A casing on a spindle of the milling machine from sozh. Milling machining centers LMW (India). Rotary pallet changer
02.11.2012
New Directions in Coolant Technology for Metalworking
1. Oil instead of emulsion
In the early 90s. proposals to replace coolant emulsions with pure oils were considered from the point of view of the analysis of the total cost of the process. The main objection was the high cost of anhydrous working fluids (5-17% of the total cost of the process) compared to water-based coolants.
Currently, replacing coolant emulsions with pure oils is a possible solution to many problems. When using pure oils, the advantage is not only in price, but also in improving the quality of metalworking, as well as ensuring safety in the workplace. In terms of safety, pure oils are less harmful when exposed to exposed areas of human skin than emulsions. They do not contain biocides and fungicides. Anhydrous coolants have a longer service life (from 6 weeks for individual machines to 2-3 years in centralized circulation systems). The use of pure oils has less negative impact on the environment. Pure oils provide a higher quality of metalworking at almost all stages of the process (more than 90%).
Replacing the emulsion with oils provides better lubricity of the coolant, improves the surface quality during grinding (finishing) and significantly increases the service life of the equipment. The price analysis showed that in the production of a gearbox, the cost of almost all stages is halved.
When using anhydrous coolants, the service life of equipment for CBN (cubic boron nitride) peeling and hole broaching increases by 10-20 times. In addition, when machining cast iron and mild steels, additional corrosion protection is not required. The same applies to equipment, even if the protective paint layer is damaged.
The only disadvantage of anhydrous coolants is the release of a large amount of heat during the metalworking process. Heat dissipation can be reduced by a factor of four, which is especially important in operations such as drilling in hard, high-carbon materials. In this case, the viscosity of the oils used should be as low as possible. However, this leads to a decrease in operating safety (oil mist, etc.), and the volatility depends exponentially on the decrease in viscosity. In addition, the flash point is reduced. This problem can be solved by using unconventional (synthetic) oil bases that combine high flash point with low volatility and viscosity.
The first oils to meet these requirements were mixtures of hydrocracked oils and esters, which appeared in the late 80s. XX century, and pure essential oils that entered the market in the early 90s.
The most interesting are oils based on esters. They have very low volatility. These oils are products of various chemical structures derived from both animal and vegetable fats. In addition to low volatility, essential oils are characterized by good tribological properties. Even without additives, they provide friction and wear reduction due to their polarity. In addition, they are characterized by a high viscosity-temperature index, explosion-fire safety, high biostability and can be used not only as coolants, but also as lubricating oils. In practice, it is better to use a mixture of essential oils and hydrocracked oils, since the tribological characteristics remain high, and their price is much lower.
1.1. Family of multifunction coolants
A decisive step in optimizing the cost of lubricants in metalworking processes was the use of pure oils. When calculating the total cost of coolant, the impact of the cost of lubricants used in metalworking was underestimated. Studies in Europe and the USA have shown that hydraulic fluids are mixed with cutting fluids three to ten times per year.
On fig. 1 these data are shown graphically over a 10-year period in the European automotive industry.
In the case of water-based coolants, the ingress of significant amounts of oils into the coolant leads to a serious change in the quality of the emulsion, which worsens the quality of metalworking, causes corrosion and leads to an increase in cost. When using pure oils, contamination of the coolant with lubricants is imperceptible and becomes a problem only when machining accuracy begins to decrease and equipment wear increases.
The trend towards using pure oils as cutting fluids opens up a number of opportunities to reduce costs. An analysis by German machine builders showed that, on average, seven different types of lubricants are used in each type of machine tools. This in turn raises the issues of leakage, compatibility and cost of all lubricants used. Incorrect selection and use of lubricants can lead to equipment failure, which is likely to result in a production stop. One possible solution to this problem is the use of multifunctional products that satisfy a wide range of requirements and can replace lubricants for various applications. An obstacle to the use of universal fluids is the requirements of the standard ISO to hydraulic fluids VG 32 and 46 because modern hydraulic equipment is designed to meet the viscosity values given in these standards. On the other hand, metalworking requires low viscosity cutting fluid to reduce losses and improve heat dissipation during high-speed metal cutting. These inconsistencies in viscosity requirements for different lubricant applications are resolved by the use of additives to reduce overall cost.
Advantages:
. the inevitable loss of hydraulic and break-in oils does not impair the coolant;
. invariability of quality, which eliminates complex analyses;
. the use of coolants as lubricating oils reduces the overall cost;
. improving reliability, process results and equipment durability significantly reduces the overall cost of production;
. versatility of application.
