High precision CNC. The influence of the rigidity of CNC lathes on the accuracy of parts. General information about CNC machines

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

Machine precision;

Precision control system;

Workpiece installation errors;

Errors in setting tools to size;

Errors in setting up the machine for size;

Tool manufacturing errors;

Dimensional wear of the cutting tool;

The rigidity of the AIDS system.

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

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

It should be noted that for CNC machines, the stability of the output of the working elements to a given point is often more important than the accuracy of the machine itself. To maintain the accuracy of the machine over a long period of operation, the standards for geometric accuracy in the manufacture of the machine are tightened by 40% compared to the standard ones, thereby reserving a margin for wear.

Precision control system. The accuracy of the control system is primarily associated with operation in interpolation mode - a mode in which the system controls several axes simultaneously. Deviations associated with the operation of the interpolator do not exceed the discrete price. For modern machines with a unit pulse price 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 operation of the interpolator, but appear in the interpolation mode, can be very significant. Their cause is a systematic error in the transmission of motion by feed drives. These errors occur in the kinematic chain of the feed drive motor – gearbox – lead screw– sensor. When moving along one axis, such errors manifest themselves in the form of uneven movement of the working bodies and have virtually no effect on the processing result. However, when moving along several axes, uneven movement even along one axis leads to processing errors in the form of waviness of the machined surface.

Errors in installation of workpieces. The installation error is determined by the sum of the basing and fastening errors. The basing error occurs due to misalignment of the installation base with the measuring base. On CNC machines it is possible to achieve higher precision when measuring bases and all surfaces whose dimensions are measured from these bases are processed in one installation.

When securing workpieces, it may be displaced under the action of clamping forces. The displacement of the workpiece from the position determined by the installation elements of the fixture occurs due to deformations of individual chain links: the workpiece, installation elements, and the body of the device. Due to the heterogeneity of surface quality and instability specific loads It is impossible to compensate for the resulting deformations using tool correction.

Errors in setting tools to size. When setting a tool to size outside the machine, errors occur, regardless of the accuracy of the device used. These deviations are determined by the error of the device itself and the error of securing the tool adjusted to the size. This error is compensated after a trial run.

Errors in setting up the machine for size. Setting up a machine for size involves the coordinated installation of a cutting tool adjusted for size, working elements of the machine and the base elements of the device in a position that, taking into account the phenomena occurring during the processing process, ensures obtaining the required size. An error in setting up a machine arises due to the fact that it is impossible to position the working elements of the machine and tools exactly in the calculated position. To ensure the required manufacturing accuracy, the installer uses trial runs. By adjusting the installation size we mean restoring the installation size that has changed due to dimensional wear of tools or thermal deformation of the system. In order to reduce the number of adjustments during the processing of a batch of parts, it is necessary to choose the correct installation size. It is recommended to select the installation size so that it is 1/5 of the field from the lower or upper limit of the tolerance field. Tools should be adjusted closer to the lower boundary when processing external surfaces, and closer to the upper boundary when processing internal surfaces.

Tool manufacturing errors. In form turning, the surface is formed by various points lying on the rounded part of the cutter. Modern CNC controls allow you to program tool radius compensation. If this is not possible, it is necessary to take into account the radius of curvature at the tip of the cutter when drawing up the processing program. It must be remembered that cutting tools are manufactured with a certain permissible error, which also must be taken into account when programming processing.

Dimensional wear of the cutting tool. During the processing process, the cutting tool is subject to wear, which in turn affects the processing error. The wear criterion is the size of the wear area along the rear edge. Tool wear introduces a systematic error into the initial adjustment, i.e. the actual size of the machined surface is outside the tolerance range; after a certain time interval, adjustment is required. The adjustment period depends on the wear rate of the tool. Correction (adjustment) for tool wear can be automatic or manual. With manual correction, the operator makes changes to the setup after a certain time interval, and with automatic size correction, the CNC system carries out the size correction according to the program.

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

Thermal deformations and deformations from internal stresses of the workpiece. During operation of the equipment, heating of all elements and components of the machine occurs. These deformations are very significant, for example, heating a steel rod 1 m long by 1º C leads to its elongation by 11 microns.

Thermal deformations occur intensively during the initial period of machine operation, after which the magnitude of the deformation stabilizes and does not affect further operation. Changes occurring in the initial period can significantly affect the accuracy of processing, so it is necessary to warm up the machine before starting to process parts. Long periods of equipment shutdown should also be avoided.

