Dry Machining

Dry machining is a processing method that consciously does not use cutting fluid and performs machining without cold liquid, mainly to protect the environment and reduce costs.

Dry Machining

Dry machining is not simply to stop using cutting fluid, but to ensure high efficiency, high product quality, high tool durability and reliability of the cutting process while stopping the use of cutting fluid.

This requires the use of dry machining tools, machine tools and auxiliary facilities with excellent performance to replace the cutting fluid in traditional cutting to achieve true dry machining.

Dry cutting technology background

First, understand the role of cutting fluids in traditional cutting processes.

Cutting fluid is usually one of the indispensable production factors in most machining, and plays an important role in ensuring machining accuracy, surface quality and production efficiency.

As global environmental awareness increases and environmental regulations become more stringent, the negative effects of cutting fluids on the environment are becoming more apparent.

According to statistics, twenty years ago, the cost of cutting fluid was less than 3% of the cost of the workpiece.

At present, 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 the tool costs only account for 2%-5%.

About 22% of the total cost associated with cutting fluids is the processing cost of cutting fluid.

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 manufacturing model for sustainable development.

Dry cutting without any cutting fluid during processing is a green manufacturing process that controls the source of environmental pollution.

It can obtain clean, non-polluting chips, eliminating the need for cutting fluids and their disposal, which can further reduce production costs.

Therefore, the direction of future machining is to use as little cutting fluid as possible.

With the development of high temperature tool materials and coating technology, dry machining has become possible in the field of machine building.

Dry machining technology emerged in this historical context and has developed rapidly since the mid-1990s.

Its development history has only been ten years, and it is a frontier research topic of advanced manufacturing technology.

Three main functions of cutting fluid

Cooling effect

Take away the heat generated by cutting, reduce tool wear and prevent oxidation of the workpiece surface;

Lubrication

Reduce friction, reduce cutting force, and ensure smooth cutting;

Chip removal

Quickly remove chips from the surface of the workpiece and prevent chips from scratching the surface of the workpiece.

Negative effects of cutting fluid

However, from the perspective of environmental protection, the negative effects of cutting fluid are becoming more and more obvious, mainly in the following aspects:

1) The high temperature generated during the processing causes the cutting fluid to form a misty volatilization, pollute the environment and threaten the health of the operator;

2) Certain cutting fluids and the cutting chips adhere to the cutting fluid must be treated as toxic and hazardous materials, and the processing cost is very high;

3) Leakage and overflow of cutting fluid have a great impact on safe production;

4) Additives for cutting fluids (such as sulfur, chlorine, etc.) can cause harm to the operator’s health and affect the quality of processing.

In addition, a large number of studies on the cutting process have also shown that the traditional cooling, lubrication and chip evacuation of 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 condition with or without cutting fluids to accommodate clean production processes and reduced production costs.

Dry machining technology is an advanced processing method generated in this case.

The dry machining technology not only reduces the environmental pollution of the cutting fluid, improves the working conditions of the operator, but also saves the cost associated with the cutting fluid and reduces the cost of the chip recycling process.

Dry machining technology places higher demands on machine tools and tooling technology.

In recent years, industrialized countries have paid great attention to dry machining research.

This new processing method – dry machining is one of the development trends of metal cutting in the future.

Dry machining features

(1) Chips are clean, non-polluting, easy to recycle and handle

(2) Eliminating the cutting fluid transfer, recovery, filtration and other equipment and corresponding costs, simplifying the production system and reducing production costs

(3) The separation device for cutting fluid and chips and the corresponding electrical equipment are omitted. Compact machine structure, reducing floor space

(4) No environmental pollution

There will be no safety accidents and quality accidents 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 emergence of high hardness materials has made it possible for dry cutting.

Dry machining not only requires the tool material to have extremely high red hardness and thermal toughness, but also must have good wear resistance, thermal shock resistance and adhesion resistance.

The tool materials currently 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 can improve the toughness of ordinary cemented carbide, has good wear resistance and high temperature resistance, can produce deep hole drills and inserts with large rake angle, and is used for dry machining of milling and drilling.

The hardness of materials such as ceramic cutters (Al203, Si3N4) and cermets (Cennet) is also rarely lowered at high temperatures, that is, it has good red hardness, so it is suitable for general purpose dry cutting without coolant.

However, such materials are generally brittle, that is, the thermal toughness is not good, so it is not suitable for interrupted cutting.

In other words, ceramic tools are more suitable for dry turning than for dry milling.

The hardness of cubic boron nitride (CBN) material is very high, reaching HV3200~HV4000, second only to diamond. The thermal conductivity is good, up to 1300W/MK, with good high temperature chemical stability, and the thermal stability is very good at 1200 °C.