Rational use of universal fluids is preferable to the consumer. An example of this is the engine industry. The same oil can be used in the initial processing of the cylinder block and in their honing. This technology is very efficient.
1.2. Washing lines
In these lines of cleaning operations, water-based cleaning solutions must be avoided to avoid the formation of undesirable mixtures with hydrophilic oils. Solid contaminants are removed from oils by ultrafiltration, and detergents (energy costs for cleaning and pumping water, analyzing the quality of waste water) can be eliminated, which will reduce the overall cost of production.
1.3. Removing oil from scrap metal and equipment
The correct selection of additives allows the oils extracted from metal waste and equipment to be recycled back into the process. The volume of recirculation is up to 50% of the losses.
1.4. Perspectives on universal fluids - " Unifluid»
The future lies in low viscosity oil, which will be used both as a hydraulic fluid and as a cutting fluid for metalworking. Universal fluid " Unifluid» developed and tested in a German research project sponsored by the Ministry of Agriculture. This fluid has a viscosity of 10 mm2/s at 40°C and performs excellently in automotive engine manufacturing plants for metalworking, lubrication and power lines including hydraulic systems.
2. Minimize the amount of lubricants
Changes in legislation and increasing demands for environmental protection also apply to the production of coolants. Given the international competition, the metalworking industry takes all possible measures to reduce the cost of production. An analysis of the automotive industry published in the 1990s showed that the main cost problems are caused by the use of working fluids, with the cost of the coolant playing an important role in this case. The real cost comes from the cost of the systems themselves, the cost of labor and the cost of keeping the fluids in working condition, the cost of cleaning both fluids and water, and disposal (Figure 2).
All this leads to the fact that much attention is paid to the possible reduction in the use of lubricants. A significant reduction in the amount of coolant used, as a result of the use of new technologies, makes it possible to reduce the cost of production. However, this requires that coolant functions such as heat dissipation, friction reduction, removal of solid contaminants be solved using other technological processes.
2.1. Analysis of coolant needs in various metalworking processes
If coolants are not used, then, naturally, the equipment overheats during operation, which can lead to structural changes and tempering of the metal, changes in dimensions, and even equipment failure. The use of coolant, firstly, allows heat to be removed, and secondly, it reduces friction during metal processing. However, if the equipment is made of carbon alloys, then the use of coolant can, on the contrary, lead to its breakdown and, accordingly, reduce the service life. And yet, as a rule, the use of coolants (especially due to their ability to reduce friction) leads to an increase in the life of the equipment. In the case of grinding and honing, the use of coolant is extremely important. The cooling system plays a huge role in these processes, as it maintains the normal temperature of the equipment, which is very important in metalworking. Chip removal generates about 80% of the heat, and the coolants perform a dual function here, cooling both the cutter and the chip, preventing possible overheating. In addition, part of the small chips leaves with the coolant.
On fig. 3 shows the coolant requirements for various metalworking processes.
Dry (without coolant) metal processing is possible in processes such as crushing, and very rarely in turning and drilling. But it should be noted that dry machining with a geometrically inaccurate end of the cutting tool is not possible, since in this case heat removal and liquid irrigation have a decisive influence on the quality of the product and the service life of the equipment. Dry processing in the crushing of iron and steel is currently used with the help of special equipment. However, the removal of chips must be carried out either by simple cleaning or by compressed air, and as a result, new problems arise: increased noise, additional cost of compressed air, and the need for thorough dusting. In addition, dust containing cobalt or chromium nickel is toxic, which also affects the cost of production; the increased explosion and fire hazard during dry processing of aluminum and magnesium cannot be ignored.
2.2. Low Coolant Systems
By definition, the minimum amount of lubricant is an amount not exceeding 50 ml/h.
On fig. 4 shows a schematic diagram of a system with a minimum amount of lubricant.
With the help of a dosing device, a small amount of coolant (maximum 50 ml/h) is supplied in the form of fine sprays to the metalworking site. Of all the types of dosing devices on the market, only two types are successfully used in metalworking. The most widely used are systems operating under pressure. Systems are used where oil and compressed air are mixed in containers, and the aerosol is supplied by a hose directly to the place of metalworking. There are also systems where oil and compressed air, without mixing, are supplied under pressure to the nozzle. The volume of fluid supplied by the piston in one stroke and the frequency of the piston are very different. The amount of compressed air supplied is determined separately. The advantage of using a dosing pump is that it is possible to use computer programs that control the entire workflow.
