Abstract:
The application of Cryogenic Minimum Quantity Lubrication (CMQL) technology, which replaces traditional oily cooling and lubricating cutting fluids, in the green processing technology of small modulus gears on remanufactured gear shaping machines. Experiments have proven that this method not only improves processing accuracy and reduces processing costs but also achieves green and environmentally friendly manufacturing.
1. Introduction
In the process of gear shaping, due to the plastic deformation and intense friction caused by the gear shaping cutter on the metallic material during cutting, a high-temperature and highly harmful cutting heat is generated instantaneously between the tool edge and the metallic material. If this cutting heat cannot be quickly removed, the machined surface of the workpiece and the cutting edge of the tool will expand and deform as the temperature in the cutting zone rises sharply, and the strength and hardness of the cutting edge of the tool will also rapidly decline, leading to the formation of built-up edge and scoring. This will directly result in a rapid increase in the surface roughness of the machined part, a significant reduction in processing accuracy, and poor stability of the processed dimensions. Therefore, it is necessary to perform forced cooling on the cutting zone to quickly remove this highly harmful cutting heat.
In production processing, conventional pour-type oily or chemical cooling and lubricating cutting fluids are used to quickly reduce the cutting heat generated during the machining of parts through heat conduction between the liquid and the solid.
2. Advantages and Disadvantages of Using Cutting Fluids
2.1 Advantages
The use of oily lubricating cutting fluids in gear shaping machines prolongs the lifespan of cutting tools, improves the processing accuracy and production efficiency of parts, and its advantages are obvious.
2.2 Disadvantages
There are also many disadvantages to using oily lubricating cutting fluids for gear shaping, mainly in the following aspects:
- During the cutting process, conventional pour-type oily cooling and lubricating cutting fluids penetrate in a single pouring method. However, to improve production efficiency and meet processing cycle requirements, gear shaping machines often adopt rapid processing methods combining high stroke, large circumferential feed speed, and small radial feed speed. During the generative cutting process, a considerable portion of the oily cooling and lubricating cutting fluid is blocked by the back of the gear shaping cutter’s blade and hub or carried out of the processing area by the high-speed stroke motion of the arbor, resulting in limited actual penetration ability and efficiency of the cutting fluid when the gear shaping cutter and workpiece are engaged in cutting without clearance. This affects the lubricating and cooling heat conduction effects.
- Oily or chemical cooling and lubricating cutting fluids use a circulating pouring method to achieve lubrication and cooling of the cutting zone through heat exchange between the liquid and the solid. As processing time increases, the heat exchanged cutting fluid retains heat, which is difficult to evaporate and dissipate at room temperature, and the cooling effect is significantly reduced, forming a new heat source. This heat source causes local thermal deformation of tools, workpieces, and machine tool components such as the bed, leading to deviations in the processing dimensions and accuracy of parts from the tolerance zone requirements.
- To improve the performance of oily or chemical cooling and lubricating cutting fluids, extreme pressure additives containing sulfur, phosphorus, and chlorine are often added. The mist generated during use not only poses a hazard to the health of operators but also causes severe and sustained damage to the air, soil, and groundwater. Moreover, the used cutting fluids must be collected according to environmental protection regulations for hazardous waste and disposed of by professional environmental recycling agencies at a cost. This results in a significant increase in costs related to the purchase, storage, use, recycling, and safety supervision of cutting fluids. Studies have shown that the comprehensive use cost of cutting fluids accounts for 7% to 17% of the total processing cost of part manufacturing.
3. Cryogenic Cold Air and Minimum Quantity Lubrication Technology
In today’s era of advocating green and clean manufacturing, it has become a top priority for enterprises to find alternatives to oily or chemical cooling and lubricating cutting fluids and reduce the consumption of fossil energy and hazardous chemicals. Based on this environmental protection concept, we successfully provided green remanufacturing services for a foreign brand gear shaping machine.