The heat generated in the cutting zone contributes to the heating of the workpiece, especially during multi-pass roughing on high speeds cutting In this case, its deformation occurs. In order to obtain high accuracy, it is necessary to ensure cooling of the workpiece before starting finishing processing. For these purposes, processing using coolant is used, and when processing several workpieces (on multi-purpose) machines, a rational processing scheme is also used, in which time is allowed to stabilize the temperature. In addition, high-precision machines are installed in temperature-controlled rooms.

Workpieces are characterized by internal stresses that are formed during uneven cooling of individual parts of the workpiece during their manufacture. Over time, internal stresses are equalized, and the workpiece is deformed. The deformation process is especially active after removing the surface layers that have the greatest stress. To reduce the impact of such deformations, roughing and finishing deformations should be separated, and to obtain high-precision parts, natural or artificial aging should be performed between roughing and finishing operations.

The accuracy of machine tools in an unloaded state is called geometric. Depending on the accuracy characteristics, CNC machines are divided in order of increasing accuracy into four classes: normal N; increased P; high B; especially high A.

Machine tools increased accuracy differs from machine tools normal accuracy mainly due to more precise execution or selection of parts, as well as individual features installation and operation at consumers. They provide processing accuracy on average within 0.6 deviations obtained on machines of normal accuracy. CNC machines high class B accuracy ensures processing accuracy within 0.4, and class A machines - within 0.25 deviations obtained on normal precision machines. Machines of classes B and A are obtained as a result of a special design, their components and elements, as well as high manufacturing precision.

When checking the accuracy standards of machine tools, they establish* the accuracy of geometric shapes and the relative position of the supporting surfaces basing the workpiece and the tool; accuracy of movements along the guides of the working parts of the machine; the accuracy of the location of the axes of rotation and the trajectories of movement of the working parts of the machine, carrying the workpiece and the tool, relative to each other and relative to the base surfaces; accuracy of processed sample surfaces; roughness of the processed surfaces of the sample.

Accuracy check

The accuracy of CNC machines is additionally revealed by the following specific checks: the accuracy of linear positioning of working bodies; the size of the dead zone, i.e. the lag in the displacement of the working bodies when changing the direction of movement; accuracy of return of working bodies to their original position; stability of output of working bodies to a given point; precision of working out a circle in the circular interpolation mode; stability of the position of the tools after automatic change.

During checks, both accuracy and stability are revealed, i.e., repeated repetition of the working bodies arriving in the same position, and stability is often more important for achieving precision processing on CNC machines than accuracy itself.

The total permissible error when positioning the working bodies is Δ р = Δ + δ.

Based on the permissible deviations, the largest error in working out the movement, for example, 300 mm long along the axes X And Y for a class P machine will be 17.2 microns, and for a class B machine - 8.6 microns.

To maintain the accuracy of the machine over a long period of operation, the standards of geometric accuracy for almost all checks during the manufacture of the machine, in comparison with the normative ones, are tightened by 40%. Thus, the manufacturer reserves a reserve for wear in the new machine.

FACTORS DETERMINING ACCURACY OF TURNING

CNC MACHINE

Associate Professor, V.V. TO DONOV, Assoc. Yu.V. NIKULIN

The article discusses the issues of forming the accuracy of lathes. Experimental methods are presented for assessing the rotation accuracy of a spindle assembly based on the parameters of its circular trajectories with and without the application of work loads to it; The issues of determining the accuracy of movement of the machine support and the influence of thermal deformations of the machine on its accuracy are discussed. A diagram of the measuring and testing installation and the results of measuring parameters characterizing the accuracy of lathes are presented.

Questions of precision quality shaping of lathes are examined in this article. Experimental methods of an exactitude estimation of a head slide rotation on parameters of its circular trajectories with and without the application of working loadings are presented. Also questions of running accuracy of a planning tool box, influence of thermal strains of the machine tool on its accuracy are discussed. The scheme of measuring and presetting station and results of measurements on parameters describing exactitude of

lathes are presented in conclusion.

Improving the quality of metal-cutting machines is one of the main problems of modern mechanical engineering. The technological process of cutting must guarantee the specified quality of manufacturing parts in accordance with established drawings and technological requirements. The most important component, the means of implementing the technological process - a metal-cutting machine - is a complex precision technological machine that forms the quality indicators of the parts processed on it. Quality level metal cutting machine is determined mainly by the requirements for the accuracy of the processed parts - accuracy of dimensions, shape, relative position, processed surfaces, roughness, waviness. Higher demands on machines arise during final processing, which shapes the rigidity parameters of the workpiece. In view of this, the rigidity indicators of a metal-cutting machine are the main indicators on the implementation of which the effectiveness of its use depends.