Since very small quantities of lubricant are used, the supply directly to the work station must be done with great care. There are two types of coolant supply, which are quite different: internal and external. With an external supply of liquid, the mixture is sprayed by nozzles onto the surface of the cutting tool. This process is relatively inexpensive, easy to perform and does not require much labor. However, with external coolant supply, the ratio of the tool length to the hole diameter should be no more than 3. In addition, when changing the cutting tool, it is easy to make a positional error. With internal coolant, the aerosol is fed through a channel inside the cutting tool. The ratio of length to diameter must be greater than 3, and positional errors are excluded. In addition, the chips are easily removed through the same internal channels. The minimum tool diameter is 4 mm, due to the presence of a coolant channel. This process is more costly as coolant is supplied through the machine spindle. Systems with low coolant supply have one thing in common: the liquid enters the working area in the form of small droplets (aerosol). At the same time, toxicity and maintaining the hygiene standards of the workplace at the proper level become the main problems. Modern developments of coolant aerosol supply systems make it possible to prevent flooding of the workplace, reduce losses during spraying, thereby improving the air quality in the workplace. A large number of low coolant supply systems leads to the fact that although it is possible to select the required droplet size, many indicators, such as concentration, particle size, etc., are not well understood.
2.3. Coolant for low flow systems
Along with mineral oils and water-based cutting fluids, oils based on esters and fatty alcohols are used today. Since low coolant systems use flow lubrication oils sprayed into the work area in the form of aerosols and oil mist, occupational health and safety (OHS) issues become a priority. In this regard, the use of lubricants based on esters and fatty alcohols with low toxicity additives is preferable. Natural fats and oils have a big drawback - low oxidation stability. When using lubricants based on esters and fatty acids, deposits do not form in the working area due to their high antioxidant stability. In table. Table 1 shows data for lubricants based on esters and fatty alcohols.
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For systems with low coolant supply, the correct selection of lubricant is of great importance. To reduce emissions, the lubricant used must be low-toxic and dermatologically safe, while maintaining high lubricity and thermal stability. Lubricants based on synthetic esters and fatty alcohols are characterized by low volatility, high flash point, low toxicity and have proven themselves in practical applications. The main indicators in the selection of low-emission lubricants are the flash point ( DIN EN ISO 2592) and evaporative loss according to Noack ( DIN 51 581T01). t vsp should not be lower than 150 °C, and evaporation losses at a temperature of 250 °C should not exceed 65%. Viscosity at 40 ° C> 10 mm 2 / s.
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For the same viscosity, fatty alcohol based lubricants have a lower flash point than ester based lubricants. Their volatility is higher, so the cooling effect is lower. The lubricating properties are also relatively low compared to ester-based lubricants. Fatty alcohols can be used where lubricity is not essential. For example, when processing gray cast iron. Carbon (graphite), which is part of cast iron, itself provides a lubricating effect. They can also be used when cutting cast iron, steel and aluminum, as the working area remains dry as a result of rapid evaporation. However, too high evaporation is undesirable due to air pollution in the working area with oil mist (should not exceed 10 mg / m 3). Ester based lubricants are useful when good lubrication is required and high chip flow occurs, such as threading, drilling and turning. The advantage of ester-based lubricants is high boiling and flash points at low viscosities. As a result, volatility is lower. At the same time, a corrosion-preventing film remains on the surface of the part. In addition, ester-based lubricants are readily biodegradable and have class 1 water pollution.
In table. 2 shows examples of the use of lubricants based on synthetic esters and fatty alcohols.
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The main considerations when designing coolants for low flow systems are listed below. The main thing to pay attention to when developing coolants is their low volatility, non-toxicity, low effect on human skin, combined with a high flash point. The results of new research on the selection of optimal coolants are shown below.
2.4. Investigation of factors affecting the formation of oil mist in coolant systems with low flow
When a low coolant system is used in the metalworking process, aerosol formation occurs when fluid is introduced into the work area, with a high concentration of aerosol observed when using an external spray system. In this case, the aerosol is an oil mist (particle size from 1 to 5 microns), which has a harmful effect on human lungs. The factors contributing to the formation of oil mist were studied (Fig. 5).
Of particular interest is the effect of lubricant viscosity, namely the decrease in oil mist concentration (oil mist index) with increasing lubricant viscosity. Studies have been conducted on the effect of anti-fog additives in order to reduce its harmful effects on the human lungs.
