Dry machining is a processing method that intentionally performs machining without the use of cutting fluid and avoids the use of coolants, primarily to protect the environment and reduce costs.
Dry machining is not just about eliminating the use of cutting fluid, but rather, it aims to achieve high efficiency, high product quality, and high tool durability and reliability in the cutting process without relying on cutting fluid. This can only be achieved through the use of advanced dry machining tools, machine tools, and auxiliary facilities that are capable of replacing the cutting fluid used in traditional cutting processes.
Dry cutting technology background
First, let’s understand the role of cutting fluids in traditional cutting processes. Cutting fluid is an essential component in most machining operations and plays a crucial role in maintaining accuracy, surface quality, and production efficiency.
However, with increasing global environmental awareness and stricter regulations, the negative effects of cutting fluids on the environment are becoming increasingly apparent.
Statistics show that two decades ago, the cost of cutting fluid was less than 3% of the cost of the workpiece. Currently, in high-productivity manufacturing enterprises, the supply, maintenance, and recycling costs of cutting fluids account for 13-17% of the manufacturing cost of the workpiece, while tool costs only account for 2-5%. Approximately 22% of the total cost associated with cutting fluids is the processing cost.
It is estimated that if 20% of the cutting process uses dry machining, the total manufacturing cost can be reduced by 1.6%.
Green manufacturing that is environmentally friendly is considered a modern and sustainable model for development. Dry machining, which eliminates the use of cutting fluid during processing, is a green manufacturing process that reduces the source of environmental pollution. It results in clean, non-polluting chips and eliminates the need for cutting fluids and their disposal, thereby reducing production costs.
Therefore, the future trend in machining is to minimize the use of cutting fluid. With the advancement of high temperature tool materials and coating technology, dry machining has become a reality in the field of machine building.
Dry machining technology emerged in the mid-1990s and has developed rapidly in recent years. Although it is a relatively new field of research, it is considered a frontier in advanced manufacturing technology.
Three main functions of cutting fluid
Removes the heat generated during cutting, reducing tool wear and preventing oxidation on the surface of the workpiece.
Reduces friction, reduces the cutting force, and ensures smooth cutting.
Efficiently removes chips from the surface of the workpiece to prevent scratching.
Negative effects of cutting fluid
However, from an environmental perspective, the negative effects of cutting fluid are becoming increasingly apparent, mainly in the following ways:
- The high temperature generated during processing causes the cutting fluid to volatilize and form mist, which contaminates the environment and poses a threat to the operator’s health.
- Certain cutting fluids and cutting chips are considered toxic and hazardous materials, resulting in high processing costs.
- Leaks and overflow of cutting fluid can have significant impacts on safety in production.
- Cutting fluid additives (such as sulfur and chlorine) can harm the operator’s health and affect the quality of processing.
In addition, research on the cutting process has shown that traditional cooling, lubrication, and chip removal methods using cutting fluids are not fully and effectively utilized in many machining processes, especially in high-speed cutting.
Therefore, attempts have been made to change this situation and adopt clean production processes with reduced costs, either with or without cutting fluids.
Dry machining technology is a solution that emerged from this need. It not only reduces the environmental pollution caused by cutting fluid but also improves working conditions for operators and saves costs associated with cutting fluid and chip recycling.
Dry machining technology has higher demands for machine tools and tooling technology. In recent years, industrialized countries have shown great interest in research on dry machining. This new processing method, dry machining, is one of the future trends in metal cutting.
Dry machining features
(1) Clean and non-polluting chips that are easy to recycle and handle
(2) Elimination of cutting fluid transfer, recovery, filtration, and other equipment, as well as associated costs, simplifying the production system and reducing costs
(3) The omission of cutting fluid and chip separation devices and corresponding electrical equipment results in a more compact machine structure and reduced floor space
(4) No environmental pollution
There will be no safety or quality incidents related to the cutting fluid.
Dry machining implementation conditions
Dry machining tool technology
(1) The tool should have excellent heat resistance (high temperature hardness) and wear resistance
(2) Minimize the friction coefficient between the tool and the chip
(3) Reducing the dependence on cutting fluid chip removal
Dry cutting machine technology
The heat transfer from the cutting and the discharge of chips and dust should be rapid.
Dry machining technology
Particular attention should be paid to the proper matching between the tool material and the workpiece material.
Using new tool materials
In the past decade, the development of high hardness materials has made dry cutting a possibility. Dry machining requires not only the tool material to have high red hardness and thermal toughness, but also good wear resistance, thermal shock resistance, and adhesion resistance.
The current tool materials used for dry machining are mainly ultra-hard materials such as ultra-fine cemented carbide, ceramics, cubic boron nitride, and polycrystalline diamond.
