Reducing the use of cutting fluids provides considerable cost savings opportunities. Tool life may even increase.
We recently visited a factory and found that the performance leap from the use of cutting fluids was surprising. This discovery is purely accidental. The shortcomings of cutting fluids force people to shift the production quota of machined parts to dry machining. Need is the mother of the invention, and employees have experimented to determine if it can maintain effective production. As a result, they found that investment in cutting fluids did not necessarily pay off.
The economics of using cutting fluids have changed dramatically over the past two decades. As early as the early 1980s, the cost of purchasing, managing, and processing cutting fluids for most machining accounts for less than 3% of the total cost today. On average, the cost of cutting fluid (including management and handling) accounts for 16% of the total cost. %. Since the cutting tool only accounts for 4% of the total cost of the machining project, it may become less expensive to accept the cost savings of replacing the cutting fluid with a shorter tool life and the headaches associated with maintaining the cutting fluid. select.
Moreover, the tool life may not even be shortened. Because coated cemented carbide, ceramics, cermets, cubic boron nitride (CBN) and polycrystalline diamond are brittle, they are sensitive to micro-cracking and cracking caused by thermal stress, especially in face turning and milling. At the time, the intervention of the coolant will exacerbate this tendency.
For example, in milling, when the cutting edge cuts in and cuts out the workpiece, the cutting edge is heated and cooled. The expansion and contraction caused by these temperature fluctuations can cause fatigue. Eventually a series of comb-like hot cracks perpendicular to the cutting edge will be formed and cause cracking. Adding a cutting fluid usually makes the situation worse, and the reason is simple. Most of the coolant acts on the workpiece at a much lower temperature than the cutting zone. Experts are still arguing about whether a cutting fluid has reached the cutting area (the area between the chip and the workpiece) to control the heat source of the process. The coolant typically only cools the surrounding area (previously hotter areas), thus enhancing the temperature gradient and increasing thermal stress.
Tapping, reaming, and drilling do require the aid of cutting fluid, but not necessarily for cooling purposes. Especially for drilling, it is for the lubrication of the drill tip, and also for the removal of the discharged chips from the hole. Without cutting fluid, the chips can become clogged in the holes and the average roughness of the machined surface is twice that of wet machining. Lubricating the point of contact between the drill bit and the wall of the hole also reduces the torque requirements of the machine.
The end mill as shown has an extra large radial and axial positive rake angle. The large positive rake angle reduces power consumption and helps prevent chip fusion.
Which materials are suitable for dry cutting?
In addition to cost and tool life, another factor that affects dry cutting is the workpiece. Sometimes the cutting fluid can stain parts or cause contamination. Think about it, for medical implants, such as implanting a spherical joint for the hip. In such places where contamination is not allowed, cutting fluids are undesirable.
The suitability of a workpiece for a dry cutting process also depends on the material itself. For example, for most cast iron, carbon steel, and alloy steel cuts, the cutting fluid is redundant. These materials are relatively easy to machine and have good thermal conductivity, allowing the chips to carry away most of the heat generated. There are some exceptions, low carbon steel, which become more viscous when the carbon content drops. These alloys require a cutting fluid to lubricate to prevent fusion welding.
Normally, when processing most aluminum alloys, cutting fluid is not necessary because the cutting temperature is relatively low. These materials usually solve this problem when a chip fusion weld does occur, with a large positive rake angle and a sharp cutting edge. However, when cutting aluminum alloys at high speeds, high-pressure coolant is helpful because chip breaking and chip evacuation with simple compressed air is not enough.
Dry cutting stainless steel is more difficult. Among these materials, heat causes some problems. For example, it can over-temper martensitic stainless steel. For many austenitic stainless steels, the heat flow from the cutting area to the chips is not good because the thermal conductivity is often low. The cutting edge is then overheated and the tool life is shortened to an unacceptable level. The use of cutting fluids in the process of cutting stainless steel is necessary, and because many stainless steels are viscous, which means that they have a tendency to build up in the direction of the cutting edge, resulting in poor surface finish.