It was necessary to find out how the pressure applied in the coolant system affects the amount of oil mist generated. In order to assess the generated oil mist, a device based on the “Tyndall cone” effect, a tyndallometer, was used (Fig. 6).
To assess the oil mist, the tindallometer is placed at some distance from the nozzle. Further, the obtained data is processed on a computer. Below are the results of the assessment in the form of graphs. From these graphs, it can be seen that the formation of oil mist increases with increasing pressure during spraying, especially when using low-viscosity liquids. A doubling of the spray pressure causes a corresponding doubling of the mist volume. However, if the spray pressure is low and the starting characteristics of the equipment are low, then the period for which the amount of coolant reaches the required rates for normal operation increases. At the same time, the oil mist index increases significantly with a decrease in the viscosity of the coolant. On the other hand, the start-up performance of spray equipment is better with low viscosity fluids than with high viscosity fluids.
This problem is solved by adding anti-fog additives to the coolant, which makes it possible to reduce the amount of fog formed for liquids with different viscosities (Fig. 7).
The use of such additives makes it possible to reduce the formation of mist by more than 80%, without compromising either the starting characteristics of the system, or the stability of the coolant, or the characteristics of the oil mist itself. As studies have shown, mist formation can be significantly reduced with the right choice of spray pressure and viscosity of the applied coolant. The introduction of appropriate anti-fog additives also leads to positive results.
2.5. Optimization of low coolant systems for drilling equipment
The tests were carried out on materials used in systems with low coolant supply (deep drilling (length / diameter ratio more than 3) with external coolant supply), on drilling equipment DMG(Table 3)
In a workpiece made of high-alloy steel (X90MoSg18) with high tensile strength (from 1000 N / mm 2), it is required to drill a blind hole. High carbon steel drill SE— a stem with a cutting edge that has a high resistance to bending, coated PVD-TIN. Coolants were selected in order to obtain optimal process conditions, taking into account external supply. The influence of the viscosity of the ether (the base of the coolant) and the composition of special additives on the service life of the drill was studied. The test bench allows you to measure the magnitude of cutting forces in the direction of the z-axis (in depth) using the Kistler measuring platform. Spindle performance was measured over the entire time required for drilling. The two methods adopted for measuring the loads during a single drilling made it possible to determine the loads during the entire test. On fig. 8 shows the properties of two esters, each with the same additives.
Roman Maslov.
Based on materials from foreign publications.
The primary task of modern machining on machine tools is tool lubrication, as well as the rapid removal of chips from the cutting zone. Failure to do so can result in problems leading to premature tool wear or damage, and even machine failure.
A standard feature on Haas and VM series machines is an annular coolant supply that provides coolant by spraying into the cutting area while removing chips that form during cutting.
This concept, compared to the traditional one, which uses hoses, is significantly improved. Precise adjustment of the tips of the easily movable nozzles of the ring allows you to direct the coolant jet at the tool at different angles. Ergonomic ring setting provides ease of use and maximum clearance.
In addition to the main coolant supply system, there are other ways of cooling. One of them is the use of programmable coolant nozzles (P-Cool), which, depending on the tool, automatically adjust to its length.
Coolant system through the spindle
Another effective way is to supply coolant through the tail of the tool holder and through the channels of the cutting tool under high pressure. The TSC (Through-Spindle Coolant) coolant system is available in 2 pressure configurations: 300 or 1000 psi (20 or 70 bar). Its efficiency is especially high when drilling deep holes and milling deep recesses.
Air jet system through the tool
When using modern carbide tools with advanced coatings for cutting in a dry environment, there is a high probability of re-cutting chips that are not removed from the cutting zone in a timely manner. This is the main reason for increased tool wear. To solve the problem, Haas Automation developed a system that blows air through the tool (an addition to the TSC system) that immediately removes chips from the cutting area before they enter the cutting tool again. This method is important in the process of processing deep cavities.
The same function is performed using the Haas automatic air gun. The system is perfect for using small instruments that are not suitable for air supply through the instrument port. An automatic air gun is a great addition to a through-tool air supply system. The gun is used when it is impossible to use a liquid cooling system and when it is necessary to supply significant volumes of air.
Minimum Coolant System
In cases where it is not possible to use a cutting fluid, but it is necessary to ensure tool lubrication, a system for supplying a minimum amount of lubrication is used. The innovative Haas system sprays a moderate amount of lubricant onto the cutting edges of the tool using an air jet. The amount of coolant used is so small that it cannot be seen.