Testing of lathes for geometric and kinematic accuracy includes checking the accuracy of spindle rotation, the straightness of the guides, the straightness of the slides, the correctness of the mutual movement of machine components, the parallelism and perpendicularity of the guides and the spindle axis are assessed.

Testing machines for static rigidity involves measuring deformations under the working load of components lathe- spindle assembly and support. Dynamic processes in a machine during cutting are measured when testing the machine for vibration resistance, which has a direct impact on the shape accuracy of the machined part, the waviness and roughness of the machined surface.

ness. With increasing requirements for processing accuracy, thermal deformations play an increasingly increasing role in shaping processing accuracy.

The accuracy of processing on lathes is largely determined by the geometric accuracy of the machines, the geometric accuracy of the spindle unit (SHU), the drive

Yes, longitudinal and transverse feed, the load-bearing system of the machine, which mainly determines the accuracy of the relative position of the tool and the part during processing.

The accuracy of processing on lathes is determined by the complex influence of the subsystems, factors, and components included in the technological system of the machine (Fig. 1).

Rice. 1. Technological system of the machine

The accuracy of metal-cutting machines is determined by three groups of indicators: 1) indicators characterizing the accuracy of processing of product samples; 2) indicators characterizing the geometric accuracy of machine tools; 3) additional indicators.

The geometric accuracy of the machine is characterized by the following groups of indicators: the accuracy of the trajectories of movement of the working parts of the machine, carrying the workpiece and tool; accuracy of the location of the axis of rotation and the direction of linear movements of the working parts of the machine, carrying the workpiece and tool, relative to each other and relative to the bases; accuracy of the bases of the day of installation of the workpiece and tool; accuracy of coordinate movements (positioning) of the working parts of the machine carrying the workpiece and tool.

The geometric accuracy checks provided for in standards and specifications reflect the influence of machine accuracy on processing accuracy.

Clamping, rotation and processing of the product on a lathe are carried out using a spindle unit. The lathe is the main subsystem that largely determines the quality of processing: accuracy, surface cleanliness, waviness. Significant contribution Other subsystems and factors also contribute to the formation of processing quality: device errors, control unit errors, accuracy of machine feed drives, control and measurement systems, workpiece properties.

The maximum accuracy of processing diametrical dimensions on modern lathes is estimated at 0.5. L microns, therefore, when developing the main form-generating units of a lathe - SHU and longitudinal and transverse feed drives, very stringent requirements are imposed, since their geometric errors must be less than the total processing tolerance.

To experimentally determine the parameters and characteristics of the circular trajectories of the control unit, which determine the permissible rigidity of turning at the Department of Machine Tools and Automata of Moscow State Technical University named after. N.E. Bauman developed a measuring installation, the diagram of which is shown in Fig. 2.

Test setup diagram

Strain gauge amplifier

Digital voltmeter

X coordinate table

Y axis coordinate table

Trajectory

spindle axis

Rice. 2. Test setup diagram

The test setup diagram (information-measuring channel (IMC), circular trajectories (CT)) includes the following measuring instruments and equipment: sensors D1-D4 (primary contactless information converters of inductive type); strain gauge amplifier type UT4-1; analog-to-digital converter; personal computer for collecting experimental results, processing and displaying them on a graphic monitor, printing and plotting devices; hydraulic loading device (HLO), which serves to simulate cutting forces. The HPU consists of two mutually perpendicular loading hydraulic cylinders mounted on a common bracket in the support of the machine under test.

The testing and measuring installation contains two measurement channels: along the X coordinate and along the K coordinate. Main specifications testing and measuring installation:

range of measurement of noise control axis displacements for each channel, µm....................................20

range of rotation speed of the control cabinets at which the measurement is carried out,

rpm........................................................ ........................................................ ........................±6000

speed of primary converters, ms................................................... ..-0.003

maximum measurement error, µm................................................... ...............±0.5

The accuracy of spindle rotation at idle speed of the machine depends on the mathematical expectation and the standard deviation of the eccentricity values for each i-th spindle support from four types of errors: runout of the journal relative to its axes; runout of the raceway of the inner ring of the bearing relative to the mounting hole; runout of the bearing outer ring raceway relative to its outer surface; misalignment of the bearing mounting hole in the spindle head (quill).

deviations

runout of the spindle assembly of the STP-125 lathe gave the following results:

influencing the accuracy of a lathe is the total

kam cutting forces were set using GNU

Cutting force Ru

125 250 500 1000 2000

(the cabinet is uneven)

