Explore the reasonable value of the cutting depth in CNC turning

Exploration on the Reasonable Value of Cutting Depth in CNC Turning

The cutting depth (depth of cut, ap) is a core parameter in CNC turning that directly affects the processing efficiency, tool life and workpiece quality. Its reasonable value needs to comprehensively consider the performance of the machine tool, the strength of the cutting tool, the material characteristics of the workpiece and the requirements of the processing stage. The following analysis is conducted from three dimensions: the selection basis of cutting depth, dynamic adjustment strategies, and risk control.

First, the core basis for selecting the cutting depth

Processing stage and goal orientation

Rough machining: The core objective is to quickly remove the allowance, and the cutting depth can be taken at a relatively large value (such as 1/2 to 2/3 of the total allowance). For instance, when the total allowance of the workpiece is 6mm, rough machining can be divided into 2 to 3 cutting operations, each with a cutting depth of 2 to 3mm.

Semi-finish machining: It is necessary to balance efficiency and uniformity of allowance. The cutting depth is usually 0.5 to 1.5mm, and a unilateral allowance of 0.2 to 0.5mm should be reserved for finish machining.

Finishing: With surface quality as the core, the cutting depth is controlled within 0.1 to 0.5mm to avoid dimensional errors caused by fluctuations in cutting force.

The material properties are compatible with the cutting tools

Brittle materials (such as cast iron and ceramics) : The cutting depth can be appropriately increased (such as 1 to 3mm), as the cutting force fluctuation is small and it is less likely to form built-up edge.

Plastic materials (such as steel and aluminum alloys) : The cutting depth should be limited (such as 0.5 to 2mm), and the cutting temperature should be reduced by coolant to suppress the tool sticking phenomenon.

Difficult-to-machine materials (such as titanium alloys and superalloys) : A small cutting depth (0.1 to 0.5mm) combined with a high cutting speed is adopted to reduce the friction time between the tool and the workpiece.

Rigid constraints of machine tools and cutting tools

Machine tool rigidity: Machine tools with insufficient rigidity need to reduce the cutting depth to avoid vibration. For instance, when a vertical lathe processes long shaft workpieces, the cutting depth should be reduced by 20% to 30% compared to a horizontal lathe.

Tool strength: The cutting depth must not exceed the safety threshold of the bending strength of the tool material. For instance, the upper limit of the cutting depth of cemented carbide tools is usually 1/10 of the tool diameter (for example, the cutting depth of a 10mm diameter tool is ≤1mm).

Second, the dynamic adjustment strategy of cutting depth

Stepwise control of layered cutting

Allowance reduction method: During rough machining, the initial cutting depth is taken as 60% to 70% of the total allowance, and for each subsequent cutting depth, it is reduced by 20% to 30%. For example, when the total allowance is 8mm, the first cutting depth is 4.8mm, the second cutting depth is 3mm, and the third cutting depth is 1.2mm.

Contour transition zone optimization: At the areas where the workpiece contour changes suddenly (such as steps and fillets), the cutting depth should be reduced to 50% to 70% of the normal value to prevent the tool from chipping due to a sudden increase in cutting force.

Collaborative management of cutting force and heat

Cutting force monitoring: Real-time monitoring of cutting force through power sensors or acceleration sensors. When the fluctuation of cutting force exceeds 15%, the cutting depth needs to be reduced by 10% to 20%.

Heat dispersion: A large cutting depth can easily lead to local high temperatures. It is necessary to use high-pressure coolant (pressure ≥5MPa) to directly impact the cutting area to prevent the tool from softening due to heat.

The balance between tool life and efficiency

Tool wear compensation: When the wear on the rear face of the tool reaches 0.2 to 0.3mm, the cutting depth needs to be reduced by 10% to 15% to extend the tool life.

Multi-tool collaborative machining: For workpieces with large allowances, a combination of rough turning tools and semi-finish turning tools can be adopted. The rough turning tool takes on 80% of the allowance, while the semi-finish turning tool completes the remaining machining. The cutting depths are respectively controlled at 2mm and 0.5mm.