The main advantage of the method is the low consumption of lubricant. The amount of air and coolant supplied is independently adjustable, i.e. in each specific operating mode, you can independently make adjustments for optimal cooling.
Metalworking production can only be considered effective when the number of unpleasant surprises that appear in the process of manufacturing parts is minimized.
Efficient production cannot afford to increase the cycle time of a part, to obtain a repairable or irreparable defect. Most often this happens due to improper clamping of the workpiece, improper use of the tool, heating of the workpiece during processing, etc. In addition, you need to pay attention to the causes associated with the failure of machine tool spindles.
In production, especially those engaged in the manufacture of high-precision parts, when ordering equipment, care must be taken to install the most suitable spindles. During the operation of the machine, it is important that the spindle does not overheat, that there are no collisions with workpieces and machine tools, and that coolant and metal chips do not seep through the seals and do not damage the spindle components.
SOLID BODIES EXPAND WHEN HEATED
From the heat released during the processing process, not only the workpieces, but also the spindle itself can expand. This usually occurs in high-speed processing and processing that requires high power for a long period of time. If the spindle extension is large enough, it can move out of its normal position, and this, in turn, will lead to the output of the dimensions of the part outside the tolerance zone.
With linear expansion, the timing wheel can move so much relative to the machine sensors that the machine does not know the exact position of the spindle, and therefore the tool. As a result, it is quite likely that the machine will stop, which is especially unpleasant when it is working in an automatic cycle. Another possible problem is the loss of binding of the position of the tool to the position of the arm of the manipulator for changing tools. The manipulator arm works in unison with the spindle pull to secure the tool. If their movements are not coordinated, then the manipulator may crash into the tool, and the manipulator, the tool, as well as the spindle, may be damaged.
Spindle linear expansion can be controlled in several ways. The first method is to supply cooling to it. The working fluid is a mixture of water and glycol. It passes through the cooling jacket, its temperature is maintained by the cooling station. The second method is to design the spindle in such a way that when heated, it expands not forward, but backward. Therefore, the dimensional accuracy of the part will not be affected.
COOLANT MUST BE IN THE WORK AREA
The spindle can also be damaged by cutting fluid penetrating the seals and reaching the bearings. The penetration of coolant into the spindle is one of the main causes of its breakdown. In this case, the spindle has two main enemies - high pressure coolant systems and coolant systems with a large number of nozzles. The nozzles must be precisely adjusted to ensure that the minimum amount of coolant enters the machine spindle. In any case, coolant will get onto the spindle, so additional screens, mechanical or labyrinth seals may be needed. These seals must not interfere with the automatic tool change. Another way to help keep coolant out of the spindle is to use a spindle air purge system. It turns on when changing the tool, increasing or decreasing the spindle speed. When the spindle speed is changed, air currents and heat released from the spindle cause coolant mist to enter the spindle. The air purge system removes coolant and thus protects the spindle from damage. The use of an air purge system is not necessary for all applications, but it will be cheaper to install it as an option and save on spindle repairs. When grinding, the air cleaning system also protects the spindle from fine metal dust.
HOW TO AVOID COLLISIONS
Spindle breakage as a result of a collision is a fairly common occurrence. Collisions occur due to various reasons. For example, the operator may accidentally enter the wrong value, forgetting to put a separator, and press the button. Even if he immediately realizes the error, there may not be enough time to stop the machine. One way to deal with this kind of problem is to use machining simulation software. The graphical interface allows you to follow the entire process step by step and see the points of possible collision with the workpiece, fixture or the machine itself.
Often it is necessary to carry out processing close enough to the machine equipment. For example, when milling or drilling - close to the vise. The result is increased rigidity and, consequently, manufacturing accuracy. Vibrations are dealt with in the same way. The proximity of the tool to the machine tool during simulation can result in a collision in reality. In this case, after simulation, programmers must necessarily warn operators about possible collision sites, and then the latter will be ready to go through dangerous sections when debugging the program at minimum speed.
The spindle can be adversely affected by vibrations that occur when the system machine - fixture - tool - part is insufficiently rigid. Some applications may require anti-vibration tools and fixtures that provide high tool clamping rigidity.
Manufacturer: Sunmill, production: Taiwan
General Information of JHV-710 CNC Vertical Machining Center
Linear Guides (Standard):
The spindle uses special high precision bearings to withstand 8000 rpm (BT-40) and optionally 10000 and 12000.