3.1 Experimental Process
Through testing different processing parameters, it was found that when using only compressed air for dry cutting, the gear shaping cutter undergoes relatively rapid wear during processing, and the surface roughness, dimensional accuracy, and stability of the processed parts are difficult to meet technical requirements. Analysis revealed that this is because under the constraint of the processing cycle, high cutting heat is generated due to instantaneous friction between the cutting edge of the gear shaping cutter and the metallic material during the high stroke, large circumference, and small feed shaping process. Using only room-temperature compressed air cannot quickly remove the cutting heat, with limited cooling effects, leading to thermal expansion and deformation of the workpiece and tool, rapid decline in the strength and hardness of the cutting edge of the tool, dulling of the cutting edge, and the formation of built-up edge and scoring on the cutting edge of the tool and the tooth surface of the workpiece. This greatly affects the detection of main accuracy indicators for gear processing (individual pitch error fp, total pitch error Fp, pitch circle runout error Fr, tooth profile angle error fHβ, cumulative helix angle error Fβ, and form error ffβ), with actual measured values far exceeding the tolerance values of DIN 7. At the same time, it also significantly affects the stability of processing dimensions, often requiring continuous size compensation and correction to meet the tolerance requirements for batch part processing, and it is impossible to meet the technical requirements of the Capability Index (Cm) and the Machine Capability Index (CMK) values ≥ 1.67.
After querying and analyzing relevant dry and near-dry cutting materials, the cryogenic cold air technology with vortex tube effect and minimum quantity lubrication technology (MQL) were introduced, combining them into a cryogenic minimum quantity lubrication system (CMQL). The test of near-dry cutting technology using cryogenic cold air and minimum quantity of lubricating cutting fluid was a complete success.
The vortex tube is a very unique phenomenon in gas dynamics called the Ranque-Hilsch effect or vortex effect, and the vortex tube is a concrete embodiment of this theory. The vortex tube is a simple, stable, reliable, and low-cost energy separation device consisting of a compressed air inlet, a separation orifice plate, a vortex tube, a hot end tube, an adjusting valve, a hot air nozzle, a cold end tube, and a cold air nozzle.
During operation, compressed air expands within the inlet and then enters the vortex tube at a high speed in a tangential direction, forming a vortex along the inner wall of the vortex tube after high-speed rotation. It is then separated into low-energy gas molecules and high-energy gas molecules by the separation orifice plate. The low-energy gas molecules are on the inside, and the high-energy gas molecules are on the outside. That is, low-temperature airflow is in the center of the vortex tube, while high-temperature airflow is on the outside. After being separated, one end produces cold air with a minimum temperature of -45°C, and the other end produces hot air with a maximum temperature of 120°C.
During the test, the vortex tube effect’s cryogenic cold air technology was used for application testing. The comparison of the vortex tube test on the gear shaping machine.
To further reduce energy consumption and improve the stability of processing accuracy for workpieces during mass production, a minimum quantity lubrication (MQL) system was added to the cryogenic cold air, combining them into a new CMQL system, and a third cutting test was conducted.
The new CMQL uses low-temperature cold air below 0°C at 0.4 to 0.6 MPa as the carrier for specially formulated minimum quantity lubricating cutting fluid, which is mixed and then directly sprayed onto the cutting edge of the tool and the workpiece through the nozzle, promoting the formation of a stable and highly reliable strong oil film by the minimum quantity lubricating cutting fluid, thereby improving cutting conditions and production efficiency.
Table 1: Trial Cutting Processing Parameters
| Trial Cutting Condition | First Cutting | Second Cutting |
|---|---|---|
| Tool Stroke Speed (str/min) | 550 | 700 |
| Circumferential Feed Speed (mm/str) | 0.400 | 0.350 |
| Radial Feed Speed (mm/str) | 0.0050 | 0.0050 |
| Radial Cutting Depth (mm) | 1.000 | 0.200 |
Experimental results showed that under gas-liquid two-phase jet flow, although the lubricating oil passes through the throttling control of the minimum quantity lubrication control system, the sprayed minimum quantity liquid has a very small particle size with a flow rate of only 0.03 to 0.4 L/h. However, when the low-temperature misty gas-liquid with high kinetic energy, fast flow speed, and strong penetration enters the cracks on the cutting surface, part of the minimal lubrication liquid, upon encountering higher-temperature metals, will rapidly vaporize after absorbing the cutting heat and be carried away along with the low-temperature cold air. Another part will adhere to the cutting edge of the tool, forming a strong lubricating oil film to reduce the friction generated during tool cutting. The working efficiency of the low-temperature minimal lubrication system can fully replace the cooling and lubricating effect of the original machine tool’s 0.9kW oil cooling system.
