MQL Technology for 304 Stainless Steel Milling: Improving Tool Life, Surface Quality, and Sustainability

Thanks to its excellent corrosion resistance, mechanical stability, and formability, 304 stainless steel is widely used in high-end manufacturing sectors such as aerospace, medical devices, and food processing equipment.

However, its milling process faces inherent challenges: the material’s poor thermal conductivity leads to localized heat buildup, while its high toughness can cause tool adhesion wear, directly affecting machining efficiency and surface quality.

Although traditional emulsion lubrication is the standard approach, it has significant drawbacks.

A single machine consumes 8–15 L/min of coolant, and the cost of treating oily waste liquid accounts for 12%–18% of total processing costs.

This not only causes water pollution but also contradicts the green manufacturing philosophy under the “dual carbon” goals.

Furthermore, long-term use can lead to secondary issues such as machine tool corrosion and excessive oil mist in the work environment.

Minimum Quantity Lubrication (MQL) technology centers on the combination of “environmentally friendly micro-quantity oil + compressed air.”

By precisely applying micron-level oil mist to the cutting interface, it achieves synergistic optimization of lubrication and cooling, offering the dual advantages of low consumption and zero waste liquid.

While this technology has demonstrated effectiveness in the machining of carbon steel and aluminum alloys, research on its suitability for 304 stainless steel milling remains insufficient.

There is a lack of data to support the selection of lubricants and their matching to specific operating conditions, and the mechanisms by which system parameters influence machining performance are unclear.

Based on this, this paper constructs an MQL system tailored for 304 stainless steel milling.

Through comparative experiments, it quantifies its machining performance and environmental benefits, providing technical support for the green transformation of stainless steel machining.

Applications of MQL Technology

• Principles of Micro-Lubrication Technology

Micro-lubrication technology (MQL) utilizes environmentally friendly lubricants and compressed air as media.

Through the synergistic action of the system, it achieves efficient lubrication and cooling in the cutting zone.

MQL pressurizes environmentally friendly lubricants, such as vegetable oil-based fluids, which are then converted into micron-sized oil mist particles by a precision atomization device.

These particles are precisely delivered to the contact interface between the cutting tool, workpiece, and chips via high-velocity compressed air.

Through molecular adsorption, the oil mist particles form a continuous and stable lubricating film, significantly reducing interfacial friction and mitigating adhesive wear between the tool and the workpiece.

Simultaneously, the high-speed airflow rapidly dissipates the concentrated heat generated during the cutting process, suppressing temperature rise in the cutting zone.

It also assists in dislodging chips from the machining area, preventing scratches on the machined surface, and achieving synergistic optimization of lubrication, cooling, and chip removal.

• Design of a Minimal Lubrication Technology Solution

Lubricant Selection and Precise Parameter Matching

For the turning of 45 steel, this article selects an ester-based synthetic lubricant as the lubricating medium.

It has a kinematic viscosity of 12.5 mm³/s at 40°C and a flash point of ≥220°C, and possesses excellent extreme-pressure and anti-wear properties as well as biodegradability.

The lubricant supply system employs a positive displacement metering pump, with single-dose accuracy controlled within ±0.01 mL.

Three supply modes are set based on machining conditions: during rough machining, the lubricant flow rate is 0.08 mL/min;

During semi-finishing, it is reduced to 0.05 mL/min; and during finishing, it is optimized to 0.03 mL/min.

Compressed air is used as the carrier, with the operating pressure maintained at 0.45–0.55 MPa.

After passing through a three-stage filtration system, the dust particle size is ≤0.3 μm and the moisture content is ≤0.01 g/m³, ensuring that the oil atomization process is not interfered with by impurities.

The mixing ratio of oil and air is precisely controlled via a proportional control valve, maintaining a dynamic adjustment range of 1:1200 to 1:1800 across different processing stages to achieve a balance between lubrication efficiency and cost-effectiveness.

Atomizing Nozzle Structure and Layout Design

The nozzle features a double-layer structure, with an outer air channel diameter of 6 mm and an inner oil channel diameter of 0.8 mm.

The inner wall roughness of the channels is Ra ≤ 0.4 μm to reduce flow resistance.

A 15° conical constriction is provided at the nozzle outlet to form a high-speed jet of the gas-liquid mixture, achieving optimal atomization 15–20 mm from the nozzle.