The guides of the three axes are connected by a ball screw pair through a coupling with a servomotor. This allows you to achieve the highest precision in your work. Bearings of the highest class C3 allow you to achieve thermal stability during operation.
To maintain a constant temperature inside the control, a heat exchanger is installed on the machine. This provides exceptional protection for the control and electrical components on the machine.
It avoids the destruction of the spindle due to thermal loads, and also allows you to maintain high accuracy and speed of the spindle.
Specifications of JHV-710 CNC Vertical Machining Center
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Options, descriptions
Every SUNMILL machine is tested:
BALL BAR TEST
Using the ball bar test, roundness, out-of-shape and backswings (actuator mismatch) are checked.
Laser check
Additional options:
4th and 5th axis processing (option) :
On a CNC milling machine, it is possible to install a 4th / 5th axis, and, accordingly, create a 4th / 5th coordinate machining center. Both a vertical rotary table (4th axis) and a swivel axis (5th axis) can be installed on the table of the machining center. When installing the 4th or 5th axis, it is recommended to use the FANUC 18iMB control system.
Coolant supply through the spindle:
Supplying coolant through the spindle using a special tool allows better heat dissipation when machining blind holes and avoids overheating of the tool and workpiece. Supplied complete with filtration system.
High-speed spindle that allows you to withstand the parameters: 10000, 12000, 15000 rpm.
Tool magazine for 20 or 24 positions.
Complete set of this machine.
- CNC system Fanuc 0i-MD controller.
- Fourth axis interface.
- Spindle BT40 10,000 rpm
- Motor power 5.5 / 7.5 kW
- Spindle drive
- Spindle taper blowing system
- Automatic lubrication system
- Carousel magazine ATC 16-tools, BT40
- Complete cutting area enclosure
- Machine lighting
- Toolbox and Documentation Kit
- Oil cooled spindle
- Chip screw conveyor
Equipment for an additional fee:
| Tool magazine drum type ATC 24-tools, BT40 * | 5 600 USD |
| Coolant supply through the spindle 20 bar * | 7 600 USD |
| Chip conveyor belt + tank * | 3 800 USD |
| Machine power increase up to 7.5 / 11 kW | 1000 USD |
| 4th axis, rotary table, faceplate 200 mm | 16 800 USD |
| 5th axis, swivel table, faceplate 175 mm | 36 000USD |
| Renishaw TS27R tool setting probe | 4000 USD |
| Renishaw NC4 proximity sensor | 13 000USD |
| Renishaw OMP60 Moment of Touch Probe | 17 000USD |
| Carousel tool magazine 20 tools VT40 | 800USD |
| Increase in spindle speed up to 12,000 rpm (belt drive) | 2 700USD |
| Spindle speed increase up to 15,000, 24,000, 30,000, 36,000 rpm | On request |
The benefits of dry machining or dry machining sound appealing: savings in production costs for coolant and its cleaning, increasing productivity. However, it is not enough to simply close the coolant valve. To carry out dry processing, the machine must be functionally modified.
In normal cutting, the coolant performs the following main functions: cooling, lubrication, chip evacuation and removal of contaminants. With the exception of the use of coolant, these functions must be compensated by the machine and tool.
Lubrication compensation
The lubricating action of the coolant extends in two directions. On the one hand, the friction surface between the part and the tool is lubricated, and on the other hand, the movable elements and seals in the working area are lubricated. The working area of the machine, the moving elements located here and the removal of chips must be designed to work with dry chips. However, when cutting, it is not possible in all cases to refuse lubrication, for example, when drilling aluminum alloys as a whole. This type of processing requires the supply of lubricant in minimal dosed quantities in the form of oil mist, which is supplied under pressure to the cutting edges and flutes of the drill. Such a lubricant effectively reduces heat generation during cutting and material sticking to the tool, which leads to a decrease in its performance. With a dosed supply of lubricant, its consumption is 5..100 ml / min, so the chips are slightly moistened with oil and can be removed as dry. The oil content in the chips sent for remelting, with the correct system settings, does not exceed the permissible value - 0.3%.
The metered supply of lubricant causes an increase in contamination of the part, fixture and machine as a whole and can lead to a decrease in the reliability of the machining process. To improve the lubrication of the cutting edges of the drill, machines used for dry machining should be equipped with an internal oil mist supply system through the hole in the spindle. Further, the aerosol is fed through the channel in the cartridge and tool directly to its cutting edges. A key requirement for metered coolant systems is fast and precisely controlled oil mist preparation. Not only the protection of the tool depends on this, but also the cleanliness in the working area.
