CNC Machining surface integrity how machining parameters affect fatigue resistance
Content Menu
● Surface Integrity Components Relevant to Fatigue
● Influence of Depth of Cut and Width of Cut
● Tool Edge Preparation and Geometry
● Real-World Examples from Published Work
● Practical Optimization Approaches
● Frequently Asked Questions (FAQs)
Extended Introduction
Surface integrity determines whether a machined component survives its intended service life or fails prematurely under cyclic loading. In fatigue-critical applications — turbine disks, landing gear components, crankshafts, orthopedic implants — the way a CNC tool removes material often matters more than the bulk material properties themselves. Small changes in cutting speed, feed per tooth, depth of cut, or tool edge preparation can shift residual stress from compressive to tensile, alter surface roughness by a factor of three, or create subsurface damage that cuts fatigue strength in half.
The connection between machining parameters and fatigue resistance has been studied for decades, yet many shops still select speeds and feeds based primarily on tool life or metal removal rate. When fatigue becomes the limiting factor, that approach costs money and reputation. Research consistently shows that properly controlled machining can raise the fatigue limit of nickel alloys by 25–40 %, extend the life of titanium components past 10⁷ cycles, and eliminate crack initiation sites that would otherwise appear after only a few thousand hours in service.
This article examines the mechanisms that link everyday CNC parameters to surface integrity and, in turn, to fatigue performance. The discussion draws directly from peer-reviewed work on high-strength alloys and hard-to-machine materials, with specific numbers and conditions taken from experiments rather than general statements.
Surface Integrity Components Relevant to Fatigue
Three main aspects of surface integrity control fatigue behavior: topography, residual stress state, and near-surface microstructure/microhardness.
Topography includes roughness (Ra, Rz, Rsm), waviness, and lay direction. Peaks act as stress concentrators; the stress concentration factor Kt can reach 2.5–3.0 for Ra values above 2 µm in milled surfaces.
Residual stress profiles extend 50–300 µm below the surface depending on the alloy and conditions. Compressive stress delays crack initiation; tensile stress accelerates it.
Microstructural changes include work hardening, phase transformations, grain refinement, and (in extreme cases) white-layer formation or microcracks. These changes alter local yield strength and crack growth resistance.
Influence of Cutting Speed
Cutting speed controls the temperature rise in the shear zone and the rate of tool wear. In nickel-based superalloys such as Inconel 718, increasing speed from 40 m/min to 80 m/min during end milling typically reduces surface roughness from Ra 1.4 µm to Ra 0.6 µm and shifts residual stress from +150 MPa (tensile) to –350 MPa (compressive). The higher speed shortens the time available for heat conduction into the workpiece, limiting the depth of the tensile layer.
In Ti-6Al-4V, speeds above 120 m/min often produce a thin nano-grained layer with hardness 30–40 % higher than the bulk. This layer resists crack initiation even when surface roughness remains moderate. However, speeds beyond 200 m/min can generate adiabatic shear bands and microcracks that outweigh the compressive stress benefit.
Influence of Feed Rate
Feed per tooth is the strongest single driver of surface roughness in milling. The theoretical relationship Ra ≈ fz²/(8 × nose radius) holds well in practice. Doubling feed from 0.05 mm/tooth to 0.10 mm/tooth routinely increases Ra from 0.4 µm to 1.2 µm in aluminum and from 0.7 µm to 2.2 µm in hardened steel.
Higher feed also increases specific cutting force, which drives plastic deformation deeper into the subsurface. In 300M high-strength steel, feed rates above 0.15 mm/tooth have been shown to flip hoop-direction residual stress from –400 MPa to +200 MPa, reducing fatigue limit by 28 % at 10⁷ cycles.
Lower feeds (0.02–0.05 mm/tooth) combined with wiper geometry or variable helix tools routinely achieve Ra below 0.3 µm with strongly compressive stress fields, making them standard practice for aerospace finishing passes.
Influence of Depth of Cut and Width of Cut
Depth of cut affects tool engagement time and cutting forces. Shallow depths (0.1–0.3 mm) produce lower forces and more stable cutting, favoring compressive stress. Larger depths increase the volume of deformed material and can generate tensile stress at the transition zone between cut and uncut surface.