Ultra-fine cemented carbide has improved toughness compared to regular cemented carbide, good wear resistance and high temperature resistance, and can be used to produce deep hole drills and inserts with large rake angles for dry milling and drilling.
Materials like ceramic cutters (Al203, Si3N4) and cermets (Cennet) also retain their hardness at high temperatures, making them suitable for general purpose dry cutting without coolant. However, these materials are generally brittle, meaning they have poor thermal toughness and are not suitable for interrupted cutting. This means that ceramic tools are better suited for dry turning rather than dry milling.
Cubic boron nitride (CBN) has a very high hardness of HV3200-HV4000, second only to diamond, and good thermal conductivity of up to 1300W/MK. It also has good high temperature chemical stability and thermal stability at 1200°C. The use of CBN tools for machining cast iron can greatly increase the cutting speed, and it is used for machining hardened steel as a replacement for turning.
Polycrystalline diamond (PCD) tools have extremely high hardness of HV7000-HV8000 and high thermal conductivity of up to 2100W/MK, along with a low coefficient of linear expansion. The thermal energy generated during cutting with a PCD tool can be quickly transferred from the tool tip to the tool body, reducing machining errors caused by thermal deformation of the tool. PCD tools are suitable for dry machining of copper, aluminum, and aluminum alloy workpieces.
Coating the tool is a crucial method for improving tool performance. In the past decade, tool coating technology has advanced rapidly, with up to 15 coating materials and some tools having as many as 13 layers on their body. The coating process has also become more mature, and technological advancements have solved the problem of low bonding strength between the coating and substrate material.
There are two main categories of coated tools: “hard” coated tools such as TiN, TIC, and Al203, which have high surface hardness and good wear resistance.
TIC coated tools have exceptional resistance to flank wear, while TiN coated tools have higher resistance to “crater” wear. The other category is “soft” coated tools such as MOS2 and WS, which are also known as “self-lubricating tools.” This type of coated tool has a low friction coefficient with the workpiece material, around 0.01, effectively reducing cutting force and temperature.
For example, the “MOVIC” coated tap developed in Switzerland is coated with MOS2, and cutting experiments show that while uncoated taps can process 20 tapped holes, TiAlN coated taps can process 1000, and MoS2 coated taps can process up to 4000.
High-speed steel and cemented carbide can be used for dry cutting after PVD coating, and CBN tools that were originally only suitable for dry cast iron can also be used for processing steel, aluminum alloys, and other superhard alloys after coating.
The coating functions similarly to a coolant, creating a protective layer that isolates the tool from the heat of the cut, keeping heat from reaching the tool and maintaining its hardness and sharpness for a longer period of time. A smooth surface coating also reduces friction, reducing cutting heat and protecting the tool material from chemical reactions, as high temperatures have a significant catalytic effect on chemical reactions in most high-speed dry cuts.
TiAlN coating and Mo2 soft coating can also be alternated to create a multi-coated tool, combining high hardness and good wear resistance with a low friction coefficient and easy chip flow, making it an excellent alternative to a cooling liquid. Tool coating plays a critical role in dry cutting technology.
Tool geometry design
Dry machining tools often experience crater wear as the primary cause of failure due to the lack of cutting fluid and the increase in temperature in the tool-chip contact area. To counter this, the tool should have a large rake angle and blade inclination. However, increasing the rake angle can impact the blade strength.
To address this, a suitable negative chamfer or rake face reinforcement unit should be provided to ensure the tool tip and edge have adequate material volume and can withstand the cutting heat and forces. This also reduces the impact and expansion of craters on the tool, allowing for long-lasting structural strength in the tool tip and cutting edge.
In recent years, advancements have been made in turning inserts with large angles, such as the ME-13 new carbide insert by Carboloy in the US with a front angle of up to 34°. There’s also a spiral-edged milling insert with a positive rake angle, providing a constant rake angle along the cutting edge and the ability to change the back or side rake angle to reduce cutting force and temperature during dry cutting.
Japan’s Mitsubishi Metal Corporation has developed a “slewing turning tool” for dry machining, which uses a round superhard blade and bearings in the support part that allow for automatic blade rotation during machining, resulting in a sharp cutting edge and increased efficiency, quality, and tool life.
Another solution for dry cutting is the heat pipe cutter, similar in structure to a conventional turning tool with the addition of a heat pipe within the shank body. The heat pipe, made of a highly efficient heat transfer element such as acetone, ethanol, or distilled water, has several hundred times the thermal conductivity of silver or copper and functions as a self-cooling tool without the need for external cutting fluid, making it ideal for CNC machine tools, machining centers, and automatic production lines.