The influence of feed rate in CNC turning on processing efficiency

In CNC turning, feed rate is one of the core parameters that affect processing efficiency, surface quality and tool life. Its adjustment needs to achieve a dynamic balance among cutting force, material removal rate and processing stability. The following analysis is carried out from three aspects: efficiency improvement mechanism, potential risks and optimization strategies:

First, the direct driving effect of feed rate on processing efficiency

The linear growth of material removal rate (MRR)

The feed rate (f), the depth of cut (ap), and the cutting speed (vc) jointly determine the material removal rate. When the feed rate is increased from 0.1mm/r to 0.3mm/r, under the same cutting speed and depth of cut, the amount of material removed per unit time can increase by 200%. For example, when turning shaft workpieces with a diameter of 100mm, for every 0.05mm/r increase in the feed rate, the single tool pass time can be shortened by 8% to 12%.

Implicit compression of non-cutting time

Increasing the feed rate can reduce the proportion of idle travel in the tool path. For example, in the processing of workpieces with complex contours, when the feed rate is increased from 0.2mm/r to 0.4mm/r, the proportion of idle travel time of the tool can be reduced from 35% to 25%. In addition, high-speed feed can shorten the waiting time for auxiliary actions such as tool retraction and tool change, and is particularly suitable for batch production scenarios.

The contradictory balance between cutting force and efficiency

An increase in feed rate will lead to an approximately linear growth in the cutting force (especially the feed force Ff). For example, when the feed rate increases from 0.1mm/r to 0.3mm/r, the feed force may increase by 50% to 80%. If the rigidity of the process system is insufficient, the sudden increase in cutting force may cause vibration, which in turn reduces the actual cutting efficiency. Therefore, the upper limit of the feed rate needs to be comprehensively evaluated by combining the power of the machine tool spindle, the strength of the cutting tool and the stability of the fixture.

Second, the potential risk of excessive feed rate

The dual deterioration of surface quality and tool life

When the feed rate exceeds the critical value, the surface roughness (Ra) will deteriorate significantly. For example, when the feed rate increases from 0.3mm/r to 0.5mm/r, the Ra value may rise from 1.6μm to 3.2μm, resulting in an increase in the subsequent finish machining allowance. Meanwhile, an excessive feed rate will accelerate the wear of the tool's rear face, reducing the tool's lifespan by 40% to 60%.

Uncontrolled chip morphology and difficulty in chip breaking

High feed rates are prone to generating long strip-shaped or spiral chips, especially when processing plastic materials such as stainless steel and aluminum alloy. Chips entangling around workpieces or tools can cause surface scratches and even lead to safety accidents. For example, when the feed rate exceeds 0.4mm/r, the aluminum alloy chips may be over 50mm long, and it is necessary to control them by adjusting the parameters of the tool's chip breaking groove (such as width and depth) or adding a chip breaker.

Vibration of the process system and loss of dimensional accuracy

When the feed rate is too large, the fluctuation of the cutting force intensifies, which may cause self-excited vibration. For example, when processing slender shafts (with a length-to-diameter ratio > 10), a feed rate exceeding 0.25mm/r may result in a diameter dimension error of more than 0.05mm. In addition, vibration can also accelerate tool chipping and increase unplanned downtime.

Third, the optimization strategy of feed rate

Segmented control based on material properties

Brittle materials (such as cast iron and ceramics) : A larger feed rate (0.3 to 0.8mm/r) can be selected because the cutting force fluctuation is small and it is less likely to form built-up edge.

Plastic materials (such as steel and copper alloys) : The feed rate should be limited (0.1 to 0.4mm/r), 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 low feed rate (0.05-0.2mm/r) combined with a high cutting speed is adopted to reduce the friction time between the tool and the workpiece.

Collaborative matching of tool geometric parameters

Rake Angle: Increasing the rake Angle (10° to 20°) can reduce the cutting force and allow for a higher feed rate, but it is necessary to pay attention to the risk of a decrease in the strength of the cutting edge.

Main deflection Angle: A small main deflection Angle (30° to 45°) can disperse the cutting force and is suitable for processing with large feed rates. A large principal deflection Angle (75° to 90°) is suitable for fine machining.

Chip breaking groove: By adjusting the width (0.5-2mm) and depth (0.3-1mm) of the chip breaking groove, controllable chip breaking can be achieved at high feed rates.

Dynamic compensation of the process system

Machine tool rigidity: For machine tools with poor rigidity, the cutting force needs to be dispersed by reducing the feed rate or increasing the depth of cut.

Fixture design: The use of hydraulic chucks or expansion clamps can enhance the clamping stiffness and allow for a higher feed rate.

Vibration monitoring: The cutting vibration is monitored in real time through an acceleration sensor. When the vibration amplitude exceeds the threshold, the feed rate is automatically reduced.

Multi-objective optimization and experimental verification

Through orthogonal experiments or finite element simulations, the relationship models between feed rate, cutting speed, depth of cut and machining efficiency, surface quality and tool life are established. For example, when turning 45 steel, it was found through experiments that the combination of a feed rate of 0.25mm/r, a cutting speed of 120m/min, and a depth of cut of 2mm could maximize the material removal rate (MRR=1800mm³/min) and the surface roughness Ra≤1.6μm. The final parameters need to be verified through trial cutting and fine-tuned according to the actual processing results.

Fourth, the boundary conditions for feed rate optimization

Machine tool power limit: An increase in feed rate leads to an increase in cutting power. It is necessary to ensure that it does not exceed 80% of the rated power of the machine tool.

Tool strength limit: The feed rate must not exceed the safety threshold of the bending strength of the tool material (for example, the upper limit of the feed rate for carbide tools is usually 0.8mm/r).

Surface quality requirements: The feed rate can be appropriately relaxed during rough machining, but sufficient allowance should be reserved for finish machining (usually 0.5 to 1mm on each side).

Through the comprehensive application of the above strategies, the feed rate can be optimized to a reasonable range while ensuring the processing quality, achieving a 15% to 30% increase in processing efficiency and a 10% to 20% reduction in tool wear at the same time. In practical applications, dynamic adjustments need to be made in combination with the specific material of the workpiece, the performance of the machine tool and the requirements of the production cycle.