For many materials, dry cutting is hardly chosen. Cutting fluids are required for all types of superalloys. In particular, when cutting nickel-based and chromium-based superalloys, extremely high temperatures are generated, and cutting fluid is required to remove heat. At the same time, the lubricating fluid of the cutting fluid keeps the heat generated to a minimum.
Cutting fluid is mandatory when cutting titanium alloys. Although researchers are studying the way to dry-cut titanium alloys, the nature of this material creates a significant barrier to this work. It is viscous, has a low thermal conductivity and (and some alloys) has a very low flash point. Therefore, the iron filings cannot carry away the heat and the workpiece becomes hot enough to be ignited and burned. (Although magnesium chip breaking is easy, it is easy to burn.) The cutting fluid prevents this problem by lubricating the cutting edge, washing away chips and cooling the workpiece. To ensure that the cutting fluid performs these functions, when machining titanium alloys, people tend to use cutting fluids at high pressures, typically in the range of 4,000 to 7,000 psi.
Sometimes, powder alloys also require a cutting fluid to produce a thin layer of oil film as a rust inhibitor.
The tool must control the heat
Some companies have happened to get the value of dry cutting, and some factories have tried their purpose, but they can't see the benefits. The reason for this is that the successful application of dry cutting not only requires the elimination of coolant, but also requires a systematic method of controlling heat throughout the process.
The most important way a tool affects heat transfer is by producing good chips. The chips carry away 85% of the heat generated by the cutting and allow only 5% of the incoming workpiece, while 10% of the heat is transferred to the tool and elsewhere. The new chipbreaker presses into the tool surface, which is very beneficial for controlling the shape and size of the chip.
Because the chips are hotter, the ductility is better than the corresponding wet processing, making the chip breaking more difficult and more likely to produce dangerous chip windings that result in poor surface finish. The use of a chipbreaker designed to cut the material of the strip-shaped chip helps to solve this problem. Although such cutting edges have a larger positive rake angle, they are not as brittle and brittle as in wet processing. The high cutting temperatures inherent in dry cutting typically soften the hard alloy, increase its toughness, reduce the likelihood of micro-cracking, and improve tool reliability and tool life.
For the same reason, changing to a harder tool during dry cutting will hardly reduce tool life or cutting consistency. In fact, the opposite is true. A harder matrix ensures the integrity of the cutting edge at high temperatures, while a slight softening prevents it from becoming too brittle. As a result, the user can specify a harder carbide grade to improve resistance to deformation and crater wear (chemical dissolution of the cutting edge). Otherwise, tool life is greatly reduced during dry cutting.
The tools designed for dry cutting can be sharper and lighter than wet machining, which actually produces less friction and contributes to heat control. Studies on drilling have shown that reducing the cutting edge to create a sharper drill can reduce cutting temperatures by 40%. The sharp cutting edge not only keeps the temperature low, but also reduces radial runout and improves surface finish.
Another way to help break up and discharge chips during the cutting process is to replace the liquid with a gas, with compressed air being the most common. Although there is no effect when cooling, sometimes a burst of compressed air is sufficient to blow the chips out during the cutting process, preventing secondary cutting and preventing excess heat from being transferred to the workpiece and the machine. When lubrication is necessary, the user applies a highly efficient lubricant that creates a mist that is consumed during the cutting process. Sometimes the most effective method is a relatively new technology called Minimum Quantity Lubrication (MQL), which is lubricated by injecting a small amount of coolant into the tool.
Coating keeps the tool insulated
Tool coatings also play an important role in protecting the cutting edge. For dry machining, the most efficient cutting tool, combined with a special industrial coating system, is coated with a cobalt-rich zone matrix to stiffen both the interior and the surface. A very thick composite coating is produced using conventional processes and a medium temperature chemical vapor deposition process to a thickness of 20 microns. The first layer of titanium carbonitride coating produces the necessary support for the substrate and edge toughness.
The next layer of fine-grained alumina provides an effective thermal barrier for dry cutting and high cutting speeds. The second layer is wear-resistant titanium carbonitride, which helps to control the flank and crater wear, while the top layer of titanium nitride can resist the built-up edge and make it easier to determine the degree of tool wear. .