Width of cut (radial depth) matters equally. Full-slot milling (ae = tool diameter) creates the most severe thermal-mechanical conditions and usually yields tensile stress. Step-over values of 5–15 % of tool diameter in finishing operations keep forces low and maintain compression.
Tool Edge Preparation and Geometry
Edge hone radius between 10 µm and 25 µm typically gives the best fatigue results in hard alloys. A sharp edge (<5 µm) tears the surface and leaves tensile stress; an overly large hone (>50 µm) burns the surface and creates heat-affected zones.
Positive rake angles (5–10°) reduce cutting forces and favor compression. Large nose radii (0.8–1.6 mm) smooth the feed marks and lower peak-to-valley height.
Real-World Examples from Published Work
End milling of Inconel 718 at 80 m/min, 0.05 mm/tooth, 0.2 mm axial depth with a 20 µm hone produced Ra 0.55 µm and –380 MPa maximum compressive stress. Four-point bending fatigue tests reached 10⁶ cycles at 800 MPa with no failures originating from the machined surface.
Face milling of Ti-6Al-4V with 0.03 mm/tooth feed and 10 % radial engagement achieved Ra 0.25 µm and –450 MPa subsurface stress. Rotating-beam tests showed a fatigue strength of 620 MPa at 10⁷ cycles, compared to 450 MPa for conventionally milled surfaces.
Turning of AISI 52100 bearing steel at 60 m/min, 0.08 mm/rev, 0.15 mm depth with CBN inserts generated –600 MPa compressive stress to 80 µm depth. Subsequent rolling contact fatigue life exceeded 50 million cycles, roughly double the life of ground references.
Practical Optimization Approaches
Design of experiments (Taguchi L9 or full factorial) remains the most reliable method to map parameter space. Response variables usually include Ra, maximum compressive stress, and affected layer depth. Grey relational analysis or desirability functions combine multiple objectives.
In-production monitoring with dynamometers and acoustic emission sensors allows real-time adjustment of feed when force spikes indicate edge deterioration.
CAM systems now incorporate surface integrity modules that recommend finishing strategies based on material databases and fatigue requirements.
Detailed Conclusion
The fatigue performance of a CNC-machined component is not an accidental outcome; it is a direct consequence of the chosen speeds, feeds, depths, and tool preparation. Cutting speed governs thermal penetration and stress sign, feed rate dominates topography and stress magnitude, depth and width control force levels, and edge geometry fine-tunes the deformation mode.
When all parameters work together — moderate to high speed, low feed, light finishing passes, and controlled edge hone — the result is a surface under strong compression with minimum stress raisers and a hardened subsurface that resists crack growth. Published studies on Inconel 718, Ti-6Al-4V, and high-strength steels repeatedly demonstrate fatigue strength gains of 25–50 % and life extensions beyond 10⁷ cycles.
Manufacturing engineers who treat surface integrity as a primary output rather than a side effect consistently produce parts that meet or exceed fatigue specifications without post-machining treatments. The data are available, the mechanisms are understood, and modern tooling makes the required conditions achievable on the shop floor.
Frequently Asked Questions (FAQs)
Q1: Will increasing cutting speed always improve fatigue performance in titanium alloys?
A: No. Speeds up to 120–150 m/min usually help; above 200 m/min often create shear bands and microcracks that hurt fatigue.
Q2: What is the single most effective change for better fatigue life in milled nickel alloys?
A: Reduce feed per tooth to 0.04–0.06 mm/tooth in the finishing pass while keeping light radial engagement.
Q3: Do coated carbide tools give better surface integrity than uncoated ones for fatigue parts?
A: Yes in most cases. Coatings maintain a sharp edge longer, reducing tearing and tensile stress buildup.
Q4: How deep do beneficial compressive stresses typically extend in optimized machining?
A: 50–150 µm for nickel alloys, 100–250 µm for steels and titanium with proper parameters.
Q5: Is dry machining acceptable for fatigue-critical components?
A: Only with MQL or cryogenic assistance in most hard alloys; flood coolant or MQL is still safer for consistent compression.