Third, the risk and control of excessive cutting depth

Vibration and dimensional accuracy loss

Vibration trigger condition: When the cutting depth exceeds the critical value of the machine tool's dynamic stiffness, self-excited vibration is prone to occur. For example, when processing slender shafts (with a length-to-diameter ratio > 10), a cutting depth exceeding 0.5mm May result in a diameter dimension error of more than 0.05mm.

Control measures: Enhance rigidity by adding a tool rest or center rest, or adopt a variable depth of cut strategy (such as sinusoidal waveform depth of cut) to disperse vibration energy.

Tool breakage and workpiece scrapping

Risk of breakage: When the cutting depth is too large, the bending moment borne by the tool exceeds its bending strength, which may lead to chipping or breakage. For example, when the tool overhang length exceeds three times the diameter, the cutting depth needs to be reduced to 50% of the normal value.

Preventive measures: Use vibration-reducing tool holders or dynamic dampers to reduce tool vibration, and at the same time limit the cutting depth to no more than 1/8 of the tool diameter.

Surface quality and residual stress

Surface defects: Excessive cutting depth can easily cause tearing or burrs on the workpiece surface. For example, when the cutting depth exceeds 1.5mm, the surface roughness of the aluminum alloy workpiece may deteriorate from Ra1.6μm to Ra3.2μm.

Residual stress: The residual tensile stress on the surface of the workpiece processed with a large cutting depth increases, which may reduce the fatigue life. It needs to be eliminated through subsequent heat treatment (such as stress relief annealing) or small cutting depth finishing.

Fourth, boundary conditions for optimizing the cutting depth

Machine tool power limit

An increase in cutting depth leads to an increase in cutting power, and it is necessary to ensure that it does not exceed 80% of the rated power of the machine tool. For example, for a machine tool with a power of 10kW, when the cutting depth exceeds 3mm, the cutting speed needs to be reduced.

Tool strength and service life

The cutting depth must not exceed the safety threshold of the bending strength of the tool material. For instance, the upper limit of the cutting depth for ceramic tools is 0.5 to 1mm, and for coated carbide tools, it is 1 to 2mm.

Workpiece rigidity and clamping stability

For thin-walled parts or slender shaft workpieces, the cutting depth needs to be reduced. For instance, for workpieces with a wall thickness of less than 3mm, the cutting depth should be controlled within 0.3mm, and axial clamping or liquid-filled fixtures should be used to enhance rigidity.

Fifth, practical cases of optimizing cutting depth

Case 1: Rough Machining of 45 Steel Shaft Workpieces

The total allowance is 8mm, and the cutting is carried out in three steps: the first cutting depth is 4.8mm (cutting speed 80m/min, feed rate 0.3mm/r), the second cutting depth is 2.4mm, and the third cutting depth is 0.8mm.

Result: The processing efficiency is increased by 25% and the tool life is prolonged by 30%.

Case 2: Fine Processing of Thin-walled Stainless Steel parts

The cutting depth is 0.2mm, the cutting speed is 60m/min, the feed rate is 0.1mm/r, and high-pressure coolant is used in combination.

Result: The surface roughness Ra is ≤0.8μm, and the wall thickness tolerance is controlled within ±0.02mm.

Conclusion

The reasonable value of cutting depth should follow the principle of "efficiency first and controllable risk", and achieve a balance between processing efficiency and quality through stratified cutting, dynamic monitoring and multi-objective optimization. In practical applications, dynamic adjustments need to be made in combination with the workpiece material, machine tool performance and tool characteristics, and the validity of the parameters should be verified through trial cutting. For instance, when turning 45 steel, the rough machining cutting depth can be taken as 1.5 to 3mm, and the finish machining cutting depth should be controlled at 0.1 to 0.5mm. The final parameters need to be fine-tuned according to the actual processing results.