Cooling compensation
The rejection of the cooling effect of the coolant must also be compensated by design changes in the machine.
During cutting, mechanical work is almost completely converted into heat. Depending on the cutting parameters and the tool used, 75:95% of the thermal energy remains in the chips that are removed from the part. During dry processing, it performs the function of removing the resulting heat from the working area. Therefore, it is important to minimize the effect of this heat transport on machining accuracy. The uneven temperature field in the working area of the machine and the point transfer of thermal energy to the part, fixture and machine as a whole affect the accuracy.
The possibility of chip accumulation on the fixture and machine parts should be excluded. From this it is clear that processing from above is an unfavorable option. In order to limit the harmful effects of thermal energy as far as possible, the machine must be designed in such a way that the thermal deformations of individual machine components and parts do not affect the position of the tool relative to the part.
Coolant flush compensation
Since no coolant is used, when machining materials such as cast iron or light metals, dust and tiny chips are formed, which are no longer bound by the liquid. Seals and protective devices must be additionally protected against abrasive attack.
Since the direction of the trajectory of the chips is not unambiguous, the action of gravity should be used. To do this, it is necessary to ensure unimpeded fall of chips on the discharge conveyor, located in the lower part of the working space. Any horizontal plane becomes a chip accumulator and can affect the reliability of the machining.
Vacuum suction systems are another means of chip removal. The main requirement here will be to place the suction nozzle as close as possible to the working area in order to increase the reliability of chip trapping. It is possible to recommend systems in which the nozzle is mounted on a spindle or tool, as well as
in which the nozzle is installed with a programmable rotation in the servo mode. In some cases, for example, when milling planes with a face mill, the suction effect can be enhanced by using a bell-shaped cutter guard. Without it, a powerful air stream would be required to capture chips flying at high speed.
The suction system must, first of all, remove dust and excess oil mist, and the removal of large chips is the task of the chip conveyor. The suction of the smallest particles is very important, because, mixing with the aerosol, they form a durable mud layer. The air from the suction system is returned to the environment and must be thoroughly cleaned from the suction products.
Safety aspects in dry processing
In dry processing, the possibility of a dust explosion in the working area must be taken into account. Therefore, the dust extraction nozzle must be positioned in such a way that areas with a critical concentration of dust do not occur.
The risk of ignition of oil aerosols, as shown by studies carried out at the Institute of Machine Tools and Technological Equipment of the University of Karlsruhe, is extremely unlikely. When working with suction systems and shop air conditioners, this danger can be neglected. All these statements can scare off small-scale manufacturers and manufacturers of individual parts. Many imagine the transition from wet to dry machining to be much easier.
The path to the dry process multi-purpose machine
The machine tool company that knows exactly where to go is Hüller Hille. This supplier of complete systems is required to provide high quality processing in automatically operating installations. The same requirements should apply to all dry technology machines. As an example, Fig. 1 shows the production module of a technological system designed to process a car wheel bracket. On each of the two machines included in the module, during 3-shift operation, 1400 pairs of brackets are processed with a metered coolant supply. The processed material - aluminum.
Dosed lubrication supply when cutting light alloys
Whereas completely dry machining can be achieved in the machining of gray cast irons over a wide range, drilling, reaming and threading in aluminum and magnesium alloys require metered coolant supply to ensure process reliability. Otherwise, due to clogging of the flutes, there is a risk of frequent tool breakages and the formation of built-up edges that prevent high-quality machining.
The main aspect is the supply of a lubricating medium. With a dosed supply of coolant, it is an air-oil mixture (aerosol).
According to the type of aerosol supply, the systems currently in use are divided into external and internal. If, with an external supply, an aerosol or individual drops of oil can be brought directly to the cutting edges of the tool, then with an internal supply, metered oil is supplied through the spindle and the channel in the tool to the cutting zone. There are also 2 technical solutions here: 1-channel and 2-channel supply. With a 2-channel supply, air and oil are supplied to the spindle separately and mixed immediately before being supplied to the tool. This allows you to quickly deliver the mixture to the work area and shorten the path of aerosols inside the fast-rotating parts, thereby reducing the risk of separation.
On fig. 2 shows the solution used by Huller Hille for the separate supply of aerosol components through a rotary distributor to the spindle. The oil enters the dosing device, which pushes it into the body, made by powder metallurgy. The housing is a reservoir for oil and a mixer with the supplied air. The aerosol is formed immediately before entering the instrument channel. This creates a minimal path to the cutting edge, where the delamination effect is possible. The device allows you to precisely adjust the oil content in the aerosols and thus more accurately adapt to the working conditions of various tools.