The average droplet size is controlled between 5 and 8 μm, with a droplet size uniformity of ≥92%.

For the spindle structure of horizontal lathes, a symmetrical dual-nozzle layout is adopted.

The nozzles are mounted at a 30° angle to the workpiece axis, positioned 8 mm above the tool’s cutting edge, with a spacing of 45 mm between them, ensuring that the mist droplets fully cover the contact area between the front and rear faces of the turning tool.

The relative position of the nozzles to the tool is fixed via a three-dimensional adjustment bracket with an adjustment accuracy of 0.1 mm.

This allows for rapid adaptation to different tool models (such as the CNMG120408 turning tool), ensuring that the lubricant is precisely applied to the cutting zone.

• Intelligent Control and Operating Condition Adaptation

The system is equipped with an STM32H743 microcontroller as its control core, integrating vibration sensors, temperature sensors, and an infrared temperature measurement module to collect real-time data on spindle vibration frequency (measuring range:

0–5000 Hz, accuracy: ±1 Hz), tool temperature (measuring range: 0–500°C, accuracy: ±2°C), and cutting force signals (collected via piezoelectric sensors, range: 0–5000 N).

Based on a fuzzy PID control algorithm, when the tool temperature exceeds 350°C or the vibration frequency exceeds 2000 Hz, the system automatically increases the coolant flow rate by 15% and raises the air pressure by 0.05 MPa.

When the cutting force drops below 60% of the rated value, the system automatically activates energy-saving mode, reducing the coolant flow rate by 20%.

The control module interfaces with the machine tool’s CNC system via an RS485 communication port, with a response delay of ≤50 ms, enabling synchronized adjustment of machining and lubrication parameters.

Additionally, the system is equipped with a fault diagnosis unit;

When the remaining coolant level falls below 50 mL or the air pressure drops below 0.3 MPa, it immediately triggers an audible and visual alarm and sends a shutdown signal, ensuring the continuity and safety of the machining process.

Analysis of Case Study Results

• Case Overview

In this test, 304 stainless steel bar stock (Φ120 mm × 300 mm, hardness 180–200 HB) was used as the workpiece.

Face milling tests were conducted using a VMC850 vertical machining center, with a coated carbide end mill selected as the cutting tool (Model RCMX1204MO, 4 flutes, TiAlN coating, 0.4 mm cutting edge radius).

In the traditional emulsion lubrication scheme, with an emulsion concentration of 5% and a consumption rate of 8 L/min, the wear on the tool’s rake face reached 0.32 mm after machining 10 workpieces, requiring a shutdown for replacement.

The average surface roughness (Ra) of the machined workpieces was 1.8 μm, and approximately 20 L of oily waste liquid was generated, with disposal costs exceeding 16 yuan.

To address these issues, MQL technology was introduced with the goal of “performance optimization and environmental improvement.”

A comparative test was conducted between traditional emulsion lubrication (control group) and MQL lubrication (test group), with consistent cutting parameters maintained: spindle speed of 1,500 rpm, feed rate of 80 mm/min, and cutting depth of 2 mm.

The system evaluated the effectiveness of the MQL technology.

• Test Environment Setup

The test site is located within the temperature-controlled workshop of the Mechanical Manufacturing Laboratory, where the ambient temperature is maintained at 22±2°C and relative humidity at 45%–60% to prevent fluctuations in temperature and humidity from affecting measurement accuracy.

The machining equipment consists of a VMC850 vertical machining center, which features X/Y/Z-axis positioning accuracy of ±0.005 mm/300 mm, repeatability of ±0.003 mm, and a maximum spindle speed of 8,000 rpm, meeting the stability requirements for test machining.

The measurement system consists of five sets of specialized equipment:

① Olympus GX51 metallographic microscope, with a magnification range of 50–1000×, used to observe the wear patterns on the tool’s rake face and measure wear depth, with a measurement accuracy of 0.001 mm;

② Taylor-Hobson Surtronic S-100 surface roughness tester, with a measurement range of Ra 0.01–20 μm, a resolution of 0.001 μm, a sampling length of 0.8 mm, and an evaluation length of 4 mm, used to measure the surface roughness of the machined workpiece;

③ The Kistler 9257B force transducer mentioned earlier, paired with a 5070A charge amplifier, with a data acquisition frequency of 1000 Hz, used to record changes in cutting force in real time;

④ A German Testo 350 flue gas analyzer, with a measurement accuracy of ±1 ppm, used to monitor MQL oil mist concentration and ensure the work environment complies with the requirements of GBZ 2.1—2019 “Occupational Exposure Limits for Hazardous Factors in the Workplace”;

⑤ An electronic balance (accuracy 0.001 g) and a measuring cup (accuracy 1 mL), used respectively to measure oil consumption and waste liquid generation.