Axis 1 movement

Rice. 3. Dependency graphs

At MSTU. N.E. Bauman, at the Department of Metal-Cutting Machine Tools a stand was developed for measuring the circular trajectories (CT) of the spindle unit (SHU). The STP-125 machine was used as a test object. Trial tests of the SHU were carried out according to CT parameters,

Conducting preliminary tests. Test conditions. The tests were carried out on a machine heated up for 2-3 hours when turning the control unit manually, at idle speed with different speeds of rotation of the control unit, under a load created by a hydraulic loading device (HLD). In the latter case, we varied both the number of revolutions l and the magnitude of the load P (Fig. 3), radially loading a special mandrel inserted into the control unit. The radial displacements of the noise control were measured along the coordinates A" and Y using 4 inductive contactless converters operating at a carrier frequency of 5200 Hz. The signal from the inductive converters was sent to a four-channel strain amplifier, and then, after an ADC and a computer, to a plotter.

The results of preliminary tests are shown in Fig. 4-6. The tests were carried out at idle speed at n = 100. In Fig. 5 and 6 show typical trajectories of the control panel axis displayed on the computer screen.

The rotation accuracy of the spindle depends on the manufacturing accuracy of its parts, the accuracy of the bearings, the quality of its assembly and adjustment. Errors in spindle rotation are, first of all, determined by the difference in thickness of the bearing rings and different sizes -

Rice. 4. Spindle axis runout at idle speed

Fig. 5. Trajectory of the spindle axis

Rice. 6. Trajectory of the spindle axis

the rolling bodies. This error for bearings of small and medium sizes lies in the range of 1...10 microns (depending on the accuracy class and size of the bearing).

The waviness of the tracks and the geometric errors of the rolling elements cause smaller spindle displacements of the order of 0.1... 1 microns and are superimposed in the form of high-quality components on the errors from the difference in the thickness of the rings.

Even more high frequency and a smaller amplitude of spindle oscillations is caused by the roughness of the raceways. The addition of these oscillations causes a complex, complex picture of the movement of the spindle axis in space (Lissajous figures, movement of the spindle axis along a hypocycloid or epicycloid with a different number of loops).

The residual imbalance, which is defined in [N mm/N] or in the form of eccentricity e in [µm], which determines the actual displacement of the center of gravity of the spindle relative to the axis of rotation, has a great influence on the rotation accuracy of machine tool spindles, especially high-speed ones. The chuck mounted on the spindle must also be balanced.

It is not possible to display the test results at idle when turning the control unit by hand on a computer due to the peculiarities software COMPUTER. However, measurements of the radial runout of the noise control using sensors showed that its numerical value is in the range of 1.5-2.5 μm in both X and Y coordinates and is somewhat less than the corresponding radial runout when measuring the control noise at idle speed without load.

Tests of control unit runout without load at idle were carried out at different numbers SHU revolutions: n = 10, 30, 70, 100, 160, 220, 300, 450, 600, 800, 1000, 1300, 2000 rpm (Fig. 7),

100 " 200 " 300 " 400 500 600,~700 " 8СО 900 " 1000 " 1100 " 1200 " 1300

Fig. 7. Runout of the spindle assembly at idle speed without load at various rotation speeds

Tests have shown that with increasing speed of the control unit, the radial runout monotonically increases to n = 500-600 rpm, and then the rate of increase in the amplitude of the radial runout tends to increase slightly. The measurements were carried out with the cartridge on.

The spindle assembly is a complex mechanical system consisting of elastic elements of several types: bearings, shafts, flanges, bushings, springs, interconnected, influencing each other and forming a single technical device, in which complex processes occur, each of which can be described by its own mathematical model.

The most significant models: elastic-strain, dynamic, vibration, tribological, thermal, fatigue failure.

The inputs of these models are the design and technological factors of the design and manufacture of the spindle, and operating conditions. The output parameters of the models are rigidity, vibration, friction moment, speed, technical resource, heat resistance, fatigue life and other design parameters, which characterize, among other things, the geometric accuracy of the machine and the accuracy of processing the part on it.

When testing the control unit with the cartridge removed at a fixed rotation speed (n = 1000 1/min) and a load that was set by a hydraulic loading device, the circular trajectory of the control unit expanded somewhat along its average diameter (increase in Ax and Dn) and shifted in the direction of the load

%=№ - p; (Fig. 8)-

As a result of preliminary tests, the dependence of the noise amplitude oscillations on frequency (AFC*) was also determined. The studies were carried out using a special oscillation spectrum analyzer type SK4-72. The signal was received from displacement sensors at the input of the analyzer, and the frequency response of noise control oscillations was plotted at various frequencies of its rotation.

