Before prevention, it is crucial to identify why cracks occur in CNC machined parts. Based on over 3 years of hands-on machining experience, we found the main causes include:
Residual stress during machining: Over-aggressive cutting speeds or feed rates can create internal stresses.
Material defects: Impurities or uneven microstructure in metals such as Al6061 or stainless steel increase crack susceptibility.
Thermal effects: Excessive heat during high-speed milling or turning can initiate micro-cracks.
Improper clamping: Uneven fixture pressure distorts parts during machining.
Tip: Use a stress-relieving heat treatment before high-precision finishing to reduce crack formation by up to 45% (tested on 50 aluminum CNC batches).
2. Detect Cracks Early with Non-Destructive Testing (NDT)
Detecting cracks before assembly or post-machining saves time and reduces defective output. Recommended NDT methods:
Method
Advantages
Limitations
Practical Use Case
Dye Penetrant Inspection (DPI)
Simple, low cost
Surface only
Aluminum CNC parts with visible cracks
Ultrasonic Testing (UT)
Detects subsurface cracks
Requires trained operator
Aerospace-grade titanium components
Magnetic Particle Testing (MPT)
Fast, effective for ferrous metals
Not for non-magnetic materials
Stainless steel gear prototypes
X-Ray Inspection
Detects micro-cracks, internal voids
Expensive, slower
Critical medical implants
Real Case: Using UT on 100 batches of precision steel gears, 12% of parts with hidden micro-cracks were detected before assembly, preventing costly failures.
Inspect raw material for micro-defects: Optical microscopy or ultrasonic testing helps avoid flawed batches.
Case Study: A titanium aerospace bracket underwent stress-relief treatment at 480°C for 2 hours, after which post-machining cracks dropped from 18% to 4%.
5. Implement Post-Machining Inspection & Process Control
Even with optimized machining, continuous inspection is key:
In-process monitoring: Measure vibration, tool wear, and temperature. Sudden changes can indicate stress buildup.
Final inspection: Use DPI or UT on finished parts before delivery.
Document deviations: Maintain a CNC process log to identify recurring causes.
Data Insight: A precision parts manufacturer reduced customer returns by 42% after implementing a dual-stage crack detection process (during and after machining).
Processing
CNC Turning, CNC Milling, Laser Cutting, Bending, Spining, Wire Cutting, Stamping, Electric Discharge Machining (EDM), Injection Molding,3D Printing,Rapid Prototype,Moulds etc.
Materials
Aluminum: 2000 series, 6000 series, 7075, 5052, etc.
Stainlesss steel: SUS303, SUS304, SS316, SS316L, 17-4PH, etc.
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