• Analysis of Test Results

Analysis of Tool Wear Test Results

Tool wear is a key indicator that determines machining continuity and cost.

At high temperatures, 304 stainless steel tends to adhere to the tool, accelerating wear.

In this test, the number of machined workpieces served as the time dimension.

After every two workpieces, the wear on the tool’s rake face, the depth of the crescent-shaped groove, and the condition of the cutting edge were recorded.

The control group used a 5% concentration emulsion (8 L/min), while the test group used ester-based MQL (finishing flow rate 0.03 mL/min, air pressure 0.5 MPa).

Cutting parameters were kept consistent, and wear morphology was observed using a metallographic microscope. The test results are shown in Table 1.

> Comparison of Tool Wear Performance

As shown in Table 1, throughout the entire machining process, the tool wear metrics of the test group were significantly better than those of the control group, and the gap tended to widen as the number of machined workpieces increased.

After machining 2 workpieces, the rake face wear of the test group (0.04 mm) was 50% of that of the control group (0.08 mm), and the crescent-shaped pit depth (0.02 mm) was 40% of that of the control group (0.05 mm).

There was no obvious welding on the cutting edge, whereas the control group had already exhibited slight welding.

After machining 10 workpieces, the rake face wear of the test group (0.18 mm) was 43.75% lower than that of the control group (0.32 mm), and the depth of the crescent-shaped groove (0.10 mm) was 52.38% lower than that of the control group (0.21 mm).

The cutting edge of the control group was no longer usable, while the test group still retained machining capability.

> Wear Progression During Machining

Tool wear in both groups increased with the number of workpieces machined, but the wear rate in the control group was significantly faster.

The rake face wear in the control group increased from 0.08 mm after 2 workpieces to 0.32 mm after 10 workpieces, a 300% increase; localized chipping appeared after 6 workpieces and expanded after 8 workpieces.

In contrast, the rake face wear of the test group increased from 0.04 mm to 0.18 mm, a 350% increase; however, the cutting edge remained stable throughout, with no chipping observed even after machining 8 parts.

The primary reason lies in the difference in lubrication mechanisms:

The micron-level oil mist of MQL forms a stable lubricating film at the cutting interface, while the extreme-pressure components of ester-based oil suppress adhesive wear, and the high-speed airflow carries away heat to prevent tool softening.

In contrast, the emulsion lubricating film in the control group is easily disrupted by centrifugal force, and chips entrained with impurities cause abrasive wear, leading to rapid tool failure.

Table 1. Analysis of Tool Wear Changes (mm)

Analysis of Workpiece Surface Quality Test Results

Surface roughness is a key indicator of machining quality.

During the milling of 304 stainless steel, surface scratches and adhesion marks can easily lead to increased roughness.

In this experiment, after each workpiece was machined, five measurement points were selected at evenly spaced intervals on the circumferential surface.

The Ra value was measured using a surface roughness tester, and surface defects were observed using a metallographic microscope.

The control group used emulsion lubrication, while the test group used MQL lubrication.

With identical machining parameters, the analysis focused on the changes in Ra values and differences in surface defects between the two groups.

The test results are shown in Table 2.

> Comparison of Surface Roughness and Defects

As shown in Table 2, the average Ra value for the control group was 1.8 μm, whereas that for the test group was only 0.9 μm, representing a 50% reduction;

The control group showed a steady increase in Ra values, rising from 1.5 μm for Workpiece No. 1 to 2.0 μm for Workpiece No. 4.

By contrast, the experimental group maintained Ra values within the 0.8–1.0 μm range throughout the test.

The experimental group also exhibited much better stability, with fluctuations measuring only one-third of those observed in the control group.

Furthermore, it was observed that surface defects in the control group showed a clear trend of deterioration: Part 1 had only minor scratches;

Part 2 exhibited adhesion marks; Part 3 showed an increase in scratches;

Part 4 developed combined defects of adhesion and scratches; and Part 5 developed localized indentations due to accelerated tool wear.