The analysis of experimental results reveals the following:
- Improved Quality and Efficiency, and Achievement of Green Production: The low-temperature cold air effectively increases the dynamic heat exchange area in the cutting zone, enhancing heat dissipation efficiency and significantly reducing the temperature in the cutting zone. The workpiece material undergoes local cold brittleness under constant low-temperature cold air, strengthening the shear force of the tool during cutting, enabling rapid separation of the chip from the workpiece. This reduces thermal deformation and residual stress on the machined surface of the workpiece, improving the stability of part processing accuracy. Minimal lubrication significantly decreases the friction between the tool and the workpiece during cutting, improves cutting conditions, accelerates the rapid conduction of cutting heat among the tool, workpiece, and chips, and continuously forms fresh oil film layers, effectively protecting the cutting performance of the tool and extending its service life. Therefore, the cooling and lubrication effects obtained using low-temperature minimal lubrication are many times better than those achieved with traditional oil-based or chemical cooling and lubricating cutting fluids. With a processing cycle time of 120s, the gear shafts processed were tested by the customer’s German-made WENZEL WGT400 gear measuring center, and the processing accuracy was stable, meeting the DIN 7 accuracy requirement. Through measurement and statistical analysis of the processed parts, the customer determined that the Capability Index (CM) and the Process Capability Index (CMK) were both ≥1.67. This demonstrates that after remanufacturing, the equipment is capable of meeting the precision requirements of part processing, achieving the goals of improving part processing quality and extending tool life under near-dry cutting conditions, and realizing green manufacturing.
- Reduced Costs: Compared to the original machine tool design’s pour-type cooling and lubricating cutting fluid processing method, the use of low-temperature minimal lubrication significantly lowers costs. The original machine tool required 170kg of oil-based cooling and lubricating cutting fluid at a cost of over 3,000 yuan each time it was filled. Every six months, the fluid needed to be drained for sedimentation and initial filtration, then further processed using a dedicated oil filtration machine to remove moisture at high temperatures, and finally finely filtered under pressure using consumable filter paper to remove impurities before reuse. After two years of use, the oil-based cooling and lubricating cutting fluid would deteriorate due to chemical and physical property degradation and natural loss, requiring complete replacement. At the same time, the waste oil, classified as hazardous waste, needed to be paid for professional disposal by a specialized factory. Additionally, the machine tool’s original lubrication pump and cleaning pump combined had a total power of 1.8kW, consuming 14kWh of electricity per shift. Parts with cooling and lubricating cutting fluid also needed to be sent to a cleaning line for oil and chip removal using a dedicated chemical water-based cleaning fluid, with a 170kg bucket costing over 1,000 yuan. After cleaning, the processed parts were transferred to a high-power warm air drying line for rapid drying and rust prevention. The disposal costs for the used and deteriorated chemical water-based cleaning fluid, also classified as hazardous waste, were also considerable. The online practical application of CMQL shows that it generates no waste and has low operating costs. It merely utilizes the low-cost, renewable compressed air and centralized air supply system commonly used on the production line more fully and efficiently, thereby greatly expanding the use of air energy and enhancing its value. Therefore, the successful application of CMQL not only reduces environmental pressure on enterprises but also significantly lowers part processing costs, demonstrating a company’s sense of social responsibility and mission.
- Lowered Environmental and Health Hazards: During the experiment, it was also found that although most of the lubricating oil mist sprayed from the CMQL nozzles reached the cutting zone, a small portion dispersed into the air within the protective hood. Although the machine tool was originally equipped with an oil mist filtration device, to avoid potential harm to operators and the environment from any minor oil mist that might escape the protective hood, a vegetable oil-based cutting fluid with good atomizing cooling, lubricating, rust-preventing properties, and easy biodegradability was selected. This cutting fluid minimizes environmental and health hazards.
- Additional Rust Prevention Function: Since conventional pour-type cooling and lubricating cutting fluids were not used in the processing, under the action of the low-temperature minimal lubricating oil mist sprayed by CMQL, the finished workpiece surface was clean and free from obvious oil stains and chip adhesion. At the same time, the lubricating oil mist adhering to the workpiece formed an ultra-thin conformal oil film, providing rust prevention for a certain period and eliminating the need to transfer the workpiece to a cleaning line for cleaning off oil stains, chips, drying, and rust prevention before proceeding to the next process as before.
In conclusion, this successful case truly achieves the goals of improving processing accuracy, lowering processing costs, and realizing green and clean manufacturing. It also creates favorable conditions for enterprises to obtain ISO14001 environmental management system certification. This experiment lays the foundation for extending the green processing technology of low-temperature minimal lubrication to other types of machine tools in the future.