In addition, the device allows you to quickly turn on and off the metered coolant supply. Depending on the design of the channel in the instrument, the response time can be 0.1 s. This allows you to turn off the oil supply during the positioning process, which helps to reduce oil consumption and machine contamination.
As a result, during the experimental treatment of the cylinder head, the average oil consumption was 25 ml/h, while during the treatment with free irrigation, the consumption reaches 300:400 l/min.
Currently, to eliminate dead zones, test tests of the metered coolant supply system are being carried out, aimed at increasing the uniformity of the aerosols, reducing the oil content and optimizing the design of the aerosol supply through the shank type<полый конус>. Solving these problems will reduce oil consumption and machine contamination. The possibility of adaptive control of the lubricant jet depending on the set and measured values of the volumetric flow is investigated. This will allow maintaining constant lubrication conditions with changes in temperature, viscosity, internal geometry of the tool.
Optimization of the working area of the machine
In addition to the spindle, designed in accordance with the requirements of metered lubrication through the internal cavity, Huller Hille has released a multi-purpose machine designed for the processing of parts using dry technology. The basis for reliable chip removal was the design of the working area. This eliminates all sorts of edges and planes on which chips can accumulate. The sizes of windows for the free passage of falling chips, which are limited by steep walls (the angle of inclination is more than 55 0), have been increased. Unpainted steel guardrails minimize chip adhesion and scorch marks.
The installation of a fixture with a workpiece on a vertical wall is important for unimpeded fall of chips (Fig. 3). On the machine for changing satellites with parts, an internal manipulator swivel around a horizontal axis is used. In the change position, the part assumes its usual vertical position and can be changed manually or automatically by an external manipulator that connects the machine to the transport system.
When removing chips from the working area, a dust extraction system is used. As prescribed in the EU countries, the suction nozzle is located under the chip conveyor mesh. It picks up dust particles, aerosol residues and small chips. Large chips are caught by the conveyor mesh and removed. This solution reduces the power of the dust extraction system.
Despite the best option for fixing the part, in some cases the chips are not removed by free fall, for example, when machining body parts that have internal cavities where they can accumulate. For such cases, the machine is equipped with a high speed round table - 500 min -1 compared to 50 min -1 on conventional machines. With rapid rotation, the chips are ejected from the cavities of the part, especially if, when changing, it is periodically set to a horizontal position.
An important aspect is the contamination of the machine. Small chips moistened with oil cover the machine components in the working area with a rather thick layer. If, due to the high kinetic energy, it is difficult to remove large chips by suction, then small ones, which are the main component of pollution, are easily removed. Therefore, the use of a dust extractor is a major component of pollution control.
A current subject of research is the search for universally usable dust extraction solutions for different types of tools or the possibility of using the magazine and manipulator of the automatic tool changer for automatic change of suction devices.
thermal effect
Thermal problems concern both the fixtures and the machining process, as well as the machine as a whole. The machine must have a thermosymmetrical design. 3-coordinate nodes, which are equipped with machines of the Specht range, satisfy these conditions. The inner manipulator for the satellite with the part, which is rotatable in the vertical plane, is mounted on two supports in a frame-type rack, which also ensures the thermal symmetry of the structure. Thus, uniformity of thermal deformations of the machine is ensured perpendicular to the surface of the part. In the upper part of the rack is connected with a 3-coordinate node. Together with the tie at the bottom of the bed, the design prevents tipping. A net translational displacement occurs, which can be taken into account by introducing compensation.
Thermosymmetry, however, does not prevent errors along the Z axis, in the honesty of the lengthening of the spindle and machine components. In general, machining operations that require precise positioning along the Z axis are not as common. However, Hüller Hille offers additional options for active error compensation on this axis. So, the Specht 500T machine is equipped with a laser tool breakage control system. The position of the control marks on the spindle and on the device is recorded by a laser beam, by means of which the change in positions is determined and a correction is introduced.
The construction of the machining process determines the accuracy
Still, process design is critical to achieving accuracy. The sequence of operations for dry processing compared to wet processing has been significantly changed. In most cases, a direct transfer of the sequence of operations from wet to dry processing is not desirable. On the other hand, the sequence used in dry technology is not harmful in wet technology. Therefore, dry processing concepts can be adopted in all cases.