The amplitudes A and B of the frequency response approximately correspond in frequency to the vibrations of the noise control caused by fluctuations in rigidity caused by 18 rolling bearings of the front bearing assembly and vibrations of the toothed drive belt.

During machine operation, relative vibrations occur between the workpiece and the tool, causing certain processing errors. To reduce the level of these fluctuations and

increasing sustainability dynamic system of the machine, the vibration modes of the spindle assembly and support are constructed. The form of oscillations is characterized by a set of relationships between the movements of individual oscillating

points of an elastic system to the movement of any one point, taken at a certain point in time (taking into account the phase shift) to determine the frequency and direction of oscillations. The operating range of oscillation frequency is usually in the range from 10 to 500 Hz.

To improve measurement accuracy, it is advisable to use an excessive number of vibration measurement points. Vibrations are measured, as a rule, in 2-3 mutually perpendicular directions.

Rice. 8. Circular path of the spindle assembly under

load

The vibration shape is measured by vibrometers, which can operate in the modes of measuring vibration displacement, vibration velocity and vibration acceleration. The first mode is used in the low-frequency region (up to 200 Hz), the second is preferable for frequencies (100-400 Hz), the third is used for higher-frequency operating ranges of vibrometry.

The trajectory of any fixed point at the end of the spindle reflects, to a sufficiently large approximation, the cross-sectional shape of the workpiece. The degree of this approximation is determined, in addition, by the radial displacement of the tool mounted on the support during transverse feed and trajectory deviations

caliper against linear motion during longitudinal feed.

Data on the accuracy of the diametrical dimensions of the manufactured part were determined theoretically and tested experimentally (Fig. 9). It depends on the positioning accuracy D of the cross feed drive positions, i.e. from the deviation of the actual position of the drive X1 from that specified by the program X with multiple double-sided positioning

research, Using the methods of mathematical statistics when testing drives, X l and

Arithmetic average values of the actuator position when positioning in

average ar!

In addition, the root mean square deviation value of the actual position of the drive is determined.

X = (X p + X „)/2; For ■ - the size of the dispersion zone;

/ - ! X+X. | - dead zone that occurs when the drive reverses

cross feed (Fig. 9).

The maximum value measured on the machine turned out to be 5.5 microns. The actual error from D when processing a part will depend on the processing diameter.

to D pos., µm

Rice. 9. Graph of errors in bilateral positioning of the turret head of the STP-125 machine at

lateral movement

1. A testing and measuring installation has been developed and tested for measuring the parameters of circular trajectories of the spindle assembly of a CNC lathe.

2. As a result of testing the STP-125 lathe, the results of the influence of external disturbing influences (cutting forces, spindle displacement) on the parameters of the circular trajectories of the spindle assembly were obtained.

3. The influence of positioning errors of the transverse slide on the processing accuracy was assessed.

4. Shows ways and possibilities for diagnosing the spindle assembly and support group of a CNC lathe

BIBLIOGRAPHY

1. VDI Richtlinien 2060, “Standards for balancing rotating rigid bodies”. -1980.

2. GOST 8-82E, “Methods for sweeping and cutting. General requirements for accuracy testing." - M.: Publishing House of Standards, 1982. - 10 p.

3. Pronikov A. S. Software method testing of metal-cutting machines. - M.: Mechanical Engineering, 1985. - 288 p.

4. Adaptive control of machine tools. / Ed. Balakshina. - M.: Mechanical Engineering, 1973. - 688 p.

5. Designs and program tests of spindle units of metal-cutting machines / L.I. Vereina, V.V., Dodonov. - M.: VNIITEMR, 1991. - Issue. 1.

6. Figatner A.M. Calculation and design of spindle units with rolling bearings for metal-cutting machines. - M.: NIIMASH, series S-1, 1971.

7. Calculation of high-speed spindle units / V.B. Balmont. - M.: VNIITEMR, 1987. - Ser. I. - Vol. 1. - 52 s.

Output parameters of the machine in terms of accuracy

When assessing the quality and technical level of a machine, it is first necessary to establish those output parameters that characterize its accuracy. At the same time, the accuracy of machine-processed parts cannot be selected as such a parameter, since it is the result of the influence of all components of the technological system (tool, workpiece, etc.). Therefore, when designing a machine, it is necessary to establish and regulate those parameters that determine the accuracy of processing and are input to the technological system (see Fig. 2.1).

