XI Международная студенческая научная конференция Студенческий научный форум - 2019

The duration of cutting with a new or reground cutting tool before it fails, that is, until the maximum permissible wear is reached, is called the period of resistance T. Sometimes, to express technological capabilities, the period of tool resistance is given in meters of the cutting path (TL) and in the number of parts machined between two regrints. The greater the wear rate, the shorter the tool life. It serves as a quantitative expression of the wear rate and varies greatly depending on the cutting conditions — cutting conditions, geometrical parameters, the cutting part, and so on. One of the main factors determining the durability of tools is the cutting speed. This is because, depending on the speed, the temperature in the cutting zone changes. The analysis of dependences T = f (V) leads to the conclusion that for each type of machining, the machined tool. materials, there is a maximum achievable limit of durability period Tpr.T = CT / vμ or T = CV / Tm: where CT and CV are constant values for given cutting conditions, depends on the tool material, material being processed, cut, tool geometry, cooling conditions μ = 1 / m or m = 1 / μ.

m = 0.1-0.12 - steel turning with fast-cutting cutters without cooling, m = 0.2 - with cooling, m = 0.08-0.1 - turning of cast iron with high-speed cutting tools, m = 0.2-0.3 - turning of iron and steel with carbide cutters, m = 0.3-0.5 - threading, punching.

Vμ * T = CT = const;

Precision machining on CNC machines

Working in automatic or semi-automatic mode, the CNC machine must first of all ensure the accuracy of the parts being manufactured, which depends on the total error. The total error in turn consists of a number of factors:

- accuracy of the machine;

- accuracy of the control system;

- error installation billet;

- errors of setting up tools for size;

- errors of adjustment of the machine on the size;

- manufacturing errors of the tool;

- dimensional wear of the cutting tool;

- rigidity of the AIDS system.

The precision of a machine means, first of all, its geometrical accuracy, i.e. accuracy in the unloaded condition. Distinguish machines of four accuracy classes: N (normal), P (enhanced), B (high), A (especially high). When checking the machines for compliance with the standards of accuracy, they reveal the accuracy of geometric shapes and positions of base surfaces, the accuracy of movements along guides, the accuracy of the location of the axes of rotation, the accuracy of machined surfaces, the roughness of machined surfaces.

The accuracy of CNC machines is additionally characterized by the following specific manifestations: the linear positioning accuracy of the working bodies, the size of the deadband, i.e. lag when changing the direction of movement, return accuracy, stability of output to a given point, accuracy in the circular interpolation mode, stability of the tool position after the automatic change.

It should be noted that for CNC machines, the stability of the output of the working bodies at a given point is often more important than the accuracy of the machine itself. To preserve the accuracy of the machine over a long period of operation, the norms of geometrical accuracy in the manufacture of the machine as compared to regulatory standards are tightened by 40%, thereby reserving the stock for wear.

Accuracy control system. The accuracy of the control system is primarily associated with work in the interpolation mode - the mode in which the system is controlled simultaneously by several axes. Deviations associated with the work of the interpolator do not exceed the price discretes. For modern machines with a price of single impulses of 0.001-0.002 mm, the error is insignificant, but manifests itself in the form of deviations in microgeometry, i.e. roughness.

Errors that do not depend on the work of the interpolator, but are manifested in the interpolation mode, can turn out to be very significant. They are caused by a systematic error in the transfer of motion drives feed. These errors occur in the kinematic chain of a feed drive motor - gearbox - spindle screw - sensor. When moving along one axis, such errors appear as irregularities in the movement of the working bodies and practically do not affect the result of processing. However, when moving along several axes, the unevenness of movement, even along one axis, leads to an error in processing the appearance of a waviness of the treated surface.

Error installation billets. The error of installation is determined by the sum of the errors of basing and fixing. The error of the base occurs due to the misalignment of the installation base with the measuring. On CNC machines, there is a possibility of achieving higher accuracy when measuring bases and all surfaces, measured from these bases, are processed in one machine.

When securing blanks, it can be displaced by the action of clamping forces. The displacement of the workpiece from the position determined by the installation elements of the device occurs due to the deformations of individual chain links: the workpiece, the installation elements, the housing of the device. Due to the non-uniform quality of the surfaces and the instability of the specific loads, it is impossible to compensate for the resulting deformations using tool correction.

Error setting tools for size. When adjusting the tool to the size outside the machine, regardless of the accuracy of the device used, errors occur. These deviations are determined by the error of the device itself and the error of fixing the adjusted tool size. This error is compensated after the test run.

Errors of adjustment of the machine on the size. Adjustment of the machine to the size consists in coordinating the installation of the cutting tool adjusted to the size, the working elements of the machine and the fixing elements of the device in a position that, taking into account the phenomena occurring during the processing, ensures the required size. The error in setting up the machine arises due to the fact that it is impossible to locate the working elements of the machine and tools exactly in the design position. To ensure the required manufacturing accuracy, the adjuster uses test runs. Under the adjustment of the installation size understand the restoration of the installation size, which has changed due to dimensional wear tools or thermal deformation of the system. In order to reduce the number of sub-adjustments during the processing of a batch of parts, it is necessary to choose the correct installation size. It is recommended to choose the installation size so that it is separated from the lower or upper limit of the tolerance field for 1/5 of the field. Closer to the lower boundary should be adjusted tools when machining exterior surfaces, and closer to the top when machining internal surfaces.