The use of CBN tools for the machining of cast iron can greatly increase the cutting speed and is used to machine hardened steel, which can be replaced by turning.

Polycrystalline diamond (PCD) tools have very high hardness, up to HV7000~HV8000, thermal conductivity up to 2100 W/MK, and low coefficient of linear expansion.

The thermal energy generated by the cutting of the PCD tool can be quickly transferred from the tool tip to the tool body, thereby reducing the machining error caused by the thermal deformation of the tool.

PCD tools are suitable for dry machining of copper, aluminum and aluminum alloy workpieces.

Coating technology

Coating the tool is an important way to improve tool performance.

In the past decade, tool coating technology has developed very rapidly, with up to 15 coating materials and some tools with up to 13 layers on the body.

The coating process is also becoming more and more mature. With the development of technology, the technical problem of low bonding strength between the coating and the substrate material has been solved.

There are two broad categories of coated tools: one is “hard” coated tools such as TiN, TIC and Al203.

These tools have high surface hardness and good wear resistance.

Among them, TIC coated tools have a particularly strong resistance to flank wear, while TiN coated tools have a higher resistance to “crater” wear.

The other type is “soft” coated tools such as MOS2, W S and other coated tools.

This type of coated tool is also called “self-lubricating tool”. Its friction coefficient with the workpiece material is very low, only about 0.01, which can effectively reduce the cutting force and reduce the cutting temperature.

For example, the “MOVIC” coated tap developed in Switzerland is coated with a layer of MOS2.

Cutting experiments show that only 20 tapped holes can be processed for uncoated taps; 1000 tapped holes can be processed with TiAlN coated taps, while 4000 tapped holes can be processed with MoS2 coated taps.

High speed steel and cemented carbide can be used for dry cutting after PVD coating.

CBN tools that were originally only suitable for dry cast iron castings can also be used to process steel, aluminum alloys and other superhard alloys after coating.

In fact, the coating has a function similar to a coolant.

It creates a protective layer that isolates the tool from the heat of the cut, so that heat is rarely transmitted to the tool, allowing the tip to be hard and sharp for a longer period of time.

A smooth surface coating also reduces friction to reduce heat of cutting and keeps the tool material from chemical reactions, as high temperatures have a large catalytic effect on chemical reactions in most high speed dry cuts.

TiAlN coating and Mo2 soft coating can also be alternately coated to form a multi-coated tool, which has the characteristics of high hardness and good wear resistance, small friction coefficient and easy chip flow, and excellent alternative material to replace the cooling liquid.

Tool coating plays a very important role in dry cutting technology.

Tool geometry design

Dry machining tools usually have crater wear as the main cause of failure because there is no cutting fluid in the machining, and the temperature of the tool and chip contact area rises.

Therefore, the knife should usually have a large rake angle and a blade inclination.

However, after the rake angle is increased, the blade strength will be affected.

In this case, a suitable negative chamfer or rake face reinforcement unit should be provided so that the tool tip and the edge have a sufficient volume of material and a reasonable way to withstand the cutting heat and cutting forces.

At the same time, it reduces the adverse effects of impact and crater expansion on the tool.

This allows the tool tip and cutting edge to maintain sufficient structural strength over long cutting times.

In recent years, many large-angle turning inserts have been developed abroad (such as the front angle of a ME-13 new carbide insert introduced by Carboloy in the United States up to 34°) and a spiral-edged milling insert with a positive rake angle ( The blade has a nearly constant rake angle along the cutting edge, and the back rake angle or side rake angle can be changed from negative to small or from small to large, in order to reduce the driving power of the machine tool. And by reducing the cutting force and reducing the cutting temperature to meet the requirements of the tool during dry cutting.

Japan’s Mitsubishi Metal Corporation has developed a “slewing turning tool” for dry machining.

The tool uses a round superhard blade.

The support part of the blade is equipped with bearings, and the blade can automatically rotate during machining, so that the cutting edge is always sharp, with high work efficiency, good processing quality and long tool life.

There is also a heat pipe cutter that can achieve better dry cutting results.

Their structure is basically the same as that of a conventional turning tool, except that a heat pipe is formed inside the shank body.

The working medium in the heat pipe is generally acetone, ethanol and distilled water.

The heat pipe is a highly efficient heat transfer element that utilizes the two strongest heat transfer mechanisms of boiling heat absorption and condensation heat release.

The thermal conductivity of a heat pipe is several hundred times that of a silver or copper rod.

The heat pipe tool is a self-cooling tool, so there is no need to cast the cutting fluid from the outside, especially for CNC machine tools, machining centers and automatic production lines.

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