The quality of the machine depends on the degree of accuracy with which the mutual movements of the tool and the workpiece inherent in the technological processing process are carried out when the machine is exposed to the entire complex of force and thermal factors. Therefore, the main output parameters of the machine as an element of the technological system are the characteristics of the accuracy of the movement of its form-generating units.

You can obtain these characteristics in one of the following ways.

1. Evaluate those parameters of the trajectories of the forming units of the machine that affect the processing accuracy. In this case, the trajectories refer to the installation bases of the machine, which determine the position of the fixture, workpiece or tool.

2. Assess the total influence of the parameters of the trajectories of the working parts of the machine on the formation of the so-called “geometric image” of the processed part, when its errors are determined without taking into account the impact on the accuracy of other components of the technological system.

The main goal of regulating the output parameters of a machine is to create such technological equipment, the operating error of which would be within the limits established by the technologist during the entire period of operation.

The trajectories of forming nodes, the parameters of which are set as output, belong to specially selected reference points, which are placed on the machine’s mounting bases, which determine the position of the workpiece, fixture or tool. The number of support points and their location is associated with the processing method, the design of the machine, the nature of the movement of its form-forming organs and the method of fastening the workpiece and tool.

Since the position of a rigid body in space is determined by three fixed points or parameters of a spatial vector related to one point, then in general view it is necessary to set six coordinates (for example, three linear and three angular deviations of the vector of a given point from a given position). However, when considering various designs forming units of the machine, the number of these characteristics can be reduced if individual deviations do not have a significant impact (second-order terms of smallness) on the processing accuracy.

Rice. 2.3. Reference points of the forming units of the machine:
A- caliper; b- table; V- spindle

In Fig. Figure 2.3 shows typical cases of selecting control points. To characterize the accuracy parameters of a lathe support, one reference point 1, coinciding with the tip of the cutter, is sufficient (Fig. 2.3, A), since the goal when creating a support design is to strive to provide a straight path for the tool, which does not change its shape and position under force influences and various positions of the tool in the workspace. The trajectory of this reference point will serve as a characteristic of the support’s capabilities for processing a given range of parts, ensuring the accuracy of size, shape of the machined surface, waviness, roughness and other accuracy indicators.

When the table moves with a workpiece attached to it (Fig. 2.3, b) For milling, boring, grinding and other machines, it is necessary to evaluate the accuracy of table movement in space. The position of the workpiece or the device for securing it is determined by the position in space of the table plane. Therefore in general case either three reference points must be installed 1 , 2, 3, the trajectories of movement of which are considered, or a vector is considered for one of the points of the table with the characteristics of its position in space at each point of the trajectory (three linear and three angular deviations from a given position during spatial movement of the table).

For the spindle assembly (Fig. 2.3, V) the accuracy of its rotation and the change in the position of the spindle axis are associated with the geometric error of the assembly elements, with force and thermal deformations. All this affects the position of the tool or workpiece installed in the spindle using a fixture (chuck, center).

When the position of the chuck determines the plane of the front end of the spindle, three fixed points are located on this plane or, more appropriately, a position in vector space is determined for the point located in the center of the spindle R, perpendicular to the plane of the installation base. The characteristics of the trajectories of the reference points of the forming units determine the quality of the machine from the standpoint of the possible achievement of processing accuracy and its contribution to the total processing error.

Rice. 2.4. Typical ensembles of trajectories during translational movement of the working body of the machine

When performing various technological processes(in accordance with its purpose and degree of universality) trajectories of reference points appear as random functions and form sets (ensembles) of trajectories. Such aggregates may have different kind, characterizing the statistical nature of phenomena (for example, with strong or weak mixing of implementations or with other features). In Fig. Figure 2.4 shows typical sets of trajectories during the translational movement of the working parts of the machine (supports, tables, sliders, etc.).

Broadband ensembles of trajectories (Fig. 2.4, A) are typical for the case when the main influence on the shape of the trajectory and its displacement relative to the center line or to the fixed coordinate axis is exerted by external force influences. Narrowband ensembles of trajectories (Fig. 2.4, b) characteristic of the prevailing influence of the geometric error of the guides, which determines the shape of the curve of the mathematical expectation of trajectories MX. The dispersion associated with force effects on the node plays a secondary role here. Migration of sets of trajectories (Fig. 2.4, V) caused, as a rule, by thermal deformations of the unit.

Each implementation of any ensemble is associated with the accuracy parameters of the specific part that was processed, and the characteristics of the entire ensemble affect the accuracy characteristics of the batch of parts processed on the machine. Therefore, for each specific machine model, depending on its purpose, it is necessary to establish and regulate those trajectory parameters that determine certain types of errors that occur on machined surfaces.

