance detection

●  Tools and methods:

          Naked eye observation: Check for obvious scratches, burrs, or discoloration on the part surface.

          Magnifying glass (5-20x): Confirm small defects (e.g., fine scratches) that are hard to see with the naked eye.

          Industrial endoscope: Inspect the inner surface of deep holes or complex cavities (e.g., the inner wall of a valve body) for defects.

          Surface roughness tester: Quantitatively detect the part’s surface finish (e.g., Ra value) to determine if it meets design requirements.

●  Applicable defects: Surface scratches, burrs, discoloration, oxidation, and other appearance-related issues.

●  Advantages: Low cost, simple operation, and fast detection speed—suitable for 100% inspection of parts after processing.

▶  4.2 Dimensional accuracy detection

Tools and methods:

          Basic measuring tools: Calipers (for length, outer diameter, inner diameter), micrometers (for high-precision size measurement), and depth gauges (for depth measurement).

          Coordinate Measuring Machine (CMM): High-precision detection of shape and position tolerance (e.g., coaxiality, parallelism) for complex parts.

          Dial indicators/lever indicators: Detect shape tolerances such as flatness and straightness (e.g., checking if a part’s surface is flat by measuring the indicator’s variation).

●  Applicable defects: Dimensional deviation (too large/too small), shape and position tolerance out of tolerance.

●  Advantages: High detection accuracy, suitable for both simple and complex parts; CMM can realize automated detection, improving efficiency.

▶  4.3 Internal structure detection

●  Tools and methods:

          Ultrasonic testing: Use high-frequency sound waves to detect internal cracks, pores, or inclusions (suitable for most metal materials).

          X-ray testing: Visualize internal defects (e.g., inclusions, pores) in parts—ideal for thick or complex-structured parts.

          Magnetic particle testing: Detect surface and near-surface defects (e.g., small cracks) in ferromagnetic materials (e.g., iron, steel).

          Metallographic analysis: Cut and polish the part, then observe its internal structure under a metallographic microscope to detect inclusions or structural abnormalities.

●  Applicable defects: Internal cracks, internal inclusions, pores.

●  Advantages: Can detect "invisible" internal defects, ensuring part reliability—especially important for key safety parts.

Reducing the occurrence of machined part defects requires a full-process management approach, covering raw material selection, equipment maintenance, process optimization, and operator training. The following measures are proven effective in CNC machining enterprises.

▶  5.1 Strictly control raw material quality

●  Before using raw materials, conduct comprehensive inspections: Test chemical composition (to ensure it matches the design material), mechanical properties (e.g., hardness, tensile strength), and internal structure (use ultrasonic testing to check for pre-existing cracks or inclusions).

●  Purchase raw materials from regular suppliers with qualification certificates (e.g., ISO 9001 certification) to avoid using unqualified materials that may cause subsequent defects.

●  Establish a raw material traceability system: Record the supplier, batch number, and inspection results of each batch of raw materials to facilitate tracking and accountability if defects occur.

▶  5.2 Regularly maintain and calibrate processing equipment

●  Develop a machine tool maintenance plan: Regularly check tool wear (replace worn tools in time), guide rail accuracy (adjust or repair deformed guide rails), and spindle tightness (prevent spindle vibration). For example, replace turning tools every 500 hours of use (depending on the material being cut) to ensure cutting accuracy.

●  Calibrate measuring tools and testing equipment regularly: Use standard gauges to calibrate calipers, micrometers, and CMMs every 3-6 months to ensure their accuracy meets detection requirements.

●  Keep the machine tool clean: Remove cutting chips, coolant residues, and other impurities from the machine tool regularly to prevent them from affecting processing accuracy.

▶  5.3 Optimize processing parameters and process flow

●  Customize processing parameters based on raw materials and part requirements: For example, when cutting aluminum (a soft material), use a higher cutting speed (1000-1500m/min) and a moderate feed rate (0.1-0.2mm/r) to avoid overheating; when cutting stainless steel (a hard material), use a lower cutting speed (300-500m/min) and a smaller feed rate to reduce tool wear.

●  Optimize the process flow:

          Add a deburring process after cutting (e.g., use a deburring tool or sandblasting) to remove burrs.

          Add a cleaning and anti-rust process after processing: Clean the part with a neutral cleaner to remove coolant residues, then apply anti-rust oil or spray anti-rust coating to prevent oxidation.

          For thin-walled parts, use special clamping fixtures (e.g., soft jaws made of aluminum) to reduce clamping deformation.

▶  5.4 Strengthen operator training and management

●  Provide systematic professional training for operators:

          Machine tool operation: Teach correct operation of CNC machine tools (e.g., programming, tool setting, clamping) to avoid human errors.

          Defect identification: Train operators to recognize common defects (e.g., scratches, dimensional deviation) and know how to report and handle them.

          Measuring tool use: Guide operators to use calipers, micrometers, and other tools correctly to ensure accurate self-inspection.

●  Establish a strict quality inspection system:

          Require operators to conduct self-inspection of each processed part (e.g., check size with a caliper, check appearance with the naked eye) and record inspection results.

          Arrange quality inspectors to conduct sampling inspection (sampling rate ≥10%) or full inspection (for key parts) of parts. Reject unqualified parts and analyze the causes to prevent recurrence.

Machined part defects—covering appearance, dimensional accuracy, and internal structure—have specific causes and potential impacts on product quality and safety. For CNC machining enterprises, mastering the types, detection methods, and prevention measures of these defects is key to improving product quality, reducing returns, and enhancing market competitiveness.

By implementing full-process quality control—from strictly selecting raw materials and maintaining equipment to optimizing processes and training operators—enterprises can effectively reduce defect rates, produce high-quality machined parts, and win customer trust. In the long run, this not only helps enterprises gain more market share but also promotes the sustainable development of the entire CNC machining industry.