Error manufacturing tool. When shaped turning surface is formed by various points lying on the rounded part of the tool. Modern CNC systems allow programming the tool radius compensation. In the absence of such a possibility, it is necessary to take into account the radius of rounding at the tip of the tool when drawing up a machining program. It must be remembered that cutting tools are made with a certain permissible error, which must also be considered when programming processing.

Dimensional wear of the cutting tool. During processing, the cutting tool is subject to wear, which in turn affects the machining error. The criterion of wear is the size of the wear area on the rear face. Tool wear introduces a systematic error in the initial setup, i.e. the actual size of the machined surface is outside the tolerance field, after a certain time interval, an additional adjustment is required. The sub adjustment period depends on the intensity of tool wear. Correction (adjustment) for tool wear can be automatic or manual. During a manual override, the operator makes changes to the setup after a certain time interval, and during automatic correction of the size, the CNC system performs the program.

The rigidity of the AIDS system. Elastic deformation. As noted earlier, the AIDS system is an elastic system. The stiffness of an elastic system is understood as its ability to resist a deforming action. With insufficient rigidity under the action of cutting forces, the deformation of the AIDS system occurs, which causes errors in the shape and size of the machined surface. The errors associated with insufficient rigidity of the system are the higher, the higher the load (i.e., the greater the cutting forces). To reduce these errors, it is necessary to reduce the size of the metal layer removed in a single pass. It should be noted that CNC machines usually have cruelty 40-50% higher than universal equipment, which allows processing in fewer passes.

Thermal deformations and deformations from internal stresses of the workpiece. In the process of equipment operation, all elements and components of the machine are heated. These deformations are very significant, for example, heating a steel rod with a length of 1 m to 1 ° C leads to its elongation by 11 μm.

Thermal deformations proceed intensively in the initial period of the machine operation, after which the strain value stabilizes and does not affect further work. Changes occurring in the initial period can significantly affect the accuracy of processing, therefore, it is necessary to warm up the machine before machining parts. Also avoid prolonged equipment stops.

The heat generated in the cutting zone contributes to the heating of the workpiece, especially when multi-pass roughing at high cutting speeds. When this happens its deformation. In order to obtain high accuracy, it is necessary to ensure the cooling of the workpiece before finishing. For these purposes, processing with the use of coolant is used, and when processing several blanks (on multi-purpose) machines, a rational processing scheme is also used, under which the time delay for temperature stabilization is carried out. In addition, high-precision machines are installed in thermoconstant rooms.

Internal stresses, which are formed during uneven cooling of individual parts of the workpiece during their manufacture, are inherent in blanks. Over time, the internal stresses are equalized, and the workpiece is deformed. The deformation process is especially active after the removal of surface layers with the highest stresses. To reduce the impact of such deformations, it is necessary to separate rough and finishing deformations, and to obtain high-precision parts, natural or artificial aging should be performed between the roughing and finishing operations.

Coolant (coolant).

However, all these operations are carried out with the use of coolants. When cutting, coolant should have a lubricating, cooling and cleaning effect.

Under the lubricating action, we understand the ability of coolant to form strong surface-active films on the contact surfaces of the tool, on the chips and parts, which prevent the front surface of the tool from contact with the chips and the rear surface with the cutting surface.

Surfactants (stearic acid, sodium soap) form resistant films on the surfaces of the workpiece and tool. Getting into the microcracks from the free surface of the chip, these films produce a wedging effect, embrittling the material being processed and thereby reducing the specific work of cutting.

Films on the contact surfaces of the instrument, created by oils, as well as chemical compounds based on sulfur and chlorine, are able to withstand high contact pressures and not collapse at sufficiently high temperatures. Due to these films in the contact zone hydrodynamic friction takes place, which also leads to a decrease in heat generation at the contact sites.

The cooling effect of coolant is that, having greater thermal conductivity and heat resistance than the part, tool and chips, coolant allows redistributing heat fluxes in the cutting zone, reducing the cutting temperature. The heat exchange occurs the more intensely the colder the coolant and the higher the speed of its relative movement.

Under the washing action of coolant means the ability of liquids to remove wear products from the cutting surface and the contact surfaces of the tool. Detergent action is manifested in reducing abrasive tool wear and reducing the height of asperities in finishing operations.

In addition to the considered actions, certain substances that are part of the coolant can form with the tool material connections on the contact surfaces, which play the role of protective coatings and reduce the intensity of adhesion and diffusion. This reduces the wear of the cutting tool as a result of contact interactions.

The ability to varying degrees to perform these functions depends on the composition of the coolant. In addition, a number of technical and hygienic requirements are presented to coolant, without which coolant cannot be recommended for use.