In contrast, none of the five workpieces in the test group exhibited any significant defects;

Only workpiece No. 2 showed slight sticking, while the surfaces of the remaining workpieces were smooth and uniform, with no scratches or dents.

> Surface Quality Evolution During Machining

The surface quality of the control group continued to deteriorate as machining progressed, whereas the test group maintained a consistently excellent state, primarily due to differences in lubrication and chip removal effectiveness.

The emulsion in the control group had poor lubrication stability, causing the lubricating film at the cutting interface to rupture easily.

This led to adhesion between the tool and the workpiece, with the resulting adhesive residue adhering to the cutting edge and forming

scratches on the workpiece surface during machining.

Additionally, the emulsion had weak chip removal capabilities, causing chips to accumulate in the machining area and scratch the machined surface.

In contrast, the high-speed airflow generated by MQL technology rapidly removes chips from the machining zone, preventing chip abrasion.

Micron-sized oil mist forms a stable lubricating film that reduces adhesion between the tool and the workpiece.

This lubrication mechanism keeps the cutting edge sharp throughout the machining process and generates a regular, uniform surface texture.

As a result, MQL technology significantly improves surface quality and maintains its consistency over time.

Table 2. Changes in Workpiece Surface Quality (μm)

Analysis of Cutting Force Test Results

Cutting forces directly reflect the energy consumption of the machining process and the stress conditions on the cutting tool;

The high toughness of 304 stainless steel tends to increase cutting forces.

In this test, a Kistler 9257B force transducer was used to collect real-time data on the principal cutting force (F_z), radial force (F_x), and axial force (F_y) during the milling process.

The system recorded cutting force data at one-minute intervals throughout the machining process.

For each time interval, it calculated the average of 10 data points and used this value to represent the cutting force.

The analysis then compared the trends in cutting force variation and peak force values between the control group (emulsion) and the test group (MQL).

Table 3 presents the test results.

The three cutting force metrics in the experimental group were significantly lower than those in the control group throughout the entire process, and the reduction remained consistent.

Regarding the principal cutting force, the control group increased from 1250 N to 1380 N, a 10.4% increase;

The test group increased from 1080 N to 1180 N, a 9.3% increase. At 5 minutes, the test group’s Fz was 14.5% lower than that of the control group.

The radial and axial forces followed trends similar to those of the principal cutting force.

Compared with the control group, the test group reduced the average Fx and Fy values by 16.3% and 14.9%, respectively.

The greatest reductions occurred at the 5-minute mark, where Fx decreased by 18.8% and Fy decreased by 18.2%.

> Evolution of Cutting Forces During Machining

Cutting forces in both groups increased with machining time, but the increase was more pronounced in the control group, indicating that tool wear led to an accelerated rise in cutting resistance.

> Mechanism of Cutting Force Reduction Under MQL

MQL generates a micron-sized oil mist that creates a durable lubricating film at the cutting interface.

At the same time, the extreme-pressure additives in the ester-based oil effectively prevent adhesion between the tool and the workpiece.

These combined effects reduce frictional resistance during the machining process.

The high-speed airflow rapidly dissipates cutting heat, preventing sudden spikes in cutting force caused by tool softening.

In contrast, the emulsion in the control group is easily flung away from the cutting zone by centrifugal force, resulting in poor lubrication film stability.

As tool wear intensifies, the frictional resistance between the chips and the cutting edge increases, causing cutting forces to rise continuously.

This further validates the advantages of MQL technology in stabilizing cutting loads and reducing machining energy consumption.

Conclusion

In summary, the application of MQL technology to the milling of 304 stainless steel achieves a synergistic improvement in both machining performance and environmental friendliness.

Experimental results demonstrate the effectiveness of this technology in improving machining performance.

It reduces tool rake face wear by 43.75% and lowers the workpiece surface roughness (Ra) value by 50%.

In addition, it decreases the average principal cutting force by 14.5%.

These improvements enhance both machining efficiency and product quality.

Furthermore, oil consumption is only 0.04% to 0.1% of that of conventional methods, and there is no oily waste liquid, resulting in near-zero environmental costs.

This provides reliable technical support for the green transformation of stainless steel machining.