A lubricious coating reduces heat generation by reducing friction. For example, molybdenum disulfide and cemented carbide coatings have a low coefficient of friction and can be lubricated during the cutting process. However, these coatings are very soft and the tool life is relatively short. To compensate for this limitation, these coatings are often used with hard liners such as titanium carbide, titanium aluminide, aluminum oxide or some compounds.
When turning, chip entanglement is more likely to occur during dry cutting. For turning steel, the blade referred to here is characterized by a cobalt-rich zone matrix and a 20 micron thick composite coating to form a thermal barrier.
Find the best cutting parameters
If you want to achieve good results in dry cutting, in addition to the need to specify the correct cutting tool, there are many places to pay attention to. The optimum spindle speed, feed rate and depth of cut are also important. For example, if the chip is not adequately controlled by changing the chipbreaker, try to adjust the feed rate. Increasing the feed rate usually yields the best results, and reducing the feed rate is unfavorable.
The use of appropriate cutting parameters also helps to minimize heat generation. The most obvious method is achieved with higher speed and feed, reducing the load on the chip when cutting workpiece material at a faster rate. This results in less cutting time and also reduces the amount of heat generated and the time it takes for heat to penetrate the workpiece.
But sometimes reducing the spindle speed by about 15% is the best way to reduce the cutting temperature. To prevent productivity from being affected, the user can increase the feed rate accordingly. Always refer to the machine's torque chart to ensure that lower shaft speeds and higher feed rates do not increase torque requirements and suffocate the spindle. If the torque requirement exceeds the capacity of the spindle, a tool with a smaller diameter can be used. If the higher feed rate is detrimental to the surface finish, the tool nose radius can be increased as compensation.
In milling, the depth of cut also affects the cutting temperature because it affects both the cutting force and the cooling time. When a full-cut milling cutter cuts a workpiece, it is cut and heated in half the time, and the other half is air-cooled. However, when the meshing width is only 50%, only 1/4 of the time is cutting, and 3/4 of the time is air-cooled. In other words, the workpiece uses only half the time to get the heat, and more time is used for cooling. Most tool manufacturers determine the depth of cut based on the optimum cutting temperature for different hardness values, so follow their recommendations.
The machine tool also plays a role
When the tool discharges chips from the cutting zone, the machine must do its duty to quickly remove the chips. If the chips accumulate on the machine or elsewhere, even if they accumulate for a relatively short period of time, the heat inside the chips can be transmitted to the bed, causing expansion and slight distortion, which affects the precision of precision machining.
Since no cutting fluid carries the chips and absorbs heat, the machine must rely on its own design to effectively remove the chips. For dry milling, horizontal machines are the best because they allow the chips to fall directly on the chip conveyor below the machine. In fact, some designers have designed the latest HMC to eliminate the accumulation of chips on the horizontal surface by opening holes in the center.
For turning, the preferred spindle direction is exactly the opposite. The vertical chuck is completely enclosed, and when the part is rotated, the chips are pulled onto the inner wall by inertia. The chips then fall to the conveyor belt below. Many designers have designed the latest vertical lathes with inverted spindles to take advantage of gravity.
Although there is no standard chip conveyor for all machine tools today, it is a must for dry machining. The vacuum filter unit also removes dust that is enclosed in the machine tool when machining cast iron and graphite.
Regardless of the machine's efficiency in chip removal, machine tools and workpieces are more sensitive to temperature changes when cutting fluids are not used to increase thermal stability. As a result, high-precision applications may require a machine tool with a symmetrical design and thermal compensation package for real-time adjustment compensation. The user may also consider periodically measuring the critical dimensions of the workpiece, using on-line probing or monitoring the thermal drift at the off-line measuring station and taking corrective action if necessary.
Another way to control thermal fluctuations is to minimize fluctuations in the design process. For example, after starting the machine in the morning, the trade union operates the machine for some time to idle to achieve a steady state, and uses automation to keep the machine running without machining. For applications that perform several operations at the same station, first dry machining should be planned, followed by drilling, tapping, and finally other wet machining. This precaution is taken to minimize the amount of cutting fluid and to prevent interference with dry machining.
This article is provided by Shan Gao Tool
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