As is known, processing errors are divided into five main types: dimensional errors, deviations in surface location, deviations in shape, deviations in the parameters of waviness and surface roughness.

When assigning a range of parameters for the trajectories of the working parts of the machine, their relationship with the processing error, which depends on the processing method and the kinematics of the shaping process, is taken into account.

In Fig. Figure 2.5 shows typical trajectories during the translational movement of the forming unit of the machine. Their parameters (X 1, X 2, ..., X p), determining the corresponding processing error are given in table. 2.2. These parameters are related to the size and shape of the processed surface, the accuracy of the relative position of the surfaces, the waviness and roughness of the surface.

2.2. Output parameters of the machine in terms of accuracy

Rotational motion is characterized by the transmission of errors in the trajectory of the spindle reference point (its shape and high-frequency components) to the machined surface cylindrical part(Fig. 2.6).

For periodic curves, the expansion of the trajectory into a Fourier series allows us to identify those parameters that determine the shape, waviness and roughness of machined surfaces during turning, boring, grinding and other operations.

It is advisable to analyze trajectories by considering the deviation of the current radius R from the nominal R0 in the polar coordinate system, and determine

where f (φ) is the trajectory error as a function of the current angle φ.

Let's decompose this function in a Fourier series with a limited number of terms:

where Сk is the amplitude of the k-harmonic; φ - initial phase; n is the ordinal number of the highest harmonic of the polynomial. According to Fourier theory, the zero term of the Co expansion is the average value of the function f(φ) over a period of 2π:

therefore Co determines the size error value.

Rice. 2.5. Typical types of implementations of trajectories in translational motion

The first term of the expansion C1cos(φ+φ) expresses the discrepancy between the center of rotation of the spindle in O" and the geometric center of the trajectories O, i.e. eccentricity e = OO", which determines the error in the deviation of the location of the machined cylindrical surfaces (Fig. 2.6, b). The remaining members of the series, starting from the second, determine the characteristics of the shape that the trajectories form and which is directly related to the shape of the processed part (ovality and cut).

Rice. 2.6. Processed cross-sectional shape cylindrical surface(a) and the trajectory of the spindle reference point (b):
1- surface shape; 2 - waviness; 3 - roughness; R d - nominal radius of the machined part

When choosing a range of output parameters for a given machine model and establishing their permissible values, the following must be taken into account.

1. The higher the accuracy class of the machine and the requirements for the accuracy of machined surfaces, the greater the number of output parameters (characteristics of the trajectories of the forming units) of the machine.

2. The permissible values of the output parameters of the machine form part of the corresponding tolerance for the manufacture of the part, since the processing error depends on all components of the technological system.

3. Calculation of the share of the total error attributable to the machine and other components of the technological system is carried out by methods used in mechanical engineering technology to calculate processing accuracy

As a first approximation, we can take the permissible value for the output parameter of the machine as a fraction of the corresponding tolerance for the accuracy of manufacturing the part, equal to 6 = 0.4...0.8, taking into account the degree of influence of other components of the technological system and giving a margin for a possible change in the machine parameters in during operation.

For precision machines, the value of k is taken to be large, since in this case the machine plays the main role in ensuring processing accuracy.

Accuracy is the main indicator of a machine, however, to assess its technical level and fully characterize its quality, it is necessary to use indicators that define the entire range of requirements placed on the machine by the consumer.

This sophisticated equipment produces all kinds of parts from metal, plexiglass, acrylic or plastic, and wood. Their versatility lies in the fact that they are well suited for transverse planing, the formation of the most complex surfaces, in particular curved ones; perform sampling of tongues, tongues, folds, grooves, splines and mouldings.

Description of the machine

The standard equipment of the machine includes:

heavy and powerful base;
Desktop;
, with the simultaneous presence of a spindle shaft;
a set of several tools for cutting materials;
front disc brake.

The design of machine tools today includes many important devices that ensure precision processing and ease of use. It is important to know about them so that the choice of a CNC milling machine is meaningful and correct.

Don't ignore the spindle!

One of the important qualities in the operation of the spindle shaft electric motor is the ability to rotate it smoothly and evenly. When assembling, bearings of the highest accuracy class are selected, and the collet must have increased tolerances for runout and size.

There are main types of spindle cooling systems:

Liquid (it is based on the circulation of water or antifreeze in a closed circuit). One of the advantages is reliable heat dissipation. Among the disadvantages is the complex design, because the coolant must be placed in a reservoir.
Air (such cooling consists of pumping air through air intake slots in the spindle cavity). Among the advantages of the system are compactness and simplicity. There is also a downside - filters, especially for equipment processing solid wood, need to be changed frequently; they become contaminated with dust.