 First of all, coolant should not contain substances that have a harmful effect on the human body, should not have an unpleasant odor and form a fog.

Coolant shall not cause corrosion of the material being processed, machine parts and tools. Coolant should be as stable as possible, i.e., over time and under the influence of various contaminants, not be subjected to chemical and bacteriological decomposition. It is desirable that the coolant was transparent. In addition, an important requirement is the possibility of subsequent disposal of coolant without harming the environment.

The fulfillment of all the above requirements is ensured by the introduction of appropriate anticorrosive, bactericidal, antifoaming and other additives into the coolant. Therefore, as a rule, the composition of modern coolant grades is very complicated, and the choice of the optimal grade of coolant for specific processing conditions is no less difficult than the choice of tool material.

All modern coolant can be divided into three groups.

The first group includes mineral oils of different viscosity. To improve lubricating properties, up to 30% of vegetable and animal fats are added to them, as well as up to 1.7% of sulfur, which contributes to the preservation of lubricating properties at high contact pressures and temperatures.

Oils as a coolant are used at relatively low cutting speeds and increased requirements for surface roughness (broaching, machining, thread cutting, shaped turning, etc.).

The second group includes oil emulsions ("water-soluble" oils). They are a complex system consisting of two insoluble liquids - water and oil. The oil is in the form of individual small droplets, to prevent the merging of which an emulsifier is injected into the liquid - a surfactant (water-soluble soap). The soap in the emulsion plays a mainly anti-corrosion role, and the friction on the contact surfaces is reduced by the films formed by the soap. The main advantage of emulsions is their high cooling capacity; therefore, emulsions are used in operations carried out at high cutting speeds under conditions of intense heat generation (rough turning, drilling, milling).

The third group of coolant consists of synthetic fluids that do not contain oils. They contain, as a rule, wetting agents, some types of soaps, antifriction and detergents. These types of coolant are less prone to decomposition than oils and emulsions, have enhanced cooling properties. However, as a rule, synthetic coolant systems are more expensive, and some of them lose their properties when oil enters them from the hydraulic system of the machine.

Lubricating and cooling process media are fed to the cutting zone to reduce friction, improve heat dissipation and chip removal,

Methods for supplying coolant to the cutting zone

The effectiveness of the influence of coolant on the cutting process depends not only on its properties, but also on the method of supplying coolant to the treatment area.

Watering with a free-falling stream is an old, versatile and reasonably reliable method for supplying coolant to the cutting zone (a). When cooling with a free falling jet, the liquid should be supplied with a continuous jet, starting from the moment of penetration, and the jet should be directed to the place where the chips are separated. This allows you to remove the most heat. The rate of fall of the jet is 60-80 m / min, and the flow rate depends on the type of treatment.

The advantages of the method are simplicity and reliability, and the disadvantages are the strong splashing of the liquid at high cutting speeds; inability to observe the place of treatment; high flow rate and its gradual heating.

When cooled through the internal channels of the instrument, a great effect on increasing durability is achieved. So, when machining high-strength steels, it is possible to increase the cutting speed by 25-40%. Durability of drills with internal coolant is increased by 3-10 times compared with conventional.

With this method of supply, the conditions for removing chips from the hole are improved. When using the coolant supply through the internal channels to the carbide plate of the turning tool, you can use a closed circulation system with a pump and a refrigerator, which significantly reduces the coolant flow. The same method is used for the supply of coolant to the grinding zone through the pores of the grinding wheel due to centrifugal forces from the hollow mandrel on which the wheel is fixed.

When using pressure jet cooling, the coolant is directed under the pressure of 1.5-2 MPa to the cutting edge of the tool from its rear surface. The distance from the hole to the cutting edge should be as small as possible to reduce jet dispersion. Due to the pressure of the fluid particles more intensively penetrate into the microcracks and the gaps of the contact zone.

This cooling method is especially effective for high-speed steel cutters. Their durability increases by 3-7 times in comparison with resistance when cooled with a free falling stream, and 10-20 times when cutting without cooling. The disadvantages of this method include the difficulty of ensuring the exact direction of the jet to the cutting zone, the difficulty of splash protection, the need to equip the machine with a special pump.

Liquid spray cooling. In this method of cooling, coolant is sprayed with compressed air and in the form of a mist at high speed (up to 300 m / s) is supplied to the cutting zone. The flow rate of the fluid is very small, and the tool life increases by 2–4 times as compared to cooling with a free falling stream. In addition, the exact direction of the jet to the treatment area is not required. The sprayed liquid is a transparent mixture of the smallest droplets of liquid with air. The sprayed liquid has an increased lubricating and cooling effect.

The mechanism of action of the sprayed coolant is as follows. At the nozzle exit, the mixture of liquid and air expands, therefore, its temperature decreases. The sprayed particles of the liquid, falling on the heated metal surface, easily evaporate, intensively absorbing additional heat. The sprayed liquid, having a lower viscosity, easier to penetrate into microcracks. Cooling and lubricating effect increases. The disadvantage of this method is too loud whistle flowing air stream.

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