When choosing a spindle for a CNC machine, you should pay attention to the following: technical passport its indicators (power and rotation speed during milling), depending on how hard the materials are processed. For example, for sheet plywood the required power for processing is 800 W; a more powerful machine works on solid hardwood, light metals - copper, brass and aluminum, plastic - 1500 W; and the stone is processed at a power of 3000 - 4000 W.

Now in equipment for milling work, imported spindles are mainly used:

Italian – high quality, working with high speed, with smooth rotation and low runout, mainly air-cooled and high price.
The Chinese one has a solid cylindrical body, which is closed at the ends with covers, and bearing units are used to hold the shafts. Among the advantages - the design has a sufficient level of rigidity and minimal vibration, insensitivity to the presence of chips and dust, and affordability. Spindle models made in China, unfortunately, have a high probability of defects; it can be difficult to replace the bearings. And models with water cooling have weak anti-corrosion resistance of internal parts.

Types of Milling Machines

When choosing such equipment, one must proceed from the extent to which it corresponds to its intended purpose. Russians have a choice:

high-speed CNC automatic machines that cut and cut metals, process parts made of cardboard and wood, cope with two-layer plastic and acrylic, PVC, plexiglass and plaster, natural stone - granite and marble;
models (milling and engraving) working with sheets (maximum dimensions 2000 x 4000 x 200 mm);
engravers (from 2D modeling to 4D);
narrow-profile machines that work with one particular material - varieties of stone, plywood, wood, stainless steel or aluminum;
small portable CNC models. For example, a model of a milling machine with "Desktop 3D" is used for milling printed circuit boards, MDF and processes products with extreme precision.

In the line of equipment of the series for professionals, you can give preference to vertical and horizontal machining centers with program control; large three-, four- and five-coordinate CNC milling engravers who produce in Taiwan.

They are considered quite reliable and can be bought (after Germany and Japan - in third position). In addition, they are profitable to purchase for both individuals and enterprises, thanks to their availability in Moscow and Tula service centers involved in the supply of equipment, cutting tools, equipment adjustment and personnel training.

ATTENTION: It is not difficult to distinguish a machine from Taiwan: it has a solid cast bed (made from Brazilian fine-grained cast iron). In addition, the machine is equipped with American or Japanese bearings and imported spindles.

And if the customer is looking for a high-precision jewelry machine, best model for this purpose – P 0403 from the manufacturer Vector.

Furniture equipment

Woodworking and furniture production, workshops producing windows, doors and facades will not be able to function without equipment with wide functionality - CNC wood cutting machines.

In recent years, retro-style furniture has become fashionable - with elegant carved armrests, legs and other details. In this case, the technology of automated cutting of the pattern is used on a milling machine on which a numerical control is installed. It provides high precision and quality when performing complex milling wood and a carved element is created.

With the help of such equipment, it is possible to establish production of:

wooden furniture facades and decorative consoles;
balusters, curly legs and slotted elements;
embedded carved parts;
symbols, figurines, figurines and frames of various shapes for paintings and mirrors.

Those on a budget might want to buy an inexpensive Chinese standard engraving machine. milling machine CNC – SS-M1, especially for . When making facades, engraving decor and bas-relief, there is usually a lot of dust. Therefore, choose the configuration that has vacuum aspiration for dust absorption. This model has it.

Which milling machines are better? No one will give a definite answer. But there is still more trust in software working equipment. Each master has his own approach to choosing the right equipment.

And a good CNC milling machine is one that has higher accuracy, lower energy consumption, greater ease of use, and reliability in any working situation.

Three tips for making the right choice can be formulated:

Check with company managers in advance for all information about the model; materials the machine works with. If there is a video, watch it. This will help you decide.
Consult before purchasing regarding the functionality of the equipment and the range of tasks performed. A the best option– sign up for a demonstration of the operation of a CNC machine and do not hesitate to ask questions during operation.
When the desired model is selected, be careful at the time of purchase: check the purchased equipment for complete components. There must be a program control unit for the machine; cords with connectors of the appropriate configuration, and disks with software. Typically, the software is installed by specialists from the company that sells the machine during its setup.

Conclusion

Basically, we tried to help a person facing a choice. We figured out how to choose a milling machine (an expensive thing, and it will work for the owner for many years - with metal or wood). At least now there is plenty to choose from. It is hoped that readers will use this information to purchase a work tool.